Dipolar solute

When a nonpolar solute is in solution in any solvent, either nonpolar or polar, then mainly dispersive forces operate between them, and any solvent effects are very small and bathochromic (Reichardt, 1988), increasing with the polarizability of the solvent. If the solute is dipolar in a nonpolar solvent, then both hypso- and bathochromic shifts, increasing with solvent polarizability, are possible, depending on the dipole moments of the ground and excited states. The situation becomes more complicated for a dipolar solute in a dipolar solvent. 

Whether this treatment is valid for a dipolar solute in a mixture of polar solvents is doubtful, but it would not appear to be valid for ions without intrinsic dipole moments. Nevertheless, Equation 53, as a ratio of wavelength shifts, has been applied to the analysis of CTTS spectra (19, 20, 21). 

According to Eq. (19), t, is the time scale for excited state solvation for a Debye solvent. In fact, it is the time scale for both excited state and ground solvation of dipolar solutes and ionic solutes, t, also plays a role in a broad range of reactive (Section III) and nonreactive charge transfer processes in solution. It is clearly worthwhile to establish a physical picture for this important variable. 

An important advance in the understanding of microscopic solvation and Onsager s snowball picture has recently been made through the introduction of the linearized mean spherical approximation (MSA) model for the solvation dynamics around ionic and dipolar solutes. The first model of this type was introduced by Wolynes who extended the equilibrium linearized microscopic theory of solvation to handle dynamic solvation [38]. Wolynes further demonstrated that approximate solutions to the new dynamic MSA model were in accord with Onsager s predictions. Subsequently, Rips, Klafter, and Jortner published an exact solution for the solvation dynamics within the framework of the MSA [43], For an ionic solute, the exact results from these author s calculations are in agreement with Onsager s inverted snowball model and the previous numerical calculations of Calef and Wolynes [37]. Recently, the MSA model has been extended by Nichols and Calef and Rips et al. [39-43] to solvation of a dipolar solute. 

Figure 2.5 Ordering of solvent molecules around (a) a dipolar solute molecule and (b) a solute positive ion. The orientation will be most pronounced in the innermost shell of solvent molecules and will become increasingly random as distance from the solute particle increases. The strength of the interaction will depend on the molecular sizes and shapes and on the magnitudes of the dipole moments of both solutes and solvent particles.

This longitudinal relaxation time differs from the usual Debye relaxation time by a factor which depends on the static and optical dielectric constants of the solvent this is based on the fact that the first solvent shell is subjected to the unscreened electric field of the ionic or dipolar solute molecule, whereas in a macroscopic measurement the external field is reduced by the screening effect of the dielectric [73]. 

Figure 3 Frequency-domain vibrational friction felt by diatomic solutes dissolved in molecular fluids (52). The three panels show the friction for a model dipolar solute dissolved in acetonitrile (top), and for I2 dissolved in liquid (middle) and supercritical (bottom) carbon dioxide. Each panel compares the exact molecular dynamics (MD) results with the linear INM predictions.

That electrostatic forces could be crucial to vibrational energy relaxation was amply demonstrated by the liquid water simulations of Whitnell et al. (34). They noted that since the electrostatic portion of the force between their solvent and a dipolar solute was linear in the solute dipole moment, Equations (12) and (13) implied that the electrostatic part of the friction ought to scale as the dipole moment squared. When they then found that their entire relaxation rate scaled with the square of the solute dipole moment, it certainly seemed to be convincing evidence that electrostatics forces were indeed the primary ingredients generating ultrafast relaxation. Subsequent theoretical work on relaxation rates in such manifestly protic solvents as water and alcohols has largely served to reinforce this message (37,38,60,61). 

The results of such a calculation, shown in Fig. 8 (52), seem to tell a very different story from the earliest studies. With nondipolar I2 as a solute and C02 as a solvent, the complete domination of the solvent response by the Lennard-Jones forces is impressive, but perhaps not all that startling. One might surmise that the quadupole-quadrupole forces at work in this example are a bit too weak to accomplish much. Yet, when we have a dipolar solute dissolved in the strongly polar solvent CH3CN, we get almost the same kind of complete control by Lennard-Jones forces. Electrostatics now seems totally unimportant. 

Here inv stands for an invariant in respect to transformation consistent with the symmetry of the system. For quantum mechanical operators, this means unitary transformations. The parameter Ae in Eq. [107] quantifies the extent of mixing between two adiabatic gas-phase states induced by the interaction with the solvent. For a dipolar solute, it is determined through the adiabatic differential and the transition dipole moments... 

Dipolar solute in a nonpolar solvent. In this case, the forces contributing to solvation are dipole-induced dipole and dispersion forces. If the solute dipole moment increases... 

Dipolar solute in a polar solvent. Since the ground-state solvation results largely from dipole-dipole forces in this case, there is an oriented solvent cage around the dipolar solute molecules, resulting in a net stabilization of their ground state. If the solute dipole moment increases during the electronic transition the Franck-Condon excited... 

Fig. 6-4. Schematic qualitative representation of solvent effects on the electronic transition energy of dipolar solutes in polar solvents [2, 69]. (a) i.e. the dipole moment of the Franck-...

This reaction field, caused by the solvent molecules surrounding the dipolar solute molecule, is of the order of 10 V/cm and can influence an absorption spectrum in the same manner as an externally applied electric field. The spectral changes produced by means of a homogeneous external electric field have been termed electrochromism. Thus, solvatochromism is closely related to elec-trochromism [13, 81, 82]. 

That the electronic ground-state structure of a dipolar solute is indeed affected by solvent polarity has been independently shown by NMR [20, 50, 73, 75, 78], NMR [77], and IR measurements [20] of merocyanines. Some of these results observed with the positively solvatochromic 3-(dimethylamino)propenal are presented in Table 6-2. 

Dielectric friction is the measure of the dynamic interaction of a charged or dipolar solute molecule with the surrounding polar solvent molecules. This concept has been applied, by Hynes et al. [339] and others [486], to solvent- and time-dependent fluorescence shifts resulting from the electronic absorption by a solute in polar solvents. If the solvent molecules are strongly coupled to the charge distribution in ground- and excited-state molecules, the relatively slow solvent reorientation can lead to an observable time evolution of the fluorescence spectrum in the nano- to picosecond range. This time-dependent fluorescence (TDF) has been theoretically analysed in terms of dynamic... 

Compared with the pronounced solvent-induced chemical shifts observed with ionic and dipolar solutes, the corresponding shifts of nonpolar solutes such as tetrame-thylsilane are rather small cf. Table 6-6. A careful investigation of chemical shifts of unsubstituted aromatic, as well as alternant and nonalternant, unsaturated hydrocarbons in aliphatic and aromatic non-HBD solvents by Abboud et al. has shown that the differential solvent-induced chemical shift range (relative to benzene as reference) is of the order of only —1.4...+1.0 ppm (positive values representing downfield shifts) [405]. The NMR spectra of these aromatic compounds have been shown to be sensitive to solvent dipolarity and polarizability, except in aromatic solvents, for which an additional specific aromatic solvent-induced shift (ASIS see later) has been found. There is no simple relationship between the solvent-induced chemical shifts and the calculated charge distribution of the aromatic solute molecules. This demonstrates the importance of quadrupoles and higher multipoles in solute/solvent interactions involving aromatic solutes [405]. 

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