CALCULATED UV ABSORPTION SPECTRUM FOR HOCl

$^{1}$W. C. Fergusson, L. Slotin, and D.W.B. Style, Trans. Farad. Soc. 32, 956 (1936). S. R. Langhoff: NASA-NRC Research Associate.Author Institution: NASA, Ames Research Centerinterest in the hypochlorous acid molecule (HOCl) has been revived recently with the suggestion that it might be formed as an intermediate species in the stratosphere. Based on an old UV absorption $measurement^{1}$ it was thought that HOCl would have a short lifetime for photodissociation in the stratosphere. Ab initio molecular orbital calculations have been performed for HOCl to verify the UV absorption measurements and determine the photodissociation products. A double zeta plus polarization basis set and large ($>$ 10 000 configuration) CI were used to calculate the energies and electronic transition dipole moments between the low-lying singlet states of HOCl. The resulting spectral intensities are in good agreement with the experimental data for the $2^{1}A^{\prime} \leftarrow 1^{1} A^{\prime}$ transition which peaks at 2160 \AA $(\in_{max} = 157.7 \ \ \ell M^{-1} cm^{-1})$. We also find a weaker transition at 2840 \AA $(\in_{max} = 1.48 \ \ \ell M^{-1} cm^{-1})$ which is due to the $1^{1}A^{\prime\prime} \leftarrow 1^{1}A^{\prime}$ transition. We find no evidence for a stronger transition at 3200 {\AA} which has been seen experimentally. As a result, we predict that the photodissociation lifetime of HOCl in the stratosphere will be rather long and that other removal processes should dominate the chemistry of HOCl at altitudes below 40 km. The products of the photodissociation have been found to be exclusively OH + Cl for both the $2^{1}A^{\prime} \leftarrow 1^{1}A^{\prime}$ and $1^{1}A^{\prime\prime} \leftarrow 1^{1}A^{\prime}$ transition