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 360degrees, 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 similar to78 cm(-1) compared to that of 1. (C) 2003 Elsevier B.V. All rights reserved.