Solvent Effects on UV Spectra

There is a general observation that many molecules that absorb radiation due to a tt rr transition exhibit a shift in the absorption maximum to a longer wavelength when the molecule is dissolved in a polar solvent compared to a nonpolar solvent. The shift to a longer wavelength is called a bathochromic or red shift. This does not mean that the solution turns red or that the absorption occurs in the red portion of the visible spectrum. 

If the TT energy level is decreased by attractive forces in polar solvents, it should be expected that the n tt transition will also show a red shift in polar solvents. It does, but there is a much more important interaction that overcomes the red shift for the n — tt transition. 

The hypsochromic shift effect on n electrons is much larger than the tt batho-chromic shift due to lowering of the tt orbital described above. As a consequence, the absorption maximum for the tt transition in a molecule which contains a lone pair of electrons 

A hypothetical example of how we can use this solvent effect information follows. A compound with molecules that contain both tt and n electrons may exhibit two absorption maxima and may show both a red shift and a blue shift with a change in solvent polarity. In general, tt tt transitions absorb approximately 10 times more strongly than n TT transitions. A molecule that contains both tt and n electrons and is dissolved in a nonpolar solvent such as hexane might have an absorption spectmm similar to the spectrum in Fig. 5.31. By comparing the relative intensities of the two peaks, we would suspect that the absorption at 250 nm was due to a tt tt transition and the absorption at 350 nm was due 

If we now needed to conhrm these assignments, we would put the compound in a solvent such as ethanol, which is both polar and capable of hydrogen-bonding. The polar nature of the solvent would induce a red shift in the tt tt transition and hydrogen-bonding would induce a blue shift in the transition. If our assignments were 

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The least problematic issues are UV spectral changes as a function of different solvents between the reference and the test sample. Solvent effects on UV spectra in solvents of decreased dielectric constant compared with water parallel solvent effects on apparent pKa. The changes are most marked for acids, for example, leading to a numerical increase of up to two pKa units - an apparent decrease in the acidity of the carboxylic acid. Effects on bases are considerably less. The apparent pKa of a base in a reduced dielectric constant solvent might be up to about half a pKa unit numerically lower (less basic). The UV spectra of neutral compounds... 

Solvatochromic pareuaeters, so called because they were Initially derived from solvent effects on UV/visible spectra, have been applied subsequently with success to a wide variety of solvent-dependent phenomena and have demonstrated good predictive ability. The B jo) scale of solvent polarity is based on the position of the intermolecular charge transfer absorption band of Reichardt s betaine dye [506]. Et(io> values are available for over 200 common solvents and have been used by Dorsey and co-%rarkers to study solvent interactions in reversed-phase liquid chromatography (section 4.5.4) [305,306]. For hydrogen-bonding solvents the... 

A survey of older works of solvent effects on UV/Vis absorption spectra has been given by Sheppard [21]. 

Thus, solvent effects on absorption spectra can be used to provide information about solute-solvent interactions [1-4]. On the other hand, in order to minimize these effects, it would be preferable to record absorption spectra in less interacting nonpolar solvents, such as hydrocarbons, whenever solubility permits. Suitable choice of a spectral solvent may be facilitated by consulting Tables A-4 (UV/Vis), A-5 (IR), A-6 ( H NMR), and A-7 NMR) in the Appendix, which list some of the more common solvents and their absorption properties. 

In contrast to these nonpolar compounds, very dramatic solvent effects on UV/ Vis spectra have been observed for dipolar meropolymethine dyes, especially mero-cyanines, due mainly to the change in their dipole moments on electronic transition. An example is the following negatively solvatochromic pyridinium V-phenolate betaine, which exhibits one of the largest solvatochromic effects ever observed cf. Fig. 6-2 [10, 29]). 

The electron donor-acceptor molecular complexes between iodine and thiazole, benzothiazole, and some derivatives have been studied in several organic solvents by UV spectroscopy <87CJC468>. In all cases, the presence of the thiazole ring produces a displacement of the Amax iodine band at 512 nm towards shorter wavelengths and a decrease of its absorbance. Moreover, a sharp isosbestic point near 470 nm was observed for all iodine-thiazole complexes. 2-Aryl and 2-hetarylbenzothiazoles showed fluorescence, the maxima of emission being between 350 nm and 395 nm. Both substituent and solvent effects on the spectra were observed <93MI 306-02>. The photophysical properties of bis(benzothiazolylidene)squaraine dyes have also been studied <93JPC13625>. 

