Influence of Atomic Fine Structure on Bimolecular Rate Constants: The Cl(2P) + HCl Reaction

Abstract

In this paper we use quantum reactive scattering calculations to study the influence of atomic fine structure on the rate constants for atom-diatom reactions, using the C1(2P) + HC1 reaction as an example. To do this, we calculate rate constants using two different methods, first a conventional single surface reactive scattering method based on propagating coupled channel equations using hyperspherical coordinates, and second a multiple surface method in which the electronic fine structure of the C1 is explicitly included in the calculation. In this second calculation, the Hamiltonian combines the nonrelativistic potential surfaces for the 2C and 211 states of ClHCl with a spin-orbit Hamiltonian in which the spin-orbit parameter is taken to be a constant and with Coriolis-induced electronic and rotational couplings. Rate constants are evaluated using the J-shifting approximation, using J = l/2 calculations as the reference. We find that the biggest effect of the fine structure is a static one which arises because the spin-orbit term in the Hamiltonian stabilizes the reagents and products differently than the 2Z1/2 saddle point. This causes the activation energy to increase by about 29% of the atomic 2P1/2 -zP3/2 energy difference. When this effect is factored out, the single and multiple surface rate constants are found to be in very good agreement (provided that the usual electronic statistical factors are applied to the single surface rate constant), with differences of less than 20% over the temperature range that we considered. We also study how the size of the spin-orbit constant influences this result, and we find that the overall rate is relatively insensitive to spin-orbit splitting (except for the shift in activation energy). Fine structure resolved rate constants for C1(2P1/2) are found to vary by many orders of magnitude as the spin-orbit splitting is varied between zero and the experimental value, corresponding to a change in the dynamics from statistical to adiabatic limits

Reduced dimensionality spin-orbit dynamics of CH3 + HCl $\rightleftharpoons$⇌ CH4 + Cl on ab initio surfaces

“…As nonadiabatic effects have previously been shown to have little effect on bimolecular rate constants 93 and the low energy multi-state CRP is dominated by electronically adiabatic transitions (Figure 7(a) . TST rate constants were estimated using the vibrationally corrected IB barrier height.…”

A Diabatic Representation Including Both Valence Nonadiabatic Interactions and Spin−Orbit Effects for Reaction Dynamics

“…the Cl( 2 P) + HCl → HCl + Cl( 2 P) reaction 24,25 ), the elements of the SOC matrix will be different in different channels. As a consequence, the SOC matrix elements for one channel must be transformed smoothly into those for the other channels as the system evolves along the reaction coordinate from one dissociation limit through the strong-interaction region and to other dissociation limits.…”

“…Examples include F( 2 P) + H 2 → HF + H, [18][19][20][21] Cl( 2 P) + H 2 → HCl + H, 22,23 or the symmetric Cl( 2 P) + HCl → HCl + Cl( 2 P) reaction. [24][25][26] Besides the fine-structure splitting, the second important effect of SOC is that it causes spin-forbidden processes to become partially allowed through interaction and mixing of states of different spin multiplicity. 8,[27][28][29][30][31][32][33][34] The most common occurrence is the interaction between singlet and triplet states, as in the bimolecular O( 3 P, 1 D) + H 2 → OH( 2 Π) + H reaction, 37,38 and in photodissociation of systems such as HCl, 39,40 HBr, [41][42][43][44][45] CH 3 I, [46][47][48][49][50] ICN, [51][52][53][54] BrCH 2 Cl, [55][56][57][58] or BrCH 2 COCl.…”

The nonadiabatic quantum dynamics of the F(2P1/2, 2P3/2)+HD reaction on modified Alexander, Stark, and Werner potential energy surfaces

“…Our previous studies [18] predict that the resonance peak for the HF + D product on the ASW PESs is about 1.3 times larger than the experimental results compared to about two times for the single-state calculation on the SW PES. However, the position of the resonance peak moves to higher energy by 0.016 eV [19] because the ASW PESs increase the barrier height of the diagonal diabatic potentials by 0.016 eV because of spin-orbit coupling [5,31]. This leads to the position of the distinctive steplike feature in the averaged F + HD !…”