An ab initio molecular orbital study of even-membered hydrogen cluster cations: H6+, H8+, H10+, H12+, and H14+

Abstract

Geometries and energetics for even-membered Hn+ (n=6–14) clusters that have the H2+ core in the geometrical structures have been theoretically studied using ab initio molecular orbital methods. It was found that the H2+-core H6+ cluster has D2d symmetry and that the geometrical structures of H2+-core clusters of larger size are composed of unperturbed H2+-core H6+ and outer H2s weakly bound to it. It was predicted for both the H6+ and H8+ clusters that the H2+-core clusters are more stable in energy than the corresponding H3+-core ones. However, the energy difference between H2+- and H3+-core H8+s was calculated to be significantly smaller than that between H2+- and H3+core H6+s. The binding energies of outer H2 in H2+-core clusters were predicted to be 0.7, 0.4, 0.1, and <0.1 kcal mol−1 for H8+, H10+, H12+, and H14+, respectively, at the PMP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ+ZPE level, suggesting that H8+ and H10+ are stable enough to be detected, but H12+ and H14+ are less stable. This result is consistent with the experimental observation of Kirchner and Bowers [J. Chem. Phys. 86, 1301 (1987)] that only Hn+ clusters with n⩽11 are formed with sufficient intensity to be detected.