Nuclear magnetic resonance proton nmr


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Nuclear magnetic resonance proton nmr

  1. 1. Nuclear Magnetic ResonanceSpectroscopy - PMR 1
  2. 2. 2
  3. 3. CONTENTS Introduction Nuclear spin and magnetic movement Theory and principle Applied field and precession Precessional frequency Width of absorption line in NMR Shielding and DeshieldingReference standardJanuary 26, 2013 M.M.C.P. 3
  4. 4.  Chemical shift  Factor affecting chemical shift  Interpretation of PMR  Instrumentation of NMR  Splitting of the signals  Spin-spin coupling  Intensities of Multiplet Peaks  Spin DecouplingJanuary 26, 2013 M.M.C.P. 4
  5. 5. INTRODUCTIONNMR spectroscopy is different from the interaction of electromagnetic radiation with matter.In this spectroscopy the sample is subjected simultaneously with two magnetic field, One is a stationary and another is varying at same radio frequency.The particular combination of these two field energy is absorbed by sample and signal is obtained when electromagnetic field is provided to the nucleus of sample. The nucleus start to spin around the nuclear axies and generate an another magnetic field. And particular combination of this two field the energy is absorbed by nucleus this technique is called as a NMR spectroscopy. January 26, 2013 M.M.C.P. 5
  6. 6.  This transition of nucleus occurs in radio frequency region. The radio waves are considered for lowest energy and this energy is just sufficient to affect the nuclear spin of the atom in a molecule. Hence, this is a most fundamental part of NMR spectroscopy. In general, the study of radio frequency radiation by nuclei is called nuclear magnetic resonance. The method of NMR was first developed by E.M. Purcell and Felix Bloch (1946). January 26, 2013 M.M.C.P. 6
  7. 7. In synthetic organic and organometallic chemistry, solution-state NMR means a 300-500 MHz NMR spectrometer, high-precision glass sample tubes, 2 ml of deuterated solvent (typically fully deuterated chloroform, acetone, benzene, or dichlorobenzene), several milligrams of pure sample, and a reference substance, NMR experiments with several hours of spectrometer time and data interpretation.The structures of new compounds with molecular weights up to 2000 Da can be determined, especially when analyzed along with results from NMR databases and mass spectroscopy. January 26, 2013 M.M.C.P. 7
  8. 8.  It is a well known fact that spectra given by all forms of spectroscopy may be described in terms of the following three important factors. 1. Frequency of spectral lines or bands. 2. Intensity of spectral lines or bands. 3. Shape of spectral lines or bands.All above properties depends on the molecular parameters of thesystem. In case of the NMR these molecular parameters are foundto be: 1. Shielding constant of nuclei. 2. Coupling constant of nuclei. 3. Lifetime of energy level. January 26, 2013 M.M.C.P. 8
  9. 9. NUCLEAR SPIN AND MAGNETIC MOMENT Nucleus axis Nucleus Magnetic field Fig: Spinning of NucleusJanuary 26, 2013 M.M.C.P. 9
  10. 10. All nuclei carry a charge. In some nuclei this charge spins on the nuclear axis and this circulation of nuclear charge generates a magnetic dipole along the axies.The nuclei of atoms are composed of protons and neutrons. Like electrons, these particle also have the properties to spin on their own axis and each of them possesses angular momentum1/2(h/2π) in accordance with the quantum theory. The net resultant of the angular momentum of all nuclear particles is called nuclear spin.For a nucleus having a spin quantum number I, these are(2I +1) spin states. January 26, 2013 M.M.C.P. 10
  11. 11. Two properties of nuclear particles which are important in understanding of NMR spectroscopy are: • The net spin associated with the proton and neutron. • The distribution of positive charge.The net spin number or spin quantum number I of a particular nucleus can be obtained by adding spin numbers of individual proton and neutron of ½ each, assuming that neutrons cancel only neutrons and protons cancel only protons, because of pairing or spinning in opposite directions.The spin number I have values 0,1/2, 1, 3/2, 5/2 and so forth. If I=0 that represent no spin. January 26, 2013 M.M.C.P. 11
  12. 12. PRINCIPLE FOR NUCLEAR SPINIf the sum of protons and neutrons is even, I is zero or integral (0,1,2,3 …..)If the sum of proton and neutrons is odd, I is a half integral (1/2, 3/2, 5/2….)If the both protons and neutrons are even numbered, I is zero. January 26, 2013 M.M.C.P. 12
  13. 13. 35 Cl, 17 16 O 17January 26, 2013 M.M.C.P. 13
  14. 14. THEORY AND PRINCIPLE•The NMR is mostly consult with nucleus spin quantum no. (I)= ½ . The proton having a I = ½ when place in external magnetic field (Ho) it’s start to spin around the nuclear axis and generate a another magnetic field.•According to quantum mechanics there are 2I + 1 so two spin stage + ½ and - ½ for the proton. E I=-½ E2 I=½ I=+½ E1 Ho January 26, 2013 Spin state of proton M.M.C.P. 14
  15. 15. • When a charge particle place in magnetic field. It’s start to revel and therefore it’s pusses angular movement due to generation of another magnetic field. The charge particle with nucleus spin has magnitude and direction. Both this property is describe by the factor called as magnetic movement (µ).•So, when the proton take place in magnetic field . It has two spin steps + ½ and - ½ so, there are two energy level for spin steps + ½ & -½. E1 = + ½ µ Ho ………………….1 E2 = - ½ µ Ho ……………….….2 where, Ho = magnetic field strength. µ = magnetic movement ΔE = E1 – E2 …………………....3 January 26, 2013 M.M.C.P. 15
  16. 16. ΔE = µ Ho …………………………4 by Boher’s frequency eq. we can write ΔE = hv ……………………….5 v= frequency from the eq. 4 &5 hv = µ Ho ………………………… 6 so, µ Ho ………………………….7 v= hThis is a basic eq. in NMR spectroscopy. 1.41 Tesla = 60 MHz 2.35 Tesla = 100 MHz 7.05 Tesla = 300 MHz January 26, 2013 M.M.C.P. 16
  17. 17. APPLIED FIELD AND PRECESSION Spinning nuclei-magnetic moments Some elements have isotopes with nuclei that behave as though they were spinning about an axis much like the earth. The spinning of charge particle generates a magnetic field. As a consequence, the spinning nuclei behave as though they were tiny bar magnets having a north and a south pole. α ᵦ ᵦ αα ᵦ α Applied magnetic field (Ho) Nuclear magnetic movement with No magnetic field January 26, 2013 M.M.C.P. Ho 17
  18. 18. •Since a nucleus or an electron bears a charge, its spin gives rise to a magnetic field that is analogous to the field produce when an electric current is passed through a coil of wire. The resulting magnetic dipole (µ) is oriented along the axis of spin and has a value that is characteristic for each kind of particle. Applied field Ho E= µᵦHo No I= - ½ field energy 0 ∆E = µᵦHo I= + ½ E= µᵦHo 18 January 26, 2013 M.M.C.P.
