Potential energy surface for bond exchange among three hydrogen molecules

Abstract

Ab initio electronic structure calculations show that a termolecular, six-centre reaction path for bond exchange among hydrogen molecules is energetically accessible, whereas there appears to be no bimolecular, four-centre path. Our best SCF calculation for hexagonal H₆ finds the minimum potential energy lies about 39 kcal mol^–1 below the 2H₂+ 2H asymptote, or about 69 kcal mol^–1 above the 3H₂ asymptote. The optimum distance between neighbouring hydrogen atoms is only 0.99 Å. This calculation employs ten orbitals (two s, three p, five d functions) on each hydrogen atom, with configuration interaction including all single and double excitations and an approximate correction for quadruple excitations. Calculations using less extensive SCF basis sets are also reported which give the zero point energy for the eleven nondissociative vibrational modes of hexagonal H₆ and the potential energy profile along the reaction coordinate for the H₆→ 3H₂ dissociation. Other geometrical configurations of H₆ are treated using the simple “diatomics-in-molecules” approximation. This gives good results for the hexagonal isomer (∼16 kcal mol^–1 above our best ab initio energy) and indicates that only hexagonal H₆ lies low enough to serve as a transition state for the six-centre reaction.

Comparisons with approximate ab initio results for larger H_n polygons give an improved estimate for the cohesive energy of metallic hydrogen. It is also shown that H₆ is the only H_{4m+ 2} system for which a concerted bond exchange reaction can occur. This illustrates the need to supplement orbital symmetry correlations with energetic criteria.