Adiabatic Quantum Dynamics of CH(X²Π) + H(²S) Reactions on the CH₂(X̃³A″) Surface and Role of the Excited Electronic States

Abstract

We present the Born-Oppenheimer (BO) quantum mechanical (QM) dynamics of the CH decay (d) CH(X2Π) + H(2S) → C(3P) + H2(X1Σ(g)(+)) and of the H exchange reaction (e) CH(X2Π) + H′(2S) → CH′(X2Π) + H(2S) on the CH2 X̃3A″ adiabatic potential energy surface (PES) of Harding et al. (J. Phys. Chem. 1993, 97, 5472). A thorough analysis of the correlation diagram of the four lowest CH2 electronic states, as well as Renner-Teller and spin–orbit nonadiabatic test calculations on the X̃3A″, ã1A′, and b̃1A″ coupled PESs, validate the X̃3A″ BO results, confirming that these reactions occur essentially on the uncoupled X̃3A″ ground surface. We consider the CH molecule in the ground vibrational state and in the four lowest rotational states j0. Thus, we obtain initial-state resolved reaction probabilities, cross sections, and rate constants by propagating coupled-channel real wave packets and performing flux analyses. If J is the total angular momentum quantum number and K is its projection along the body-fixed z axis, CH + H gives essentially the C + H2 products via a barrierless K-inhibited insertion, CH2 resonances at low J, and large cross sections near the threshold. These cross sections decrease strongly with collision energy and depend slightly on j0. On the other hand, the small cross sections obtained for the (e) channel are nearly independent of energy. From initial-state resolved rate constants and Boltzmann populations at temperature T, we obtain QM thermal rate constants from 100 to 400 K: at 300 K, k(d) = (9.57 ± 0.96) × 10(-11) and k(e) = (1.41 ± 0.14) × 10(-11) cm(3) s(-1) for (d) and (e) reactions, respectively. The k(d) value is in good agreement with previous quasi-classical trajectory (QCT) results on the same PES, but it is larger than that observed at 297 K by a factor of 7. On the contrary, and in agreement with the small role of CH2 excited electronic states, X̃3A″ QCT and experimental rate constants agree at high temperatures. Thus, the discrepancy obtained at room T between theory and experiment should be due to an experimental error or to some theoretical effects that we have not been considered in this work. At the present state of the art, an experimental error is more likely and suggests a new measurement of the rate constant.

Quantum wave packet and quasiclassical trajectory studies of the reaction H(2S) + CH(X2Π; v = 0, j = 1) → C(1D) + H2(X1): Coriolis coupling effects and stereodynamics

“…The rate coefficients from the two different QM calculations by considering the initial population for singlet state without spin‐orbit interaction ( f el = 1/8) are given in Figure . From the curves, we can see that the values of the CC calculations are always larger than the corresponding CS rate coefficients.…”

“…[51] In Figure 3f, the CS values are almost zero as depicted in the inset, and the CC probabilities are almost 60 times larger than the CS ones. The rate coefficients from the two different QM calculations by considering the initial population for singlet state without spin-orbit interaction (f el 5 1/8) [29,30] are given in Figure 4. From the curves, we can see that the values of the CC calculations are always larger than the corresponding CS rate coefficients.…”

“…Aforementioned dynamics investigations are related to the forward insertion reaction C + H 2 → H + CH, as for the reverse reaction H + CH → C + H 2 , there are some remarkable investigations on the H( 2 S) + CH(X 2 π) → C( 3 P) + H 2 (X 1 ) reaction in theory and experiment . However, the reaction H( 2 S) + CH(X 2 π) → C( 1 D) + H 2 (X 1 ) has not been studied yet to our knowledge.…”

“…C 1 H 2 , there are some remarkable investigations on the H( 2 S) 1 CH(X 2 p) ! C( 3 P) 1 H 2 (X 1 R 1 g ) reaction in theory [21,[29][30][31][32] and experiment. [33][34][35][36] However, the reaction H( 2 S) 1 CH(X 2 p) !…”

The role of atom tunneling in gas-phase reactions in planet-forming disks

“…Arrhenius plot of the rate constants using the UMIST2012 parameters for the reaction H + CH → C + H 2 [28]. As the values of Gamallo et al (2012) obtained by quantum dynamics calculations agree very well with the rate constants using the UMIST2012 parameters, no fit was carried out. The vertical dashed line indicates the lower bound for the recommended temperature range of the UMIST2012 database, T l = 300 K.…”