Physical Chemistry Third Edition

614 13 Chemical Reaction Mechanisms II: Catalysis and Miscellaneous Topics

laser study, in which it was verified that there is a population inversion in the products,

with thev2 vibrational state more highly populated than the other states, so that

laser action can occur from thev2 state to thev1 state or thev0 state. The

next investigation was a crossed molecular beam study, in which the angular distribu-

tion of products was determined, and in which it was found that most of the products

were “back-scattered” (scattered generally in the direction from which the reactants

had come).

Next came a classical trajectory calculation, in which the large back-scattering was

also found to occur. After this, anab initiocalculation of the potential energy surface

was carried out. Next came a quantum-mechanical calculation, in which it was found

that “resonances” or “quasi-bound states” occurred at the transition state, which means

that the activated complex existed for a longer period of time than expected, as it

would if there were a “basin” in the potential energy surface at the transition state. This

phenomenon was explained by the fact that the potential energy surface has a saddle

that is fairly broad in the direction of the symmetric stretch, lowering the energy of the

activated complex and causing its relative persistence.

Finally, a detailed chemiluminescence study gave the distribution of product states,

showing the most back-scattering in the lower four vibrational states, but a significant

forward scattering in thev4 vibrational state, and determining that on the average,

66% of the energy of reaction goes into vibrational energy, 8% into rotational energy,

and the remainder into translational energy. The calculations did not agree well with

the new experimental results.

Since the publication of the book by Levine and Bernstein, detailed quantum-

mechanical calculations of state-to-state reaction probabilities have been carried out.^42

Several different potential energy surfaces were used in these calculations. Better

agreement with experiment was attained, but only states of zero angular momentum

were included. Further research is focused on finding better potential energy surfaces.

One calculation gave a barrier height of 0.089 eV, corresponding to 8.6 kJ mol−^1.^43

Gimenez et al. carried out a detailed comparison between their three-dimensional quan-

tum mechanical calculations, and found good agreement with experiment.^44 Aoiz and

coworkers carried out a quasiclassical trajectory study and found significant discrep-

ancies between previous experimental results and the theory.^45 Persky and Kornweitz

published a critical review of the experimental data on this reaction.^46 They recom-

mend the following experimental values for the Arrhenius parameters for the reaction

between 190 K and 376 K:

A(1. 1 ± 0 .1)× 10 −^10 cm^3 molecule−^1 s−^1

Ea 3. 7 ± 0 .4kJmol−^1

The F+H 2 reaction is one of the most thoroughly studied of all chemical reac-

tions, but study of it continues as both calculations and experiments become more

sophisticated.

(^42) C. Yu, Y. Sun, D. J. Kouri, P. Halvick, D. G. Truhlar, and D. W. Schwenke,J. Chem. Phys., 90 , 7608
(1989); C. Yu, D. J. Kouri, M. Zhaoo, D. G. Truhlar, and D. W. Schwenke,Chem. Phys. Lett., 157 , 491
(1989).
(^43) G. E. Scuseria,J. Chem. Phys., 95 , 7426 (1991).
(^44) X. Gimenez, J. M. Lucas, A. Aguilar, and A. Lagana,J. Phys. Chem., 97 , 8578 (1993).
(^45) F. J. Aoiz, L. Banares, V. J. Herrero, V. S. Rabanos, K. Stark, and H.-J. Werner,J. Chem. Phys., 102 ,
9248 (1995).
(^46) A. Persky and H. Kornweitz,Int. J. Chem. Kinetics, 29 , 67 (1997).