Chemical Dynamics Simulations on Association and Ensuing Dissociation of a Benzene-Hexafluorobenzene Molecular System.

Abstract

Chemical dynamics simulations are performed to study the association of benzene (Bz) and hexafluorobenzene (HFB) followed by the ensuing dissociation of the Bz-HFB complex. The calculations are done for 1000, 1500, and 2000 K with an impact parameter ( b) range of 0-10 Å at each temperature. Almost no complexes are observed to form at b = 8 and 10 Å. Following three different methods of calculation of the temperature-dependent association rate constant kasso( T), the values obtained are 1.67 × 10-10, 1.86 × 10-10, and 2.05 × 10-10 cm3/molecule·s with a standard deviation of approximately 0.1 × 10-10 cm3/molecule·s for T = 1500 K. Among those values of kasso( T), the middle one is obtained by considering a relative translational energy of 3 RT/2 at T = 1500 K, and the same is followed to calculate kasso( T) at 1000 and 2000 K. The Arrhenius parameters, using the kasso( T) values at three temperatures, are 0.203 × 10-10 cm3/molecule·s for the pre-exponential factor and -5.79 kcal/mol for the activation energy. The absolute value of the latter is similar to the Bz + HFB association energy of 5.93 kcal/mol. The ensuing dissociation dynamics of the complex is significantly different from the unimolecular dissociation dynamics, and an exponential function fits the N( t - t0)/ N( t0) curves comparatively well. The ensuing dissociation is also observed to be independent of time for a statistically large sample size.