Structures of [(CO2)n(H2O)m]- (n=1-4, m=1,2) cluster anions. I. Infrared photodissociation spectroscopy.

The infrared photodissociation spectra of (CO(2))(n)(H(2)O)(m) (n=1-4, m=1, 2) are measured in the 3000-3800 cm(-1) range. The (CO(2))(n)(H(2)O)(1) spectra are characterized by a sharp band around 3570 cm(-1) except for n=1; (CO(2))(1)(H(2)O)(1) does not photodissociate in the spectral range studied. The (CO(2))(n)(H(2)O)(2) (n=1, 2) species have similar spectral features with a broadband at approximately 3340 cm(-1). A drastic change in the spectral features is observed for (CO(2))(3)(H(2)O)(2), where sharp bands appear at 3224, 3321, 3364, 3438, and 3572 cm(-1). Ab initio calculations are performed at the MP2/6-311++G(**) level to provide structural information such as optimized structures, stabilization energies, and vibrational frequencies of the (CO(2))(n)(H(2)O)(m) species. Comparison between the experimental and theoretical results reveals rather size- and composition-specific hydration manner in (CO(2))(n)(H(2)O)(m): (1) the incorporated H(2)O is bonded to either CO(2) (-) or C(2)O(4) (-) through two equivalent OH...O hydrogen bonds to form a ring structure in (CO(2))(n)(H(2)O)(1); (2) two H(2)O molecules are independently bound to the O atoms of CO(2) (-) in (CO(2))(n)(H(2)O)(2) (n=1, 2); (3) a cyclic structure composed of CO(2) (-) and two H(2)O molecules is formed in (CO(2))(3)(H(2)O)(2).