DFT and ab initio calculations on two reactions between hydrogen atoms and the fire suppressants 2-H heptafluoropropane and CF3Br.
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.
Lee
Edmond P F
EPF
Department of Building Services Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong.|Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong.|School of Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ United Kingdom.
Dyke
John M
JM
School of Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ United Kingdom.
Chow
Wan-Ki
WK
Department of Building Services Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong.
Chau
Foo-Tim
FT
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong.
Mok
Daniel K W
DKW
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong.
D064228:Bromochlorofluorocarbons; D005617:Chlorofluorocarbons, Methane / Q000737:chemistry*; D003198:Computer Simulation; D005390:Fires; D006358:Hot Temperature; D006845:Hydrocarbons, Fluorinated / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D008956:Models, Chemical; D014732:Vibration; D005466:Fluorocarbons
D064228:Bromochlorofluorocarbons; D005617:Chlorofluorocarbons, Methane; D006845:Hydrocarbons, Fluorinated; D006859:Hydrogen; C034013:bromotrifluoromethane; C094049:apaflurane; D005466:Fluorocarbons