Theoretical study of the photodissociation of HOCl

Abstract

A theoretical study of the uv photodissociation of HOCl is presented based on ab initio SCF-CI calculations. Results are reported for the low-lying singlet and triplet states of the HOCl molecule using both double-zeta and double-zeta-plus-polarization basis sets of contracted Cartesian Gaussian functions. The calculated equilibrium geometry for the ground electronic state [(1)1A′] is in excellent agreement with experiment. The vertical excitation energies to the two lowest singlet states are determined to be: (1)1A′→ (1)1A″ =4.50 eV and (1)1A′→ (2)1A′=5.66 eV. Electronic transition dipole matrix elements have been calculated for these two excitations. The (1)1A′→ (2)1A′ transition moment is more than a factor of 10 larger than that for the (1)1A′→ (1)1A″ transition. The photoabsorption spectrum for HOCl (vapor) between 200 and 400 nm has been computed based on the ab initio potential energies and transition moments. This spectrum exhibits a single peak, at 220 nm, with an extinction coefficient of 123 l mole−1 cm−1. The only previous experimental measurement is that of Fergusson, Slotin, and Style, who reported two peaks at 220 and 320 nm. Possible explanations for this discrepancy are discussed in light of recent unpublished experimental data. By examination of the topographies of the (1)1A″ and (2)1A′ potential energy surfaces, the photodissociation products are predicted to be Cl+OH. Since HOCl does not have significant photoabsorption cross section in the wavelength range 300–500 nm we conclude that it would be stable to photodissociation if formed in the stratosphere. The implications of HOCl as a reservoir for stratospheric chlorine are discussed.