  19. 19. PRECESSIONPRECESSIONAL MOTION•Because the proton is behaving as a spinning magnet, it can align itself either with or opposed to an external magnetic field. It canalso move a characteristic way under the influence of the external magnet.•Considering the behavior of a spinning top, the top has a spinning motion around its axis. It also performs a slower waltz like motion in which the spinning axis of top moves slowly around the vertical. Thus is called “precessional motion” and the top is said to be precessing around the vertical axis of the gravitational force of the earth. January 26, 2013 M.M.C.P. 19
  20. 20. •Precession arises due to the interaction of spin and the gravitational force acting downwards. This is the reason why only a spinning top will precess; where as a static top will topple over.•Since the proton is a spinning magnet, it will precess around the axis of an applied external magnetic field. It will precess in two main orientations.•Aligned or parallel with the field-low energy.•Opposed or anti parallel to the field-high energy. January 26, 2013 M.M.C.P. 20
  21. 21. PRECESSIONAL FREQUENCYThe precessional frequency of the nucleus is directly proportional to the strength of external field and also depends on the nature of the nuclear magnet.Magnetic nuclei different atoms have different characteristic precessional frequency. according to Larmor precessional theory ω = γH0 ………………1 where, ω= Larmor precessional frequency. ω = 2 πV ………………2 2 πV = γH0 ………………..3 γ H0 V= 2π ………………….4 V α H0 ..................................5 Intrinsic magnetic dipole momentum Where, γ = is gyromagnetic ratio = Spin angular momentum M.M.C.P. 21 January 26, 2013
  22. 22. ENERGY TRANSITIONS A proton when kept in an external magnetic field will precess and can take one of the two orientations with respect to the axisof the external field. Either aligned or opposed.If a proton is precessing in the aligned orientation it can absorbed energy and pass into the opposed orientation and vice versa by losing energy.If we irradiate the precessing nuclei with a beam of radio frequency, the low energy nuclei may absorb this energy andmove to a higher energy state. January 26, 2013 M.M.C.P. 22
  23. 23. The precessing proton will absorb energy from the radio frequency source, if the precessing frequency same as the frequency of the radio frequency beam.When this occurs, the nucleus and radio frequency beam are said to be resonance, hence the term “ nuclear magnetic resonance”. January 26, 2013 M.M.C.P. 23
  24. 24. WIDTH OF ABSORPTION LINES IN NMRThe separation of two absorption lines depends on how close they are to each other and the absorption line width.The width of the absorption line is affected by a number of factors, only some of which we can control.These are the factors:I.The homogeneous fieldII.Relaxation timeIII.Magic angle NMRIV.Other source of line broadening January 26, 2013 M.M.C.P. 24
  25. 25. 1. THE HOMOGENEOUS FIELDThe most important factor controlling the absorption line width is the applied magnetic field H0.It is very important that this field be constant over all parts of the sample, which may be 1-2 inch long. If it is not, H0 is different parts of the sample and therefore v, the frequency of the absorbed radiation, will vary in different parts of sample.This variation results in a wide absorption line. For qualitative or quantitative analysis a wide absorption line is very undesirable, since we may get overlap between neighbouring peaks. January 26, 2013 M.M.C.P. 25
  26. 26. 2. RELAXATION TIMEThe second important feature that influences the absorption line width is the length of time that an excited nucleus stays in the exited state. ΔE Δt = constant where, ΔE is the uncertain in the value of E Δt is the length of time a nucleus of time a nucleus spends in the excited state.Since ΔE Δt is a constant, when Δt is small, ΔE is large. But we know that E = hv and that h is constant.Therefore any variation in E will result in a variation in v. If E is not an exact number but varies over the range E+ ΔE, then v will not be exact but vary over the corresponding range v + Δv. Then we have E + ΔE = h(v + Δv) January 26, 2013 M.M.C.P. 26
  27. 27. We can summarize this relationship by saying that when Δt is small, ΔE is large and therefore Δv is large. If Δv is large, then the frequency range over which absorption takes place is wide and a wide absorption line results.There are two principle modes of relaxation, 1. Longitudinal relaxation /spin lattice 2. Transverse relaxation /spin spin January 26, 2013 M.M.C.P. 27
  28. 28. A. LONGITUDINAL RELAXATIONWhen the nucleus loses its excitation energy to the surrounding molecules, the system becomes warm as the energy is changed to heat.This process is quite fast when the molecules are able to move quickly. This is the state of affairs in liquid.The excitation energy becomes dispersed throughout the whole system of molecules in which the sample finds it self. No radiation energy appears, no other nuclei become exited. Instead, as numerous nuclei lose their energy in this fashion, the temperature of the sample goes up. This process is called longitudinalrelaxation T1. January 26, 2013 M.M.C.P. 28
  29. 29. B. TRANSVERSE RELAXATIONAn excited nucleus may transfer its energy to an unexcited nucleus of a similar molecules that is nearby.In this process, the nearby unexcited nucleus becomes excited a the previously excited nucleus become unexcited.There is no net change in energy of the system, but the length of the time that one nucleus stays excited is shortened. This process, which is called transverse relaxation T2. January 26, 2013 M.M.C.P. 29
  30. 30. 3. MAGIC ANGLE NMRA problem with the examination of solids that the nuclei can be frozen in space and cannot freely line up in magnetic field.The NMR signals generated are dependent among other things, on the orientation of the nuclei. The randomly oriented nuclei therefore give broad band spectra which are not very useful analytically.It can be shown that when one rotates a solid sample such that its axis of rotation 54.70 to the direction of the applied magnetic field, the broadening caused by random nuclear orientations tends to be average out, resulting in narrower spectra. January 26, 2013 M.M.C.P. 30
  31. 31.  This is more useful analytically because it allow better resolution and therefor better measurement of chemical shift and spin-spin splitting. In turn, this is very informative of the functional group and their positions relative to each other in the sample molecule. January 26, 2013 M.M.C.P. 31
  32. 32. 4. OTHER SOURCES OF LINE BROADENINGAny process of deactivating, or relaxing, an excited molecule results in a decrease in a lifetime of the excited state. This is turn causes line broadening.Other causes of deactivation include 1. The presence of ions 2. Paramagnetic molecules 3. Nuclei with quadrupole moment January 26, 2013 M.M.C.P. 32
  33. 33. SHIELDING AND DESHIELDING1.INDUCED MAGNETIC FIELD :-In the applied magnetic field, the valence electrons around thenucleus are cause to circulates and they generates their ownsecondary magnetic field is known as induced magnetic field.2.SHIELDING:-The circulation of electron around the protons itself generates fieldin a such way that , it oppose the applied field.The field felt by the protons is thus diminished and the proton issaid to be shielded and the absorption said to be upfield. January 26, 2013 M.M.C.P. 33
  34. 34. DESHIELDING:- DESHIELDINGIf the induced magnetic field reinforced the applied magneticfield ,then the field felt by the proton is augmented and the protonis said to be deshielded and the absorption is known as downfield.January 26, 2013 M.M.C.P. 34
  35. 35.  Compare with naked proton, a shielded proton required higher applied field strength than the deshielded protons.Shifts in the position of NMR absorption arising fromshielding and deshielding by electron, due to different chemicalenvironments around protons are called chemical shift.Generally chemical shift measured from the signal ofreference standard such as TMSJanuary 26, 2013 M.M.C.P. 35
  36. 36. The extent of shielding is represented in terms of shielding parameter α. When absorption occurs, the field H felt by the proton is represented as, H = H0 (1 - α )………………. 1 where, H0 = applied field strength.Greater value of α, greater will be the value of applied strength which has to be applied to get the effective field required for absorption and vice versa. ………………. 2 From 1 and 2 H0 January 26, 2013 M.M.C.P. 36
  37. 37. It is clear that the proton with different electronic environments or with shielding parameter can be brought into resonance in two ways 1. The strength of external field is kept steady and the radio frequency is constantly varied 2. The radio frequency is kept steady and strength of the applied field is constantly varied.Clearly at constant radio frequency, shielding shift the absorption upfield in the molecules where these is a spherical distribution of electrons around the proton, It is called positive shielding.January 26, 2013 M.M.C.P. 37
  38. 38. January 26, 2013 38 M.M.C.P.
