Method of Diatomics-in-Molecules. IX. Ground and Excited States of H4 and the H2,H2 Bimolecular Exchange Reaction

The semiempirical diatomics-in-molecules theory is used to predict potential-energy surfaces for the ground and several excited electronic states of the system H4. Results are presented for linear (D∞h), rectangular (D2h), square (D4h), “perpendicular” (D2d), tetrahedral (Td), and rhombic (D2h) conformations. Results are compared with those from previous ab initio calculations. The lowest energy state of square H4 is singlet; the optimum internuclear distance is R = 2.0 bohrs, for which the energy is 67.8 kcal/mole higher than 2E(H2) (fixed nuclei energies). Vibrational frequencies for this square are computed: 2180, 1210, 1990, 1470, 1470, and 2280i cm−1, the latter corresponding to the reaction coordinate for the H2, H2 bimolecular exchange reaction. The specific rate constant for this reaction is computed according to the absolute rate theory.