Chiroptical spectra

The lower intensity, long wavelength absorption bands of a, P-unsaturated ketones vary considerably with the polarity of the solvent. The same is also true for anomalous ORD spectra and CD spectra both of which have absorbance by a chromophore as a pre-requisite. The chiroptical spectra however are much more sensitive than absorbance spectra to the interaction between the solvent and the solute. 

Linear response polarizability theory of spectral bandshapes was applied to the numerical analysis of the chiroptical spectra obtained for DNA-acridine orange complexes [85]. After analysis of various models of conformation, it was concluded that a dimer-pairs repeating sequence model was best able to account for the observed spectral trends. In another work, the CD induced in the same band system was studied at several ionic strengths [86]. The spectra were able to be interpreted in terms of the long-axis-polarized electronic transitions of the dyes, with the induced CD being attributed to intercalated and non-intercalated dye species superimposed by degenerate vibronic exciton interactions between these. 

Lanthanide(III) complexes demand special attention in view of the specific spectra-structure relationship for biological applications, chiral catalysis, molecular magnetism and luminescence. One unique chiral stereochemistry is realized by the combination of labile Ln complexes and weak Na+-fluorocarbon interactionwhich show intense CD (circular dichroism) with variation of Ln(III) and/or M(I) ions to chiroptical spectra-structure relations and an important role in configurational chirality for chemical sensors, NMR shift reagents or chiral catalysis. Trivalent lanthanides are also found to be incorporated into heterobimetallic complexes showing intramolecular energy transfer processes. 

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]. 

The analysis of a series of chiroptical spectra and recovery of systematic trends in a given set can be carried out in several ways. In the past, the results strongly depended on the spectroscopist s personal experience actually, this was the least objective part of the circular dichroism application. Nowadays, we can rely on general procedures of statistical data treatment like singular value decomposition, factor analysis (especially its first part, analysis of the correlation matrix and the projection of the experimental spectra onto the space of orthogonal components), cluster analysis and the use of neural networks. This field has been pioneered by Pancoska and Keiderling [72-76], and also by Johnson [77] when analyzing the chiroptical properties of biopolymers. 

This excursion into electronic versus vibronic rotatory strengths and the adiabatic versus the nonadiabatic approximation may be highly relevant to the detailed interpretation of transition metal complex chiroptical spectra. In most optically active transition metal complexes, the symmetry of the metal ion-donor atom cluster remains rather high (nearly 0 or nearly D h) and, as a result, it is common to find many near-degeneracies (or even exact degeneracies) among the spectroscopic states of interest. Even in... 

As was mentioned previously, two computational studies of vibronic effects in chiral trigonal systems have been reported (18, 28). Both of these studies were addressed to the general problem of vibronic perturbations on the chiroptical spectra of trigonal metal complexes. However, the calculations reported in... 

Vibronic coupling effects on the chiroptical spectra of optically active metal complexes tend to obscure the inherent relationships between the CD observables and structural features such as absolute configuration, ligand conformation, and ligand spatial distributions. For this reason, spectra-structure relationships based on the "fixed-nuclei" approximation and on the assumption of well defined "electronic" transitions must be applied with considerable caution. A great wealth of spectroscopically and structurally important information may be obtained from detailed vibronic analyses of CD spectra. However, very few analyses of this sort have been reported to date. 

Polavarapu PL, Scalmani G, Hawkins EK, Rizzo C, Jeirath N, Ibnusaud I, Habel D, Nair DS, Haleema S. Importance of solvation in understanding the chiroptical spectra of natural products in solution Phase garcinia acid dimethyl ester. J. Nat. Prod. 2011 74(3) 321-328. 

In addition to these findings suggesting that such Ln(III)-based compounds can serve as probes for recognition/sensing of cations, the observation of differing CPL activities with variation of alkali metal ions and solvents opens new perspective for the studies aimed at understanding the chiroptical spectra-structure relationships and, in particular, their... 

Lunkley JL, Shirotani D, Yamanari K, Kaizaki S, Muller G. Chiroptical Spectra of a Series of Tetrakis((-i-)-3-heptafluatDbutylyrylcamphorato)lanthanide(in) with an Encapsulated Alkali Metal Ion Circularly Polarized Luminescence and Absolute Chiral Structures for the Eu(III) and Sm(ni) Complexes. Inorg Chem 2011 2011 (50) 12724—12732. 