  39. 39. REFERENCE STANDARDS CHARACTERISTICS:-  Chemical inertness  Magnetically neutral  Gives single sharp peak  Easily recognizable peak  Miscible with wide range of solvents  Volatility –to facilitate recovery from valuable samplesJanuary 26, 2013 M.M.C.P. 39
  40. 40. 1.TMS(Tetra Methyl Silan):- It is generally used as internal standard for measuring the position of 1H,13C, 29Si in NMR TMS at 0.5% concentration is used normally TMS has 12 protons which are uniformly shielded because of highly electro-positive nature of silicon at centre Hence this 12 protons gives single sharp peak at oδ which require maximum magnetic field than protons of the most organic compounds It is chemically inert and miscible with large range of solvents Highly volatile and easily removed to get back the sampleJanuary 26, 2013 M.M.C.P. 40
  41. 41. It does not take part in intermolecular association with the sample It’s all protons are magnetically equivalent TMS can be used as an external reference also.January 26, 2013 M.M.C.P. 41
  42. 42. 2.Sodium salts of 3-(trimethyl silyl)propane sulphonate:- It is a water soluble compound. It is used as internal standard for running PMR spectra of water soluble substances in the duterium oxide solvent.January 26, 2013 M.M.C.P. 42
  43. 43. CHEMICAL SHIFT“Chemical shift is the difference between the absorption positionof the sample proton and the absorption position of referencestandard” Variations of the positions of NMR absorptions due to the electronic shielding and deshielding.January 26, 2013 M.M.C.P. 43
  44. 44. Chemical Shifts…. • Measured in parts per million (ppm). • It is the ratio of shift downfield from TMS (Hz) to total spectrometer frequency (MHz). • The chemical shift is independent of the operating frequency of the spectrometer. • Same value for 60, 100, or 300 MHz machine. • Common scale used is the delta (δ) scale.January 26, 2013 M.M.C.P. 44
  45. 45. MEASUREMENT OF CHEMICAL SHIFTEach proton in a molecule has slightly different chemical environment and consequently has a slightly different amount of electronic shielding, which results in a slightly different resonance frequency. These differences in resonance frequency are very small.For example, the difference between the resonance frequencies of the protons in Chloromethane and in Fluromethane is only 60 MHz when the applied field is 1.41 Tesla.January 26, 2013 M.M.C.P. 45
  46. 46. Since the radiation used to induce proton spin transitions at that magnetic field strength is of a frequency near 60 MHz, the difference between Chloromethane and Fluoromethane represents a change in frequency of only slightly, not more than one part per million.It is very difficult to measure the exact resonance frequencies to that precision. Hence instead of measurement of the exact radio frequency of any proton, a reference compound is placed in the solution of the substance to be measured and the resonance frequency of each proton in the sample is measured relative to the resonance frequency of the protons of the substance.January 26, 2013 M.M.C.P. 46
  47. 47. The standard reference substance used universally is TETRAMETHYLSILANE (TMS), the standard reference is also known as Internal Standard. Shift from TMS in HzChemical Shift, ppm δ = X 106 Spectrometer frequency (MHz) Eg: for CH3Br protons, chemical shift from TMS = 162Hz in a 60 MHz instrument and 270Hz in a 100 MHz instrument. Calculate δ value. δ = 162Hz / 60 MHz = 270 / 100 = 2.7ppm Hence, δ Value remains same irrespective of the spectrometer. January 26, 2013 M.M.C.P. 47
  48. 48. Chemical shift is measure in three major spectra. Delta(δ) Tau scale(τ) Hertz (Hz) Up field shielding Down field shielding5 4 3 2 1 0 δ scale5 6 7 8 9 10 Τ scale1000 800 400 100 HZ January 26, 2013 M.M.C.P. 48
  49. 49. -5 1 ppm = 60 Hz = 6 ͯ 10MHz -4 1 ppm = 300 Hz = 3 ͯ 10 MHz Each δ unit is 1 ppm difference from TMS 60Hz and 300HzJanuary 26, 2013 M.M.C.P. 49
  51. 51. BASIC CONCEPTS….1) The chemical shift or position of line in NMR spectrum gives information on molecular environment of the nuclei from which it arises.2) The chemical shift of nuclei in the different molecules are similar. If the molecular magnetic environment are similar.3) The intensity of lines gives directly the relative number of magnetically active nuclei undergoing the different chemical shift.4) The chemical shift is used for the identification of functional groups and as an aid in determining structural arrangement of groups.January 26, 2013 M.M.C.P. 51
  52. 52. 5)Greater is the deshielding of proton  higher will be the value of delta. Greater is the shielding of proton  lower will be the value of delta.6)Electron withdrawing substituents like halogens which deshielded the protons. Electron releasing substituents like alkyl groups  which shielded the protons.7)The delta unit is independent of shield strength. Chemical shift position measured in the Hz are field dependent.January 26, 2013 M.M.C.P. 52
  53. 53. FACTORS INFLUENCING CHEMICAL SHIFT • Electronegativity Effects. • Van der Waal’s Deshielding. • Hydrogen Bonding. • Magnetic Anisotropy. • Concentration, Tempareture and Solvent Effect. January 26, 2013 M.M.C.P. 53
  54. 54. ELECTRONEGATIVITY EFFECTS :-• The chemical shift simply increase as the electronegativity of the attached element increases.• Following table illustrates this relationship for several compounds of the type CH3X.January 26, 2013 M.M.C.P. 54
  55. 55. • Multiple substituents have a stronger effect than a single substituent. The influence of the substituent drops off rapidly with distance, an electronegative element having little effect on protons that are more than three carbons distant. This effect is illustrated in the following table.January 26, 2013 M.M.C.P. 55
  56. 56. Electronegative substituents attached to a carbon atom reduces that valence electron density around the protons attached to that carbon due to their electron withdrawing effects.Electronegative substituents on carbon reduce the diamagnetic shielding in the neighborhood of the attached protons because they reduce the electron density around those protons.The greater the electronegativity of the substituents, the more deshielding of protons and hence the greater is the Chemical Shift of those protons.January 26, 2013 M.M.C.P. 56
  57. 57. January 26, 2013 M.M.C.P. 57
  58. 58. HYDROGEN BONDING :-Hydrogen atom exhibit property of hydrogen bonding in a compound which absorbs at a low field in comparison to the one which does not shows hydrogen bonding.Hydrogen bonded proton being attached to a highly electronegative atom will have smaller electron density around it.  less shielded  resonance will occurs downfield and downfield shift depends up on the strength of hydrogen bonding.Intramolecular and Intermolecular hydrogen bonding can be easily distinguished as the latter does not show any shift in absorption due to change in concentration.January 26, 2013 M.M.C.P. 58
  59. 59.  In case of phenols. Absorption occurs between 4-8 δ but if the concentration is decrease and volume of carbon tetrachloride is increase then absorption of OH proton occurs upfield, Exchangeable Hydrogen: protons that exhibit hydrogen bonding ( eg. Hydroxyl or amino protons ) show resonance over a wide range. These protons are usually found to attached to a heteroatom. The more hydrogen bonding that takes place, the more deshielded proton becomes. January 26, 2013 M.M.C.P. 59
  60. 60. MAGNETIC ANISOTROPY :-Circulation of electrons, especially the π electrons near by nuclei generates an induced field which can either oppose or reinforced the applied field at proton, depending upon location of proton or space occupied by the protons.In case of alkynes, shielding occurs but in case of alkenes, benzene and aldehydes deshielding takes place.The occurrence of shielding and deshielding can be determined by the location of proton in the space and so this effect is known as space effect.January 26, 2013 M.M.C.P. 60
  61. 61. • There are some types of protons whose chemical shifts are not easily explained by simple consideration of the electronegativity of the attached groups.• For example, when benzene is placed in magnetic field, the π electrons in the aromatic ring system are induced to circulate around the ring. This circulation is called as Ring current. The moving electrons generate a magnetic field which influence the shielding of the benzene hydrogens. January 26, 2013 M.M.C.P. 61
