Ab Initio Reaction Rate Constants Computed Using Semiclassical Transition-State Theory: HO + H₂ → H₂O + H and Isotopologues

Abstract

A new algorithm [Nguyen, T. L.; Stanton, J. F.; Barker, J. R. Chem. Phys. Lett. 2010, 9, 499] for the semiclassical transition-state theory (SCTST) formulated by W. H. Miller and co-workers is used to compute rate constants for the isotopologues of the title reaction, with no empirical adjustments. The SCTST and relevant results from second-order vibrational perturbation theory (VPT2) are summarized. VPT2 is used at the CCSD(T) level of electronic structure theory to compute the anharmonicities of the fully coupled vibrational modes (including the reaction coordinate) of the transition structure. The anharmonicities are used in SCTST to compute the rate constants over the temperature range from 200 to 2500 K. The computed rate constants are compared to experimental data and theoretical calculations from the literature. The SCTST results for absolute rate constants and for both primary and secondary isotope effects are in excellent agreement with the experimental data for this reaction over the entire temperature range. The sensitivity of SCTST to various parameters is investigated by using a set of simplified models. The results show that multidimensional tunneling along the curved reaction path is important at low temperatures and the anharmonic coupling among the vibrational modes is important at high temperatures. The theoretical kinetics data are also presented as fitted empirical algebraic expressions.

WATER FORMATION THROUGH A QUANTUM TUNNELING SURFACE REACTION, OH + H2, AT 10 K

“…which has almost the same activation barrier and effective mass as reaction (12) (Nguyen et al 2011, Table 2). In the OH/H 2 co-deposition experiment described in the previous section, the reaction products were basically the same as those of the H 2 O-fragment deposition except for the presence of O 3 .…”

“…For convenience, hereafter, these three groups are denoted A, B, and C, respectively. Although the difference in the barrier height (ΔE a ) among the reactions listed in Table 2 is small (ΔE a < ∼400 K; Talukdar et al 1996;Nguyen et al 2011), it is remarkable that a considerable isotope effect was clearly observed in groups A and B. The reactions differed intrinsically depending on whether the reaction involved H-or D-atom abstraction, and the effective masses were comparable within a reaction group, as shown in Table 2.…”

“…These reactions are associated with nearly identical barrier heights (Nguyen et al 2011) whereas the effective masses differed considerably from one another (0.48 for reaction (18) and 0.90 for reaction (19)). The experimental results shown in Sections 3.1-3.4, therefore, predict that reaction (18) is more efficient than reaction (19).…”

“…Previous experimental and theoretical studies proposed that quantum tunneling plays a significant role in reaction (12) in the gas phase, particularly at low temperatures (e.g., Talukdar et al 1996;Nguyen et al 2011). Watanabe & Kouchi (2008) described the importance of quantum tunneling in various chemical reactions on low-temperature grain surfaces in molecular clouds.…”

“…c Compilation of literature values (Atkinson et al 2004). d Computed reaction barrier heights including harmonic zero-point vibrational energy (Nguyen et al 2011). e Calculated by using Equation (14) (Johnston 1966 …”