ISSN:
1572-8951
Keywords:
Spin catalysis
;
radical recombination
;
H2−O2 reaction
;
singlet-triplet mixing
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
Notes:
Abstract Recombination of the triplet state radical pair consisting of two hydrogen atoms catalysed by molecular oxygen is considered as a simulating example of a paramagnetic-exchange catalytic process. Intermolecular exchange interaction in the collision complex between the H2 and O2 molecules is calculatedab initio in STO-6G and 6–31 G* basis sets with complete active space configuration interaction. Calculations are done at a fixed O−H distance (≃3 Å), scanning the H−H bond length from 0.6 till 12 Å at the linear geometry of collision. The mixture of the triplet (‘T’)3∑ u + and singlet (‘S’)1∑ g + states of the hydrogen moiety is possible because both states have the same triplet symmetry in the ‘collision’ complex with O2 (3∑ g − ). A strong mixture of the ‘S’ (1∑ g + , H2 +3∑ g − , O2) and ‘T’) and ‘T’ (3∑ u + , H2 +3∑ g − , O2) states is actually obtained even at large H−H distances. The quintet and singlet states5,1(3∑ u + , H2 +3∑ g − , O2) are also considered for comparison of the exchange potentials. Atr(H−H)≥4.4 Å the ‘S-T’ splitting is approximately constant (12 cm-1 in the STO-6G basis set; 55.5 cm-1 in the 6–31 G* basis set) and is determined by the exchange interaction between O2 and the nearest hydrogen atom in the O−O...H fragment. The paramagnetic catalyst can accelerate radical recombination through the ‘triplet-singlet’ nonadiabatic transition to the lowest ‘S’ reactive state when the radical encounter takes place in the vicinity of the catalyst. Though we do not consider the radical dynamics in a real solvent, which modulates the exchange potentials and the ‘T-S’ transitions, the nature of this mechanism of spin catalysis is obvious. The electric polarization and charge transfer are important in the analysis of the exchange interaction and radical recombination potentials for all multiplets. In accordance with the concept of spin catalysis, the electronic spin-uncoupling mechanism, induced by O2 perturbation, has the same nature as other known catalytic processes of paramagnetic-exchange type.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01886375
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