b S Supporting Information W hen the internal energy in argon-bearing cluster ions (hereafter referred to as argon-tagged) is adequately low, there can be insufficient kinetic energy to overcome energetic barriers to rearrange into a lower or global-minimum-energy conformer. As a result, the cluster ion beam can contain multiple conformers, including kinetically trapped, high-energy clusters in addition to minimum-energy clusters.Previous studies of cluster ion conformers have been performed using a two-laser IRÀIR double resonance technique. 1,2 A recent study on tagged H þ (H 2 O) 6 cluster ions has revealed that the relative populations of the Zundel and Eigen forms are affected by the type of species used for tagging. 3 We have discovered that by monitoring different photofragmentation channels, selective IRPD spectra of kinetically trapped, high-energy con-

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Infrared predissociation (IRPD) spectra of Li(+)(CH(4))(1)Ar(n), n = 1-6, clusters are reported in the C-H stretching region from 2800 to 3100 cm(-1). The Li(+) electric field perturbs CH(4) lifting its tetrahedral symmetry and gives rise to multiple IR active modes. The observed bands arise from the totally symmetric vibrational mode, v(1), and the triple degenerate vibrational mode, v(3). Each band is shifted to lower frequency relative to the unperturbed CH(4) values. As the number of argon atoms is increased, the C-H red shift becomes less pronounced until the bands are essentially unchanged from n = 5 to n = 6. For n = 6, additional vibrational features were observed which suggested the presence of an additional conformer. By monitoring different photodissociation loss channels (loss of three Ar or loss of CH(4)), one conformer was uniquely associated with the CH(4) loss channel, with two bands at 2914 and 3017 cm(-1), values nearly identical to the neutral CH(4) gas-phase v(1) and v(3) frequencies. With supporting ab initio calculations, the two conformers were identified, both with a first solvent shell size of six. The major conformer had CH(4) in the first shell, while the conformer exclusively present in the CH(4) loss channel had six argons in the first shell and CH(4) in the second shell. This conformer is +11.89 kJ/mol higher in energy than the minimum energy conformer at the MP2/aug-cc-pVDZ level. B3LYP/6-31+G* level vibrational frequencies and MP2/aug-cc-pVDZ level single-point binding energies, D(e) (kJ/mol), are reported to support the interpretation of the experimental data.

Toluene photosensitizes the racemization of optically active penta-2,3-diene through a triplet energytransfer mechanism.