In this respect, the solvatochromic approach developed by Kamlet, Taft and coworkers38 which defines four parameters n. a, ji and <5 (with the addition of others when the need arose), to evaluate the different solvent effects, was highly successful in describing the solvent effects on the rates of reactions, as well as in NMR chemical shifts, IR, UV and fluorescence spectra, sol vent-water partition coefficients etc.38. In addition to the polarity/polarizability of the solvent, measured by the solvatochromic parameter ir, the aptitude to donate a hydrogen atom to form a hydrogen bond, measured by a, or its tendency to provide a pair of electrons to such a bond, /, and the cavity effect (or Hildebrand solubility parameter), S, are integrated in a multi-parametric equation to rationalize the solvent effects. 

The dramatic influence of solvent effects on the UV and visible spectra of certain pyridine compounds, known generally as a solvatochromic effect, has been much utilized in the expression of solvents effects. The polarity parameter. Z or ET is defined (58JA3253, B-68MI204002) from the longest wavelength charge transfer band of 1-ethyl-4-methoxycar-bonylpyridinium iodide (equation 3). 

The solvatochromic effects on UV/visible spectra of certain solutes are so large, that they can conveniently be employed as probes for certain solvating properties of the solvents. 

There are many more solvent effects on spectroscopic quantities, that cannot be even briefly discussed here, and more specialized works on solvent effects should be consulted. These solvent effects include effects on the line shape and particularly line width of the nuclear magnetic resonance signals and their spin-spin coupling constants, solvent effects on electron spin resonance (ESR) spectra, on circular dichroism (CD) and optical rotatory dispersion (ORD), on vibrational line shapes in both the infrared and the UV/visible spectral ranges, among others. 

The correlations shown on Figs. 1 and 2 are particularly remarkable when other factors which influence the spectra are considered. A primary complication is the effect of solvent polarity. Ideally, UV spectra should be recorded in nonpolar hydrocarbon solvents to minimize the effect of... 

The solvatochromic effects on UV/visible spectra of certain solutes are so large, that they can conveniently be employed as probes for certain solvating properties of the solvents. Those that have enjoyed widespread application in this capacity are discussed in Chapter 4. They include 2,6-diphenyl -4-(2,4,6-triphenyl-l-pyridino)-phenoxide, 4-methoxynitrobenzene, 4-(dimethylamino)-nitrobenzene, for the estimation of the polarity of solvents, acetylacetonato-N,N,N, N -tetramethylethylenediamino-copper(II) perchlorate, 4-nitrophenol, and 4-nitroaniline, for the estimation of the electron pair donicity of solvents, 4-carboxymethyl-l-ethylpyridinium iodide, 4-cyano-l-ethylpyridinium iodide, and bis-c/.v-1, lO-phenanthrolinodicyano-iron(II) for the estimation of the hydrogen bond donation abilities of solvents (Marcus 1993). 

The development of the MPE method opened an avenue to the theoretical analysis of solvent effects on chemical and physico-chemical properties. The method was intensively applied to spectroscopical properties in the 1980s [28] including NMR nuclear quadrupole coupling [29,30], spin-spin coupling constants [31], IR spectra [28,32-34] vibrational polarizabilities [35], as well as UV-V and circular dichroism spectra [36-38],... 

Good test cases would be the solvent effects on the UV-vis absorption spectra of formaldehyde and acetone that have been the subject of innumerous theoretical studies. Innovative theoretical methods have been applied to formaldehyde (see also the compilation of results in [20,32,113,114,115,116]). Unfortunately the experimental result for formaldehyde in water is not clear because of chemical problems mostly associated to the aggregation and formation of oligomers. Therefore a better test case is the UV-vis spectra of acetone, because reliable experimental solvent shifts and several theoretical results are available (see the compilation of results in [117]). The Stokes shift of the n-rr transition of acetone has been critically discussed by Ohrn and Karlstrom [118], Grozema and van Duijnen [17] studied the solvatochromic shift of the absorption band of acetone in as much as eight different solvents. Acetone is known to shift the maximum of the n-rr band by 1500-1700 cm 1 when immersed in water [119,120,121], Using the conventional HF/6-31 G(d) point charges, Coutinho and Canuto [54] simulated acetone in water and performed INDO/CIS... 

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