Proximity effects on nuclear spin–spin coupling constants. Part 2.—The electric field effect on 1J(CH) couplings

Abstract

The electric field effect on ¹J(CH) for C(sp³)—H and C(sp)—H bonds is studied by means of the IPPP-CLOPPA method at an ab initio level. Opposite trends for the dominant Fermi contact, FC, term in both types of bonds are found. It is shown that these opposite trends originate in the π-electronic system associated with the carbon atom in C(sp)—H bonds. In fact, while the σ-transmitted component of the FC term behaves like the FC term in a C(sp³)—H bond, the π-transmitted component shows an opposite electric field effect. Results are compared with the proximity effects that an atom bearing lone pairs exerts on these couplings, which were studied in Part 1 of this series of papers. The influence of an electric field on the bond lengths of these two types of bonds is also analysed.