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By Prashant J Patel
(Department of pharmaceutical technology)
 Indukaka Ipcowala College of Pharmacy
        New vidhyanagar,anand

Why C13-NMR is required though we
         have H1-NMR?
 1H nmr spectroscopy - The powerful and useful tool
 a tool for structural analysis.
Useless when portions of a molecule lack C-H
 bonds, no information is forthcoming.
Ex: polychlorinated compounds such as
 chlordane, polycarbonyl compounds such as
 croconic acid, and compounds incorporating triple
 bonds (structures below, orange colored carbons).
 Even when numerous C-H groups are present, an
 unambiguous interpretation of a proton nmr spectrum
 may not be possible.
The following three pairs of isomers (A & B) which
 display similar proton nmr spectra. Although a careful
 determination of chemical shifts should permit the first
 pair of compounds (blue box) to be distinguished, the
 second and third cases (red & green boxes) might be
 difficult to identify by proton nmr alone.
Natural Abundance
                          
 Since the major isotope of carbon (12C) has no
  spin, this option seems unrealistic.
 Fortunately, 1.1% of elemental carbon is the 13C
  isotope, which has a spin I = 1/2, so possible to
  conduct a carbon nmr experiment.
 It is worth noting here, that if much higher
  abundances of 13C were naturally present in all
  carbon compounds, proton nmr would become
  much more complicated due to large one-bond
  coupling of 13C and 1H.
Many obstacles needed to be overcome before
       carbon nmr emerged as a routine tool :

                           
 As noted, the abundance of 13C in a sample is very low
  (1.1%), so higher sample concentrations are needed.
 The 13C nucleus is over fifty times less sensitive than a
  proton in the nmr experiment, adding to the previous
  difficulty.
 Hydrogen atoms bonded to a 13C atom split its nmr signal
  by 130 to 270 Hz,
 1H-13C splitting is overcome by using an instrumental
  technique that decouples the proton-carbon interactions, so
  that every peak in a 13C NMR spectrum appears as a singlet.
 The two features of a 13C NMR spectrum that
 provide the most structural information are the
 number of signals observed and the chemical
 shifts of those signals.
13C   NMR—Number of Signals
The number of signals in a 13C spectrum gives the number
of different types of carbon atoms in a molecule.
Because 13C NMR signals are not split, the number of
signals equals the number of lines in the 13C spectrum.
In contrast to the 1H NMR situation, peak intensity is not
proportional to the number of absorbing carbons, so 13C
NMR signals are not integrated.
13C   NMR—Position of Signals
In contrast to the small range of chemical shifts in 1H
NMR (1-10 ppm usually), 13C NMR absorptions occur over a
much broader range (0-220 ppm).
The chemical shifts of carbon atoms in 13C NMR depend
on the same effects as the chemical shifts of protons in 1H
NMR.
13C   Chemical shifts are mainly most affected
                        by:
                         
 Electronegativity of groups attached to the Carbon
 Hybridization state of Carbon
  sp3 hybridized carbon is more shielded than sp2
  sp hybridized carbon is more shielded than sp2, but
  less shielded than sp3
 Anisotropy
 All affect 13C Chemical shifts in nearly same fashion
  as they affect 1H chemical shift
Types of Carbons

               Classification   Chemical shift,
                                1H       13C

CH4                             0.2      -2
CH3CH3         primary          0.9        8
CH3CH2CH3      secondary        1.3      16
(CH3)3CH       tertiary         1.7      25
(CH3)4C        quaternary                28
Replacing H by C (more electronegative) deshields
C to which it is attached.
Electronegativity effects on CH3

               Chemical shift,
                 1H      13C

  CH4            0.2      -2
  CH3NH2         2.5      27
  CH3OH          3.4      50
  CH3F           4.3      75
Electronegativity effects and chain length


      Cl   CH2    CH2    CH2     CH2    CH3

Chemical   45     33      29      22     14
shift,

   Deshielding effect of Cl decreases as
   number of bonds between Cl and C increases.
Corrrelation chart for C13-NMR chemical
               shift(ppm)
Spin-Spin Splitting
              
 Homonuclear spin-spin splitting:
       Because of its low natural abundance there is a
low probability of finding two C13 atoms next to each
other in a single molecule.
       C13-C13 coupling negligible.
 Hetronucler spin-spin splitting:
       C13 will magnetically couple with attached
protons and adjacent protons. N+1 rule is obeyed.
Off-Resonance Decoupling