  62. 62. Circulating π electron Secondary magnetic field generated by circulating π electrons which deshields aromatic protons Applied field B0 Diamagnetic anisotropy in BenzeneJanuary 26, 2013 M.M.C.P. 62
  63. 63. The benzene hydrogens are said to be deshielded by thediamagnetic anisotropy of the ring.In electromagnetic terminology; an Isotropic field is oneof either uniform density or spherically symmetricdistribution.Anisotropic field is nonuniform. In case of benzenelabile electrons in the ring interact with the applied fieldand thus rendered it anisotropic.January 26, 2013 M.M.C.P. 63
  64. 64. Thus a proton attached to a benzene ring is influenced by threemagnetic fields:1)The strong magnetic field applied by the electromagnets of theNMR spectrophotometer.2)Weak magnetic field due to shielding by the valence electronsaround the proton.3)Anisotropy generated by the ring-system π electrons.So anisotropic effect gives the benzene protons at higherresonance δ value.January 26, 2013 M.M.C.P. 64
  65. 65. • All groups in a molecule that have π electrons generate secondary anisotropic fields.• In Acetylene the magnetic field generated by induced circulation of the π electrons has a geometry such that the acetylenic hydrogens are shielded. Hence acetylenic hydrogens have resonance at higher field. Diamagnetic anisotropy in Acetylene πJanuary 26, 2013 M.M.C.P. 65
  66. 66. VAN DER WAAL’S DESHIELDING :-In the overcrowded molecules. It is possible that some proton may be occupying stearic hindered position.Clearly electron cloud of bulky group or hindering group will tend to repel the electron cloud surrounding the proton and such proton will shielded and will resonate at slightly higher value of δ than expected in the absence of this effect.January 26, 2013 M.M.C.P. 66
  67. 67. CONCENTRATION, TEMPERATURE AND SOLVENT EFFECT :-In ccl4 and cdcl3 chemical shift of proton attached to carbon is independent of concentration and temperature, while proton of -OH, –NH2, –SH groups exhibits a substantial conc. and temperature effects due to the hydrogen bonding The intermolecular hydrogen bonding is less affected than intramolecular bonding by concentration changeBoth type of hydrogen bonding affected by the temperature variationJanuary 26, 2013 M.M.C.P. 67
  68. 68. INTERPRETATION OF PMR SPECTRANMR spectrum of a substance gives very valuable information aboutits molecular structure. This information is gathered as follows :(1)The number signals in PMR spectrum tell us how many kindsof protons in different chemical environments are present instructure under examination(2)The position of signal tell us about the electronic environmentof each kind of proton(3)The intensities of different signals tell us about the relativenumber of protons of different kind(4)The splitting of signals tell us about environment of theabsorbing protons with respect to the environments of neighboringprotons January 26, 2013 M.M.C.P. 68
  69. 69.  Triplet = δ-1.7,3H (CH3 )  Quartet= δ-3.4, 2H (CH2 )January 26, 2013 M.M.C.P. 69
  70. 70.  Doublet = δ-2.5,3H (CH3 )  Quartet= δ-5.8,1H (CH )January 26, 2013 M.M.C.P. 70
  71. 71.  Triplet = δ-1.2,3H (CH3 )  Quartet= δ-3.6, 2H (CH2 )  Singlet= δ-4.8, 1H (OH)January 26, 2013 M.M.C.P. 71
  72. 72.  Doublet = δ-2.2,3H (CH3 )  Quartet= δ-9.8,1H (CHO )January 26, 2013 M.M.C.P. 72
  73. 73.  Singlet= δ-2.3, 3H (CH3) Doublet = δ-7.4,2H (CH3 ) Doublet = δ-8.2,2H (NO2 )January 26, 2013 M.M.C.P. 73
  74. 74. INSTRUMENTATION OF NMRJanuary 26, 2013 M.M.C.P. 74
  75. 75. CLASSIFICATION OF THE NMR SPECTROPHOTOMETERS 1. Conventional/Continuous NMR spectrophotometer Minimal type. Multiple type. Wide line. Or It can also be classified as a. Single coil spectrophotometer b. Two coil spectrophotometer 2. Pulsed Fourier transforms NMR spectrophotometerJanuary 26, 2013 M.M.C.P. 75
  76. 76. COMPONENTS OF THE SPECTROPHOTOMETERBasically NMR instrumentation involves the following units.1.A magnet to separate the nuclear spin energy state.2. Two RF channels, one for the field/frequency stabilization and one to supply RF irradiating energy.3. A sample probe containing coils for coupling the sample with the RF field; it consists of Sample holder, RF oscillator, Sweep generator and RF receiver.4. A detector to process the NMR signals.5. A recorder to display the spectrum. January 26, 2013 M.M.C.P. 76
  77. 77. M.M.C.P. 77January 26, 2013 77
  78. 78. MAGNETS•It is used to supply the principal part of the field Ho, which determines the Larmer frequency of any nucleus.•The stronger the magnetic field, the better the line separation of chemically shifted nuclei on the frequency scale.•The relative populations of the lower energy spin level increases with the increasing field, leading to a corresponding increase in the sensitivity of the NMR experiment.FEATURES:1. It should give homogeneous magnetic field i.e.; the strength and direction of the magnetic field should be constant over longer periods.2. The strength of the field should be very high at least 20,000 gaus. January 26, 2013 M.M.C.P. 78
  79. 79. TYPES OF MAGNETS: 1. PERMANENT magnets 2. ELECTRO magnets and 3. SUPER CONDUCTING magnetsMAGNETIC COILSIt is not easy or convenient to vary the magnetic field of large stable magnets, however this problem can be overcome by superimposing a small variable magnetic field on the main field.Using a pair of Helmholtz coils on the pole faces of the permanent magnet does this. These coils induce a magnetic field that can be varied by varying the current flowing through them.The small magnetic field is produced in the same direction as the main field and is added to it. The sample is exposed to both fields, which appear one field to the nucleus. January 26, 2013 M.M.C.P. 79
  80. 80. THE PROBE UNITIt is a sensing element of the spectrophotometer system. It is inserted between thepole faces of the magnet in X-Y plane of the magnet air gap an adjustable probeholder.So the sample in NMR experiment experiences the combined effect of twomagnetic fields ie Ho and RF (EMR).The usual NMR sample cell is generally made up of the glass, which is strong andcheap. It consist of a 5 mm outer diameter and 7.5 cm long glass tube containing0.4 ml of liquid.The sample tube in NMR is held vertically between the poles faces of the magnet.The probe contains a sample holder, sweep source and detector coils, with thereference cell. The detector and receiver coils are orientated at 90 to each other.The sample probe rotates the sample tube at a 30-40 revolutions on the longitudinalaxis. Each part of the sample tube experiences the same time average the field. January 26, 2013 M.M.C.P. 80
  81. 81. THE RADIOFREQUENCY GENERATORUsing an RF oscillator creates the radio frequency radiation, required to induce transition in the nuclei of the sample from the ground state to excited states.The source is highly stable crystal controlled oscillator. It is mounted at the right angles to the path of the field of wound around the sample tube perpendicular to the magnetic field to get maximum interaction with the sample. The oscillator irradiates the sample with RF radiation.Radio frequencies are generated by the electronic multiplication of natural frequency of a quartz crystal contained in a thermo stated block.In order to generate radiofrequency radiation, RFO is used. To achieve the maximum interaction of the RFradiation with the sample, the coil of oscillator is wound around the sample container. January 26, 2013 M.M.C.P. 81
  82. 82. The RFO coil is installed perpendicular (90 ºC) to the applied magnetic field and transmits radio waves of fixed frequency such as 60,100,200 or 300 MHz to a small coil that energies the sample in the probe.This is done so that the applied RF field should not change the effective magnetic field in the process of irradiation. January 26, 2013 M.M.C.P. 82
