Solute-solvent clusters

CaineUi, G. Galletti, P. Giacomini, D. Gualandi, A. Quintavalla, A. Helv. Chim. Acta 2003, 86, 3548-3559. Variable-temperature C NMR experiments show that the solute-solvent clustering effect manages the chemical and enzymatic enantioselectivity. 

In this case water is effectively acting as a catalyst for the reaction by lowering the energy of activation. These catalytic water molecules are more likely to participate in the reaction under supercritical conditions because their high compressibility promotes the formation of solute-solvent clusters. 

The photodimerization reaction of anthracene in supercritical CO2 was studied systematically at different CO2 densities. Unlike in normal liqnid solvents, the reaction in supercritical CO2 is significant even at anthracene concentrations as low as a few micromolar. At comparable anthracene concentrations, the photodimerization reaction is one order of magnitude more efficient in CO2 than in normal liqnid solvents. The results also show that the efficient photodimerization reaction of anthracene is hardly affected by the local density augmentation (or solute-solvent clustering) in supercritical CO2 (Bunker et al., 1997). 

R2-R2 was 25%, 50%, and 25%, respectively, and did not vary with pressure. The fact that a statistical 1 2 1 ratio of bibenzyls is produced, at all pressures examined including near the critical pressure, suggests that these products are produced by diffusive encounters of (Ri.)free and (R2.)free rather than by in-cage coupling (which would yield solely Ri R2). Thus, these authors found no evidence for an enhanced cage-effect that might be attributable to solute solvent clustering. 

As in the case of a microsolvation calculation itself, in a mixed explicit-implicit description the (explicit) solvent molecules and the solute often play different roles and require different levels of theory. Ideally one would use a hybrid method for the solute-solvent cluster, which is then embedded in a continuum. This is illustrated in Figure 4.9 for a chloride anion, methylchloride, and one explicit water molecule. With this in mind, we developed the ONIOM-PCM method [41],... 

The clearest use of ONIOM-PCM is for solute-solvent clusters embedded in a continuum. The method can also be used to partition the solute itself into layers that are each treated at a different level of theory. An example is the study of NMR shielding in a merocyanine in solution (Figure 4.10) [41], We looked at the shielding on the nitrogen center. Nuclear shielding is a relatively local property, and previous gas-phase studies showed that ONIOM can accurately reproduce target values [43], We investigated several... 

In order to generate an appropriate number of solute-solvent clusters to be used in the NMR calculations a series of classical MD simulations of pyrazine, pyrimidine or pyridazine in aqueous solution has been carried out. All the details of the force fields used for the diazines and water as well as computational details of the MD simulation can be found in Ref. [31]. Every 1 ps an MD configuration was dumped so as to obtain 600 different molecular configurations. Then, a spherical cut-off distance equal to 12 A was applied so as to obtain the final cluster including 230-240 water molecules together with the solute. 

The theory reflects the solvent properties through the thermody-namic/hydrodynamic input parameters obtained from the accurate equations of state for the two solvents. However, the theory employs a hard sphere solute-solvent direct correlation function (C12), which is a measure of the spatial distribution of the particles. Therefore, the agreement between theory and experiment does not depend on a solute-solvent spatial distribution determined by attractive solute-solvent interactions. In particular, it is not necessary to invoke local density augmentation (solute-solvent clustering) (31,112,113) in the vicinity of the critical point arising from significant attractive solute-solvent interactions to theoretically replicate the data. 

Fig. 31 ORD of methyloxirane in benzene solute-solvent cluster and solvent imprint. Experimental ORD, computed ORD for gas phase and continuum solvent for (5)-methyloxirane, and computed gas phase ORD of (/ )-methyloxirane shown for comparison. Data to prepare the figure were taken from [156]...

Figure 4.2 Schematic representation of solute-solvent clustering in an SCF, eom-pared with liquid-phase solvation and lack of solvation in the gas phase.

Most investigations of photoinduced electron transfer have been performed in condensed phases. Much less is known about conditions that permit the occurrence of intramolecular ET in isolated (collision-free) molecular D-A systems. A powerful method for this kind of study is the supersonic jet expansion teehnique (which was originally developed by Kantrowitz and Grey in 1951 [66]) combined with laser-induced fluorescence (LIF) spectroscopy and time-of-flight mass spectrometry (TOF-MS). On the other hand, the molecular aspects of solvation can be studied by investigations of isolated gas-phase solute-solvent clusters which are formed in a supersonic jet expansion [67] (jet cooling under controlled expansion conditions [68] permits a stepwise growth of size-selected solvation clusters [69-71]). The formation of van der Waals complexes between polyatomic molecules in a supersonic jet pro-... 

Experiments with clusters in a supersonic jet can advantageously be employed to study the effects of solvation of a chromophore on its emission spectrum. Hence it is desirable to characterize the composition and the structure of the solute-solvent clusters as precisely as possible. The cluster size distributions can be determined by TOF-MS after resonant two-photon ionization, R2PI [84, 92a,c]. This allows for a... 

In current studies of supercritical mixtures, there is an outstanding hypothesis in the search for a molecular-scale mechanism that underlies all the unusual behavior associated with nearness to the CP. This hypothesis is the clustering of solvent molecules around the solute molecules. A surprising result from a recent fluorescence spectroscopy study shows the possibility of formation of solute-solute aggregates that occur near the CP. We propose in this study to clarify the understanding of solute-solvent clustering and solute-solute aggregation near CPs in supercritical solutions. 

The fluorescence data, specifically ratios of emission intensities, I1/I3, were used to show that solute-solvent clustering increases as the temperature approaches the critical temperature (I1/I3 is a measure of solvent strength). In liquid organic solvents, it is known that the ratio I1/I3 may be correlated linearly with another solvent strength parameter, it, by the relationship... 

Solute-solvent clustering, as determined both from V2, and solvatochromic shifts for UV-visible and fluorescent probes, has been described by a unified theoretical result, eq 2. The degree of clustering is related quantitatively to the solute-solvent interaction strength multiplied by the isothermal compressibility. Large pressure effects on chemical potentials, solubilities, rate constants, and equilibrium constants are all a manifestation of and can be related to the isothermal ccHnpressibility. 

Many of the experimental studies of solute-solute and solute-solvent clustering by fluorescence spectroscopy in SCFs have used pyrene as the probe molecule [23-25] because the liquid-phase behavior of this molecule is well under-... 

A plot of the product ratio (4+5) 6 as a function of pressure is presented in Figure 4.4-5. This plot exhibits a dramatic spike at pressures near the critical pressure which the authors attribute to the onset of solute-solvent clustering disintegration of the caged pair is inhibited because the viscosity at the molecular level is much greater than the bulk viscosity. 

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