Rate coefficients of the CF₃CHFCF₃ + H → CF₃CFCF₃ + H₂ reaction at different temperatures calculated by transition state theory with ab initio and DFT reaction paths

Abstract

The minimum energy path (MEP) of the reaction, CF3CHFCF3 + H → transition state (TS) → CF3CFCF3 + H2, has been computed at different ab initio levels and with density functional theory (DFT) using different functionals. The computed B3LYP/6‐31++G**, BH&HLYP/cc‐pVDZ, BMK/6‐31++G**, M05/6‐31+G**, M05‐2X/6‐31+G**, UMP2/6‐31++G**, PUMP2/6‐31++G**//UMP2/6‐31++G**, RCCSD(T)/aug‐cc‐pVDZ//UMP2/6‐31++G**, RCCSD(T)/aug‐cc‐pVTZ(spd,sp)//UMP2//6‐31++G**, RCCSD(T)/CBS//M05/6‐31+G**, and RCCSD(T)/CBS//UMP2/6‐31++G** MEPs, and associated gradients and Hessians, were used in reaction rate coefficient calculations based on the transition state theory (TST). Reaction rate coefficients were computed between 300 and 1500 K at various levels of TST, which include conventional TST, canonical variational TST (CVT) and improved CVT (ICVT), and with different tunneling corrections, namely, Wigner, zero‐curvature, and small‐curvature (SCT). The computed rate coefficients obtained at different ab initio, DFT and TST levels are compared with experimental values available in the 1000–1200 K temperature range. Based on the rate coefficients computed at the ICVT/SCT level, the highest TST level used in this study, the BH&HLYP functional performs best among all the functionals used, while the RCCSD(T)/CBS//MP2/6‐31++G** level is the best among all the ab initio levels used. Comparing computed reaction rate coefficients obtained at different levels of theory shows that, the computed barrier height has the strongest effect on the computed reaction rate coefficients as expected. Variational effects on the computed rate coefficients are found to be negligibly small. Although tunneling effects are relatively small at high temperatures (∼1500 K), SCT corrections are significant at low temperatures (∼300 K), and both barrier heights and the magnitudes of the imaginary frequencies affect SCT corrections. © 2012 Wiley Periodicals, Inc.

A theoretical investigation of the atmospherically important reaction between chlorine atoms and formic acid: determination of the reaction mechanism and calculation of the rate coefficient at different temperatures

“…Consequently, it is difficult to benchmark lower levels in the present study, as has been pointed out before [36,38]. Also, similar to our recent findings [36], employing different lower levels with the same higher level improved relative energies gives spreads in computed rate coefficients of at least an order of magnitude, which may be considered generally as uncertainties associated with any lower level employed.…”

“…Also, similar to our recent findings [36], employing different lower levels with the same higher level improved relative energies gives spreads in computed rate coefficients of at least an order of magnitude, which may be considered generally as uncertainties associated with any lower level employed. In addition, in the present study with more extensive investigations at dual levels than in our previous works [36,37,38], basis size effects at higher levels were found to be related to geometry effects from lower levels (i.e. different basis size effects at different geometries).…”

“…The geometry optimisation, transition state (TS) search, vibrational frequency and intrinsic reaction coordinate (IRC) calculations were performed by employing the GAUSSIAN09 programs [46] with the 6-31 + G * * and 6-31 ++ G * * (5D) basis sets. The choices of the functionals and basis sets used follow from the theoretical considerations of our previous work [36][37][38]. Initially, these calculations were performed with appropriate symmetry constraints, as the use of symmetry speeds up the computation.…”

“…This study is part of a wider project to study the choices of the ab initio level and density functionals used to compute electronic energy surfaces of atmospherically important reactions, which are then used to calculate rate coefficients with VTST, in order to establish a consistent, reliable and practical methodology for the calculation of their k [36][37][38].…”

Water OH Bond Activation by Gas‐Phase Plutonium Atoms: Reaction Mechanisms and Ab Initio Molecular Dynamics Study

“…Moreover, log 10 k versus 1000/T plots, which are used commonly in experimental rate studies, [63] are also reported. [64] We examined the maxima of both classical potential energy, V MEP , and the ground-state adiabatic potential energy, V a G , which had no shifting. [64] We examined the maxima of both classical potential energy, V MEP , and the ground-state adiabatic potential energy, V a G , which had no shifting.…”