Structure and structural transitions of size-selected clusters: (CH3CN)n and (CH3OH)6

Abstract

Photodissociation spectra of acetonitrile clusters (CH₃CN)_n from n= 4 to n= 13, n= 24 ± 3, n= 33 ± 8 have been measured near the monomer 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 size selected by momentum transfer in a scattering experiment with He, Ne and Ar atoms, respectively. The spectra near the CC-stretch mode are 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 odd-sized clusters n= 5, 7, 13 and n= 8 showing two peaks, respectively. The results are explained by the pairwise antiparallel arrangement of these molecules with a strong dipole moment. For the methanol hexamer a transition between the two energetically lowest-lying isomers has been investigated by measuring their IR spectra.