STRUCTURE AND STRUCTURAL TRANSITIONS OF SIZE-SELECTED CLUSTERS - (CH3CN)(N) AND (CH3OH)(6)

Photodissociation spectra of acetonitrile clusters (CH3CN)(n) from n = 4 to n = 13, n = 24 +/- 3, n = 33 +/- 8 have been measured near the m onomer frequencies of the CC-stretch (920.3 cm(-1)) and the CH3-rock ( 1041.8 cm(-1)) mode using a line-tunable CO2 laser. The clusters are s ize selected by momentum transfer in a scattering experiment with He, Ne and Ar atoms, respectively. The spectra near the CC-stretch mode ar e slightly red-shifted and do not depend on the cluster size. Near the CH3-rock mode there are two series of identical spectra for the even- sized clusters n = 4, 6, 12 and n = 9 exhibiting three peaks and the o dd-sized clusters n = 5, 7, 13 and n = 8 showing two peaks, respective ly. The results are explained by the pairwise antiparallel arrangement of these molecules with a strong dipole moment. For the methanol hexa mer a transition between the two energetically lowest-lying isomers ha s been investigated by measuring their IR spectra.