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

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Presentation transcript:

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.

Calculation of 13C shifts Well defined for acyclic, saturated hydrocarbons d = -2.5 + ∑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.

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 27.7 + 5.2 + (2 x 8.9) + -1.2 = 49.5 (obs. 49.9)

g a terminal b d = 16 + 31 – 4 = 43 (obs. 42) internal g d = 27 + 8 = 35 (obs. 34) a b Starting with benzene = 128.7 128.4 149.8 148.3 123.4 Starting with ethene = 123.3 =CH2 104-115; =CHR 120-140; =CR2 140-165 129.5 134.7

Distortionless Enhancement by Polarization Transfer CH CH, CH3 CH2

COrrelation SpectroscopY Also H,H-COSY

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