Electronic and Vibrational Circular Dichroism

If we mutually compare chiroptical methods, we find a remarkable complementarity that can be used as advantage when a particular system is investigated by the selection of chiroptical procedures. ECD [1, 24], which is a far reaching technique, tends to see a molecule as a whole and provides chiral information via chromophores and their properties [25-27]. This far-reaching nature of ECD can be utilized, for example, in supramolecular chemistry, where chirality is introduced into the system by a chiral, but spectroscopically neutral matrix. However, it is observed on an inherently nonchiral chromophore, like, for example, porphyrine [28-36] 

Commercial instrumentation is available for both the ECD and VCD experiments and the commercial ROA spectrometer was introduced last year. To measure chiroptical spectra is relatively easy, provided that a good laboratory routine is worked out and consistently maintained. Electronic CD is easily measurable from 175 nm to more than 1 pm (see Section 8.2.1), and custom-built instruments can go even further. At present, VCD can be measured from 4000 to about 900 cm, routinely from 2000 cnr to about 900 cm (see Section 8.2.2) measurements were reported below 600 cnr [39] and in the near IR up to 6150 cm [40, 41]. The ROA spectra can be reasonably obtained from 2000 cm up to a region close to the Rayleigh line [42]. 

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Molecular dynamics (MD) simulations fill a significant niche in the study of chemical structure. While nuclear magnetic resonance (NMR) yields the structure of a molecule in atomic detail, this structure is the time-averaged composite of several conformations. Electronic and vibrational circular dichroism spectroscopy and more general ultraviolet/visible and infrared (IR) spectroscopy yield the secondary structure of the molecule, but at low resolution. MD simulations, on the other hand, yield a large set of individual structures in high detail and can describe the dynamic properties of these structures in solution. Movement and energy details of individual atoms can then be easily obtained from these studies. 

Baumruk V. Pancoska P. Keiderling TA. Prediction of secondary structure using statistical analysis of electronic and vibrational circular dichroism and Fourier transform infrared spectra of proteins in H2O. J Mol Biol 1996 259 774-791. 

Abbate S, Lebon F, Gangemi R, Longhi G, Spizzichino S, Ruzziconi R (2009) Electronic and vibrational circular dichroism spectra of chiral 4-X-[2.2]paracyclophanes with X containing fluorine atoms. J Phys Chem A 113 14851-14859... 

Dukor, R.K. and Keiderling, T.A. Mutarotation studies of poly-L-proline using FTIR, electronic and vibrational circular dichroism. Biospectroscopy, 2, 83, 1996. 

Brizard A, Berthier D, Aime C, Buffeteau T, Cavagnat D, Ducasse L, Hue 1, Oda R (2009) Molecular and supramolecular chirality in gemini-tartrate amphiphQes studied by electronic and vibrational circular dichroisms. Chirality 2LS153-S162... 

Electronic and vibrational circular dichroism spectroscopy With commercially available instruments the accessible spectral region of BCD and VCD spectroscopies lies nowadays between 800 (1.25 pm) and 62 500cm (160nm). Brom the spectroscopic point of view the chosen solvents should be free of absorption and in order to have only a small solvent/ solute interaction they should be as nonpolar as possible. Acyclic or cyclic hydrocarbons are a good choice for the UV/vis absorption region if sufficient solubility is guaranteed. As a compromise, dioxane. 

At present, the density functional theory (DFT) prevails in calculations of the structural and spectral parameters of transition metal complexes [18]. For this reason, and thanks to our previous positive experience with simulation and interpretation of the UV-Vis, IR, electronic and vibrational circular dichroism (BCD and VCD) spectra of chiral vanadium complexes [19], we decided to use the DFT methods as the main tool in the present work as weU. We shall present the calculated molecular stractures, vibrational and electronic spectra, as well as the V chemical shifts of the chosen anions of the vanadium(V) tartrato complexes. Where applicable, results are confronted with the available experimental data, with the aim to assess the reliability of the individual methods for future calculations of a similar kind. 

OPTICAL ROTATION, OPTICAL ROTATORY DISPERSION, ELECTRONIC CIRCULAR DICHROISM, AND VIBRATIONAL CIRCULAR DICHROISM... 

Optical rotation (OR), optical rotatory dispersion (ORD), electronic circular dichroism (BCD), and vibrational circular dichroism (VCD) provide spectral information nniqne to enantiomers, allowing for the determination of absolute configuration. Recent theoretical developments in DFT, using time-dependent density fnnctional theory (TD-DFT), provide the means for computing OR, ORD, and Similar theoretical development with coupled-cluster theory,... 