  83. 83. SWEEP GENERATOR Resonance This can achieved by two methods•Frequency sweep methodIf the applied magnetic field is kept constant, the precession frequency is fixed. In order to bring about resonance, the frequency of the RF field should be changed so that it is becomes equal to the resonance frequency.Thus resonance condition is reached by the holding the applied magnetic field Ho constant and scanning the Rf transmitter through the frequencies, until the various nuclei come to resonance in turn as their precessional frequency matched by the scanning source. January 26, 2013 M.M.C.P. 83
  84. 84. •Field sweep method•There is a relationship between the resonance frequency of the nucleus and the strength of the magnetic field in which the sample is placed.•If the RF radiation is constant, in order to bring their resonance, the precession of the nucleus is to be changed by changing the applied magnetic field.•Generally the field sweep method is regarded as better because it is easier to vary the magnetic field than the RF radiation so as to bring about resonance in nuclei. January 26, 2013 M.M.C.P. 84
  85. 85. •Practically it is not very easy to vary the magnetic field of a large stable magnet. This is technical problem is solved by superimposing a small variable magnetic field on the main field.•Helmholtz coils January 26, 2013 M.M.C.P. 85
  86. 86. RADIO FREQUENCY RECEIVER OR DETECTORA few turns of wire is wound around the sample tube lightly. The receiver coil is perpendicular to both the external magnetic and radiofrequency transmitter coil.When RF radiation is passed through the magnetised sample, resonance occurs which cause the current voltage across the coil to drop.This electrical signal is small and is usually amplified before recording.Detection of NMR.When the radiofrequency radiation is passed through the magnetised sample two phenomena namely absorption and dispersion may occur.The absorption of either signal will enable the resonance frequency to be determined. It is found that the interpretation of absorption spectrum is easier as compared to the dispersion spectrum.The detector should be capable of separating absorption signal from dispersion signals. January 26, 2013 M.M.C.P. 86
  87. 87. THERE ARE TWO WAYS OF DETECTING THE NMR PHENOMENA 1. Radio frequency bridge (single coil detection) 2. Nuclear detection (crossed coil detection)SINGLE COIL METHODSingle coil probe has one coil that not only supplies the RF radiation to the sample but also serves as part of the detector circuit for the NMR absorption signal. To detect the resonance absorption and to separate the NMR signal from the imposed RF field, a RF bridge is used.At the fixed frequency the current flowing through the coils wrapped around the pole pieces of the magnet is varied. At the resonance there is a imbalance generated in this coil by virtue of the developing magnetization of the sample and this out of balance is detected in RF circuit.This technique is widely used in modern NMR spectrophotometer. January 26, 2013 M.M.C.P. 87
  88. 88. January 26, 2013 M.M.C.P. 88
  89. 89. CROSSED COIL PROBESNuclear induction has two coils, one for the irradiating the sample and second coil mounted orthogonally for the signal detection.The irradiating coil oriented with its axis perpendicular to the magnetic field (i.e. along the x-axis). The detector coil is wound around the sample tube with its axis is the (y-axis) perpendicular to the both Ho (z-axis).The RF current in the first coil wound around the x-axis excites the nuclei. The nuclei induction in the second coil wound around the y-axis is detected. The number of turns in the coil determines the particular frequency involved.The RF detector can be tuned to detect either a signal in the absorption mode or in the dispersion mode. Phase sensitive detector is used which helping the operator to select the phase of the signal to be detected. January 26, 2013 M.M.C.P. 89
  90. 90. January 26, 2013 M.M.C.P. 90
  91. 91. THE CONTINUOUS –WAVE (CW) INSTRUMENTJanuary 26, 2013 M.M.C.P. 91
  92. 92. WORKINGIn the CW spectrometers the spectra can be recorded either with field sweep or frequency sweep.Keeping the frequency constant, while the magnetic field is varied, (swept) is technically easier than holding the magnetic field constant and varying the frequency.The sample (0.5 mg) is dissolved in a solvent containing no interfering protons usually CCl4 or CdCl3 0.5 ml and a small amount of TMS is added to serve as an internal reference.The sample cell is a small cylindrical glass tube that is suspended in the gap between the faces of the pole pieces of the magnet. The sample cell is rotated around its axis to ensure that all parts of the solution experience a relatively uniform magnetic field. This increases the resolution of the spectrum. January 26, 2013 M.M.C.P. 92
  93. 93. Also in the magnetic gap, the radio frequency oscillator coil is installed perpendicular (90˚) to the applied magnetic field.This coil supplies the electromagnetic energy used to change the spinorientations of the protons.Detector coil is arranged perpendicular to the RF oscillator coil. As the magnetic field strength is increased, the precessional frequencies of all the nucleus increases (a peak or series of peaks)As the magnetic field strength is increased linearly, a pen travels from left to the right on a recording chart.As each chemically distinct type of proton comes into resonance, it is record as a peak on the chart. The peak δ=0 ppm is due to the internal reference compound TMS. January 26, 2013 M.M.C.P. 93
  94. 94. Since highly shielded protons precess more slowly than relatively deshielded protons. Hence highly shielded protons appear to the right of the chart, and less shielded or dishelded protons appear to the left.The region of the chart to the left is sometimes said to be downfield and that to the right is said to be upfield.Instruments which vary the magnetic field in a continuos fashion scanning from the downfield end to upfield end of the spectrum, are called continuous wave instruments.Because the chemical shifts of the peaks in this spectrum are calculated from the frequency differences from the TMS, this type of spectrum is said to be frequency domain spectrum. January 26, 2013 M.M.C.P. 94
  95. 95. Peaks generated by a CW instrument have ringing. Ringing occurs because the excited nuclei do not have time to relax back to their equilibrium state. And pen of the instrument have advanced to a new position. Ringing is most noticeable when a peak is a sharp singlet. January 26, 2013 M.M.C.P. 95
  96. 96. TYPES OF CONTINUOUS –WAVE (CW) INSTRUMENT1. Minimal-type NMR spectrometer This basic instrument often utilizes a permanent of 14, 21 or 23 K gaus field strength and RF fields of 60, 90 or 100 MHz respectively. Each frequency needed for the selected magnetic nuclei is synthesized from a suitable harmonic of a 15 MHz crystal oscillator and mixed with the output of an appropriate low frequency incremental oscillator. The minimal type has, 1. Stressed reliability 2. Ease of operation 3. High performance 4. Low cost January 26, 2013 M.M.C.P. 96
  97. 97. 2. Multipurpose NMR spectrometersThese instruments are designed primarily for research, high performance,expensive and versatility better than minimal type.The high precision comes through the use of homonuclear and heteronuclearlock systems and frequency synthesizers.They are also characterized by high intrinsic sensitivity and the ability to study avariety of nuclei.The strength of the magnetic field is quite important since sensitivity, resolutionand the separation of chemically shifted peaks increase as the field strengthincreases.These instruments uses RF field of 220,300 or even 500MHz. January 26, 2013 M.M.C.P. 97
  98. 98. 3. Wild-line CW NMR spectrometerThe wild line NMR spectrometer uses a frequency synthesizer to generate the RF field and a permanent magnet or a compact lightweight electromagnet.Slowly varying scan voltages are directly injected in the regulator for the magnet power supply for the electromagnet. Sample probe temperatures may be varied over the range 170 to 2000 ºC.Sample tubes are 15-18mm in outer diameter. The std magnetic field is 9.4 K gaus for protons and 10 K gaus for F19;the RF field is 40 MHz.Instruments are also available in which RF applied field is continuously adjustable over a basic frequency range of 300 Hz to 31MHz usually in steps of 10 Hz.For signal detection a sweep unit generates audio-modulation voltages that have selectable frequencies of 20,40,80,200 and 400 MHz.The output is amplified for simultaneous application to the probe modulation coils and to the oscilloscope. January 26, 2013 M.M.C.P. 98
