Gas-phase alkaline earth halide anions, MgX3(-) and CaX3(-) (X = Cl, Br), were produced using electrospray and investigated using photoelectron spectroscopy at 157 nm. Extremely high electron binding energies were observed for all species and their first vertical detachment energies were measured as 6.60 +/- 0.04 eV for MgCl3(-), 6.00 +/- 0.04 eV for MgBr3(-), 6.62 +/- 0.04 eV for CaCl3(-), and 6.10 +/- 0.04 eV for CaBr3(-). The high electron binding energies indicate these are very stable anions and they belong to a class of anions, called superhalogens. Theoretical calculations at several levels of theory were carried out on these species, as well as the analogous BeX3(-). Vertical detachment energy spectra were predicted to compare with the experimental observations, and good agreement was obtained for all species. The first adiabatic detachment energies were found to be substantially lower (by about 1 eV) than the corresponding vertical detachment energies for all the MX3(-) species, indicating extremely large geometry changes between MX3(-) and MX3. We found that all the MX3(-) anions possess D3h ((1)A1') structures and are extremely stable against dissociation into MX2 and X-. The corresponding neutral species MX3, however, were found to be only weakly bound with respect to dissociation toward MX2 + X. The global minimum structures of all the MX3 neutrals were found to be C2v ((2)B2), which can be described as (X2(-))(MX+) charge-transfer complexes, whereas the MX2...X (C2v, (2)B1) van der Waals complexes were shown to be low-lying isomers.

We have studied the influence of hydrogenation on the relative stability of the low-lying isomers of the anionic B7- cluster, computationally. It is known that the pure-boron B7- cluster has a doubly (sigma- and pi-) aromatic C6v(3A1) quasi-planar wheel-type triplet global minimum (structure 1), a low-lying sigma-aromatic and pi-antiaromatic quasi-planar singlet C2v (1A1) isomer 2 (0.7 kcal mol(-1) above the global minimum), and a planar doubly (sigma- and pi-) antiaromatic C2v (1A1) isomer 3 (7.8 kcal mol(-1) above the global minimum). However, upon hydrogenation, an inversion in the stability of the species occurs. The planar B7H 2- (C2v, 1A1) isomer 4, originated from the addition of two hydrogen atoms to the doubly antiaromatic B7- isomer 3, becomes the global minimum structure. The second most stable B7H2- isomer 5, originated from the quasi-planar triplet wheel isomer 1 of B7- , was found to be 27 kcal mol(-1) higher in energy. The inversion in stability occurs due to the loss of the doubly aromatic character in the wheel-type global minimum isomer (C6v, 3A1) of B7- upon H2-addition. In contrast, the planar isomer of B7- (C2v, 1A1) gains aromatic character upon addition of two hydrogen atoms, which makes it more stable.