Mechanisms of 1H,1H; 1H,13C; and 13C,13C spin–spin coupling constants in benzyl cyanide and some derivatives. Experimental and theoretical estimates of internal rotational potentials

Abstract

The mechanisms of long-range spin–spin coupling constants involving the methylene protons and the 13C nucleus of the cyano group are discussed for benzyl cyanide. Analysis of the 1H nmr spectrum of benzyl cyanide-8-13C in benzene-d6 solution yields nJ(H,CH2) and nJ(H,13CN) for n = 4–6. Similar data are reported for the 2,6-dichloro and 2,6-difluoro derivatives, together with some sign determinations. nJ(13C,13CN), n = 1–5, are given for the three compounds. It is shown that all these parameters are consistent with a small barrier to internal rotation about the [Formula: see text] bond in benzyl cyanide in solution. Computations at various levels of molecular orbital theory agree that this barrier is small. The nJ(13C, 13CN) imply a stabilization in polar solvents of the conformation in which the cyano group of benzyl cyanide lies in a plane perpendicular to the benzene plane. The molecular orbital calculations indicate a predominantly twofold nature of the internal barrier, although a significant fourfold component is also present. The coupling constants cannot discern the presence of the fourfold component for benzyl cyanide nor for its 2,6-difluoro derivative. 1J(13C,13CN) is solvent dependent. A table of the computed sidechain geometries is appended.