Intramolecular hydrogen bonds: ab initio Car–Parrinello simulations of arylamide torsions

Gas-phase, room temperature Car–Parrinello molecular dynamics simulations using the HCTH density functional are reported for the arylamides acetanilide (1) and ortho-methylthioacetanilide (2). The simulations show that in 1, rotation around the ring–amide bond is relatively unrestricted. By contrast, in 2 the methylthio side chain encourages the amide to be directed with N–H pointing toward S, not to flip by 360°, and furthermore to remain close to coplanar with the benzene ring. Because of an intramolecular N–H⋯S hydrogen bond, the N–H stretch frequency of 2 is red-shifted by ∼78 cm−1 compared to that of 1.