Solvent Shift Parameter

There have been several other attempts to define solvent polarity parameters, among the more successful being those related to solvatochromic shifts the shift in wave-length/frequency of a band in the spectrum of a suitable absorbing species resulting from its interaction with the molecules of a series of different solvents. Particularly large shifts were observed with the zwitterion (51),... 

For an organic compound (Q) in dipolar aprotic solvents, the half-wave potential ( 1/2) of the first reduction step tends to shift to the positive direction with an increase in solvent Lewis acidity (i.e. acceptor number). This is because, for the redox couple Q/Q, the reduced fonn (Q ) is energetically more stabilized than the oxidized fonn (Q) with increasing solvent acidity. The positive shift in E1/2 with solvent acceptor number has been observed with quinones [57 b], benzophenone [57 a, c] and anthracene [57 c], With fullerene (C60), the positive shift in E1/2 with solvent acidity parameter, ET, has been observed for the reductions of C60 to Qo, Qo to Clo, and Cf)0 to Cli, [54c], However, the positive shift in E1/2 is not apparent if the charge in Q is highly delocalized, as in the cases of perylene and fluoren-9-one [57 c]. 

The core of the model is then the definition of the Q matrix, which in the most recent implementations of PCM depends only on the electrostatic potentials, takes into the proper account the part of the solute electron density outside the molecular cavity, and allows the treatment of conventional, isotropic solutions, and anisotropic media such as liquid crystals. Furthermore, analytical first and second derivatives with respect to geometrical, electric, and magnetic parameters have been coded, thus giving access to proper evaluation of structural, thermodynamic, kinetic, and spectroscopic solvent shifts. 

As shown in Figure 2.5, continuum solvent models (PCM) reproduce satisfactorily solvent effects on the aN parameter only for aprotic solvents (bulk effects), whereas there is a noticeable underestimation of solvent shifts for protic solvents (methanol and water). In these media also specific solute-solvent interactions have to be taken into account. 

As we compare with the experimental results for the shift in the electronic excitation energy (1500-1600 cm-1), it is clear that for the three most reliable methods we obtain a slight underestimation of the solvent-induced shifts in excitation energy. However, the agreement between experimental and DFT- or CCSD- based solvent shifts is overall good. The remaining discrepancies could potentially be attributed to the possible imperfections in the force field parameters used for acetone and water in the QM/MM calculations and, especially, in the MD simulations. 

Differential solvent interactions with ground- and excited-state molecules not only lead to shifts in the fluorescence maxima but also to perturbation of the relative intensities of the vibrational fine structure of emission bands. For instance, symmetry-forbidden vibronic bands in weak electronic transitions can exhibit marked intensity enhaneements with increasing solute/solvent interaction [320, 359]. A particularly well-studied ease is the solvent-influenced fluorescence spectrum of pyrene, first reported by Nakajima [356] and later used by Winnik et al. [357] for the introduction of an empirical solvent polarity parameter, the so-called Py scale cf. Section 7.4. 

The solvent dependence of the n n transition energies of two meropoly-methine dyes was used by Brooker et al. [77] to establish the solvent polarity parameters /r and Xb ( / Table 7-2). is based on the positively solvatochromic merocyanine dye no. 1 in Table 6-1 of Section 6.2.1 (red shift with increasing solvent polarity), while Xb represents the transition energies of the negatively solvatochromic merocyanine dye no. 13 in Table 6-1. 

If X—H B vibrations are being examined, v refers to its gas-phase value. The value for fl is a measure of solvent susceptibihty of a particular IR vibration, and G is a function of the solvent only. Since solvent shifts of vc-o and vs-o are proportional to solvent shifts of vx-h b, G values are calculated from the solvent shifts of the carbonyl bands of A,iV-dimethyIformamide and benzophenone and the sulfonyl band of dimethyl sulfoxide. An arbitrary value of 100 was assigned to dichloromethane to fix the scale [G = 0 for the gas phase) [85]. Values of G are given in Table 7-2. Further G values have been determined by Somolinos et al. [240] and their relationships to other solvent polarity parameters have been investigated [241]. 

Interesting solvent scales based on NMR measurements have been proposed by Taft et al. [90] and by Gutmann, Mayer et al [91]. A solvent polarity parameter, designated as P, has been defined by Taft et al [90] as the F chemical shift (in ppm) of 4-fluoro-nitrosobenzene in a given solvent, relative to the same quantity in the reference solvent cyclohexane cf. Table 6-6 and the discussion in Section 6.5.1). These parameters define a scale ranging from P = 0.0 in cyclohexane to P = 2.7 in sulfolane, and can easily be measured in a wide variety of solvents. The P values appear to be related to the ability of the solvents to form specific 1 1 complexes with the nitroso group of the standard compound. A compilation of P values can be found in reference [92], In addition, chemical shifts of (trifiuoromethyl)benzene and phenylsulfur pentafiuoride have been used by Taft et al. to study nonspecific dipolar interactions with HBD solvents and utilized to define n values of solvent dipolarity/polarizability for protic solvents [249]. 

The polarity of the solvent exerts an appreciable effect on some of the hfc constants for pyridinyl radicals The effects have been correlated with the solvent polarity parameters, Z-value and Er(30)-value and a theory relating the shifts to the permittivity of the solvent has been published... 

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