The interplay of thio(seleno)amide/vinylogous thio(seleno)amide "resonance" and the anisotropic effect of thiocarbonyl and selenocarbonyl functional groups

Amino-substituted thio(seleno)acrylamides 1-4 were synthesized and their 1H and 13C NMR spectra assigned. Both the NMR data and the results of theoretical calculations at the ab initio level of theory were employed to elucidate the adopted structures of the compounds in terms of E/Z isomerism and s-cis/s-trans configuration. In the case of the asymmetrically N(Me)Ph-substituted compounds, ab initio GIAO-calculated ring current effects of the N-phenyl group were applied to successfully determine the preferred conformer bias. The restricted rotations about the two C-N partial double bonds were studied by DNMR and the barriers to rotation (DeltaG(c)++) determined at the coalescence temperatures, and these were discussed with respect to the structural differences between the compounds. The barriers to rotation were also calculated at the ab initio level of theory where the best results (R(2) = 0.8746) were obtained only with inclusion of the solvent at the SCIPCM-HF/6-31G* level of theory. The calculations also provided means of assessing structural influences which were not available due to inaccessible rotation barriers. By means of natural bond orbital (NBO) analysis of 1-4, the occupation numbers of nitrogen lone pairs and bonding/antibonding pi/pi orbitals were shown to quantitatively describe thio(seleno)amide/vinylogous thio(seleno)amide "resonance". Finally, the thio(seleno)carbonyl anisotropic effect was quantitatively calculated by the GIAO method and visualized by isochemical shielding surfaces (ICSS). Only marginal differences between the two anisotropic effects were calculated and are therefore of questionable utility for previous and future applications with respect to stereochemical assignments.