Pseudohalogen chemistry. Part 12. NMR and IR studies of substituent effects in 4-substituted thiocyanatobenzenes

Abstract

1H and 13C NMR chemicals shifts for 15 4-X-thiocyanatobenzenes have been measured at high dilution in CDCl 3 solution. Substituent chemical shifts for the SCN group were derived from the data for thiocyanatobenzene. Excellent substituent-shift correlations ( r > 0.995) were obtained for the SCN carbon using single and dual substituent parameter (SSP and DSP) approaches, and for the ring carbons using SSP, modified DSP, and triple substituent parameter (TSP) approaches. The SCN carbon experiences a reverse substituent effect through both polar and resonance components of the electronic effect of X. Reverse resonance effects were also observed for C-2(6) and C-4, and reverse polar effects for C-2(6) and C-3(5). The SCN group causes a 50% enhancement of the normal para SCS values of X at C-1. SSP and DSP substituent-shift correlations were successful for H-3(5) but only fair or poor for H-2(6). IR SCN stretching frequencies were also measured and correlated successfully with substituent parameters using SSP and DSP methods: in contrast to δSCN, νSCN shows a normal substituent effect.