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Chemical shift with c13 nmr
 

Chemical shift with c13 nmr

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Chemical shift with c13 nmr Chemical shift with c13 nmr Presentation Transcript

  • Presented by: Naveen Kadian K.L.E.S’s College of Pharmacy, BELAGAVI.
  • CONTENTS:  INTRODUCTION  PRINCIPLE OF 13 C NMR SPECTROSCOPY  IMPORTANCE  DIFFICULTIES ENCOUNTERED IN 13 C NMR  13 C CHEMICAL SHIFT  APPLICATIONS  REFERENCES
  • INTRODUCTION:  The first NMR observation regarding 13 C nuclei were reported in 1957.  The experiment concluded that the direct observation of carbon nuclei had greater utility over the equivalent protons studies.  The study of carbon nuclei through magnetic resonance spectroscopy is important technique for determining the structure of organic compounds,using it with proton NMR and IR spectroscopy organic chemist can often determine the complete structure of unknown compounds.  Thus, carbon NMR provides direct information about the carbon skeleton of the molecule.
  • INTRODUCTION: contd…. 13 C NMR ( CMR) Proton NMR ( PMR) It is study of spin changes of carbon nuclei. It is study of spin changes of proton nuclei. Chemical shift range is 0-240 ppm. Chemical shift range is 0-14 ppm. Fourier transform Technique is used. Continuous wave method is used Very fast process. Gyromagnetic ratio is 1.4043 slow process. Gyromagnetic ratio is 5.5854 Coupling constant range is 125-250Hz. Coupling constant range is 0-15Hz. Solvent peak is observed. Solvent peak is not observed. Area under the peak is not considered. Area under the peak is considered TMS peak is quartet. TMS peak is singlet. Effect of substitute on adjacent carbon atom cannot varies chemical shift. Effect of substituent on adjacent carbon atom can varies chemical shift.
  •  Any nucleus with odd mass number spins on its own axis  By the application of an external magnetic field (Ho), the nucleus spins on its own axis and a magnetic moment is created,. In this ground state the magnetic field caused by a spin of nuclei is aligned with external magnetic field. When the energy in the form of radio frequency is applied and when applied frequency is equal to processional frequency, absorption of energy occurs and NMR signal is recorded. Because of absorption of energy, the nucleus moves from ground state to excited state, which results in spin reversal or anti-parallel orientation in which magnetic field caused by the spin of nucleus opposes the external applied magnetic field.
  • Nuclear SpinNuclear Spin A spinning charge, such as the nucleus of 1 H or 13 C, generates a magnetic field. The magnetic field generated by a nucleus of spin +1/2 is opposite in direction from that generated by a nucleus of spin –1/2. + +
  • + + + + + The distribution of nuclear spins is random in the absence of an external magnetic field.
  • + + + + + An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field. HH00
  • Energy Differences Between Nuclear Spin StatesEnergy Differences Between Nuclear Spin States no difference in absence of magnetic field proportional to strength of external magnetic field + + ∆∆EE ∆∆EE '' increasing field strengthincreasing field strength
  • Radio Wave Transceiver Radio Wave Transceiver A Modern NMR Instrument
  • IMPORTANCE OF 13 C NMR  CMR is nondestructive and noninvasive method.  CMR of biological materials allows for the assessment of the metabolism of carbon.  CMR, chemical shift range is wider than PMR.  The low natural abundance of 13 C nuclei (1.1%) can be made use of tagging the specific carbon position by selective 13 C enrichment.  13 C nucleus is a stable isotope, hence not subjected to dangers related to radiotracers.  Homo nuclear coupling of 13 C provides novel biochemical information.
  • DIFFICULTIES ENCOUNTERED IN 13 C NMR  The 13 C nucleus is magnetically active and which is similar to 1 H nucleus.  Recording of CMR nucleus is difficult due to the following reasons: 1. Natural abundance. 2. Gyro magnetic ratio. 3. Coupling phenomenon.
