A THEORETICAL STUDY FOR H₂ + CN ↔ HCN + H REACTION AND ITS KINETIC ISOTOPE EFFECTS WITH VARIATIONAL TRANSITION STATE THEORY

Abstract

We present variational transition state theory (VTST) calculations for the H 2 + CN → HCN + H (R1) and D 2 + CN → DCN + D (R2) reactions and their reverses based on a global many-body expansion potential energy surface (PES) for ground-state H 2 CN (ter Horst MA, Schatz GC, Harding LB, J Chem Phys105:558, 1996). It is found that the tunneling effects are negligible over the 200–2000 K temperature range and non-negligible over 100–200 K for R1 and R2 reactions. The C–N bond acts almost as a spectator for both reactions. The present VTST rate constants are in good agreement with the available experimental results and the previous theoretical predictions for R1 and R2 reactions except for the overestimation of rate constants by VTST at lower temperatures that may be caused by recrossing effect. Additionally, the kinetic isotope effects are important for the forward R1 and R2 reactions, but not for the reverses of R1 and R2.

The Complex (Organic) Puzzle of the Formation of Hydrogen Cyanide and Isocyanide on Interstellar Ice Analogues

“…The H 2 + (CN–CO) VdW reaction is likely accessible thanks to the mild barrier height (12.0 kJ mol –1 ) and its strong quantum tunneling effect, with rate constants around 10 4 –10 5 s –1 between 10 and 30 K, which would form only HCN. Note that the barrier height on CO is close to the gas phase value (13.3 kJ mol –1 , as reported by Ju et al). However, one should not overlook the - albeit limited - presence of atomic hydrogen, with fractional abundances around ∼10 –5 –10 –2 relative to the global proton abundance .…”

Global dynamics and transition state theories: Comparative study of reaction rate constants for gas‐phase chemical reactions

“…[101][102][103][104][105][106][107][108][109][110][123][124][125] The VTST rate constants on the ter HorstSchatz-Harding (TSH3) PES 101 (its classical barrier height is 3.19 kcal/mol, and the geometry of TS is collinear) are calculated by the present authors 109 with the POLYRATE program. 126 The VTST results 109 together with the available thermal rate constants calculated by QCT 101 Figures 6 and 7, respectively. As shown in Figure 6, the results of the five-dimensional quantum calculations involving all other internal DOFs but with fixed CN bond are in good agreement with those of the fulldimensional QD calculations, and both of which are comparable to those predicted by QCT method.…”

“…Comparison of rate constant by various approaches for OH 1 H 2 ? H 2 O 1 H reaction: (a) the solid square is the QCT result, 100 the solid line is the VTST result with the ICVT/uOMT method, 97 101 The solid square is the QCT result, 101 and the solid line and the dashed line are the VTST results with the ICVT method and ICVT/SCT one, 109 respectively, the dotted line and the dashed-dotted line are the results with the QD approaches. 110,123 of ICVT and ICVT/SCT are in good agreement with each other.…”

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

“…11. Difference in HCN density between the reference model (n sp,1 ) and the recent literature values of Ju et al (2006) for the reaction H 2 + CN → HCN + H (n sp,2 ) according to Table 2 ( reaction [24]). The black dashed line indicates the surface of the ice reservoir (one monolayer) excluded from the statistical analysis.…”

“…Reaction [24] has already been discussed in the screening approach. For this reaction, Ju et al (2006) published rate constants down to 100 K using variational transition state theory in combination with the small-curvature tunneling scheme. Even though the authors of this computational study state that "the tunneling effects are [...] non-negligible over 100-200K," the reaction rate constants provided by Ju et al (2006) are even slightly smaller than the UMIST2012 values in this temperature regime (see Fig.…”