  99. 99. THE PULSED FOURIER TRANSFORM (FT ) INSTRUMENT•The continuous wave type of NMR spectrometer operates by exciting the nuclei of the isotope under observation one type at a time.•In the case of H1 nuclei each distinct type of proton (phenyl, vinyl, methyl and so on) is excited individually and its resonance peak is observed and recorded, independently of all the others. As we look at first one type of hydrogen and then another scanning until all of the types have come into resonance.•An alternative approach common to modern sophisticated instrument is to use a powerful but short burst of energy called a pulse that excites all of the magnetic nuclei in the molecule simultaneously and all the signals are collected at the same time with a computer.•In an organic molecule for instance all of the H1 nuclei are induced to undergo resonance at the same time. January 26, 2013 M.M.C.P. 99
  100. 100. •The pulse actually contains a range of frequencies centered about the hydrogen in the molecule at once this signal burst of energy.•When the pulse is discontinued the excited nuclei begin to lose their excitation energy and return to the original state or relax. As each excited nucleus relaxes it emits EMR.•Since the molecule contains many different nuclei many different frequencies of EMR are emitted simultaneously. This emission is called a free-induction decay (FID) signal.•The intensity of FID decays with the time as all of the frequencies emitted and can be quite complex. We usually extract individual frequencies due to differentnuclei by using a computer and a mathematical method called a Fourier-transform analysis.•The Fourier transform breaks the FID into its separate since or cosine wave components. This procedure is too complex to be carried out by eye or by hand; it requires a computer. January 26, 2013 M.M.C.P. 100
  101. 101. ADVANTAGES OF FT-NMRFT-NMR is more sensitive and can measure weaker signals.The pulsed FT-NMR is much faster (seconds instead of min) as compared to continuous wave NMR.FT-NMR can be obtained with less than 0.5 mg of compound. This is important in the biological chemistry, where only μg quantities of the material may be available.The FT method also gives improved spectra for sparingly soluble compounds.Pulsed FT-NMR is therefore especially suitable for the examination of nuclei that are magnetic or very dilute samples. January 26, 2013 M.M.C.P. 101
  102. 102. January 26, 2013 M.M.C.P. 102
  103. 103. COMPONENTS OF FT-NMRA simplified form of the block diagram showing the instrument components of a typical Fourier transform NMR spectrometer.The central component of the instrument is a highly stable magnet in which the sample is placed.The sample is surrounded by the transmitter/receiver coil.A crystal controlled frequency synthesizer having an output frequency of Vc produces radio-frequency radiation.This signal passes into a pulse switch and power amplifier, which creates an intense and reproducible pulse of RF current in the transmitter coil.Resulting signal is picked up by the same coil which now serves a as receiver. January 26, 2013 M.M.C.P. 103
  104. 104. The signal is then amplified and transmitted to a phase sensitive detector .The detector circuitry produced the difference between the nuclear signals Vn and the crystal oscillator output Vc which leads to the low frequency time-domain signal as shown in the fig.This signal is digitalized and collected in the memory of the computer for analysis by a Fourier transform program and other data analysis software.The output from this program is plotted giving a frequency domain spectrum. January 26, 2013 M.M.C.P. 104
  105. 105. SAMPLE HANDLING TECHNIQUES IN NMR SPECTROSCOPYThe sample is placed in the probe, which contains the transmitter and receiver coilsand a spinner to spin the tube about its vertical axis in order to average out field inhomogeneities. In the electromagnet, the tube spins at right angles to the Z axis,which is horizontal, where as in the superconducting magnet, the tube fits in thebore.A routine sample for proton NMR on a scanning60 MHz instrument consists about 5 – 20mg of the sample in about 0.4ml of thesolvent in a 5mm glass tube.500MHz instrument consists about less than 1μg of the sample of modestmolecular weight in a microtube.IDEAL SAMPLE SIZE For continuous wave spectra – less than 50mg. For FT spectra 1 – 10mg January 26, 2013 M.M.C.P. 105
  106. 106. IDEAL SOLVENTS Inert Non polar Low boiling point Inexpensive Should contain no protonsCOMMONLY USED SOLVENTS CCl4 CdCl3 DMSO D2 O Cd3OD January 26, 2013 M.M.C.P. 106
  107. 107. SPLITTING OF THE SIGNALS• Each signal in an NMR spectrum represents one kind or one set of protons in a molecule.• It is found that in certain molecules, a single peak (singlet) is not observed, but instead, a multiplet (groups of peaks) is observed.January 26, 2013 M.M.C.P. 107
  108. 108. E.g. A molecule of CH3CH2Br, ethyl bromide.January 26, 2013 M.M.C.P. 108
  109. 109. SPIN-SPIN COUPLING• The interaction between two or more protons, most often through the bonds, results in splitting of the spectral lines.• It is related to the number of possible combinations of the spin orientations of the neighboring protons.• The magnitude of the spin coupling interaction between protons in general decreases as the number of bonds between the coupled nuclei increases.January 26, 2013 M.M.C.P. 109
  110. 110.  Consider a molecule of ethyl bromide (CH3-CH2-Br).the spin of two protons (-CH2-) can couple with the adjacent methyl group (-CH3-) in three different ways relative to the external field . The three different ways of alignment are ;  Thus a triplet of peaks results with the intensity ratio of 1 : 2 : 1 which corresponds to the distribution ratio of alignment .January 26, 2013 M.M.C.P. 110
  111. 111.  Similarly the spin of three protons (CH3-) can couple with the adjacent methylene group (-CH2-) in four different ways relative to the external field Thus a quartet of peaks results with an intensity ratio of 1:3:3:1 which corresponds to the distribution ratio of all the alignment.January 26, 2013 M.M.C.P. 111
  112. 112. • The relative intensities of the individual lines of a multiplet corresponds to the lines in the binomial expression .• If n=1, then (1+x)n = 1 + x.• If n=2, then (1+ x )2 = 1+2x + x2, thus the lines of triplet have relative intensities 1: 2 :1.• If n=3, then ( 1 + x )3 = 1 +3X + 3X + X3, the lines of quartet have relative intensities 1 : 3: 3 : 1.January 26, 2013 M.M.C.P. 112
  113. 113. Often a group of hydrogens will appear as a multiplet rather than as a single peak. Multiplets are named as follows: Singlet Quintet Doublet Sextet Septet Triplet Octet Quartet Nonet This happens because of interaction with neighboring hydrogens and is called, SPIN-SPIN SPLITTING. M.M.C.P.January 26, 2013 113
  114. 114. 1,1,2-Trichloroethane The two kinds of hydrogens do not appear as single peaks, rather there is a “triplet” and a “doublet”. integral = 2 Cl H H C C Cl integral = 1 Cl H The sub peaks are due to triplet doublet spin-spin splitting and are predicted by the n+1 rule.January 26, 2013 M.M.C.P. 114
  115. 115. n + 1 RULEJanuary 26, 2013 M.M.C.P. 115
  116. 116. 1,1,2-Trichloroethane integral = 2 Cl H H C C Cl integral = 1 Cl H Where do these multiplets come from ? ….. interaction with neighborsJanuary 26, 2013 M.M.C.P. 116
  117. 117. this hydrogen’s peak these hydrogens are MULTIPLETSis split by its two neighbors split by their single neighbor singlet doublet H H H H triplet C C C C quartet H H quintet sextet two neighbors n+1 = 3 one neighbor septet triplet n+1 = 2 doubletJanuary 26, 2013 M.M.C.P. 117
  118. 118. EXCEPTIONS TO THE n+1 RULE IMPORTANT ! 1) Protons that are equivalent by symmetry usually do not split one another X CH CH Y X CH2 CH2 Y no splitting if x=y no splitting if x=y 2) Protons in the same group usually do not split one another H H C H or C H H 118January 26, 2013 M.M.C.P.
