Journal of Physical Chemistry A, Vol.101, No.41, 7513-7521, 1997
Quasiclassical trajectory study of molecular alignment effects on the dynamics of the reactions of Cl, Br, and I with H-2
The X + H-2 (X = Cl, Br, and I) reactions may be taken as models for endoergic triatomic reactions with heavy-light-light kinematics and collinear saddle point. The dependence of scalar and two-vector properties, angular distributions (k, k'), (k, j'), (k', j'), and (l', j'), on (E-T, v, j), as well as the effect of considering initial parallel (II), perpendicular (perpendicular to), and random (null) k-j alignment has been studied using the quasiclassical trajectory (QCT) method. The threshold energy for II alignment is always higher than the ones for I and null alignments, but for high enough E-T values sigma(II) becomes larger than sigma(perpendicular to) and sigma(null), and the same occurs for the j-dependence. In the v range of values explored sigma(II) is in general equal or larger than sigma(perpendicular to) and sigma(null). The expression 1/3 sigma(II) + 2/3 sigma(perpendicular to) provides a very good estimate to sigma(null) if the system is not in the vicinities of the threshold region, and some useful relations to simplify the QCT calculations for II alignment have also been given. For the two-vector properties considered the results obtained for perpendicular to alignment are in general closer to the ones for null alignment than the results obtained for II alignment. The angular correlations that result from the calculation are not a trivial result coming from a kinematic constraint; being particularly remarkable the role played by the rotation of the H-2 molecule. These results may be explained taking into account the saddle point properties, the ''effective molecular size'' of the rovibrationally excited H-2 molecule, and the geometrical implications of the alignments.