Non-adiabatic pathways

Variation in the metal surface composition is, then, generally expected to yield large variations in the observed rate constant for inner-sphere pathways since the reaction energetics will be sensitive to the chemical nature of the metal surface. For outer-sphere reactions, on the other hand, the rate constants are anticipated to be independent of the electrode material after correction for electrostatic work terms provided that adiabatic (or equally non-adiabatic) pathways are followed. Although a number of studies of the dependence of the rate constants for supposed outer-sphere reactions on the nature of the electrode material have been reported, relatively few refer to sufficiently well-defined conditions where double-layer corrections are small or can be applied with confidence [111-115]. Several of these studies indeed... 

A DFT-based computational study has established the thermal and photochemical isomerisation mechanisms from the terminal P-bound phosphinidene oxide complex [Ru(tpy)(bpy)(POPh)] to the corresponding O-bound, [Ru(tpy)(bpy)(OPPh)] . Thermal isomerisation was found to be both kinetically and thermodynamically unfavourable, while photoisomerisation can readily take place by either adiabatic or non-adiabatic pathways. The different absorption spectra of the two isomers and the bi-stability of the system make this complex a good candidate for photochromism. ... 

This again illustrates how excited states in metal systems can lead to new products and can also influence the types of product formed. Another interesting point to note is that the non-adiabatic pathway is similar between this system and the binary carbonyls in terms of the coordination sphere of the metal. This lends credence to our belief that the binary carbonyls can act as model complexes in terms of photo-induced chemistry. 

Zurek JM, Paterson MJ (2012) Photoracemization and excited state relaxation through non-adiabatic pathways in chromium (ID) oxalate ions. J Chem Phys 137... 

Based on this physical view of the reaction dynamics, a very broad class of models can be constructed that yield qualitatively similar oscillations of the reaction probabilities. As shown in Fig. 40(b), a model based on Eckart barriers and constant non-adiabatic coupling to mimic H + D2, yields out-of-phase oscillations in Pr(0,0 — 0,j E) analogous to those observed in the full quantum scattering calculation. Note, however, that if the recoupling in the exit-channel is omitted (as shown in Fig. 40(b) with dashed lines) then oscillations disappear and Pr exhibits simple steps at the QBS energies. As the occurrence of the oscillation is quite insensitive to the details of the model, the interference of pathways through the network of QBS seems to provide a robust mechanism for the oscillating reaction probabilities. 

Reactions of ascorbic acid have been intensively studied in relation to the behavior of this familiar compound in biological systems. Most of the studies treated ascorbic acid as an simple outersphere reducing reagent, until Creutz published an article concerning the complexity of the reaction pathways of ascorbic acid and related radicals. The authors recently demonstrated that the oxidation reactions of ascorbic acid in acidic aqueous solutions are not adiabatic from the volume analysis of the reactions. As the non-adiabaticity of the ascorbate reactions implies the involvement of the proton dissociation and/or ring-closure processes at the rate-determining step, it is expected that Ae reactions of ascorbic acid in dipolar aprotic solvents such as dimethyl sulfoxide (DMSO) are adiabatic. ... 

Pre-association and limiting electron-transfer behaviour is also observed in the oxidation of stellacyanan, St(i), by [Co(edta)] at pH 7.0. The large negative entropy of activation for electron transfer is interpreted as indicative of a non-adiabatic mechanism. When binding of [Co(edta)] to the protein is prevented, either at pH 10.0 or by the addition of edta to block the reactive site, electron transfer takes place through an alternative adiabatic pathway. Under these conditions a Marcus self-exchange rate constant of 3 x 10 s ... 

Molecular tunnelling processes have been detected in the recombination of HbCO after flash photolysis at low temperature ( < 10 K) and attempts to analyse the data using non-adiabatic molecular group transfer theory have met with reasonable success. At higher temperatures, (< 20 K) a non-exponential Arrhenius pathway is detected suggesting a distribution of activation enthalpies depend-... 

A very useful starting point for the study of non-adiabatic processes, which are common in photochemistry and photophysics, is the vibronic coupling model Hamiltonian. The model is based on a Taylor expansion of the potential surfaces in a diabatic electronic basis, and it is able to correctly describe the dominant feature resulting from vibronic coupling in polyatomic molecules a conical intersection. The importance of such intersections is that they provide efficient non-radiative pathways for electronic transitions. Not only is the position and shape of the intersection described by the model, but it also predicts which nuclear modes of motion are coupled to the electronic transition which takes place as the system evolves through the intersection. 

In non-adiabatic dynamics it is necessary to treat the nuclei as moving over a set of coupled potential energy surfaces rather than the single surface of classical molecular dynamics. The surfaces can then approach to form avoided crossings or meet as conical intersections that provide pathways where the initially excited molecule can cross back to the ground electronic state in a non-radiative manner. This crossing is particularly efficient at a conical intersection, which is why these features play a central role in the mechanistic description of photochemistry, in a similar way to the role played by the transition state in thermal chemistry. 

FIGURE 1. Potential energy profiles for the two lowest = J potential energy surfaces for the coUinear H + F2. The non-adiabatic coupling strength Z13 for motion along the minimum energy pathway is shown below. 

FIGURE 2. Contour map of non-adiabatic coupling strength Z13 for extension of the H-F bond as a fimction of H-F and F-F bond lengths for H + Minimum energy pathway is shown by a broken curve. 

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