Dipolar solvation

Initial supercritical fluid work was on C02 (21-23) and indicated weak interactions between the fluid and solute. Additional work has appeared on Xe, SF6, C2H, and NH3 (24,25). For all fluids, spectral shifts were observed with fluid density. Yonker, Smith and co-workers (24-26,28) compared their results to the McRae continuum model for dipolar solvation (56,57), which is based on Onsanger reaction field theory (58). Over a limited density range, there was agreement between the experimental data and the model (24-26,28), but conditions existed where the predicted linear relationship was not followed (28). At low fluid densities, this deviation was attributed (qualitatively) to fluid clustering around the solute (28). 

While none of the calculated quantities exhibits an overall monotonic trend with increasing polarity, they all show the same nonmonotonic pattern, in which the trend of dipole shifts is inversely correlated with that for HDA, p,12, and AEn. A key feature governing this behavior is the fact that inertial solvation is equilibrated to the site with less access to solvent (i.e., AC, the site of the initial state hole) in comparison with the more accessible ABP site (where the charge resides in the final state). As a result, there is a mismatch in dipolar solvation in the vertical CSh absorption, such that within the solvent sequence (e0 = 1-8, 7.0, and 37.5), increasing the polarity actually decreases the degree of charge localization (i.e., smaller A/xn and A/zda, an effect dominated by the oxidized D site in the final state), and hence increases the D/A coupling (as reflected in HDA and also p.n). 

There are also a number of theories taking into account dipolar solvation dynamics. These theories use the solvent s dielectric response function as the dynamical input and also include effects due to the molecular nature of the solvent. The most sophisticated of these theories, by Raineri et al. [136] and by Friedman [137], uses fully atomistic representations for both solute and solvent and recent comparisons have shown it to be capable of quantitatively reproducing both the static and dynamic aspects of solvation of C153 [110]. In these cases the theoretical nature of solvation dynamics is fully understood. However, it must be remembered that much of the success of these theories rests on using the dynamical content of the complicated function, dielectric response function, determined from experiment. Although there... 

T. Higuchi, A. Michaelis, and A. Hnrwitz, Ion pair extraction of pharmaceutical amines. Role of dipolar solvating agents in extraction of dextromethorphan, Ana/. Chem. 39 (1967), 91A-919. 

A rather simple experimental teehnique involving measurement of the time-dependent fluorescence Stokes shift (TDFSS) after an initial exeitation has been applied to measure SD in a large number of liquids. TDFSS oceurs due to dipolar solvation of the excited probe and thus gives an estimate of the solvation timeseales. In an important paper, Jimenez et al. reported the results of SD of the exeited state of the dye coumarin 343 (C343) in liquid water [14]. Their result is shown in Figure 3.13. The initial part of the solvent response of water was found to be extremely fast (few tens of femtoseconds) and it constituted more than 60% of the total solvation energy relaxation. The subsequent relaxation was found to occur in the picosecond timescale. The decay of the solvation time correlation function, S t)y was fitted to a function of the following form... 

This leads to a Sehrodinger equation whieh eigenvalue is direetly related to the total eleetrostatie (dipolar) solvation energy, E ],... 

Going back to the interpretation of correlated rotations, the issues resumed above give us a picture of short-range order where a central molecule is surrounded by those of its first coordination shell, reminiscent of molecular aggregates that are known to occur about electrons, radicals or ions, and thus refe-red by analogy as a "dipolar solvation" process. At present, the way the dynamics would be affected by dipolar self-solvation deserves further research concerning mechanisms that entail rotational coherence. 

This appears to be a better scheme than distinguishing between truly nonpolar and nondipolar solvents. A polar molecule can be defined as having a strongly polar bond, bnt need not necessarily be a dipole. In this framework, the solvating power of the nondipolar solvents need no longer be viewed as anomalous or as essentially dependent on specific solvation effects. Beyond this it shonld be emphasized that matty liquids have both a dipole moment and a quadmpole moment, water for example. However, for dipolar solvents snch as acetonitrile, acetone, and dimethyl snlfoxide, the dipolar solvation mechanism will prevail. For less dipolar solvents like tetrahydrofurane, quadrapoles and dipoles might equally contribute to the solvation energetics. ... 

In fee absence of fee solvation typical of protic solvents, fee relative nucleophilicity of anions changes. Hard nucleophiles increase in reactivity more than do soft nucleophiles. As a result, fee relative reactivity order changes. In methanol, for example, fee relative reactivity order is N3 > 1 > CN > Br > CP, whereas in DMSO fee order becomes CN > N3 > CP > Br > P. In mefeanol, fee reactivity order is dominated by solvent effects, and fee more weakly solvated N3 and P ions are fee most reactive nucleophiles. The iodide ion is large and very polarizable. The anionic charge on fee azide ion is dispersed by delocalization. When fee effect of solvation is diminished in DMSO, other factors become more important. These include fee strength of fee bond being formed, which would account for fee reversed order of fee halides in fee two series. There is also evidence fiiat S( 2 transition states are better solvated in protic dipolar solvents than in protic solvents. 

The remarkable enhancement of anion nucleophilicity in Sn2 reactions carried out in dipolar aprotic solvents is a solvation effect.Solvents like DMF and DMSO are very polar owing to the charge separation indicated in 1 and 2. 

Semiempirical MNDO (85JOC4894, 95ZOR1422) and AMI, PM3 (95ZOR1422), and ab initio (95JPC12790) calculations conclude that the unfavorable tautomer 26c (R = R = H) has a substantially higher dipole moment than 26a 5.86 D and 3.01 D, respectively (95JPC12790). This factor provides for better solvation of 26c in dipolar solvents and leads to a... 

The same conclusion was reached in a kinetic study of solvent effects in reactions of benzenediazonium tetrafluoroborate with substituted phenols. As expected due to the difference in solvation, the effects of para substituents are smaller in protic than in dipolar aprotic solvents. Alkyl substitution of phenol in the 2-position was found to increase the coupling rate, again as would be expected for electron-releasing substituents. However, this rate increase was larger in protic than in dipolar aprotic solvents, since in the former case the anion solvation is much stronger to begin with, and therefore steric hindrance to solvation will have a larger effect (Hashida et al., 1975 c). 

Recent studies have found enhanced substituent solvation assisted resonance effects in dipolar non-hydrogen bonding solvents131. For several +R substituents acidities of phenols in DMSO are well correlated with their gas-phase acidities. The substituents include m- and p-SOMe, m- and p-S02Me, m-S02CF3 and m-N02. But there is very considerable enhancement of the effect of p-S02CF3, p-N02 and various other para-substituents in DMSO solution. 

Nevertheless, as in many previous observations, the clathrate formation by dipolar host compounds could not have been predicted in advance. In fact, there are no channels in the crystal structures of hydrated moxnidazole hydrochloride (closely related species to furaltadone hydrochloride) and of hydrated furaltadone base (Fig. 13)37). Rather, the latter two structures are best described as solvates with the H20 molecules contained in local voids between adjacent moieties of the host. 

---