Nuclear Magnetic Resonance Spectroscopy: Theory and Applications
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compound has distinct chemical shift and provide useful information even in the
absence of predictive rules.
7 13 C-NMR Spectroscopy
1 H-NMR spectrum of compound provides detailed information of set of proton nuclei
(equivalent proton), number of protons in each set of proton-nuclei, chemical environment of each set of nuclei and spatial position of nuclei.
13 C-NMR is used to
determine number of non-equivalent carbons and also types of carbon-atoms present
in compounds [21].
7.1 Characteristic Features of 13 C-NMR.
• All nuclei having non-zero spin quantum number are NMR active.
12 C has zero
spin quantum number, but
13 C naturally occurring isotope of carbon having spin
quantum number equal to half (I = ½) and hence NMR-active.
• The natural abundance of
13 C is only 1.1% and also possesses very low magnetogyric ratio (γ). Magnetogyric ratio of
13 C is only 1/4th
1 H-proton. A 1.4 T
NMR-spectrometer, if excited at 300 MHz radiofrequency to record
1 H-NMR
spectrum then only 75 MHz transmission will be required to record
13 C-NMR
spectrum.
• The chemical shifts of
13 C-NMR are spread over chemical shift range of δ-value
0–220 ppm relative to TMS reference than
1 H-NMR (δ = 0–15 ppm).
• 13 C–
13 C coupling is negligible because of low natural abundance of
13 C isotopes
and hence each magnetically non-equivalent carbon gives only a single sharp
peak.
• There is possibility of
13 C–
1 H coupling that originate splitting of magnetically
equivalent carbon-peak by coupling of protons directly bonded to carbon-atoms
and hence n + 1 rule is applied.
13 C-NMR spectra can be simplified by broad-band
decoupling or off-resonance decoupling techniques.
• Similar to
1 H-NMR, chemical shift is observed in
13 C-NMR which depends upon
various factors. The chemical shift of
13 C NMR is greater than
1 H-NMR due to
direct attachment of the substituent that affect chemical shift to
13 C atom.
7.2 Chemical Shift
Chemical shift in
13 C-NMR similar to
1 H-NMR are expressed in ppm using TMS
as a reference at δ-value at zero-ppm. The range of chemical shift lie between 0 and
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compound has distinct chemical shift and provide useful information even in the
absence of predictive rules.
7 13 C-NMR Spectroscopy
1 H-NMR spectrum of compound provides detailed information of set of proton nuclei
(equivalent proton), number of protons in each set of proton-nuclei, chemical environment of each set of nuclei and spatial position of nuclei.
13 C-NMR is used to
determine number of non-equivalent carbons and also types of carbon-atoms present
in compounds [21].
7.1 Characteristic Features of 13 C-NMR.
• All nuclei having non-zero spin quantum number are NMR active.
12 C has zero
spin quantum number, but
13 C naturally occurring isotope of carbon having spin
quantum number equal to half (I = ½) and hence NMR-active.
• The natural abundance of
13 C is only 1.1% and also possesses very low magnetogyric ratio (γ). Magnetogyric ratio of
13 C is only 1/4th
1 H-proton. A 1.4 T
NMR-spectrometer, if excited at 300 MHz radiofrequency to record
1 H-NMR
spectrum then only 75 MHz transmission will be required to record
13 C-NMR
spectrum.
• The chemical shifts of
13 C-NMR are spread over chemical shift range of δ-value
0–220 ppm relative to TMS reference than
1 H-NMR (δ = 0–15 ppm).
• 13 C–
13 C coupling is negligible because of low natural abundance of
13 C isotopes
and hence each magnetically non-equivalent carbon gives only a single sharp
peak.
• There is possibility of
13 C–
1 H coupling that originate splitting of magnetically
equivalent carbon-peak by coupling of protons directly bonded to carbon-atoms
and hence n + 1 rule is applied.
13 C-NMR spectra can be simplified by broad-band
decoupling or off-resonance decoupling techniques.
• Similar to
1 H-NMR, chemical shift is observed in
13 C-NMR which depends upon
various factors. The chemical shift of
13 C NMR is greater than
1 H-NMR due to
direct attachment of the substituent that affect chemical shift to
13 C atom.
7.2 Chemical Shift
Chemical shift in
13 C-NMR similar to
1 H-NMR are expressed in ppm using TMS
as a reference at δ-value at zero-ppm. The range of chemical shift lie between 0 and