Does GaH5 exist?

Abstract

The existence or nonexistence of GaH(5) has been widely discussed [N. M. Mitzel, Angew. Chem. Int. Ed. 42, 3856 (2003)]. Seven possible structures for gallium pentahydride have been systematically investigated using ab initio electronic structure theory. Structures and vibrational frequencies have been determined employing self-consistent field, coupled cluster including all single and double excitations (CCSD), and CCSD with perturbative triples levels of theory, with at least three correlation-consistent polarized-valence-(cc-pVXZ and aug-cc-pVXZ) type basis sets. The X (1)A(') state for GaH(5) is predicted to be weakly bound complex 1 between gallane and molecular hydrogen, with C(s) symmetry. The dissociation energy corresponding to GaH(5)-->GaH(3)+H(2) is predicted to be D(e)=2.05 kcal mol(-1). The H-H stretching fundamental is predicted to be v=4060 cm(-1), compared to the tentatively assigned experimental feature of Wang and Andrews [J. Phys. Chem. A 107, 11371 (2003)] at 4087 cm(-1). A second C(s) structure 2 with nearly equal energy is predicted to be a transition state, corresponding to a 90 degrees rotation of the H(2) bond. Thus the rotation of the hydrogen molecule is essentially free. However, hydrogen scrambling through the C(2v) structure 3 seems unlikely, as the activation barrier for scrambling is at least 30 kcal mol(-1) higher in energy than that for the dissociation of GaH(5) to GaH(3) and H(2). Two additional structures consisting of GaH(3) with a dihydrogen bond perpendicular to gallane (C(3v) structure 4) and an in-plane dihydrogen bond [C(s)(III) structure 5] were also examined. A C(3v) symmetry second-order saddle point has nearly the same energy as the GaH(3)+H(2) dissociation limit, while the C(s)(III) structure 5 is a transition structure to the C(3v) structure. The C(4v) structure 6 and the D(3h) structure 7 are much higher in energy than GaH(3)+H(2) by 88 and 103 kcal mol(-1), respectively.