1H and 13C nuclear magnetic resonance spectra of benzylidenemalononitriles : a method for the determination of σ+ substituent constants

Abstract

The ¹H and ¹³C n.m.r. spectra of 15 substituted benzylidenemalononitriles (IIa–o) are reported in detail. The ¹H chemical shift of the vinylic hydrogen changes by 0.67 p.p.m. over the range of substituents and may be correlated with either σ or σ⁺ substituent constants with almost the same degree of confidence (r0.989 for σ and 0.994 for σ⁺). The ¹³C chemical shift of the β-vinylic carbon is, however, much more sensitive to substituent effects (range of δ= 17.2 p.p.m.) and shows a clear correlation with the σ⁺ substituent constants (r 0.996) rather than the corresponding σ values (r 0.968). Three hitherto unreported σ⁺ values are derived from the data.