                        
 13C nuclei are split only by the protons attached
  directly to them.
 The N + 1 rule applies: a carbon with N number of
  protons gives a signal with
  N + 1 peaks.
13C   Off-resonance decoupled spectrum


                  
Proton-decoupled spectra

                        
 A common method used in determining a carbon-C13
  NMR spectrum is to irradiate all of the hydrogen
  nuclei in the molecules at the same time the carbon
  resonances are being measured.
 Thins required a second radiofrequency(RF) source
  (the decoupler) tuned to the frequency of the
  hydrogen nuclei, while the primary RF source is
  tuned to the C13 frequency.
 In this method the hydrogen nuclei are “saturated”, a
  situation where there are as many downward as there are
  upward transition, all occurring rapidly.
 During time the C-13 spectrum is being determined, the
  hydrogen nuclei cycle rapidly between their two spin state
  (+1/2 and -1/2) and the carbon nuclei see an average
  coupling (i.e. zero) to the hydrogen.
 The hydrogen are said to be coupled from the carbon-13
  nuclei.
 You no longer see multiples for the c13 resonances. Each
  carbon gives a singlet, and the spectrum is easier to
  interpret.
Nuclear Over Hauser enhancement effect
 When we obtain a proton-decoupled c13 spectrum, the
  intensities of many of the carbon resonances increase
  significantly above those observed on a proton-coupled
  experiments.
 Carbon atoms with hydrogen atoms directly attached are
  enhanced the most, and the enhancement increases as more
  hydrogen are attached. This efface is called the Nuclear Over
  Hauser enhancement (NOE).
 Shown when two different type of atoms are irradiated
  while NMR spectroscopy of other type is determined.
 The effect can be either positive or negative, depending on
  which atom types are involved.
 In case od c-13 interacting with H-1 the effect is positive.so,
  Intensities of signals increases.
Magnetogyric
ratio of nucleus
being irradiated

 Magnetogyric
 ratio of nucleus
 being observed
1H   & 13C NMR: 1,1,2-trichloropropane


                  
DEPT spectra (Distortionless Enhancement by
               Polarization Transfer)
 Useful method for determining the presence of
  primary, secondary and tertiary carbon atoms.
 The DEPT experiment differentiates between CH, CH2 and
  CH3 groups by variation of the selection angle parameter (the
  tip angle of the final 1H pulse.
 45° angle gives all carbons with attached protons (regardless
  of number) in phase
 90° angle gives only CH groups, the others being suppressed
 135° angle gives all CH and CH3 in a phase opposite to CH2
 Signals from quaternary carbons and other carbons with no
  attached protons are always absent (due to the lack of
  attached protons.
DEPT Spectrum
                                           O

                                           CCH2CH2CH2CH3

                    CH CH
                                                           CH3
                   CH


CH and CH3 unaffected
C and C=O nulled
CH2 inverted
                                            CH2        CH2
                                                     CH2
 200   180   160    140   120   100   80   60   40    20     0

                   Chemical shift ( , ppm)
 blue box- cyclohexane and2,3-dimethyl-2-butene
 single sharp resonance signal in the proton nmr spectrum
  (the former at δ 1.43 ppm and the latter at 1.64 ppm).
 carbon nmr spectrum :- cyclohexane displays a single signal
  at δ 27.1 ppm, generated by the equivalent ring carbon
  atoms (colored blue) and isomeric alkene shows two signals
  1) at δ 20.4 ppm from the methyl carbons (colored brown)
  (2)at 123.5 ppm (typical of the green colored sp2 hybrid
  carbon atoms)
 The C8H10 isomers in the center (red) box have pairs of
  homotopic carbons and hydrogens, so symmetry should
  simplify their nmr spectra. The fulvene (isomer A) has five
  structurally different groups of carbon atoms (colored
  brown, magenta, orange, blue and green respectively) and
  should display five 13C nmr signals (one near 20 ppm and
  the other four greater than 100 ppm).
ortho-xylene (isomer B) will have a proton nmr very similar to isomer
A, it should only display four 13C nmr signals, originating from the four
different groups of carbon atoms (colored brown, blue, orange and green).
The methyl carbon signal will appear at high field (near 20 ppm), and the
aromatic ring carbons will all give signals having δ > 100 ppm.
Finally, the last isomeric pair, quinones A & B in the green box, are easily
distinguished by carbon nmr. Isomer A displays only four carbon nmr
signals (δ 15.4, 133.4, 145.8 & 187.9 ppm); whereas, isomer B displays five
signals (δ 15.9, 133.3, 145.8, 187.5 & 188.1 ppm), the additional signal
coming from the non-identity of the two carbonyl carbon atoms (one
colored orange and the other magenta).
C13 nmr