  119. 119. Con….3) The n+1 rule applies principally to protons in aliphatic (saturated) chains or on saturated rings. CH3 CH2CH2CH2CH2CH3 or H YES YES but does not apply (in the simple way shown here) to protons on double bonds or on benzene rings. H CH3 CH3 H H NO NOJanuary 26, 2013 M.M.C.P. 119
  121. 121. PASCAL’S TRIANGLE Intensities of 1 multiplet peaks singlet The interior entries are 1 1 doublet the sums of the two 1 2 1 triplet 1 3 3 1 numbers immediately quartet above. 1 4 6 4 1 quintet 1 5 10 10 5 1 sextet 1 6 15 20 15 6 1 septet 1 7 21 35 35 21 7 1 M.M.C.P.January 26, 2013 octet 121
  122. 122.  The simple rule to find the multiplicity of the signal from a group of protons, is to count the number of neighbours (n) & add 1. That is (n+1) . No coupled C A singlethydrogen C –C – C –H C J One coupled A doublet H hydrogen C- C – C –H C J J H A triplet Two coupled hydrogen H - C –C-H J C J J H A quartet Three coupled hydrogen H - C – C- H January 26, 2013 H M.M.C.P. 122
  123. 123. THE ORIGIN OF SPIN-SPIN SPLITTING HOW IT HAPPENS ?January 26, 2013 M.M.C.P. 123
  124. 124. THE CHEMICAL SHIFT OF PROTON HA ISAFFECTED BY THE SPIN OF ITS NEIGHBOURS aligned with Bo opposed to Bo 50 % of +1/2 -1/2 50 % of molecules molecules H HA H HA C C C C Bo downfield upfield neighbor aligned neighbor opposed At any given time about half of the molecules in solution will have spin +1/2 and the other half will have spin -1/2. January 26, 2013 M.M.C.P. 124
  125. 125. SPIN ARRANGEMENTS one neighbor one neighbor n+1 = 2 n+1 = 2 doublet doublet H H H H C C C C The resonance positions (splitting) of a given hydrogen is affected by the possible spins of its neighbor.January 26, 2013 M.M.C.P. 125
  126. 126. SPIN ARRANGEMENTS two neighbors one neighbor n+1 = 3 n+1 = 2 triplet doublet H H H H C C C C H H methine spins methylene spinsJanuary 26, 2013 M.M.C.P. 126
  127. 127. SPIN ARRANGEMENTS three neighbors two neighbors n+1 = 4 n+1 = 3 quartet triplet H H H H C C H C C H H H H HJanuary 26, 2013 methyl spins M.M.C.P. methylene spins 127
  128. 128. Advanced Spin-spin Coupling NMR Spectroscopy NOMENCLATURE • The spacing between the two adjacent peaks of a multiplet is referred to as the J or coupling constant • The value of J for a given coupling is constant, regardless of the field strength or operating frequency of the instrument • Coupling between nuclei of the same type is referred to as homonuclear coupling • Coupling between dissimilar nuclei is referred to as heteronuclear coupling 128 • The magnitude of this effect is dependent on the number of bonds intervening between two nuclei – in general it is a distance effect, where one-bond couplings January 26, 2013 be the strongest would M.M.C.P. 128
  129. 129. Advanced Spin-spin Coupling NMR Spectroscopy Con…. There are many variations of the subscripts and superscripts associated with J constants In general, the superscript numeral to the left of J is the number of intervening bonds through which the coupling is taking place 3 J is a coupling constant operating through three bonds Subscripts to the right of J can be used to show the type of coupling, such as HH for homonuclear between protons or HC for heteronuclear between a carbon and proton Often, this subscript will be used to define the various J-constants within a complex multiplet: J1, J2, J3, etc. or JAB, JBC, JAC] Although J values are referred to as positive numbers, they may in actuality be positive or negative January 26, 2013 M.M.C.P. 129
  130. 130. Advanced Spin-spin Coupling NMR Spectroscopy MECHANISM OF COUPLING • The most coherent theory of how spin information is transferred from one nucleus to another is the Dirac vector model • In this model, there is an energetic relationship between the spin of the electrons and the spin of the nuclei • An electron near the nucleus has the lowest energy of interaction if its spin is opposite to that of the nucleus Nuclear spin electron spin Energy Nuclear spin electron spin January 26, 2013 M.M.C.P. 130
  131. 131. Advanced Spin-spin Coupling NMR Spectroscopy MECHANISM OF COUPLING – ONE BOND COUPLINGS, 1J • Here, a single bond (two electrons) joins two spin-active nuclei – such as 13C- 1 H • The bonding electrons will tend to avoid one another, if one is near the 13C nucleus the other will be near the 1H nucleus • By the Pauli principle, these electrons must be opposite in spin • The Dirac model then predicts that the most stable condition between the two nuclei must be one in which they too are opposite in spin: C spin 13 H spin 1 electrons opposite in spinJanuary 26, 2013 M.M.C.P. 131
  132. 132. Advanced Spin-spin Coupling NMR Spectroscopy MECHANISM OF COUPLING – ONE BOND COUPLINGS, 1J • These alignments can be used for any heteronuclear pair of spin-active nuclei – 13 P-13C, etc. • When two nuclei prefer an opposed alignment, as in this example, the J is positive • If the two nuclei have parallel spins, the J will be negative (remember spin information is transferred through the electrons!) January 26, 2013 M.M.C.P. 132
  133. 133. Advanced Spin-spin Coupling NMR Spectroscopy MECHANISM OF COUPLING – ONE BOND COUPLINGS, 1J • The Dirac model predicts the observed spin-spin coupling for the methine 13C- 1 H system • It is important to note that the electron spins must be opposite 13 C 1 H 13 C 1 H Excited state is 13 C nuclear resonance of lower energy 13 C 1 H 13 C 1 H Dirac model Dirac model favored ground less-favored state ground state January 26, 2013 M.M.C.P. 133
  134. 134. Advanced Spin-spin Coupling NMR Spectroscopy MECHANISM OF COUPLING – ONE BOND COUPLINGS, 1J • It is these two upper energy states, and the two DEs that generated them that result in the doublet for an undecoupled methine in a 13C spectrum 13 C 1 H 13 C 1 H 13 C 1 H 13 C 1 H January 26, 2013 M.M.C.P. 134
  135. 135. Advanced Spin-spin Coupling NMR Spectroscopy MECHANISM OF COUPLING – TWO BOND COUPLINGS, 2J • As the bond angle H-C-H decreases, the amount of electronic interaction between the two orbitals increases, the electronic spin correlations also increase, and J becomes larger. They are sometimes called geminal coupling, because the two nuclei that interact are attached to the same central atom(Latin gemini = “twins”) H H-C-H 109o In general: 2J H HH = 12-18 Hz 40 JHH H H-C-H 118o 2J 20 HH = 5 Hz H H H-C-H 120o 90 100 110 120 2J HH = 0-3 Hz H January 26, 2013 M.M.C.P. 135
  136. 136. Advanced Spin-spin Coupling NMR Spectroscopy MECHANISM OF COUPLING – TWO BOND COUPLINGS, 2J • Variations in J also result from ring size • As ring size decreases, the C-C-C bond angle decreases, the resulting H-C-H bond angle increases, – J becomes smaller H H H H H H H H H C H H H 2 JHH (Hz) = 3 5 9 11 13 9 to 15 January 26, 2013 M.M.C.P. 136
  137. 137. Advanced Spin-spin Coupling NMR Spectroscopy MECHANISM OF COUPLING – THREE BOND COUPLINGS, 3J • These couplings are the one most common to introductory studies in NMR, and are observed as the coupling through a C-C bond between two C-H bonds - vicinal coupling. • Observe the two possible spin intra C-C cations: -1/2 +1/2 +1/2 +1/2 January 26, 2013 M.M.C.P. 137
  138. 138. Advanced Spin-spin Coupling NMR SpectroscopyMECHANISM OF COUPLING – THREE BOND COUPLINGS, 3J Observe that the orbitals must overlap for this communication to take place The magnitude of the interaction, it can readily be observed, is greatest when the orbitals are at angles of 0o and 180o to one another: 0o dihedral angle 180o dihedral angle Maximum overlap January 26, 2013 M.M.C.P. 138
