Halogen monoxides

The importance of ozone in the stratosphere has been stressed in Section 9.3.8. The fact that ozone can be decomposed by the halogen monoxides CIO, BrO and 10 means that their presence in the stratosphere contributes to the depletion of the ozone layer. For example, iodine, in the form of methyl iodide, is released into the atmosphere by marine algae and is readily photolysed, by radiation from the sun, to produce iodine atoms which can react with ozone to produce 10 ... 

In Cycle I, the rate-limiting step involves reaction (2.82), the self- or cross-reaction of the halogen monoxide radicals. Cycle I has been identified to be the prime cause for polar boundary layer ozone destruction. The second-ozone destruction cycle, which is more prevalent at low halogen levels, has the form Cycle II ... 

Reactive species such as NOx molecules, chlorine radicals, and hydrogen radicals present in the stratosphere can also cause the catalytic conversion of ozone to oxygen, in competition with the last reaction shown above. Reactions such as the following, therefore interfere with the equilibrium photochemical processes, reducing the concentration of ozone in the stratosphere. The net reaction is one where ozone is converted to oxygen and catalytic Cl species survives for further reaction. Particularly effective in this regard are halogen monoxides (CIO and BrO). 

Experimentally deteraiined RKR potentials for the Xi( Il3/2) and XiCllm) states of the halogen monoxides FO, CIO, BrO and 10 are compared to available ab initio potentials. The results suggest that fully relativistic ab initio calculations have the capability to reproduce the experimental bond lengths, harmonic vibrational frequencies and fine structure intervals of the XO series with reasonable accuracy. The quest for spectroscopically accurate XO potentials will provide an excellent benchmark for future theoretical methods. 

This paper addresses the question of what it means to have a spectroscopic quality potential energy surface. The literature of the last few years contains numerous references to spectroscopic quality ab initio molecular potentials, yet the definition of spectroscopic quality necessarily depends on the resolution of the experimental spectrum and the theoretical calculation. The accuracy of a molecular potential is perhaps better characterized by asking how well it reproduces experimental observables. This topic is explored by comparing the 1( 113/2) and X2 Yl rd potentials of the halogen monoxides determined from high-resolution spectroscopic studies with potentials computed using ab initio methods. The RKR potentials determined from spectroscopic data provide... 

The intrinsically relativistic nature of the electronic structure of the halogen monoxides is dictated by the presence of two spin-orbit components in the ground electronic state. Any accurate characterization of the XO Xj and X2 rii/2 potentials must therefore treat relativistic effects explicitly. However, diis requirement greatly increases the cost and complexity of the ab initio effort(26), resulting in few relativistic potential surface calculations such as the 10 study by Roszak et al.(21) One more frequently finds relativistic effects incorporated as corrections to non-relativistic energies or treated through the use of effective core potentials (23). 

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