Solvent shift electronic spectra

Those solutes for which the solvent shifts are particularly large have been used in the specification of solvent properties, such as electron-pair donation ability, Lewis basicity, or softness. For the former property, the solvent shifts of deuteromethanol or of phenol have served as suitable scales. For the latter property the solvent shifts of the symmetrical stretch of Hg-Br in the Raman spectrum of HgBr2 and of I-CN in the infrared spectrum of ICN have been so employed (see Chapter 4). 

Since the suggestion of the sequential QM/MM hybrid method, Canuto, Coutinho and co-authors have applied this method with success in the study of several systems and properties shift of the electronic absorption spectrum of benzene [42], pyrimidine [51] and (3-carotene [47] in several solvents shift of the ortho-betaine in water [52] shift of the electronic absorption and emission spectrum of formaldehyde in water [53] and acetone in water [54] hydrogen interaction energy of pyridine [46] and guanine-cytosine in water [55] differential solvation of phenol and phenoxy radical in different solvents [56,57] hydrated electron [58] dipole polarizability of F in water [59] tautomeric equilibrium of 2-mercaptopyridine in water [60] NMR chemical shifts in liquid water [61] electron affinity and ionization potential of liquid water [62] and liquid ammonia [35] dipole polarizability of atomic liquids [63] etc. 

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 structures of the pairs have been determined by ab initio calculations. Surprisingly, while the absorption spectrum of the solvated electron presents a single band located around 2250 nm, the absorption spectra of the pairs are blue-shifted and composed of two bands (Fig. 7)7 Those spectra were interpreted as a perturbation of the solvated electron spectrum with the use of an asymptotic model. This model describes the solvated electron as a single electron trapped in a THF solvent cavity and takes into account the effects of electrostatic interaction and polarization due to the solutes that are modeled by their charge distribution. It was shown that the p-like excited states of the solvated electron can be split in the presence of molecules presenting a dipole. So, the model accounts for the results obtained with dissociated alkali and non-dissociated alkaline earth salts in THF since ionic solutes yield absorption spectra with only one absorption band, and dipolar neutral solutes yield absorption spectra with two bands (Fig. 8). ... 

The hexaphenyl derivative has a dipole moment of 6.3 0.3 D [259]. The longest wavelength absorption peak in its electronic spectrum is shifted to shorter wavelengths with increasing polarity of solvent [259]. These results imply some separation of charge in the molecule as indicated in the limit by formula XXX A. 

An important exception is the electronic spectrum for nitroxide radicals [6]. In this case, solvent shifts for vN-O in the infrared are too small to give reliable information, but shifts in the first electronic band are large enough to give separate components. These studies revealed that nitroxides in water form two... 

NMR spectroscopy has been used to characterize aluminium chromium mixed oxides.The Mo chemical shifts, electronic absorption bands, and solvent donor numbers have been examined for [Mo2(02CR)4L2] solvent adducts. NMR spectroscopy has been used to confirm the presence of three different types of oxygen atoms in [ (ri -4-MeC6H4Pr )Ru 4Mo40i6]. The NMR spectrum of [HPW9034] is a five line, 1 2 2 2 2, spectrum. has... 

Diiodine is chosen as the reference Lewis acid. The standard conditions are T = 25 °C and an alkane (e.g. n-heptane or cyclohexane) is the solvent. Diiodine provides a remarkable opportunity to study halogen-bonded complexes from three regions of the electronic spectrum in which the absorption is directly related either to the concentration of the complex (the charge-transfer band, often around 240-350 nm, and the blue-shifted band, around 400-510 nm) or to the concentration of free diiodine (the visible band at 520 nm in n-heptane). In alkanes, diiodine interacts with Lewis bases B to form stable molecular... 

The peak in the UV VIS spectrum of acetone [(CH3)2C=0] corresponding to the transition appears at 279 nm when hexane is the solvent but shifts to 262 nm in water Which is more polar the ground electronic state or the excited stated... 

Many other measures of solvent polarity have been developed. One of the most useful is based on shifts in the absorption spectrum of a reference dye. The positions of absorption bands are, in general, sensitive to solvent polarity because the electronic distribution, and therefore the polarity, of the excited state is different from that of the ground state. The shift in the absorption maximum reflects the effect of solvent on the energy gap between the ground-state and excited-state molecules. An empirical solvent polarity measure called y(30) is based on this concept. Some values of this measure for common solvents are given in Table 4.12 along with the dielectric constants for the solvents. It can be seen that there is a rather different order of polarity given by these two quantities. 

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