Solvent polarization effect

Treating the term in Eq. [13] as a perturbation to the solute wave-function in the gas phase 4 , AEx, can be written as  

Because both and 4 ,oi are found in the combined QM/MM Monte Carlo or molecular dynamics simulations, p can be computed exactly taking the difference between A , and E ). The p , term is further decomposed into two components a net gain in the solute-solvent interaction energy between the 

---

In summary, all the experiments expressly selected to check the theoretical description provided fairly clear evidence in favour of both the basic electronic model proposed for the BMPC photoisomerization (involving a TICT-like state) and the essential characteristics of the intramolecular S and S, potential surfaces as derived from CS INDO Cl calculations. Now, combining the results of the present investigation with those of previous studies [24,25] we are in a position to fix the following points about the mechanism and dynamics of BMPC excited-state relaxation l)photoexcitation (So-Si)of the stable (trans) form results in the formation of the 3-4 cis planar isomer, as well as recovery of the trans one, through a perpendicular CT-like S] minimum of intramolecular origin, 2) a small intramolecular barrier (1.-1.2 kcal mol ) is interposed between the secondary trans and the absolute perp minima, 3) the thermal back 3-4 cis trans isomerization requires travelling over a substantial intramolecular barrier (=18 kcal moM) at the perp conformation, 4) solvent polarity effects come into play primarily around the perp conformation, due to localization of the... 

Swinney TC, Kelley DF (1993) Proton transfer dynamics in substituted 3-hydroxyflavones solvent polarization effects. J Chem Phys 99 211-221... 

The reactant R2 can also be considered to be a solvent molecule. The global kinetics become pseudo first order in Rl. For a SNl mechanism, the bond breaking in R1 can be solvent assisted in the sense that the ionic fluctuation state is stabilized by solvent polarization effects and the probability of having an interconversion via heterolytic decomposition is facilitated by the solvent. This is actually found when external and/or reaction field effects are introduced in the quantum chemical calculation of the energy of such species [2]. The kinetics, however, may depend on the process moving the system from the contact ionic-pair to a solvent-separated ionic pair, but the interconversion step takes place inside the contact ion-pair following the quantum mechanical mechanism described in section 4.1. Solvation then should ensure quantum resonance conditions. 

Figure 1.18 Solvent polarity effect in an Sn 1 reaction. Increasing the polarity of the solvent stabihzes the charge separation formed in the transition state, lowering the activation energy and increasing the rate of reaction...

Solvent polarity effects are also seen in the formation of isomers of transition metal complexes. Reactions that give a mixture of cis and trans isomers can be tuned by careful choice of solvent to give one isomer in preference to the other. For example, with cis and trans- PUlLI.-SbCL (where H2L = A-benzoyl-A -propylthiourea), shown in Scheme 1.4 [29], the cis isomer is favoured in solvents of high polarity whereas the trans isomer is dominant in solvents of low polarity. These observations are in accordance with other related observations [30], and... 

A possible mechanism for these reactions is shown in Scheme 21 for compound 101. The absence of a solvent polarity effect on the efficiency of photoreactions of 119 and 124 might be due to a very fast rearrangement of the radical-anion 126 within solvent cages (Scheme 21, path a). In cases in which this intermediate escapes from the cage before rearrangement occurs, a significant influence of the polarity of the solvent would have been observed. This is the situation in DMA-sensitized reactions of 101,117, and 118 (Scheme 21, path b). 

The thermal decomposition of 1,2-oxathietane 2-oxides invariably leads to loss of sulfur dioxide and accompanying alkene formation (Scheme 17) (73JA3420, 78BAU142). The lack of any solvent polarity effect on rate (75CC724) as well as supporting theoretical calculations favor a concerted loss of sulfur dioxide without requiring a strained [[Pg.459]

In the present study, we have confirmed the modification of the EGL for the CS from S2 induced by the change of solvent polarity by comparing the EGL in toluene (Tol) and methylcyclohexane (MCH) solution with that in THF. Moreover, we have compared the rate constants ()>p) of the S, state formation by S2 excitation with the decay rate constants (), ) of S2 state in ZP-I series and examined also solvent polarity effects on these rate constants comparing THF with Tol or MCH solutions. 

The solvent pH and polarity will affect the absorbance and fluorescence properties of a protein. A notable example of pH effects on absorbance is seen with tyrosine residues, where a change in pH from neutral to alkaline results in a shift of the absorbance maximum to a longer wavelength and an increase in absorptivity due to dissociation of the tyrosine phenolic hydroxyl group (Freifelder, 1982 Fasman, 1989). An example of solvent polarity effects on fluorescence is observed with tryptophan, where a decrease in solvent polarity... 

An investigation into the initiation mechanism of copolymerization of ethyl vinyl ether and acrylonitrile by /-butoxyl radicals lias shown that the reaction between the two monomers competes successfully with radical trapping by the nitroxide radical trap (5).37 The /-butoxyl radicals react 3-6 times faster with ethyl vinyl ether than acrylonitrile the authors proposed that this is due to selective interaction of one monomer with the radical species rather than a solvent polarity effect. 

The relative linearity of a Stern-Volmer plot of 1/< > versus 1/[A] for the photoreaction of cyclohexenone supports this mechanism and suggests that only one excited state is involved (30). The [2+2] addition of the triplet to a ground state molecule is not concerted, and there is a large solvent polarity effect on the regioselectivity for the head-to-head (HH) versus head-to-tail (HT) photodimers, with the more polar head-to-head isomer being favored in polar media. The polarity effect is attributed to the large difference in the dipole moments of the transition states leading to the products. 

This analysis shows that in order to account properly for solvent polarity effects, a solvation model has to be characterized by a larger flexibility with respect to the same model for ground state phenomena. In particular, it should be possible to shift easily from an equilibrium to a nonequilibrium regime according to the specific phenomenon under scrutiny. In the following section, we will show that such a flexibility can be obtained in continuum models and generalized to QM descriptions of the electronic excitations. 

These findings indicate the complexity of the solvent polarization effect on the solute charge distribution, which shows differential trends depending on the nature of the solute. In conjunction with the free energy of solvation, the analysis of the solvent-induced changes in the solute s electron density should be valuable to shed light on the influence of solvation on the chemical reactivity of solutes. 

The controversy about the timing of bond formation in cycloadditions continues. Although stepwise reactions involving zwitterionic intermediates can be detected more or less reliably by solvent polarity effects on rates, the distinction between mechanisms involving diradical intermediates or no intermediates at all (concerted pathways) is a more subtle one. Whereas the articles of debate were formerly experimental data, the discussion has now expanded into the theoretical realm. 

Kinetic measurements also show that the solvolysis is of the SnI-type small solvent polarity effects were found in the correlation with the ionizing power parameter qts, with a small m value of 0.12, characteristic of a reaction of a cationic substrate to give a cationic product. Furthermore, the rate data show that the leaving group ability of the phenyliodonio group is about 10 times as great as triflate or 10 -fold higher than iodide. ... 

---