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DFT and ab initio calculations on two reactions between hydrogen atoms and the fire suppressants 2‐H heptafluoropropane and CF 3 Br
Author(s) -
Lee Edmond P. F.,
Dyke John M.,
Chow WanKi,
Chau FooTim,
Mok Daniel K. W.
Publication year - 2007
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.20695
Subject(s) - ab initio , chemistry , enthalpy , computational chemistry , molecular orbital , density functional theory , ab initio quantum chemistry methods , thermodynamics , molecule , physics , organic chemistry
Reaction enthalpies and barrier heights of the reactions CF 3 Br + H → CF 3 + HBr {reaction (1)} and CF 3 CHFCF 3 + H → CF 3 CFCF 3 + H 2 {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 CF 3 CFCF 3 calculated at the near state‐of‐the‐art level has a value of −(318 ± 3) kcal/mole. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007