Diabatic reaction

As seen previously, the chemical reactions studied most often are the exchange ones. Those requiring several potential energy surfaces of excited states (diabatic reactions) are worth special mention, since they most certainly define a domain of application with a future for classical trajectories. An electron jump from one surface to another requires either to be given a statistical probability of occurence by the Landau Zener formula (or one of its improved versions " ) or to be described by means of complex-valued classical trajectories as a direct and gradual passage in the complex-valued extension of the potential surfaces (generalization of the classical S-matrix ). 

Hynes [43] has discussed dynamic solvent effects for electron transfer reactions and described the role of solvent friction for both diabatic and adiabatic reactions. In the case of diabatic reactions the rate is strongly dependent on the coupling between the energy surfaces for the reactants and products as expressed through the parameter /j. (see section 7.8D). When is very small, dynamic... 

An example of a diabatic reaction is the oxidation of tetramethyl-p-diamino-benzene (TMDAB) to its cation radical. Kinetic data were obtained in seven solvents with Pekar factors ranging from 0.27 (chloroform) to 0.53 (acetonitrile). The Pekar factor yp is defined as... 

The observation of emission from a product B (or triplet triplet absorption by 3B in flash spectroscopy) upon excitation of a reactant A would seem to be clear evidence for an adiabatic reaction, class (b). However, this is true only when excitation of A is possible at a wavelength where B does not absorb. Otherwise, diabatic reactions can easily masquerade as adiabatic ones due to re-excitation of the photoproduct B by the light source used for excitation of A.149... 

Equation 2.53 Landau Zener model for diabatic reactions... 

Diabatic reactions are rarely initiated from triplet states, because the interaction terms V12 between states of different multiplicity, which are due to spin orbit coupling (Section 4.8) are small. Even if surface crossings with the singlet ground-state PES do occur in certain regions of the (3N — 6)-dimensional space of the internal nuclear coordinates and the molecule does explore such a region in its lifetime, it is unlikely to find the minute escape hatch to the ground state PES. 

Figure 19. Diabatic reaction probability as a function of time (in femtoseconds) for the collinear A - - BC AB + C model of Fig. 1, at an excess energy of 0.109 eV. In both panels, the full line designates the exact quantum mechanical results and the dashed lines are multiple spawning results with different initial conditions. Right panel TDB using an initial basis set with 30 basis functions and 10 seeds. Left panel Regular basis set using an initial basis set with 10 basis functions. (Figure adapted from Ref. 41.)...

The interaction between PESs in diabatic reactions is shown in Figure 5.14. 

The actual charge of the particles in this configuration is determined by the electron wavefunction. The transmitted charge in this case may be distributed practically equally between the two partners, or it can be completely concentrated on one of them (adiabatic and non diabatic reactions, see section 3.4). 

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