Nuclear Magnetic Resonance Spectroscopy: Theory and Applications
497
H 3 C
H 2
C
H 2
C
H 2
C CH 2
H
n-pentane
13.9
22.8
34.7
22.8
13.9
H 3 C
H 2
C
H 2
C
H 2
C CH 2
CH 3
n-hexane
14.1
23.1
32.2
32.2
23.1
α-carbon
13.9 → 23.1
α-effect
~9.1
β-carbon
22.8 → 32.2
β-effect
~9.4
γ-carbon
34.7 → 32.2
γ-effect
~2.5
This effect is caused due to possibilities of gauch and anti conformation of the
resonating carbon. The γ-effect in anti conformation is +1 and the pure gauche
conformation is −6. The upfield shift due to γ-substituent is average of two
conformation considering the structure mix of two conformers (+1 − 6)/2 = 2.5.
On the basis of above observation, the chemical shift value of each carbon can be
calculated in straight chain hydrocarbons using the formulaδ i = −2.6 + 9.1nα + 9.4nβ − 2.5nγ + 0.3nδ
In above equation, nα is the number of carbon atom bonded directly to the i-th
carbon atom, nβ, nγ and nδ are the number of carbon atoms from two, three and four
bonds distances respectively from i-th carbon atom.
Similarly empirical formula effect of branched chain, resonance effect, carbonyl
carbon atom etc. can be analyzed and calculated.
7.4 Simplification of 13 C-spectra
In
13 C-NMR, coupling between
13 C13 C nuclei is very rare due to very less natural
abundance of
13 C isotope; however coupling between
13 C and
1 H nuclei is very
common and ever occur when they are separated by several intervening bonds. The
spectrum obtained after such coupling interactions are very complex and it is difficult
to get information of carbon-framework and/or hydrogen nuclei of the compound
under observation [24].
497
H 3 C
H 2
C
H 2
C
H 2
C CH 2
H
n-pentane
13.9
22.8
34.7
22.8
13.9
H 3 C
H 2
C
H 2
C
H 2
C CH 2
CH 3
n-hexane
14.1
23.1
32.2
32.2
23.1
α-carbon
13.9 → 23.1
α-effect
~9.1
β-carbon
22.8 → 32.2
β-effect
~9.4
γ-carbon
34.7 → 32.2
γ-effect
~2.5
This effect is caused due to possibilities of gauch and anti conformation of the
resonating carbon. The γ-effect in anti conformation is +1 and the pure gauche
conformation is −6. The upfield shift due to γ-substituent is average of two
conformation considering the structure mix of two conformers (+1 − 6)/2 = 2.5.
On the basis of above observation, the chemical shift value of each carbon can be
calculated in straight chain hydrocarbons using the formulaδ i = −2.6 + 9.1nα + 9.4nβ − 2.5nγ + 0.3nδ
In above equation, nα is the number of carbon atom bonded directly to the i-th
carbon atom, nβ, nγ and nδ are the number of carbon atoms from two, three and four
bonds distances respectively from i-th carbon atom.
Similarly empirical formula effect of branched chain, resonance effect, carbonyl
carbon atom etc. can be analyzed and calculated.
7.4 Simplification of 13 C-spectra
In
13 C-NMR, coupling between
13 C13 C nuclei is very rare due to very less natural
abundance of
13 C isotope; however coupling between
13 C and
1 H nuclei is very
common and ever occur when they are separated by several intervening bonds. The
spectrum obtained after such coupling interactions are very complex and it is difficult
to get information of carbon-framework and/or hydrogen nuclei of the compound
under observation [24].