α-substituent effects in 13C NMR of hydrocarbons. Quantum chemical calculations

✍ Scribed by Marianne E. van Dommelen; Henk M. Buck; Jan W. de Haan

• Publisher: Elsevier Science
• Year: 1978
• Tongue: English
• Weight: 250 KB
• Volume: 57
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(78)80355-8

✦ Synopsis

The "C NMR u-substitr;cnt effects within the series methane to neopentane acd ethene to isobutene have been calculated rrdng Hartree-Foc?c perturbation theory with XlINDO/3. The sign and order of magnitude of the calculated a-effects are in agcemen: with new experimental -&dues, including effects of multipie substitution.

Recently, the 13C chemical shifts of small hydrocarbons have been measured in the gas phase [I ] _ The resulting a-substituent effects, without disturbing differences in solvent effects, are given in table 1. The largest discrepancy with the previous experimental a-effects [2] is the difference between methane and e*&ane: 14.1 ppm instead of 8.0 ppm_ The new creffects show that the consequences of multiple r _ninal methyl substitution become progressively smaller, analogous to the well-known downfield shifts of C2 in ~ .e series butane, 2-methylbutane and 2,2slimethylbutane, * o-ctTects, uicuhtcd by coupled Hartree-Fock perturbation theory [S-IO] were not incinded in table 1, because only the shift difrerence between metkane arld ethane is avvaikzble fcr comparison with experimental values.