Barrierless association of CF ₂ and dissociation of C ₂ F ₄ by variational transition-state theory and system-specific quantum Rice–Ramsperger–Kassel theory

Abstract

Bond dissociation is a fundamental chemical reaction, and the first principles modeling of the kinetics of dissociation reactions with a monotonically increasing potential energy along the dissociation coordinate presents a challenge not only for modern electronic structure methods but also for kinetics theory. In this work, we use multifaceted variable-reaction-coordinate variational transition-state theory (VRC-VTST) to compute the highpressure limit dissociation rate constant of tetrafluoroethylene (C 2 F 4 ), in which the potential energies are computed by direct dynamics with the M08-HX exchange correlation functional. To treat the pressure dependence of the unimolecular rate constants, we use the recently developed system-specific quantum Rice-Ramsperger-Kassel theory. The calculations are carried out by direct dynamics using an exchange correlation functional validated against calculations that go beyond coupled-cluster theory with single, double, and triple excitations. Our computed dissociation rate constants agree well with the recent experimental measurements.bond dissociation | barrierless reaction | variable-reaction-coordinate variational transition-state theory | falloff | system-specific quantum RRK theory B ond breaking and association of fragments to make new bonds are two of the most fundamental processes in chemistry. Gas-phase bond dissociation and radical-radical association reactions are of great importance in combustion and plasma chemistry and atmospheric chemistry. Accurately describing the energetics and kinetics of these processes in a practical way for mechanistic analysis offers great challenges to modern theoretical chemistry.The electronic structure of closed-shell singlets can often be described to a good approximation by a single configuration state function (CSF); such systems are called single-reference systems, and methods using a single CSF as a reference function are called single-reference methods. Bond dissociation is usually an intrinsically multiconfigurational problem, often called a multireference problem (1). Coupled cluster (CC) theory (2, 3), as a high-level single-reference wave function method, is able to provide fairly accurate energies for bond-dissociation processes only if a high-enough excitation level (such as quadruple excitations) is used for the cluster operatorT; however, lack of such expensive higher excitation operators in CC calculations [such as in the case of coupled-cluster theory with single and double excitations (CCSD) (4)] can lead to a significant unphysical bump (5, 6) in the potential energy curve for dissociation. Kohn-Sham density functional theory (KS-DFT) (7,8), however, with carefully chosen exchange correlation (XC) density functionals can produce smooth potential curves for dissociation with satisfactory accuracy without introducing any unphysical bump and with much smaller computational cost; however, at this time, an XC functional that can be used as a panacea does not exist. In addition to an adequate level of electronic structure...

Comparing Reaction Routes for 3(RO···OR′) Intermediates Formed in Peroxy Radical Self- and Cross-Reactions

“…For the VRC-VTST calculation, we first defined the variable reaction coordinate as explained by Bao et al 49 Here, we discuss the input setup only for the MeO...OMe system, as it was the main benchmark of our studies (and extension to the larger alkoxy radicals is straightforward). Two pivot points were symmetrically placed (Figure 1) along the MeO---OMe axis, leading to a four-face dividing surface.…”

“…Finally, the high-pressure rate constant was calculated by VRC-E, J-VT by minimizing the number of accessible transitional states N(E, J, s) with respect to s . 49 Here, s is the range of reaction coordinate (the distance between pivot points on the two fragments), for example we set the value of s as 3.49….7.19 Å with a step size of 0.1 Å for MeO…OMe. Initially, we used 384 Monte Carlo (MC) sampling points over the multifaced dividing surface, later increasing the number to 500.…”

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“…The advantage of this method is that we avoid having to choose the reactive collisional diameter of the 3 SO molecules based on a visual inspection of the potential energy surface. Instead, the rate constant is calculated with microcanonical variational transition state theory (μVTST). − …”

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“…The barriers vary from substantial values on the order of 2.5 eV down to negligibly small values (0–0.04 eV). Reactions without barriers are certainly no novelty in chemistry; − indeed, many forms of dissociative adsorption on surfaces exhibit no barrier despite the breaking of chemical bonds. However, low barriers seem surprising for thermally activated atomic rearrangements in a semiconductor.…”