CRYSTAL-STRUCTURE OF ACETANILIDE AT 15 AND 295 K BY NEUTRON-DIFFRACTION - LACK OF EVIDENCE FOR PROTON-TRANSFER ALONG THE N-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-O HYDROGEN-BOND

The crystal structure of acetanilide C8H9NO, M(r) = 135.17, orthorhombic, space group Pbca, Z = 8, has been determined from neutron diffraction data at 15 and 295 K. The following are the crystal data: at 15 K [295 K], a = 19.467(3) [19.615(3)] Angstrom, b = 9.332(1) [9.474(1)] Angstrom, c = 7.735(1) [7.979(1)] Angstrom; V = 1405.2(6) [1482.8(6)] Angstrom(3); D-n = 1.2779(1) [1.2110(5)] g . cm(-3), mu(n), = 2.14 [2.02] cm(-1), F(000) = 277.46 fm, lambda = 1.0462 Angstrom, R(F-2) = 0.047 [0.083], wR(F-2) = 0.060 [0.076], and S = 1.033 [1.000] for 2772 [1596] reflections (sin theta/lambda < 0.79 [0.68] Angstrom(-1)) and 173 variable parameters. This new investigation of the structure of acetanilide has been undertaken in order to assess a recent suggestion that conformational substates in the amide proton position may be responsible for the vibrational anomalies. We found no evidence for multiple conformations or transfer along the N-H ... O hydrogen bond of the amide proton at either temperature. However, the intramolecular O ... H6 distance from O to the nearest phenyl ring proton is unusually short and the amide proton has relatively close contacts with one of the phenyl and one of the methyl protons, which may well affect the vibrational parameters of the respective molecular groups.