Infrared photodissociation spectra of halomethane halide cluster anions and their solvation structures

Solvation structures of X-(CH3X)(n) (X = Cl and Br) were studied by use of infrared photodissociation spectroscopy (925-1090 cm(-1)) together with mas s spectroscopy. The cluster anion, X-(CH3X)(n) (n greater than or equal to 13) was found to have an absorption peak at a vibrational wavenumber, which is comparable to that of the X-C-H bending mode of a free CH,X molecule. O n the other hand, X-(CH3X)(12) did not exhibit any detectable absorption at the wavenumbers studied. It was concluded that the first solvation shell o f the cluster anion is completed at n = 12 (icosahedral structure), and tha t with a sub-shell is completed at n = 18 (capped icosahedral structure).