Calculation of structures and vibrational spectra of acetonitrile clusters

The minimum energy structures of acetonitrile clusters from n = 2 to n = 13 have been calculated using a total cluster potential which is composed from a semi-empirical intermolecular potential in combination with the intramolecular force field. Based on these results the frequency shifts of the CC stretch and the CH(3) wag modes are calculated and compared with recent measurements of size selected free clusters and clusters embedded in large helium droplets. The calculations reproduce the coarse structure of the measurements and thus demonstrate that the calculated structures with antiparallel molecules as building blocks caused by the multipolar interaction are correct. For the trimer and tetramer the spectra obtained in the ultracold helium clusters are better reproduced than those of the free clusters indicating that in this experimental arrangement specific isomeric structures are present which are not observed in the warmer free clusters. (C) 1998 Elsevier Science B.V. All rights reserved.