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C13 nmr

  • 1. By Prashant J Patel (Department of pharmaceutical technology) Indukaka Ipcowala College of Pharmacy New vidhyanagar,anand
  • 2.  Why C13-NMR is required though we have H1-NMR?
  • 3.  1H nmr spectroscopy - The powerful and useful tool a tool for structural analysis. Useless when portions of a molecule lack C-H bonds, no information is forthcoming. Ex: polychlorinated compounds such as chlordane, polycarbonyl compounds such as croconic acid, and compounds incorporating triple bonds (structures below, orange colored carbons).
  • 4.  Even when numerous C-H groups are present, an unambiguous interpretation of a proton nmr spectrum may not be possible. The following three pairs of isomers (A & B) which display similar proton nmr spectra. Although a careful determination of chemical shifts should permit the first pair of compounds (blue box) to be distinguished, the second and third cases (red & green boxes) might be difficult to identify by proton nmr alone.
  • 5. Natural Abundance   Since the major isotope of carbon (12C) has no spin, this option seems unrealistic.  Fortunately, 1.1% of elemental carbon is the 13C isotope, which has a spin I = 1/2, so possible to conduct a carbon nmr experiment.  It is worth noting here, that if much higher abundances of 13C were naturally present in all carbon compounds, proton nmr would become much more complicated due to large one-bond coupling of 13C and 1H.
  • 6. Many obstacles needed to be overcome before carbon nmr emerged as a routine tool :   As noted, the abundance of 13C in a sample is very low (1.1%), so higher sample concentrations are needed.  The 13C nucleus is over fifty times less sensitive than a proton in the nmr experiment, adding to the previous difficulty.  Hydrogen atoms bonded to a 13C atom split its nmr signal by 130 to 270 Hz,  1H-13C splitting is overcome by using an instrumental technique that decouples the proton-carbon interactions, so that every peak in a 13C NMR spectrum appears as a singlet.
  • 7.  The two features of a 13C NMR spectrum that provide the most structural information are the number of signals observed and the chemical shifts of those signals.
  • 8. 13C NMR—Number of Signals The number of signals in a 13C spectrum gives the number of different types of carbon atoms in a molecule. Because 13C NMR signals are not split, the number of signals equals the number of lines in the 13C spectrum. In contrast to the 1H NMR situation, peak intensity is not proportional to the number of absorbing carbons, so 13C NMR signals are not integrated.
  • 9. 13C NMR—Position of Signals In contrast to the small range of chemical shifts in 1H NMR (1-10 ppm usually), 13C NMR absorptions occur over a much broader range (0-220 ppm). The chemical shifts of carbon atoms in 13C NMR depend on the same effects as the chemical shifts of protons in 1H NMR.
  • 10.
  • 11. 13C Chemical shifts are mainly most affected by:   Electronegativity of groups attached to the Carbon  Hybridization state of Carbon sp3 hybridized carbon is more shielded than sp2 sp hybridized carbon is more shielded than sp2, but less shielded than sp3  Anisotropy  All affect 13C Chemical shifts in nearly same fashion as they affect 1H chemical shift
  • 12. Types of Carbons Classification Chemical shift, 1H 13C CH4 0.2 -2 CH3CH3 primary 0.9 8 CH3CH2CH3 secondary 1.3 16 (CH3)3CH tertiary 1.7 25 (CH3)4C quaternary 28 Replacing H by C (more electronegative) deshields C to which it is attached.
  • 13. Electronegativity effects on CH3 Chemical shift, 1H 13C CH4 0.2 -2 CH3NH2 2.5 27 CH3OH 3.4 50 CH3F 4.3 75
  • 14. Electronegativity effects and chain length Cl CH2 CH2 CH2 CH2 CH3 Chemical 45 33 29 22 14 shift, Deshielding effect of Cl decreases as number of bonds between Cl and C increases.
  • 15. Corrrelation chart for C13-NMR chemical shift(ppm)
  • 16. Spin-Spin Splitting   Homonuclear spin-spin splitting: Because of its low natural abundance there is a low probability of finding two C13 atoms next to each other in a single molecule. C13-C13 coupling negligible.  Hetronucler spin-spin splitting: C13 will magnetically couple with attached protons and adjacent protons. N+1 rule is obeyed.
  • 17.
  • 18. Off-Resonance Decoupling   13C nuclei are split only by the protons attached directly to them.  The N + 1 rule applies: a carbon with N number of protons gives a signal with N + 1 peaks.
