A. Rahaman et Lm. Raff, Trajectory investigations of the dissociation dynamics of vinyl bromide onan ab initio potential-energy surface, J PHYS CH A, 105(11), 2001, pp. 2156-2172
The reaction dynamics of vibrationally excited vinyl bromide have been inve
stigated using classical trajectory methods on a global, analytic potential
-energy hypersurface that is developed primarily by least-squares fitting o
f appropriately chosen functional forms to the results of ab initio electro
nic structure calculations. These calculations are carried out at the MP4 l
evel of theory with all single, double, and triple excitations included. A
6-31G(d,p) basis set is employed for the carbon and hydrogen atoms. Huzinag
a's (4333/433/4) basis set augmented with split outer s and p orbitals (433
21/3321/4) and a polarization forbital with an exponent of 0.5 is used for
the bromine atom. The present calculations focus upon the determination of
the dependence of the potential upon the stretching coordinates for the bon
ded atoms in vinyl bromide, the C-C-H and C-C-Br bending coordinates and th
e dihedral angles. The couplings between these coordinates are also investi
gated. The total ab initio database is obtained by combining these results
with previously reported studies of the vinylidene-acetylene system and our
previous calculations of saddle-point geometries and energies for the vari
ous decomposition channels, reactant and product equilibrium structures, an
d vibrational frequencies. The analytic surface fitted to this data base (P
ES 1) is then modified by adjustment of the potential curvatures at equilib
rium to provide a better fit to the measured IR and Raman vibrational frequ
encies of vinyl bromide while simultaneously holding all other topographica
l features of the surface constant to the maximum extent possible. The surf
ace so developed is labeled PES2. Finally, we have arbitrarily altered the
reaction coordinate curvatures for three-center HBr and H-2 elimination to
produce a third potential surface (PES3). The dissociation dynamics of viny
l bromide on each of these potential surfaces are investigated at several e
xcitation energies in the range 4.5 to 6.44 eV. Total decomposition rate co
efficients and product branching ratios are computed as a function of excit
ation energy. The HBr vibrational-state distribution is computed and found
to be Boltzmann with an effective vibrational temperature of 7084 K on PES1
. These results are in virtually exact agreement with recently reported mea
surements of this distribution. Finally, we have investigated the dissociat
ion mechanisms for three-center Ha and HBr elimination reactions. The resul
ts show that the dynamics are very similar on PES 1, PES2, and PES3. Conseq
uently, small variations in potential-energy curvatures at equilibrium and
along the reaction coordinates do not exert significant influence upon the
dissociation dynamics. However, some large qualitative variations between t
he dynamics on the ab initio surface and the more empirical surface previou
sly employed are found to exist. We conclude that great care must be exerci
sed when such surfaces are used to study reaction dynamics in polyatomic sy
stems.