Methyl group dynamics in paracetamol and acetanilide: probing the static properties of intermolecular hydrogen bonds formed by peptide groups

Measurements of tunnelling and librational excitations for the methyl group in paracetamol and tunnelling excitations for the methyl group in acetanil ide are reported. In both cases, results are compared with molecular mechan ics calculations, based on the measured low temperature crystal structures, which follow an established recipe. Agreement between calculated and measu red methyl group observables is not as good as expected and this is attribu ted to the presence of comprehensive hydrogen bond networks formed by the p eptide groups. Good agreement is obtained with a periodic quantum chemistry calculation which uses density functional methods, these calculations conf irming the validity of the one-dimensional rotational model used and the cr ystal structures. A correction to the Coulomb contribution to the rotationa l potential in the established recipe using semi-emipircal quantum chemistr y methods, which accommodates the modified charge distribution due to the h ydrogen bonds, is investigated. (C) 1999 Elsevier Science B.V. All rights r eserved.