Proton switch correlated with the morphological development of the hydrogen-bond network in H+ (MeOH)m(H2O)1 (m = 1-9): a theoretical and infrared spectroscopic study.

A surprising switch of the protonated site from methanol to water in protonated methanol-water mixed clusters, H(+)(MeOH)(m)(H(2)O)(1) (m = 1-9), was investigated by a joint theoretical and vibrational spectroscopic study. Extensive density functional calculations on all possible structural isomers revealed that the switch of the ion core is correlated with the size dependence and structural development of the hydrogen-bond network: (1) the CH(3)OH(2)(+) ion core is preferred for the small-sized clusters of m = 1 and 2, (2) coexistence of the H(3)O(+) and CH(3)OH(2)(+) ion cores is highly plausible for 3 < or = m < or = 7 clusters, and (3) obvious preference of the H(3)O(+) ion core appears from m > or = 8 with the appearance of the characteristic "tricyclic" structure of the hydrogen-bond network. The ion core switch at m approximately 8 is experimentally supported by the infrared photodissociation spectra of the size-selected clusters and the size dependence of the fragmentation channel following vibrational excitation.

Infrared predissociation spectroscopy of cluster cations of protic molecules, (NH3)n+, n=2-4 and (CH3OH)n+, n=2,3.

Infrared predissociation spectroscopy is carried out for the structure investigation of unprotonated cluster cations of protic molecules such as ammonia and methanol, which are generated through vacuum-ultraviolet one-photon ionization of their jet-cooled neutral clusters. The observed spectral features show that the cluster cations have the proton-transferred type structures, where a pair of a protonated cation and a neutral radical, NH(4) (+)...NH(2) or CH(3)OH(2) (+)...OCH(3), is formed. Theoretical calculations at the MP2 and B3LYP levels support the formation of the proton-transferred type structures for the cluster cations, and indicate that they are formed by proton-transfer following the photoionization of the neutral clusters.

Comprehensive analysis of the hydrogen bond network morphology and OH stretching vibrations in protonated methanol-water mixed clusters, H(+)(MeOH)1(H2O)n (n = 1-8).

Density functional theory (DFT) calculations of protonated methanol-water mixed clusters, H (+)(MeOH) 1(H 2O) n ( n = 1-8), were extensively carried out to analyze the hydrogen bond structures of the clusters. Various structural isomers were energy optimized, and their relative energies with zero point energy corrections and temperature dependence of the free energies were examined. Coexistence of different morphological isomers was suggested. Infrared spectra were simulated on the basis of the optimized structures. The infrared spectra were also experimentally measured for n = 3-9 in the OH stretching vibrational region. The observed broad bands in the hydrogen-bonded OH stretch region were assigned in comparison with the simulations. From the DFT calculations, the preferential proton location was also investigated. Clear correlations between the excess proton location and the cluster morphology were found.

Compatibility between methanol and water in the three-dimensional cage formation of large-sized protonated methanol-water mixed clusters.

Infrared spectra of large-sized protonated methanol-water mixed clusters, H(+)(MeOH)(m)(H(2)O)(n) (m=1-4, n=4-22), were measured in the OH stretch region. The free OH stretch bands of the water moiety converged to a single peak due to the three-coordinated sites at the sizes of m+n=21, which is the magic number of the protonated water cluster. This is a spectroscopic signature for the formation of the three-dimensional cage structure in the mixed cluster, and it demonstrates the compatibility of a small number of methanol molecules with water in the hydrogen-bonded cage formation. Density functional theory calculations were carried out to examine the relative stability and structures of selected isomers of the mixed clusters. The calculation results supported the microscopic compatibility of methanol and water in the hydrogen-bonded cage development. The authors also found that in the magic number clusters, the surface protonated sites are energetically favored over their internal counterparts and the excess proton prefers to take the form of H(3)O(+) despite the fact that the proton affinity of methanol is greater than that of water.