AB-INITIO DETERMINATION OF THE ENERGY BARRIERS AND RATE CONSTANTS FORHOBO-]FBO+H2O AND OBBO+HF-]FBO+HBO(HF)

We report ab initio calculations of features of the potential energy s urfaces for two reactions of potential importance for the combustion o f boron in fluorine-containing environments; HF+HOBO-->H2O+FBO and HFOBBO-->FBO+HBO. Both reactions proceed through four-center transition states and yield the product FBO, a stable molecule that appears to pl ay a similar role for B/F/O/H combustion as the isoelectronic CO2 does for hydrocarbon combustion. Multireference configuration interaction calculations with valence triple-zeta, double polarization; basis sets yield energy barriers of 27.2 and 46.2 kcal/mol, respectively, for th ese two reactions. Transition state theory calculations based on the a b initio potential energy surface information yield the following thre e-parameter fits for the temperature-dependent rate coefficients. k(HO BO)(T)=1.436x10(2) T(2.71)exp(-11917/T) cm(3)/mol s and k(OBBO)(T)=4.8 19x10(4) T(2.38)exp(-22028/T) cm(3)/mol s. Both reactions are predicte d to be orders of magnitude slower than the estimated rates used lh re cent combustion modeling efforts. (C) 1995 American Institute of Physi cs.