GAS PHASE SOLVATION OF $Na^{+}$ with $CH_{3}NH_{2}$: Spectroscopic and Computational Results

Vibrational predissociation spectra have been recorded for $Na^{+}(CH_{3}NH_{2})_{n}, n=5-20$ in the P-branch of the 9.6 $\\mu$ region of the $CO_{2}$ laser (1016-1056 $cm^{-1}$). No other predissociation was observed in any other portion of the $CO_{2}$ spectral range. At n=10 there is an abrupt $4 cm^{-1}$ blue shift in absorption to $1038 cm^{-1}$. This is indicative of a structural change in the ion cluster. At n=12 the peak shifts a second time to $1039 cm^{-1}$. There is no further change in absorption frequency seen for the remaining cluster sizes up to n=20. Monte Carlo (MC) and Molecular Dynamics (MD) simulations have been employed to help elucidate the nature of the structural changes. MC and MD results have been obtained for $Na^{+}(CH_{3}NH_{2})_{n};$ n = 6-12 at various temperatures. Both methods indicate that for n = 6-8, the dominant configuration consists of 5 solvents in the first solvation shell. At n $\\geq 9$, a new configuration appears which places 6 molecules in the first salvation shell.