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

Abstract

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, NO2 5), supported by B3LYP/cc-pVDZ calculations of 3, indicated the existence of two gauche conformers (g(1) and g(2)), the g(1) 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 g(1) 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 sigma(C-S) -> pi*(C=O) (hyperconjugative) and the pi(C=O) -> sigma*(C-S) orbital interactions contribute almost to the same extent for the stabilization of g(1) and g(2) conformers. The pi*(C=O) -> sigma*(C-S), n(S) -> pi*(C=O) and the n(S) -> pi*(C=O) orbital interactions stabilize more the g(1) conformer than the g(2) one. Moreover, the suitable geometry of the g(1) conformer leads to its stabilization through the LPO2 -> sigma*(C8-H11) orbital interaction (hydrogen bond) along with the strong O-[CO](delta-) center dot center dot center dot H-[O-Ph](delta+) electrostatic interaction. On the other hand, the appropriate geometry of the g(2) conformer leads to its stabilization by the LPO22 -> sigma*(C9-H13) orbital interaction (hydrogen bond) along with the weak O-[OMe](delta-) center dot center dot center dot H-[o'-Ph](delta+) electrostatic static interaction. As for the 4'-nitro derivative 5 the ortho-phenyl hydrogen atom becomes more acidic, leading to a stronger O-[CO](delta-) center dot center dot center dot H-[o-Ph](delta+) interaction and, thus, into a larger stabilization of the g(1) 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 pi*(C=O) orbital of the g(1) conformer relative to that of the g(2) conformer, along with the O-[CO](delta-) center dot center dot center dot H-[o-Ph](delta+) electrostatic interaction (hydrogen bond) justifies the lower carbonyl frequency of the g(1) conformer with respect to the g(2) one, in gas phase and in solution