Exit valley

Reaction 58 has been observed, and the rotational distribution measured is thermal, in marked contrast to similar measurements in H2O. Theoretical calculations suggest that this is because there is an exit valley that lies close to the bent geometry of the H2S molecule. Thus, the excited state can dissociate without producing a large amount of angular momentum in the SH fragment. 

The progress of the reaction unit A—-B—-C along the reaction coordinate, or minimum energy path, is given in terms of relative values of rAB and rBC. Figures 5.1 and 5.2 have been given in terms of a symmetrical barrier where the activated complex lies symmetrically with respect to both the entrance and exit valleys. 

In the entrance valley rAB is large and decreases towards the col. At the entrance to the exit valley rBC is large, and decreases towards the col. At the col rAB = rBC, and this holds for all points along the line, PQ, as drawn. On the profile the activated complex lies at a maximum where rAB = rBC-... 

Other situations can occur where the activated complex lies in the entrance valley, an early barrier, or in the exit valley, a late barrier. 

The activated complex and PE barrier are in the exit valley, where rAB < rBC, corresponding to a configuration A—-B------C for the activated complex. The 3-D... 

On attractive surfaces (Figure 5.7), the unit (A-—B—C) reaches the barrier or col before the bond B-C has altered much, and starts to move into the exit valley with A... 

In the exit valley C separates from B when A is still far from B. Even when C has separated from B by a very considerable extent, the AB distance remains large compared to its equilibrium internuclear distance. The reaction energy is released as the new bond is forming. Repulsion between B and C cause B and C to recoil from each other as they separate, and B is propelled towards the approaching A as the AB bond is forming. 

This places the barrier in the exit valley. In the exit valley, the AB bond is virtually formed before B and C have separated much, and energy release occurs while there is increasing separation of products, i.e. AB and C. 

If the well is in the entrance valley the reaction unit can often need vibrational energy, but if it is in the exit valley translational energy is often more effective. Such features are indicative of a collision complex which lasts sufficiently long for many vibrations and some rotations to occur. The lifetime of a collision complex is long... 

The back reaction will have the exact reverse characteristics. The activated complex will lie in the exit valley, and reaction will be enhanced by high vibrational energy. There will be high translational energy in the products, the cross section will be small, and the molecular beam contour diagram will show predominantly backward scattering, typical of a rebound mechanism. 

Since these two cross sections have similar magnitudes, this would correspond to a rebound mechanism where the two molecules have to come very close together before reaction will occur. The activated complex thus lies in the exit valley and the potential energy profile has a late barrier. 

Reaction is a rebound mechanism, with the critical configuration in the exit valley. 

If we consider the potential energy as a function of the Jacobi coordinates X and X2 and draw the energy contours in the X1-X2 plane, then the entrance and exit valleys will asymptotically be at an angle to one another and in the mass-weighted skewed angle coordinate system parallel to its axes. So the idea with this coordinate system is that it allows us to directly determine the atomic distances as they develop in time and that it shows us the asymptotic directions of the entrance and exit channels. 

Extensive use has been made of classical trajectory methods to investigate various forms of the potential-energy surface for the reaction F + H2. Muckerman [518] has recently presented a very thorough review of potential-energy surfaces and classical trajectory studies for F + H2. The calculations all correctly predict vibrational population inversion, the value of and backward scattering of the products. Most calculations overestimate (FR) and those giving the lowest values of (Fr > use a potential-energy surface that unrealistically has wells in the entrance and exit valleys [519]. 

Mok and Polanyi s work [323] indicates a correlation between the potential barrier height and its position on the energy surface. For exothermic reactions, the barrier is generally in the entry valley, that is, where rBC re BC and rAB > re AB, and it moves to progressively greater values of rAB as its height is lowered. Conversely, for endothermic reactions, the barrier moves to successively later positions in the exit valley as its height increases. 

Recently, Monte Carlo trajectory studies have been performed [325, 326] on systems with appreciable energy barriers, with a view to discovering whether excitation of particular degrees of freedom in the reagents promotes reaction. For three-atom reactions, if the barrier lies in the exit valley, vibrational (rather than translational or rotational) excitation can be used most effectively for surmounting the barrier. Conversely, if the barrier is in the entry valley, it is most easily surmounted if energy is located in the relative translation of the products rather than in vibration. For appreciably endothermic reactions, the barrier is very likely to be in the exit valley [323], and the conclusion that vibrational excitation will considerably assist the occurrence of such reactions is supported by calculations based on the applications of microscopic reversibility to the detailed rate coefficients for several exothermic reactions [327, 227]. It appears that similar criteria apply to four-center reactions of the AB + CD - AC + BD type [317], and the effect of vibrational excitation on the rate of such reactions has been investigated [316,317]. 

This experiment allowed the direct exploration of the radial part of the excited-state electron-transfer surface, here at the transition-state level and also partially in the exit valley. It has also provided further evidence of the angular dependence of the electron transfer which is addressed in the following example. This example does not take into account the correlations with excited ionic surfaces accessed by 6s electron transfer because these surfaces lie at too high energy in the case of mercury. This type of electron transfer will be described in the following section. 

The position of the maximum value of Ead(v,r) depends on v because, although Eei(x) increases toward the potential barrier, iivib(v,x) decreases, and this second contribution to f ad(v,x) assumes increasing importance as v increases. Although the v = 0 adiabatic maximum is usually located at the electronic saddle point, the maxima for higher states of bond-stretching vibration may be displaced into the entrance and exit valley on the potential energy surface. Vibrationally adiabatic motion is then expected up to the first of these maxima (r-AB > r-Bc). Trajectory calculations support this expectation [39-44]. 

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