  • 1. Natural abundance: The most abundant isotope of carbon 12 C is not detected by NMR, as it is magnetically inactive (I=0). The low natural abundant isotope 13 C is magnetically active (I=1/2). As a result of the natural abundance of 13 C is 1.1% , the sensitivity of 13 C nuclei is only 1.6% that of 1 H nuclei. The availability of FT instrumentation enhances the sensitivity of 13 C nucleus.
  • 2. Gyro magnetic ratio: The gyro magnetic ratio of 13 C is 1.4043 as compared to 5.5854 of a proton. 13 C nucleus resonance frequency is only 1/4th of PMR at a given magnetic field. Thus, CMR is less sensitive than PMR Sensitivity of CMR can be increased by adopting FT technique.
  • 3. Coupling phenomenon: Both 13 C and 1 H have I =0, so that we expect coupling in the spectrum between 13 C - 13 C and 13 C - 1 H. The probability of two 13 C nuclei adjacent to each other in the same molecule is extremely rare due to low natural abundance of 13 C. So that 13 C- 13 C coupling will not usually exist. However the 13 C - 1 H coupling have observed in CMR spectrum. As a result of coupling makes the 13 C spectrum extremely complex , consequently there is an overlap of multiplets. These 13 C - 1 H coupling can be eliminated by adopting following techniques. a) FT technique b) Decoupling technique c) Nuclear overhauser phenomenon for enrichment of the carbon signal.
  •  FT technique: Earlier, the continuous wave method is used to record 13 C spectra but it is slow procedure, require large sample and for assessing takes long time .FT technique increases activity of 13 C nuclei  FT technique permits simultaneous irradiation of all 13 C nuclei .  In this method sample is irradiated with a strong pulse of radio frequency radiation in desired range at once in a fixed magnetic field .  Advantages 1)The scanning takes place rapidly compared to continuous wave NMR. 2)The sensitivity problems are eliminated in NMR, therefore which helps in a) Analyses the sample at low conc. b) NMR studies on nuclei with low natural abundance and with low gyro magnetic ratio.
  •  Decoupling technique: Generally the probability occurrence of 13 C- 13 C coupling is rare , but the 13 C - 1 H coupling can usually observed . The problem of 13 C - 1 H coupling can be eliminated by decoupling the proton from carbon . Types of decoupling in CMR 1) Proton decoupling or noise decoupling . 2) Coherent and broadband decoupling . 3) Off resonance decoupling .
  • 1) Proton decoupling or noise decoupling:  The proton decoupled CMR spectrum can be recorded by irradiating the sample at two frequencies.  The first radio frequency signal is used to affect carbon magnetic resonance, while simultaneous exposure to second signal causes all the protons to be resonance at the same time they spin or flip very fast.  As they flip so fast, there is no coupling and each carbon appears as a single unsplit peak at corresponding chemical shift range. Ex: Proton decoupled spectra of sec-butyl bromide.
  • 2) Broadband decoupling:  In this technique, all the proton resonance can be reduced and to get sharp CMR spectral peaks, each directly reflecting a 13 C chemical shift.  The NMR spectrum of nucleus A is split by nucleus B, because A can see B in different magnetic orientation.
  • Off resonance decoupling  1000-2000 Hz above the spectral region  In this primary carbon nuclei (bearing three protons) yield a quartet of peaks, secondary carbons give three peaks, tertiary carbon nuclei appear as doublets, and quaternary carbons exhibit a single peak.
