Nuclear solvent polarization

The energies E [Pn] in Eq. [35] depend on the nuclear solvent polarization that serves as a three-dimensional (3D) nuclear reaction coordinate driving electronic transitions. The two-state model actually sets up two directions the vector of the differential field AS b and the off-diagonal field Sab-Therefore, only two projections of Vn need to be considered the longitudinal field parallel to ASab and the transverse field perpendicular to ASab- In the case when the directions of the differential and off-diagonal fields coincide, one needs to consider only the longitudinal field, and the theory can be formulated in terms of the scalar reaction coordinate... 

The reaction coordinate is now a projection of the nuclear solvent polarization on the adiabatic differential solute field... 

Observation of spin-polarized products resulting from these radical pairs by the method of chemically induced dynamic nuclear polarization (CIDNP)<67) was accomplished by photolysis in the probe of an NMR spectrometer using perfluoromethylcyclohexane as solvent. The results obtained were consistent with nuclear spin polarization steps involving radical pairs formed from dissociated radicals and also directly from excited states, although the former could not be detected in carbon tetrachloride, probably due to radical scavenging by the solvent. It was not possible to determine the fraction of the reaction proceeding by singlet and triplet radical pairs.<68)... 

The solvent reorganization term reflects the changes in solvent polarization during electron transfer. The polarization of the solvent molecule can be divided into two components (1) the electron redistribution of the solvent molecules and (2) the solvent nuclear reorientation. The latter corresponds to a slow and rate-determining step involving the dipole moments of the solvent molecules that... 

Convincing evidence was found that the majority of acyclic aldo-nitrones exist in the Z-form, by investigating the ASIS-effect (aromatic solvent induced shift effect) (399). However, in some cases, specified by structural factors and solvent, the presence of both isomers has been revealed. Thus, in C -acyl-nitrones the existence of Z -and -isomers was detected. Their ratio appears to be heavily dependant on the solvent polar solvents stabilize Z-isomers and nonpolar, E-isomers (399). A similar situation was observed in a- methoxy-A-tert-butylnitrones. In acetone, the more polar Z-isomer was observed, whereas in chloroform, the less polar E-isomer prevailed. The isomer assignments were made on the basis of the Nuclear Overhauser Effect (NOE) (398). /Z-Isomerization of acylnitrones can occur upon treatment with Lewis acids, such as, MgBr2 (397). Another reason for isomerization is free rotation with respect to the C-N bond in adduct (218) resulting from the reversible addition of MeOH to the C=N bond (Scheme 2.74). The increase of the electron acceptor character of the substituent contributes to the process (135). 

Coordinates and Potential Energy Surfaces. In any of the above formalisms one must identify the nuclear coordinates which contribute to the activation of the reactants. In the present case we confine our attention to two "active" modes — the solvent polarization mode ([Pg.262]

Nuclear magnetic resonance studies on meso-ionic l,2,4-triazol-3-ones (200) were used to examine their relationship to the alternative l,3,4-oxadiazol-2-imine structure (153). The effect of solvent polarity upon the ultraviolet spectrum of anhydro-3-hydroxy-1,4-diphenyl-1,2,4-triazolium hydroxide (200, R = = Ph, R = H) has been discussed... 

Separation of Electronic and Nuclear Motions. The polarizabilities of the ground state and the excited state can follow an electronic transition, and the same is true of the induced dipole moments in the solvent since these involve the motions of electrons only. However, the solvent dipoles cannot reorganize during such a transition and the electric field which acts on the solute remains unchanged. It is therefore necessary to separate the solvent polarity functions into an orientation polarization and an induction polarization. The total polarization depends on the static dielectric constant Z), the induction polarization depends on the square of the refractive index n2, and the orientation polarization depends on the difference between the relevant functions of D and of n2 this separation between electronic and nuclear motions will appear in the equations of solvation energies and solvatochromic shifts. 

The isomerization of donor olefins is illustrated by the reaction of chloranil with a pair of geometric isomers, cis- and trans-1 -phenylpropene. The irradiation of the quinone in polar solvents in the presence of either isomer results in nuclear spin polarization for both isomers. The key to understanding these effects lies in two observations (Fig. 9) (a) the polarization of the regenerated parent olefin is stronger than that of the rearranged olefin (b) the reaction of the ds-isomer results in stronger overall effects than does that of the trans-isomer [157, 158],... 

The solvent coordinate s measures the electric nuclear polarization in the solvent, which is not necessarily in equilibrium with the charge distribution in the reacting solute system. (We recall that the solvent s electronic polarization is assumed to be so equilibrated.) The full exposition of this coordinate [1-3] would take us a bit far afield, but the reader may think of it as qualitatively indicating whether the actual solvent polarization is more like the equilibrium polarization for the bound B state (,v 0) or like that for the dissociative A state (s 1). 

As opposed to the previous examples, the rate of the pair substitution BRi BR2 BR3 can be varied by neither the reactant concentrations nor the solvent polarity because it is intramolecular and only involves neutral species. However, the ratio of polarizations of corresponding protons in Pi and P2 exhibits a pronounced temperature dependence, " which is shown in Fig. 9.8 and can be explained in the following way. Ideally, these opposite polarizations should have exactly equal magnitudes, but their ratio deviates from 1 if nuclear spin relaxation in the paramagnetic intermediates is taken into account. Biradicals with nuclear spin states that slow down intersystem crossing of BRi live longer, so their nuclear spins suffer a stronger relaxation loss. 

The last 50 years have witnessed the establishment of a truly molecular-level description of electron transfer chemistry. From the Marcus description of how solvent polarization defines the ET reaction coordinate, to fully quantum treatments that describe electron and nuclear tunneling contributions to the kinetics, to atomistic simulations of reaction coordinate motion, a comprehensive view of biological ET is emerging (1-5). 

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