  • 19. 13C Off-resonance decoupled spectrum 
  • 20. Proton-decoupled spectra   A common method used in determining a carbon-C13 NMR spectrum is to irradiate all of the hydrogen nuclei in the molecules at the same time the carbon resonances are being measured.  Thins required a second radiofrequency(RF) source (the decoupler) tuned to the frequency of the hydrogen nuclei, while the primary RF source is tuned to the C13 frequency.
  • 21.  In this method the hydrogen nuclei are “saturated”, a situation where there are as many downward as there are upward transition, all occurring rapidly.  During time the C-13 spectrum is being determined, the hydrogen nuclei cycle rapidly between their two spin state (+1/2 and -1/2) and the carbon nuclei see an average coupling (i.e. zero) to the hydrogen.  The hydrogen are said to be coupled from the carbon-13 nuclei.  You no longer see multiples for the c13 resonances. Each carbon gives a singlet, and the spectrum is easier to interpret.
  • 22. Nuclear Over Hauser enhancement effect  When we obtain a proton-decoupled c13 spectrum, the intensities of many of the carbon resonances increase significantly above those observed on a proton-coupled experiments.  Carbon atoms with hydrogen atoms directly attached are enhanced the most, and the enhancement increases as more hydrogen are attached. This efface is called the Nuclear Over Hauser enhancement (NOE).  Shown when two different type of atoms are irradiated while NMR spectroscopy of other type is determined.  The effect can be either positive or negative, depending on which atom types are involved.  In case od c-13 interacting with H-1 the effect is positive.so, Intensities of signals increases.
  • 23. Magnetogyric ratio of nucleus being irradiated Magnetogyric ratio of nucleus being observed
  • 24. 1H & 13C NMR: 1,1,2-trichloropropane 
  • 25.
  • 26. DEPT spectra (Distortionless Enhancement by Polarization Transfer)  Useful method for determining the presence of primary, secondary and tertiary carbon atoms.  The DEPT experiment differentiates between CH, CH2 and CH3 groups by variation of the selection angle parameter (the tip angle of the final 1H pulse.  45° angle gives all carbons with attached protons (regardless of number) in phase  90° angle gives only CH groups, the others being suppressed  135° angle gives all CH and CH3 in a phase opposite to CH2  Signals from quaternary carbons and other carbons with no attached protons are always absent (due to the lack of attached protons.
  • 27. DEPT Spectrum O CCH2CH2CH2CH3 CH CH CH3 CH CH and CH3 unaffected C and C=O nulled CH2 inverted CH2 CH2 CH2 200 180 160 140 120 100 80 60 40 20 0 Chemical shift ( , ppm)
  • 28.  blue box- cyclohexane and2,3-dimethyl-2-butene  single sharp resonance signal in the proton nmr spectrum (the former at δ 1.43 ppm and the latter at 1.64 ppm).  carbon nmr spectrum :- cyclohexane displays a single signal at δ 27.1 ppm, generated by the equivalent ring carbon atoms (colored blue) and isomeric alkene shows two signals 1) at δ 20.4 ppm from the methyl carbons (colored brown) (2)at 123.5 ppm (typical of the green colored sp2 hybrid carbon atoms)
  • 29.  The C8H10 isomers in the center (red) box have pairs of homotopic carbons and hydrogens, so symmetry should simplify their nmr spectra. The fulvene (isomer A) has five structurally different groups of carbon atoms (colored brown, magenta, orange, blue and green respectively) and should display five 13C nmr signals (one near 20 ppm and the other four greater than 100 ppm).
  • 30. ortho-xylene (isomer B) will have a proton nmr very similar to isomer A, it should only display four 13C nmr signals, originating from the four different groups of carbon atoms (colored brown, blue, orange and green). The methyl carbon signal will appear at high field (near 20 ppm), and the aromatic ring carbons will all give signals having δ > 100 ppm. Finally, the last isomeric pair, quinones A & B in the green box, are easily distinguished by carbon nmr. Isomer A displays only four carbon nmr signals (δ 15.4, 133.4, 145.8 & 187.9 ppm); whereas, isomer B displays five signals (δ 15.9, 133.3, 145.8, 187.5 & 188.1 ppm), the additional signal coming from the non-identity of the two carbonyl carbon atoms (one colored orange and the other magenta).