ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
Classical trajectory calculations on the gas phase reaction F+H2 ( j)→HF+H have been carried out. Different reactivity trends were seen depending on whether there was a chemically significant and anisotropic well in the entrance channel of the potential surface. For those in which there is no such well, rotation may decrease reactivity at low values of j, but increases it thereafter. The reaction cross section SR ( j) decreases slowly from j=0, reaching a minimum near j=6 then increases again. This behavior has been reported for several systems, including H+H2, and seems to be the "canonical'' behavior for SR ( j) for most direct chemical reactions. For F+D2 the minimum does not occur until j=8. However, this does correspond to the same amount of rotational energy as the minimum for F+H2 . For potentials in which there is a deep anisotropic well, it is found that the j=0 results are dominated by the presence of the well, and that the SR ( j=0) is anomalously high. On such surfaces there is normally a sudden drop in cross section from j=0 to j=1, followed by an increase. The experimental findings of Lee's group [J. Chem. Phys. 82, 3045 (1985)] that the cross section increases on going from j=0 to j=1 probably precludes the possibility of a chemically significant well in the entrance valley. The rotational product state distribution for both types of potential is dominated by kinematics away from threshold, and does not show the same trends as the reaction cross section. The mean product vibrational quantum number 〈v'〉 can decrease at low j, then increase at higher j. This occurs only at collision energies close to threshold, and on potentials which have a tight bend force constant at the transition state. The more general case for this reaction is that 〈v'〉 increases with j. For the case in which the potential has no well the differential cross section shows precisely the same trends as does SR( j). All these trends can be explained using a simple model we have recently proposed.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.457359
