Spectroscopic and theoretical studies of some N-methoxy-N-methyl-2-[(4′-substituted) phenylthio]propanamides

The analysis of the IR carbonyl band of the N-methoxy-N-methyl-2-[(4′-substituted)phenylthio]propanamides Y-PhSCH(Me)C(O)N(OMe)Me (Y=OMe 1, Me 2, H 3, Cl 4, NO25), supported by B3LYP/cc-pVDZ calculations of 3, indicated the existence of two gauche conformers (g1 and g2), the g1 conformer being the more stable and the less polar one (in gas phase and in solution). Both conformers are present in solution of the polar solvents (CH2Cl2 and CH3CN) for 1–5 and in solution of the less polar solvent (CHCl3) for 1–4, while only the g1 conformer is present in solution of non polar solvents (n-C6H14 and CCl4) and in solution of CHCl3 for 5. NBO analysis shows that both the σCS→πCO∗ (hyperconjugative) and the πCO→σC-S∗ orbital interactions contribute almost to the same extent for the stabilization of g1 and g2   conformers. The πCO∗→σCS∗, ns→πCO∗ and the ns→πCO∗ orbital interactions stabilize more the g1 conformer than the g2 one. Moreover, the suitable geometry of the g1   conformer leads to its stabilization through the LPO2→σC8H11∗ orbital interaction (hydrogen bond) along with the strong O[CO]δ-…H[O-Ph]δ+ electrostatic interaction. On the other hand, the appropriate geometry of the g2   conformer leads to its stabilization by the LPO22→σC9H13∗ orbital interaction (hydrogen bond) along with the weak O[OMe]δ-…H[o′-Ph]δ+ electrostatic interaction. As for the 4′-nitro derivative 5 the ortho  -phenyl hydrogen atom becomes more acidic, leading to a stronger O[CO]δ-…H[o-Ph]δ+ interaction and, thus, into a larger stabilization of the g1 conformer in the whole series. This trend is responsible for the unique IR carbonyl band in CHCl3 solution of 5. The larger occupancy of the πCO∗ orbital of the g1 conformer relative to that of the g2   conformer, along with the O[CO]δ-…H[o-Ph]δ+ electrostatic interaction (hydrogen bond) justifies the lower carbonyl frequency of the g1 conformer with respect to the g2 one, in gas phase and in solution.