edited by
JOHN J. ALEXANDER University of Cincinnati
Cincinnati. OH 45221
Potential Energy Surfaces Kenneth H. Tonge Robert Gordon's Institute of Technology Aberdeen A69 IFR. Scotland Experimental chemists are becoming increasingly aware that quantum mechanical calculations can be a powerful tool in the study of chemical reactions. This changing attitude toward quantum chemistry results from the ready availability and increasing power of computers and the disseminati& at low cost, f; bophisticatedpr35 kJ mol-1. ihen the dlssociationenergy is pcaaibly about 40 kJ mol-'. A hetter estimate could be ohtained by plotting the cross-sectional Morse curve and extrapolating to large H-X distances hy hand. The energy found here is the spectroscopic dissociation energy, no account being taken of zero-point energy. ~~~~
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Question 2 (see Fig. 5a) (a) I t is important to realize that the structures A, B, C, and D are not accurately sketched and the stable conformers may not have quite the hngles indicated. The stable forms are to be found at the minima on the diagram, and it is only necessary to judge the two angles 8 and 9 on the sketches and identify the nearest minimum on the diagram. P of lb) There are a number of posrible reaction paths. T ~limits thesesre shown in Fieure 5a. The two most ohvious paths, (1) and (3,go via stab6 intermediates in two distinct stages. However, the concerted processes, (3) and (41, require no more energy since they pass through the same saddle points.
Figure 6. Acceptable solution for questlon 3. A = rhadopsin. B = barthorho. doprin, and C = stable, alecnonically exclted conformer. Volume 65
Number 1 January 1988
67
(c) The potential energy diagram to be drawn depends on the
choice of reaction coordinate. The concerted processes show only one maximum, the two-stage processes show two maxima (Fig. 5b). (d) The structures are sketched on Figure 5a. Question 3 (See Fig. 6 ) Different electronic states of a system will have their own independent surface if the Born-Oppenheimer approximation is valid. Transfer between two such surfaces will he governed by the Franck-
68
Journal of Chemical Education
Condon principle and the probability functions of the initial and final states. Mast textbook examples are restricted to diatomic systems, and the terminology used is restricted by the f a d that only one coordinate is used. In the example shown here we have an example of torsional relaxation. The thermal process has an activation energy of approximately 45 kcal mol-' with the transition state having, 81 140°, 8 2 8 0'. The photochemical process is unactivated and proceeds via electronic excitation, torsional relaxation, and fluorescence or phosphorescence (it is not possible to say which) and a further torsional relaxation. These processes are shown on Figure 6.
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