  • This division gives a number independent of the instrument used. parts per million THE CHEMICAL SHIFTTHE CHEMICAL SHIFT The shifts from TMS in Hz are bigger in higher field instruments (300 MHz, 500 MHz) than they are in the lower field instruments (100 MHz, 60 MHz). We can adjust the shift to a field-independent value, the “chemical shift” in the following way: A particular proton in a given molecule will always come at the same chemical shift (constant value). chemical shift = δ = shift in Hz spectrometer frequency in MHz = ppm
  • 01.02.03.04.05.06.07.08.09.010.0 Chemical shift (Chemical shift (δδ, ppm), ppm) measured relative to TMSmeasured relative to TMS UpfieldUpfield Increased shieldingIncreased shielding DownfieldDownfield Decreased shieldingDecreased shielding (CH(CH33))44Si (TMS)Si (TMS)
  • Factors affecting chemical shift  Electronegativity  Hybridization  Hydrogen bonding  Anisotropic
  • Applications of 13 C NMR  Metabolic studies  Metabolic studies on human 1. Brain function 2. Glucose metabolism in liver 3. Glucose metabolism in muscle 4. Determination of degree of unsaturation of fatty acids in adipose tissue 5. Characteristic of body fluids and isolated tissues 6. In diseased state  Industrial applications in solids
  • REFERENCES  Morrison RT, Boyd RN. Organic chemistry. 6th edition.2001; P.no 604-629.  Sanders FRS, Jeremy KM, Hunters BK. Modern NMR Spectroscopy. 2nd edition. 1993; P.no 46.  Skoog, Holler, Nieman. Principles of Instrumental analysis. 5th edition 1991; P.no 480-484.  Kemp W. Organic Spectroscopy 3rd edition 1991; P.no 110-130.  Silverstein RM, Webstar FX. Spectrometric Identification of Organic Compounds 6th edition. 1998; P.no 222.
  • A.The differences in the applied magnetic field strength (Angular Frequency of Precession) at which the various proton configurations in a molecule Resonate is extremely small. A.The differences amount to only a few parts per million in the Magnetic field strength. A.It is difficult to measure the precise field strength to less than a part per million. A.Measuring the difference between absorption positions is much easier using the difference between the Resonance of the sample and the Resonance of a standard reference sample. The Chemical Shift
  • The Chemical Shift (Con’t) E. The actual procedure measures the difference between nuclei resonance energies relative to the universally accepted standard - Tetramethylsilane (CH3)4Si (TMS).  All protons in TMS have chemically and electronically similar environments.  They are highly shielded - nothing about them diminishes the electron density.  They resonate at same field strength, i.e., a single reference signal is produced.  The proton resonances in Tetramethylsilane (TMS) appear at a higher magnetic field strength than proton resonances in most all other molecules.
  • Tetramethylsilane (TMS)  Chemical Shift (δ) = 0 ppm by Definition  All protons in TMS are in chemically equivalent environments  The protons are in regions of high electron density  Silicon is less electronegative than Carbon  Proton resonances appear at a higher field strength than proton resonances in most all other molecules  One signal is produced  Small amount produces large signal Reasons to use TMS as internal Standard Si H C H H H H C H H C H H H H C H
  • Observed Shift from TMS (Hz) Hz Chemical Shift (δ) = = = PPM60 MHz MHz The Chemical Shift on (Con’t) F. Thus, the protons in compounds of interest to the organic chemist resonate at frequencies greater than the protons in TMS, i.e., at lower magnetic field strengths. G. A parameter called the “Chemical Shift (δ)” has been defined to give the position of the absorption of a proton a quantitative value. H. The Chemical Shift values are reported in units of “Parts Per Million” (ppm).
  • The Chemical Shift (Con’t) A. A NMR spectrometer increases the Magnetic field strength as the pen moves from left to right on the chart. B. The TMS absorption is higher than will be obtained for just about all organic compounds. C. Thus, the absorption signal for TMS appears at the far right side of the chart. D. The Chemical Shift for TMS is arbitrarily set at “0” PPM. E. By convention, the Chemical Shift values increase from right to left, with a range of 0 (TMS) to about 13 on the far left of the chart. F. In other words: Chemical Shift values decrease with increasing Magnetic field strength!
  • OH does not have carbon ↓ no 13 C-NMR OH signal Example: HOCH2CH2CH2CH3