  139. 139. Advanced Spin-spin Coupling NMR Spectroscopy MECHANISM OF COUPLING – THREE BOND COUPLINGS, 3J 8. Examples of this effect in operation: H HH H Jdiaxial = 10-14 Hz 3 3 Jdiequitorial = 4-5 Hz α = 180ο α = 60ο H H 3 Jaxial-eq. = 4-5 Hz α = 60ο January 26, 2013 M.M.C.P. 139
  140. 140. Advanced Spin-spin Coupling NMR SpectroscopyMECHANISM OF COUPLING – LONG RANGE COUPLINGS, ≥4J • the greater the number of intervening bonds the greater the reduction in opportunity for orbital overlap – long range couplings are uncommon • In cases where a rigid structural feature preserves these overlaps, however, long range couplings are observed January 26, 2013 M.M.C.P. 140
  141. 141. Advanced Spin-spin Coupling NMR SpectroscopyMECHANISM OF COUPLING – LONG RANGE COUPLINGS, ≥4J • Examples include the meta- and para- protons to the observed proton on an aromatic ring and acetylenic systems: H H H H C C C C H H H H J = 0-1 Jz Hz 5 3 J = 7-10 Hz 4 J = 1-3 Hz 5 J = 0-1 Hz ortho meta para January 26, 2013 M.M.C.P. 141
  142. 142. Advanced Spin-spin Coupling NMR SpectroscopyMECHANISM OF COUPLING – LONG RANGE COUPLINGS, ≥4J • Rigid aliphatic ring systems exhibit a specialized case of long range coupling – W- coupling – 4JW • The more heavily strained the ring system, the less “flexing” can occur, and the ability to transmit spin information is preserved H H H O H H H J = 0-1 4 4 J =3 4 J = 7 Hz January 26, 2013 M.M.C.P. 142
  143. 143. SPIN DECOUPLING • It is a powerful tool for simplifying a spectrum & is of great value to organic chemists working with complex molecules. It helps in the identification of coupled protons in spectra that are too complex for detailed analysis. • This technique involves the irradiation of a proton or a group of equivalent proton with sufficiently intense radio frequency energy to eliminate completely the observed coupling of the neighboring protons. • The simplification of the complex spectrum for easy interpretation is done by, 1) By using an instrument with a more powerful homogeneous magnetic field, e.g. a 100 MHz instrument in preference to 60 MHz instruments. 2) By spin- spin decoupling techniques.January 26, 2013 M.M.C.P. 143
  144. 144. ISOTOPE EXCHANGE • Deuterium (2H or D ), the heavy isotope of hydrogen, has been used extensively in proton NMR spectroscopy for two reasones. First it is easily introduced into a molecule. Second, the presence of deuterium in a molecule is not detected in the proton NMR spectrum. • Deuterium has a much smaller magnetic dipole moment than hydrogen & therefore, it absorbs at different field strengths. In case of ethylbromide the deuterium replaces the methyl hydrogens & the following changes occurs. 2H 3H Br-CH2-CH3 2H 2H Br-CH2-CH2D 2H 1H Br-CH2-CHD2 2H Br-CH2-CD3January 26, 2013 M.M.C.P. 144
  145. 145. SHIFT REAGENTS• Lanthanide series of elements are used as shift reagents. A lanthanide ion can increase its co-ordination number by interacting with unshared electrons. As a result the NMR spectrum of the comp. that contains a group possessing unshared pair of electron undergoes change & large chemical shift as a difference in peaks is observed.• All the shift reagents are mild Lewis acids. Shift reagent separates NMR signals those normally overlap. Thus it gives more simplified spectrum.• Shift reagent are paramagnetic, so large chemical shift take place.• Shift reagents is normally used in non polar solvents like CdCl3, CCl4 etc.• Shift reagents, provide a useful technique for spreading out proton NMR absorption patterns which normally overlap, without increasing the strength of the applied magnetic field. January 26, 2013 M.M.C.P. 145
  146. 146. • In the proton NMR spectrum of n – hexanol, the high field triplet is distorted which represents the absorption of a methyl group adjacent to a - CH2 – group. The low field broad multiplet is due to the methylene group adjacent to the hydroxyl group. The proton of the remaining methylene groups are all burried in the methylene envelope between δ 1.2 & 1.8 .January 26, 2013 M.M.C.P. 146
  147. 147. • When the same spectrum is recorded after addition of a soluble europium (III) complex, that is the shift reagent , the spectrum is spread out over a wider range of frequencies. So that it is now simplified almost to first order. In the spectrum OH absorption signal is shifted too far to be.January 26, 2013 M.M.C.P. 147
  148. 148. COMPARISIONS BETWEEN 13C-NMR & 1H-NMR  13C-NMR 1H-NMR1. Pulse Fourier Technique is used 1. Continuous wave method is followed.2. Very fast. 2. Slow process.3. No peak overlapping observed 3. Peak overlapping observed in case of in the spectrum. complex samples.4. Sweep generator & sweep coil 4. Required. are not required in the NMR instrument.5. Chemical shift range is wide 5. δ range is very narrow (δ  0-15). (δ 0-200).6. Wide band RF is applied rather 6. Tuned to one frequency. than tuned to a precise frequency.7. Work on frequency sweep 7. Works on either field sweep technique. or frequency sweep techniques.
  149. 149. QUESTIONS :- 2o marks:- 1. (a) Explain the basic principles involved in NMR spectroscopy. (b) Write an account of NMR spectra. How its interpretation ? Explain with examples. (Sep’07)(Apr’08).1o marks:- 1. Write a note on splitting of signals in NMR spectra. (May’10). 2. Briefly indicate the functions of various units of NMR spectrometer. (Apr’08). 3. Explain shielding & deshielding effect in NMR spectroscopy. (Apr’08). 4. What is chemical shift ? Explain the factors affecting chemical shift. (Apr’08). January 26, 2013 M.M.C.P. 149
  150. 150. Con…. 5 marks:- 1. Explain chemical shifts in NMR. (‘03) 2. Explain advantages and applications of FT NMR. (‘97) January 26, 2013 M.M.C.P. 150
  151. 151. REFERENCES :-1. Sharma YR. Elementary organic spectroscopy principles and chemical applications. 1st ed. S. Chand and Company ltd; New Delhi :2008.2. Chatwal GR, Anand SK. Instrumental methods of chemical analysis. 1st ed. Himalaya Publishing house; Mumbai: 2004.3. Jag Mohan. Organic spectroscopy principles and applications. 1 st ed. Narosa publishing House; New Delhi: 2001.4. Sharma BK. Instrumental methods of chemical analysis. 24th ed. Goel Publishing house; Meerut: 2005.5. S. Ravi Shankar. Text book of pharmaceutical analysis. 3rd ed. Rx publication; Tirunelveli: 2006. January 26, 2013 M.M.C.P. 151
  152. 152. 6. O.V.K. Reddy. Pharmaceutical analysis. Pulse publication; Hyderabad.7. Willey. Handbook of spectroscopy. 2003.8. Pavia, Lampman, Kriz. Introduction of spectroscopy. 3ed edition.9. Skoog DA, West DM. principle of instrumental analysis. 2ed edition.10.Willard HH, Merritt LL, Dean JA, Settle FA. Instrumental methods of analysis. Jr CBS publishing and distributors, 7 th edition.11. Kasture AV, Mahadik KR, More HN, Wadodkar SG. Pharmaceutical analysis. Nirali Prakashan. 17th edition 2008. January 26, 2013 M.M.C.P. 152
  153. 153. 12. Silverstein R.M, Webmaster F.A, Spectrometric identification of organic compounds, 6th edition.148-150.13. Kemp W. organic spectroscopy. 3rd edition.1996.14. January 26, 2013 M.M.C.P. 153
  154. 154. Any Questions……..????? 154
  155. 155. January 26, 2013 M.M.C.P. 155