Table 22.1 Comparison of the characteristics of electronic circular dichroism (ECD) and vibrational circular dichroism (VCD).

There have been numerous applications of continuum models to equilibria and reactions in solution surveys of these and extensive listings are provided by Cramer and Truhlar.16 Other studies have focused upon the effects of solvents upon solute molecular properties, such as electronic and vibrational spectra,16 dipole moments, nuclear quadrupole and spin-spin coupling constants and circular dichroism.12... 

Over the past decade two forms of vibrational optical activity have become established. One is called vibrational circular dichroism (VCD), the extension of electronic circular dichroism into the infrared vibrational region of the spec-tram. The first measurements of VCD were reported by George Holzwarth and co-workers at the University of Chicago in 1973 for crystals (3) and 1974 for neat liquids (4). In VCD one measures the small difference in the absorption of a sample for left versus right circularly polarized incident infrared radiation. The early stages of the development of VCD have been reviewed from several perspectives (5-8). 

For our purpose, it is convenient to classify the measurements according to the format of the data produced. Sensors provide scalar valued quantities of the bulk fluid i. e. density p(t), refractive index n(t), viscosity dielectric constant e(t) and speed of sound Vj(t). Spectrometers provide vector valued quantities of the bulk fluid. Good examples include absorption spectra A t) associated with (1) far-, mid- and near-infrared FIR, MIR, NIR, (2) ultraviolet and visible UV-VIS, (3) nuclear magnetic resonance NMR, (4) electron paramagnetic resonance EPR, (5) vibrational circular dichroism VCD and (6) electronic circular dichroism ECD. Vector valued quantities are also obtained from fluorescence I t) and the Raman effect /(t). Some spectrometers produce matrix valued quantities M(t) of the bulk fluid. Here 2D-NMR spectra, 2D-EPR and 2D-flourescence spectra are noteworthy. A schematic representation of a very general experimental configuration is shown in Figure 4.1 where r is the recycle time for the system. 

Vibrational optical activity (VOA) is a relatively new area of natural optical activity. It consists of the measurement of optical activity in the spectral regions associated with vibrational transitions in chiral molecules. There are two basic manifestations of VOA. The first is simply the extension of electronic circular dichroism (CD) into the infrared region where fundamental one-photon vibrational transitions are located. This form of VOA is referred to as vibrational circular dichroism (VCD). It was first measured as a property of individual molecules in 1974 [1], and was independently confirmed in 1975 [2]. Within the past twelve years, VCD has been reviewed on a number of occasions from a variety of perspectives [3-15], and two more reviews are currently in press [16,17], The second form of VOA has no direct analog in classical forms of optical activity. Optical activity in Raman scattering, known simply as Raman optical activity (ROA), was measured successfully for the first time in 1973 [18], and confirmed independently in 1975 [19], ROA has been described in detail and reviewed several times in the past decade from several points of view [20-24], and two additional reviews [25,26], one with a view toward biological applications [25] and the other from a theoretical perspective [26], are currently in press. In addition, two articles of a pedagogical nature are in press that have been written for a general audience, one on infrared CD [27] and the other on ROA [28],... 

Shanmugam G, Polavarapu PL, Gopinath D, Jayakumar R (2005) The structure of antimicrobial pexiganan peptide in solution probed by Fourier transform infrared absorption, vibrational circular dichroism, and electronic circular dichroism spectroscopy. Biopolymers 80 636-642... 

Nafie LA (2004) Theory of vibrational circular dichroism and infrared absorption extension to molecules with low-lying excited electronic states. J Phys Chem A 108 7222-7231... 

Recenfly fhe ah initio calculation mefhod of vibrational circular dichroism (VCD), optical rotatory dispersion (ORD), and electronic circular dichroism (ECD) has been developed as fhe third nonempirical method [3, 4]. The method is applicable to compounds having no chromophore, and so should be widely used in future. 

This chapter concentrates on CD studies in the visible and ultraviolet, regions in which CD results from electronic excitations. There has been outstanding progress in extending CD measurements into the infrared, thus providing information about vibrational excitations in the form of vibrational circular dichroism (VCD). VCD instrumentation is currently available only in a few laboratories which have constructed custom-made VCD spectrometers. For a detailed discussion of VCD, the reader is referred to several reviews. " ... 

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