Structures of water-CO2 and methanol-CO2 cluster ions: [H2O•(CO2)n]+ and [CH3OH•(CO2)n]+ (n=1–7)

Abstract

Infrared photodissociation (IRPD) spectra of [H(2)O x (CO(2))(n)](+) and [CH(3)OH x (CO(2))(n)](+) (n=1-7) are measured in the 1100-3800 cm(-1) region. At the same time, the solvation characteristics in the clusters are investigated theoretically; the geometry optimization and the vibrational analysis are carried out for the [H(2)O x (CO(2))(n)](+) (n=1-4) and the [CH(3)OH x (CO(2))(n)](+) (n=1-3) ions at the MP2/6-31+G(*) level of theory. The IRPD spectrum of the [H(2)O x (CO(2))(1)](+) ion shows the free OH and the hydrogen-bonded OH stretching bands of the H(2)O(+) ion core and the antisymmetric CO stretching band of the solvent CO(2) molecule, indicating that the solvent CO(2) molecule is preferentially solvated to the H(2)O(+) ion core via the O-H...OCO hydrogen bond. In [H(2)O x (CO(2))(2)](+), the free OH stretching band is not observed; both of the OH groups of the H(2)O(+) ion core are hydrogen bonded to the solvent CO(2) molecules. Spectral features of the IRPD spectra of [H(2)O x (CO(2))(n)](+) (n=3-7) suggest that the third and the fourth CO(2) molecules are bound to the oxygen atom of the H(2)O(+) ion core, and that the first solvation shell of the H(2)O(+) ion core becomes filled with four CO(2) molecules. All the IRPD spectra of the [CH(3)OH x (CO(2))(n)](+) (n=1-7) ions display the hydrogen-bonded OH stretching band of the CH(3)OH(+) ion core, meaning that the solvent CO(2) molecule is preferentially bonded to the OH group of the CH(3)OH(+) ion core, similar to the case of [H(2)O x (CO(2))(n)](+). Quantum chemical calculations for the [CH(3)OH x (CO(2))(1-3)](+) ions demonstrate that the second and the third solvent CO(2) molecules are bonded to the oxygen atom of the CH(3)OH(+) ion core.