Vibrational spectra, solvent effects

The absorption spectrum of phenylalanine is shown in Fig. 1. The low intensity absorption peak centered slightly below 2600 A corresponds to a forbidden x it transition. Vibrational fine structure is quite evident in this region. Aside from increased blurring of the fine structure which is to be expected on passing into successively more polar media, change of solvent effects little change in the general size, shape, and location of the absorption envelope. 

These are vibrational spectra and lead to fundamental frequencies of vibration and the strength of ion-solvent interactions. When a salt is dissolved in water it will affect the frequencies of the water absorption and in favourable cases it will give rise to new peaks due to ion-solvent interactions. Alteration in the vibrational spectrum of the water due to the presence of the ion gives information regarding the effect of the ion on the water structure. The really useful information, however, comes from a study of the new lines due to the actual bonding of the ion to the solvent molecules. The frequencies and intensities of the vibrational lines give a measure of the strength of the bond between ion and solvent, and the peak areas can in favourable cases lead to hydration numbers. 

QM/EFPl scheme was used for investigating a variety of chemical processes in aqueous environment, including chemical reactions, amino acid neutral/zwitterion equilibrium, solvent effects on properties of a solute such as changes in dipole moment and shifts in vibrational spectrum, and solvatochromic shifts of electronic levels [36, 56, 59-60, 71-79]. Applications of a general QM/EFP scheme were limited so far to studies of electronic excitations and ionization energies in various solvents [56-58]. Extensions of QM/EFP to biological systems have been also developed [80-85]. 

The application of infrared spectroscopy to the solution of such solvation problems is hampered to a certain extent by the fact that complex formation leads to the appearance of new, skeletal vibrations, the coupling of which with the vibrations of the original molecules makes the vibrational spectrum more complicated. The situation may similarly be complicated by other effects, such as changes in the orbital hybridization, back-coordination, etc. In the coordination of the solvent acetonitrile to metal ions, for example, if only the effect of coordination causing a decrease in the electron density on the nitrogen is taken into consideration, the frequency of the C—vibration would be expected to decrease. In fact, in the course of this process the coordination increases the a character of the C—bond, which is accompanied by an increase in the frequency of the C—vibration [Be 61]. In many cases, it is difficult to assign the infrared bands to the corresponding vibrations the conclusions drawn from the spectral data may therefore be uncertain. 

The atomic lines in the spectrum appear as vertical lines or peaks due to the nature of the electronic transition involved. That is, in molecules an electronic transition is usually accompanied by simultaneous changes in the molecule s vibrational and rotational energy levels sometimes all the three energy types may change simultaneously in an electronic transition in a molecule. The many different transition possibilities allowed in this way and the solvent effect derived from the aggregation state of the sample (the excited sample is in liquid form) determines that in UV-Vis molecular absorption (or emission) the corresponding peaks in the spectrum are widely broadened. Typically, the half-bandwidth of an absorption band in such molecular UV-Vis spectra is around 40 nm (or 400 A), whereas in atomic lines the half-bandwidth observed as a result of pure electronic transitions is of a few hundredths of an angstrom (typically 0.03-0.05 A). 

A major advantage of Raman spectroscopy for the analysis of biomolecules stems from the fact that water has a weak Raman spectrum. Spectra can be recorded for aqueous solutes at 10 -10 M with little interference from the solvent. For a chromophore under the RR condition the accessible concentration range becomes 10 " -10 M. Moreover, the intensity enhancement associated with the RR effect confers the important advantage of selectivity, allowing one to observe selectively the vibrational spectrum of a chromophore that is just one component of an extremely complex biological system. Because many biomolecules have chromophores with an ultraviolet (UV) resonance condition, one may also selectively excite a chromophore by irradiating these molecule with UV light. This technique is known as Ultraviolet Resonance Raman Spectroscopy (UVRRS). In recent years, Raman difference spectroscopy (RDS) has been developed in... 

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