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The structure of halogen oxides, XO (X = Cl, Br, I), stability of their anions and the UV-visible absorption spectra of hypohalous acids, HOX, are studied by ab initio methods with account of spin-orbit coupling (SOC) effects. The unrestricted Hartree-Fock (UHF) method, multi-configuration self-consistent field (MCSCF) calculations with linear and quadratic response techniques and MRCI method have been implemented. The Breit-Pauli form of the SOC operator is used. NMR spectra of HOI, HOBr and IO~ species are predicted. Photodissociation of the HOX molecules, which is crucial for the catalytic ozone-destruction cycle, is critically reviewed and analyzed. The singlet-triplet (S-T) transition to the lowest triplet state T\(XlA1 —ť3 A") is responsible for the weak long-wavelength tail absorption which induces photodissociation of these molecules. These absorption bands (380, 450 and 460 nm for X = Cl, Br and I, respectively) have small cross section, about (4, 15 and 24)xlO cm , respectively, but still are important for dissociation process HOX —ť X + OH and ozone concentration, since the stratosphere is transparent for such radiation. The second, more intense band (305, 350 and 406 nm for X = Cl, Br and I, respectively) is produced by two overlapping S-S and S-T transitions. The first singlet-singlet transition, SQ — Si, is relatively weak; it is responsible for about 95% of the cross section in HOC1; in HOBr the SQ — T% transition to the second triplet state contributes to this absorption band much more (about 50%); in HOI this is mostly the So — TI absorption. This band is strongly enhanced in HOI molecule because of the T% - 84 SOC mixing and its wavelength is red-shifted. In order to check this assignment of very peculiar HOI spectrum, the ESR and UV spectral properties of IO radical have been calculated with the same approximation. A good agreement with the ESR spectrum for the calculated nuclear quadrupole coupling constant and SOC splitting makes credit to the accuracy of HOI spectral interpretation. The third, most intensive UV absorption band (242, 280 and 340 nm for X = Cl, Br and I, respectively) in hypohalous acids is determined by the O—X bond polarized S-S transition to the second singlet exited state (X A' —ť 2 A'). All excited states are dissociative. The SOC account is of crucial importance for the absorption cross section calculations of the HOX molecules in the UV-visible part of solar actinic flux and for their lifetime in lower stratosphere. Account of SOC effect is also very important for the lifetime prediction of the halogen oxides and their anions.
Rocznik
Tom
Strony
133--164
Opis fizyczny
Bibliogr. 55 poz.
Bibliografia
Typ dokumentu
Bibliografia
Identyfikator YADDA
bwmeta1.element.baztech-article-BPP1-0022-0017
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