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 , NO 2 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 (CH 2 Cl 2 and CH 3 CN) for 1 – 5 and in solution of the less polar solvent (CHCl 3 ) for 1 – 4 , while only the g 1 conformer is present in solution of non polar solvents ( n -C 6 H 14 and CCl 4 ) and in solution of CHCl 3 for 5 . NBO analysis shows that both the σ C S → π C O ∗ (hyperconjugative) and the π C O → σ C - S ∗ orbital interactions contribute almost to the same extent for the stabilization of g 1 and g 2 conformers. The π C O ∗ → σ C S ∗ , n s → π C O ∗ and the n s → π 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 LP O 2 → σ C 8 H 11 ∗ 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 g 2 conformer leads to its stabilization by the LP O 22 → σ C 9 H 13 ∗ 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 g 1 conformer in the whole series. This trend is responsible for the unique IR carbonyl band in CHCl 3 solution of 5 . The larger occupancy of the π C O ∗ orbital of the g 1 conformer relative to that of the g 2 conformer, along with the O [ CO ] δ - … H [ o - Ph ] δ + 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.