1. Diamagnetic shielding (electrons in s- and p-orbitals) and paramagnetic shielding (electrons in p-orbitals with angular momentum) contributes to the shift of C-atoms.

2. Calculation of 13C shifts
Well defined for acyclic, saturated hydrocarbons
d = ∑Aini
Replacement of hydrogen causes a relative constant shift that depends primarily on the electronegativity of X.
d Methane = -2.1 replacement of H by C (CH3, CH2, CH, C) causes a +9.1 shift in the a-position, +9.4 in the b-position, and -2.5 in the g-position.

3. The formula for alkanes is based on a weighted average for open chain conformers.
A complete new set of parameters is necessary for cyclic alkanes:
Cyclopropane = -2.6, cyclobutane = 23.3, and cyclohexane = 27.7;
all other rings have 27.7 ± 2 ppm
b-ax = 5.2
b-gem = -1.2
b-eq = 8.9
(2 x 8.9) = 49.5 (obs. 49.9)

4. g a
terminal b
d = – 4 = 43 (obs. 42)
internal g
d = = 35 (obs. 34) a b
Starting with benzene = 128.7
128.4
149.8
148.3
123.4
Starting with ethene = 123.3
=CH ; =CHR ; =CR
129.5
134.7

5. Distortionless Enhancement by Polarization TransferCH
CH, CH3
CH2

6. COrrelation SpectroscopY
Also H,H-COSY

7. HETeronuclear chemical shift CORrelation
Also HSC (heteronuclear shift correlation) or H,C COSY