Infrared spectra and density functional theory calculations for Mn+-(CH4)n (n=1-6) clusters

Infrared spectra are measured for the mass-selected Mn(CH4)(n)(+) (n = 1-6) clusters in the C-H stretch region (2700-3100 cm(-1)) by monitoring photofragment ions. Accompanying density functional theory calculations are performed for possible structural isomers, including the Mn+-(CH4)(n) ion-molecule complexes in which the ligands are attached to the Mn+ ion in an eta(3) configuration, and the inserted H-Mn-CH3+-(CH4)(n-1) complexes. The experimental infrared spectra for all species are dominated by a single band, appearing at 2836 cm(-1) for n = 1 and progressively shifting to 2880 cm(-1) for n = 6. The observed infrared spectra match predicted spectra for Mn+-(CH4)(n) ion-molecule complexes containing Mn+ in its S-7 (4s(1) 3d(5)) electronic ground state, with the dominant band corresponding to the totally symmetric C-H stretching vibrations of the attached CH4 ligands. No evidence was found for clusters containing the Mn+ cation in its S-5 (4s(1)3d(5)) electronic state. In larger clusters, the CH4 ligands are generally bound to the Mn+ ion such that adjacent Mn+center dot center dot center dot C bonds are approximately at right angles to one another taking advantage of partial 4s/4p hybridization to minimise the CH4 molecules' repulsive interaction with the half-filled 4s orbital of Mn+. (C) 2010 Elsevier B.V. All rights reserved.