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

Infrared photodissociation (IRPD) spectra of [H2O• (CO2)n]+ and [CH3OH• (CO2)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 [H2O• (CO2)n]+ (n=1–4) and the [CH3OH• (CO2)n]+ (n=1–3) ions at the MP26-31+G* level of theory. The IRPD spectrum of the [H2O・(CO2)1]+ ion shows the free OH and the hydrogen-bonded OH stretching bands of the H2O+ ion core and the antisymmetric CO stretching band of the solvent CO2 molecule, indicating that the solvent CO2 molecule is preferentially solvated to the H2O+ ion core via the O–H• • •OCO hydrogen bond. In [H2O•(CO2)2]+, the free OH stretching band is not observed; both of the OH groups of the H2O+ ion core are hydrogen bonded to the solvent CO2 molecules. Spectral features of the IRPD spectra of [H2O•(CO2)n]+ (n=3–7) suggest that the third and the fourth CO2 molecules are bound to the oxygen atom of the H2O+ ion core, and that the first solvation shell of the H2O+ ion core becomes filled with four CO2 molecules. All the IRPD spectra of the [CH3OH•(CO2)] + (n=1–7) ions display the hydrogen-bonded OH stretching band of the CH3OH+ ion core, meaning that the solvent CO2 molecule is preferentially bonded to the OH group of the CH3OH+ ion core, similar to the case of [H2O•(CO2)n]+. Quantum chemical calculations for the [CH3OH• (CO2)1–3]+ ions demonstrate that the second and the third solvent CO2 molecules are bonded to the oxygen atom of the CH3OH+ ion core.