Infrared photodissociation spectra of CH3+-Arn complexes (n=1-8)

Abstract

Infrared photodissociation spectra of the ionic complexes CH3+-Arn (n=1-8) have been recorded in the vicinity of the ν3 asymmetric stretching vibration of the CH3+ monomer. The CH3+-Ar dimer has also been investigated in the spectral range of the first CH stretching overtones, resulting in the characterization of its 2ν1, ν1+ν3, and 2ν3 vibrational states at the level of rotational resolution. The spectrum of CH3+-Ar is consistent with a pyramidal C3ν minimum structure of the complex predicted by ab initio calculations at the MP2 level, whereby the Ar atom is attached to the empty 2pz orbital of the CH3+ moiety. The rotationally resolved ν3 spectrum of the CH3+-Ar2 trimer indicates that the two Ar atoms are located on opposite sides of the CH3+ moiety on the C3 axis, with significantly differing intermolecular C-Ar bond lengths. The splittings observed in the trimer spectrum are attributed to a tunneling motion between two equivalent C3ν minimum configurations via a symmetric D3h transition state. The spectra of larger clusters (n≥3) lack rotational resolution, however the positions and profiles of the ν3 band suggest that the additional Ar atoms are weakly attached to CH3+-Ar2 trimer, which acts as the effective nucleation center for the cluster growth. The stretching fundamentals of the CH3+ ion core in the CH3+-Arn clusters are intermediate between those of the methyl radical and the methyl cation, implying a substantial charge transfer from the rare gas atoms to the unoccupied 2pz orbital of CH3+. © 1995 American Institute of Physics.