Based on solvatochromic shifts

Empirical scales of solvent polarity based on solvatochromic shifts... 

The first point to be addressed is the increase in dipole moment, Apt, on excitation of PRODAN (Formula in Figure 7.4). The determinations of A/x reported in the literature, apart from one, are based on solvatochromic shifts analyzed with the Lippert-Mataga equation. In the original paper by Weber and Farris Apt was estimated to be 20 D, but this value was later recognized to be overestimated and recalculation led to a value of 8 Da) b). Another study yielded a consistent value of 7 Dc) d). A completely different method based on transient dielectric loss measurement provided a somewhat lower value 4.4-5.0 Dd. From all these results, it can be concluded that the increase in dipole moment on excitation is not responsible for the high sensitivity of PRODAN to solvent polarity. 

Figure 6. Qustering of CHFj about (dimethylamino)benzonitrile at 50 C based on solvatochromic shifts in fluorescence (A data from ref. 26, — Onsager reaction field theory).

Empirical scales of solvent polarity based on solvatochromic shifts I 205 Tab. 7.1. Parameters of the n scale of polarity (data from Kamlet et aL, 1983)... 

S. C. Tucker, Solvent density inhomogeneities in supercritical fluids, Chem. Rev., 99 (1999) 391—418 O. Kajimoto, Solvation in supercritical fluids Its effects on energy transfer and chemical reactions, Chem. Rev., 99 (1999) 355-89 S. Nugent and B. M. Ladanyi, The effects of solute-solvent electrostatic interactions on solvatochromic shifts in supercritical C02, J. Chem. Phys., 120 (2004) 874-84 F. Ingrosso and B. M. Ladanyi, Solvation dynamics of C153 in supercritical fluoroform a simulation study based on two-site and five-site models of the solvent, J. Phys. Chem. B, 110 (2006) 10120-29 F. Ingrosso, B. M. Ladanyi, B. Mennucci and G. Scalmani, Solvation of coumarin 153 in supercritical fluoroform, J. Phys. Chem. B, 110 (2006) 4953-62 Y. Kimura and N. Hirota, Effect of solvent density and species on static and dynamic fluorescence Stokes shifts of coumarin 153, J. Chem. Phys., Ill (1999) 5474 ... 

The application of solvatochrome dyes for characterizing the strengths (polarities) of solvents was first proposed by Brooker [Br 51]. His solvent scale is based on the shifts in the absorption bands of the following two merocyanine dyes (I and II) ... 

Figure 2. Correlation of a semiempirieal polarity scale (Ep (IB DNS)), based on the shift of the solvatochromic emission band of the compound EB DNS, with the Dimroth parameter 730 For symbols of the solvents see Figure 1 (24) ethylene glycol, (25) ethyl cellosolve. EpdN DNS) (kJ/mol) is the energy of the emission maximum of DNS fluorochrome E -gQ (kcal/mol).

Laurence and coworkers (Lanrence et al., 2014) emphasize the distinction between basicity parameters of bnik solvents and those derived from measurements on substances in solntion in inert solvents. In the same paper, they discnss some difficulties in the assignment of basicity parameters based on solvatochromism, such as the Taft-Kamlet p, to amphiprotic solvents, and propose another measure of H-bond basicity, p, based on the difference between the NMR chemical shifts of 4-fluorophenol and... 

The solute descriptors such as Jt, a, and P can be extracted from the hterature [2-5] and are in most cases empirically determined by spectroscopic measurements. Their calculation is based on the shift of absorption bands of the given solutes due to varying solvatochromic effects when the dipolar or hydrogen-bond donor /acceptor properties of the solvent mixture used for spectroscopy are altered. Hence, they are not true thermodynamic data. Abraham and co-workers also determined descriptors from GC retention data and octanol/water partition coefficients [6-9]. With the help of such descriptors, the so-called solvation equation (Eq. 5) can be set out [10,11] ... 

Donor strengths, taken from ref. 207b, based upon the solvent effect on the symmetric stretching frequency of the soft Lewis acid HgBr2. Gutmann s donor number taken from ref 207b, based upon AHr for the process of coordination of an isolated solvent molecule to the moderately hard SbCL molecule in dichioroethane. ° Bulk donor number calculated as described in ref 209 from the solvent effect on the adsorption spectrum of VO(acac)2. Taken from ref 58, based on the NMR chemical shift of triethylphosphine oxide in the respective pure solvent. Taken from ref 61, based on the solvatochromic shift of a pyridinium-A-phenoxide betaine dye. 

There are single- and multiparameter approaches for determining the polarity and separation of contribution of different interactions to the total effect of polarity on spectroscopic characteristics. They are based on different theories of solvatochromic shifts of absorption and fluorescence bands. 

J. Li, C. J. Cramer, and D. G. Truhlar, A two-response-time model based on CM2/INDO/S2 electrostatic potentials for the dielectric polarization component of solvatochromic shifts on vertical excitation energies, Int. J. Quan. Chem. 77 264 (2000). 

Kosower in 1958 was the first to use solvatochromism as a probe of solvent polarity. The relevant Z-scale is based on the solvatochromic shift of 4-methoxycarbonyl-1-ethylpyridinium iodide (1). Later, Dimroth and Reichardt suggested using betain dyes, whose negative solvatochromism is exceptionally large. In particular, 2,6-... 

A solvatochromic scale, based on the ultraviolet-visible, rather than the infrared, spectral band of suitable probes is that based on the Kamlet -Taft P parameter. This is again an averaged quantity, for which the wavenumber shifts of several protic indicators relative to structurally similar but aprotic homomorphs are used (Kamlet et al. 1983 Kamlet and Taft 1976). It is assumed that the nonspecific effect of a solvent on the protic probe is the same as that on the aprotic one, and that it can be expressed in terms of the n parameter for the solvent, so that the donicity of the solvent, if it is a Lewis base, causes the difference between the responses of the two probes towards the solvent. The probes originally employed were 4-nitrophenol (vs 4-nitroanisole) and 4-nitroaniline (vs 4-nitroN,N-diethylaniline), but once a n scale is known, the need for the specific aprotic homomorph values no longer exists, since the general expression ... 

N,N-Diethyl-4-nitroaniline, has an aromatic ring but no hydrogen bond donor substituent, shows a n-jt transition based on a non-specific interaction between ions. Dipolarity/polarizability, jz, is estimated by the solvatochromic shift of N,N-diethyl-4-nitroaniline using Eq. (3.3), where Amax is the absorption maximum for N, N-diethyl-4-nitroaniline. 

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