Triatomic transition state

For a linear triatomic transition state, it is assumed that a second potential energy curve results so that the total energy is a function of two bond distances. Therefore, a diagram can be constructed that shows energy on one axis (usually chosen to be the vertical axis), one of the bond distances on another, and the second bond distance on the third axis, which generates a three-dimensional energy surface. If we suppose the reaction... 

One immediate possibility for resolving this apparent discrepancy is that of an alternate composition for the transition state. The free-radical pathway providing a triatomic transition state is of great interest. A recent shock-tube study monitored free atom concentrations as the reaction H2 + D2 2HD progressed and concluded that there was not enough H or D present to account for the entire rate of formation of HD via a free-radical pathway. However, due to the difficulties of running a clean reaction, this work is not definitive and experimental work is continuing. 

The classical counterpart of resonances is periodic orbits [91, 95, 96, 97 and 98]. For example, a purely classical study of the H+H2 collinear potential surface reveals that near the transition state for the H+H2 H2+H reaction there are several trajectories (in R and r) that are periodic. These trajectories are not stable but they nevertheless affect strongly tire quantum dynamics. A study of tlie resonances in H+H2 scattering as well as many other triatomic systems (see, e.g., [99]) reveals that the scattering peaks are closely related to tlie frequencies of the periodic orbits and the resonance wavefiinctions are large in the regions of space where the periodic orbits reside. 

Triatomic Systems Potential Energy Surface and Transition State... 

Here, the CH3-H bond is formed as the C2H5-H bond is broken. For this system, the other bond lengths and angles also affect the potential energy, and the potential energy surface therefore depends on all other coordinates (3N - 6 or 30 in all). This system, however, is similar to the triatomic case above, where A = C H, B = H, and C = CH. Again note that the transition state for the reverse reaction is the same. 

A different triatomic system with which it is instructive to contrast these systems is the XCY linear triatomic unit that features in the transition state in an Sn2 reaction [Eq. (1.3)] ... 

Figure 6.2 Triatomic model of H-transfer illustrating changes in zero-point energies of normal vibrations between the initial and transition states.

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