Ab Initio Calculation of Some Lower-Lying Excited States of H2O

Portions of the potential surfaces of the $B^1{A}_1$ excited state of the H₂O molecule and the corresponding triplet state have been calculated using the LCAO–MO–SCF method in order to investigate the origins of the $\mathrm{OH}{\mathrm{(A}}^2{\Sigma }^{+})$ fragment having an abnormal rotational distribution produced on photodissociation of H₂O. The calculated wavefunctions indicate that these states are largely Rydberg states at O–H distances close to 2.0 a.u., but at greater O–H distances the antibonding configuration predominates. In both states, the molecule shows a strong preference for the linear geometry. The calculated equilibrium O–H distance for the symmetric stretch in the linear conformation is about 0.4 Å greater than the ground‐state equilibrium distance. For the asymmetric dissociation of the bent molecule in the $B^1{A}_1$ state the cross section through the potential surface at an H–O–H angle of 135° is calculated to have a maximum as well as a minimum. The origin of the maximum is discussed in terms of the variation of the exchange integral between the singly occupied orbitals. The relevance of these results to the experimental problem is considered.