Indirect carbon-carbon spin-spin couplings across one, two and three bonds in pyridine and diazine systems: experiment and theory

An excellent linear correlation is found between a large body of experimental spin-spin carbon-carbon couplings, J(CC), across one, two and three bonds in pyridine and diazine ring systems and the corresponding B3PW91/6-311++G(d,p)//B3PW91/6-311++G(d,p) computations. The correlation does not differ significantly from the simplest relationship possible, J(CC)(exp),=J(CC)(calcd)., within a small and random spread of about 1 Hz. There are 276 experimental values considered, and 124 out of these are new and come from the present work. The aromatic carbon-carbon couplings vary from -7.6 through +78.5 Hz. It is shown that the correlation provides a reliable tool for predictions of the signs of aromatic J(CC)'s even if the magnitudes of the latter are of the order of 1 Hz. It is demonstrated, for the first time, that the relatively weak (2)J(CC) couplings, in the heteroaromatic systems studied, can bear either sign and span a considerable range of about 11 Hz. The character of the correlation indicates that rovibronic effects on aromatic J(CC)'s and those of nuclear motions on aromatic J(CC)'s are practically negligible. All of this is in a perfect agreement with our recent extensive studies on aromatic J(CC)'s in analogous benzene ring system. Substituent effects on the aromatic J(CC)'s turn out to be significant not only for (1)J(CC)'s but also for most of (3)J(CC)'s and (2)J(CC)s, and the computation neatly reciprocates these trends. Copyright (c) 2008 John Wiley & Sons, Ltd.