NMR, MP2 and DFT Study of Thiophenoxyketenimines (o-ThioSchiff bases): Determination of the preferred form

Five new thiophenoxyketinimines have been synthesized. 1H and 13C NMR spectra as well as deuterium isotope effects on 13C chemical shifts are determined and spectra are assigned. DFT and MP2 calculations of both structures, chemical shifts and isotope effects on chemical shifts are done. The combined analysis reveals that the compounds are primarily on a zwitter ionic form with an NH+ and a S- group and with a little of the neutral form mixed in. Very strong intramolecular hydrogen bonding is found and very high NH chemical shifts are observed.
The theoretical calculations show that calculations at the MP2 level are best to obtain correct “C=S” chemical shifts.