Paramagnetic spin catalysis of a radical recombination reaction

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 Hz and Oa molecules is calculated a6 initio in STOdG and 6-31 G* basis sets with complete active space configuration interaction. Calculations are done at a fixed O-H distance ( = 3 A), scanning the H-H bond length from 0.6 till 12 8, at the linear geometry of collision.

Fine and hyperfine structure in three low-lying3Σ+states of molecular hydrogen

“…Mixing between the ground singlet X ' 2; state and the first excited triplet b X i state is forbidden in the isolated H2 molecule, but is allowed when H2 interacts with a paramagnetic catalyst [13]. This mixing is induced by exchange interaction and has the same nature as the singlet-triplet excitation in electron scattering.…”

“…For the non-bound b C i state the calculated ZFS parameter has mostly theoretical interest, though its experimental test should principally be possible in the future. Since this state has a very important virtual meaning in catalysis [12,16] and in spin catalysis [13,14] theory as well as in some recent applications [14,48], we shall discuss the fine and hyperfine structure parameters of the non-bound b 3 C i state in more detail.…”

“…An account of the spin splitting turns out to be important for these states. When the radical pair is perturbed by a paramagnetic ion, the triplet b 3C: state becomes lower in energy than the singlet X ' X i state [13]. The zero-field splitting in such a weakly bound cluster is an important parameter of spin catalysis theory [13,49].…”

“…Unfortunately, the electron scattering experiments performed so far do not provide proper energy resolution to detect the activity of different spin sublevels. As indicated above, the dependence of the ZFS parameter of the b Z : state on the internuclear distance in the H2 molecule is an important factor of spin catalysis theory [13].…”

“…The triplet excited states of diamagnetic species like HZ play a central role in catalysis [12] and spin catalysis theory , and the activation of the hydrogen molecule is one of the main issues in catalysis [16] and combustion [17]. The fine structure of the Hz triplet states is also important for the catalysis of the ortho-para hydrogen conversion [14], for the general theory of magnetic field effects in chemical reactions [13,151, and for chemicaland photo-induced spin polarization [2,9] to mention a few other examples. It follows that it is an important goal to characterize and understand the triplet excited state properties of this prototype species with account of magnetic interactions, something we are trying to accomplish with the present study.…”