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One-bond C-H Coupling Constants of Acetyl Groups as Possible Monitors of CH Hydrogen Bonds and Electric Field Effects

Hansen P.

Polish Journal of Chemistry

2003

Vol. 77 / nr 11

1683-1691

C-H coupling constants of acetyl groups of a series of substituted 2-hydroxyacetophenones have been measured. These 1J(C,H) couplings are then intercompared and compared to acetophenone in order to elucidate the cause of the variation. Structures are calculated using ab initio DFT methods. A comparison of 2-hydroxyacetophenone and acetophenone shows an increase of ~0.5 Hz. An OR group in the 6-position of 2-hydroxyacetophenone leads to an increase of 1J(C,H) of ~1 Hz and substitution of an acetyl group at 3- or 5-positons likewise leads to an increase, whereas OR groups in 3-, 4- or 5-positions lead to small negative changes compared to the corresponding 2-hydroxyacetophenones. The variations in the 1J(C,H) coupling constants of 6-OR substituted 2-hydroxyacetophenones are discussed as a function of C-H hydrogen bonding and electric field effects. The former is shown not to be at play, whereas the latter is clearly active, but in some instances in an indirect fashion.