DFT and ab initio calculations on two reactions between hydrogen atoms and the fire suppressants 2‐H heptafluoropropane and CF₃Br

Abstract

Reaction enthalpies and barrier heights of the reactions CF3Br+H-->CF3+HBr {reaction (1)} and CF3CHFCF3+H-->CF3CFCF3+H2 {reaction (2)} have been calculated at the near state-of-the-art ab initio level, and also by employing the B3LYP, BH&HLYP, BB1K, MPW1K, MPWB1K and TPSS1KCIS functionals. In addition, the integrated molecular orbital+molecular orbital (IMOMO) method has been used to study reaction (2). The ab initio benchmark values of the reaction enthalpy (298 K) and barrier height (0 K) of reaction (2) are reported for the first time {-(0.7+/-0.7) and 13.3+/-0.5 kcal/mole respectively}. When density functional theory (DFT) results are compared with ab initio benchmarks for both reactions (1) and (2), the MPWB1K functional is found to have the best performance of the six functionals used. The IMOMO method with the RCCSD/aug-cc-pVTZ and/or RCCSD(T)/aug-cc-pVTZ levels, as the high levels of calculation on the model system, gives reaction enthalpies and barrier heights of reaction (2), which agree with ab initio benchmark values to within 1 kcal/mole. Computed key geometrical parameters and imaginary vibrational frequencies of the transition state structures of reactions (1) and (2) obtained at different levels of calculation are compared. The magnitudes of the computed imaginary vibrational frequencies of the transition states of both reactions considered are found to be very sensitive to the levels of calculation used to obtain them. The heat of formation (298 K) of CF3CFCF3 calculated at the near state-of-the-art level has a value of -(318+/-3) kcal/mole.

Direct dynamics studies for the reactions of CF3CHFCF3 and CF3CF2CHF2 with H atoms

“…However, our calculated result agrees well with the ab initio benchmark value (À0.7 AE 0.7 kcal mol À1 ) [13] reported by Lee et al recently. They also calculated the barrier height using several DFT and ab initio methods and obtained the benchmark value 13.3 AE 0.5 kcal mol À1 [13], which agree well with the barrier height (11.78 kcal mol À1 ) calculated at the G3(MP2)//MPW1K/6-311+G(d,p) level. Moreover, it can be found in Table 3 that the barrier heights indicate good agreement with the G2(MP2) values for all three reaction channels.…”

“…The calculated reaction enthalpy of R1 (À1.5 kcal mol À1 ) is considerably lower than the experimental value (À5.93 kcal mol À1 ) derived from the experimental standard heats of formation (CF 3 CHFCF 3 , À365.55 kcal mol À1 ; CF 3 CFCF 3 , À319.38 kcal mol À1 ; H, 52.1 kcal mol À1 ) [36,38]. However, our calculated result agrees well with the ab initio benchmark value (À0.7 AE 0.7 kcal mol À1 ) [13] reported by Lee et al recently. They also calculated the barrier height using several DFT and ab initio methods and obtained the benchmark value 13.3 AE 0.5 kcal mol À1 [13], which agree well with the barrier height (11.78 kcal mol À1 ) calculated at the G3(MP2)//MPW1K/6-311+G(d,p) level.…”

“…[12] to calculate the rate coefficient are very likely inadequate. In addition, the reaction enthalpy and barrier height of R1 were calculated by Lee et al [13] using ab initio, DFT, and IMOMO theory. However, they did not calculate the rate constants of R1.…”

Comparison of theoretical methods for assessing the heat of formation of C1 and C2 chlorofluorocarbons and hydrochlorofluorocarbons

“…Furthermore, the theoretical prediction of heat of formation continues to arouse a considerable amount of interest [10,11]. Large systematic negative errors occur in computed enthalpies when using procedures such as density functional theory (DFT), complete basis set (CBS) and Gaussian-2 (G2) methods, combined with the atomization approach.…”

Insight into the multicomponent reaction mechanisms of prop-2-en-1-amine and ethyl propiolate with alloxan derivative: A density functional theory study

“…Compared with other levels of theory, the B3LYP method was sufficiently accurate for predicting reliable geometries and frequencies of the stationary points [40][41][42][43]. Vibrational frequency calculations were performed based on the optimized geometries.…”