Shirotani D, Suzuki T, Yamanari K, Kaizaki S. Crystal Structure and Chiroptical Spectra of Sodium Tetrakis (+)-hfbc Pr(III) Complex. J AUoys Comp 2008 451 325-328. 

The binding of sulfonamides to serum albumin is thought to strongly affect the pharmacokinetics of drug action, and therefore CD spectroscopy has been used to deduce the nature of the association mechanism [71]. It was found in this study that most of the drug compounds would exhibit induced CD upon binding to either human, bovine, or rabbit serum albumin, and that the particular lineshape of the chiroptical spectrum was determined by the structural details of the bound solute. 

Gawryszewska P, Legendziewicz J, Ciunik Z, Esfandiari N, Muller G, Piguet C, et al. On the Determination of Empirical Absolute Chiral Structure Chiroptical Spectrum Correlations for D3 Lanthanide (HI) Complexes. Chirality 2006 18 406-412. 

It is interesting to note that the CD spectrum of 11 can be tuned by changing the oxidation states of the TTF units in the side-chains of 11. Polymer 11 shows reversible interconversion between three univalent and two very broad mixed-valence redox states that have different chiroptical properties. 

Daub, Grimme, and coworkers have reported chiroptical switches based on binaphthyl boron dipyrromethane (BDP) conjugates. It is known that BDP dyes with appropriate functionalization can be reversibly oxidized and reduced.58 The CD-spectroelectrochemical studies of R-56 show a decrease in the intensity of the Cotton effect at 501 nm by applying a reduction potential to the solution. The initial CD spectrum is restored completely after reoxidation thus, the CD signal intensity at 501 nm for R-56 can be switched on and off electrochemically.59... 

Circular dichroism (CD) and optical rotatory dispersion (ORD) spectra (71PMH(3)397) are very sensitive to the spatial disposition of the atoms in a molecule, and conformational changes may yield rather dramatic changes in the appearance of a CD or ORD spectrum of a chiral molecule. The analysis of the temperature dependence of the CD spectrum may give information on populations and free energy differences. Except for nucleosides, the use of the chiroptical method in conformational analysis is rather limited, which may be accounted for by the complexity of the theory for optical activity. 

Fig. 1 Top Behavior of the electronic linear chiroptical response in the vicinity of an excitation frequency. Re = real part (e.g., molar rotation [< ]), Im = imaginary part (e.g., molar ellipticity [0]). Without absorption line broadening, the imaginary part is a line-spectrum (5-functions) with corresponding singularities in the real part at coex. A broadened imaginary part is accompanied by a nonsingular anomalous OR dispersion (real part). A Gaussian broadening was used for this figure [37]. Bottom Several excitations. Electronic absorptions shown as a circular dichroism spectrum with well separated bands. The molar rotation exhibits regions of anomalous dispersion in the vicinity of the excitations [34, 36, 37]. See text for further details...

The chiroptical properties of a hybrid of paracyclophane and helicene, [2.2] paracyclophanohexahelicene (38), were reported [50]. The introduction of [2.2] paracyclophane moiety caused a red shift and intensity enhancement of the absorption in the 240-280 nm regions, which was also reflected in the CD spectrum. It is noteworthy that the (M)-helicity was evoked in the hybrid 38 when started from (/ p)-[2.2]paracyclophanecarbaldehyde. 

A racemic mixture of three-layered [3.3]paracyclophane (45) was resolved into two enantiomers by chiral HPLC (on a Daicel OD column), and their absolute configuration was determined by a comparison of the experimental CD spectrum with the theoretical one at the TD-DFT-B3-LYP/TZVP level [55]. A simple model, composed of two p-xylenes and durene (the side chains were modeled again by methyl groups), was used to explain the origin of the chiroptical properties of the three-layered cyclophane system. Due to the flexibility of the [3.3]paracyclophanes, the solvent effects on the conformer distribution and thus on the chiroptical properties were significant (Fig. 10). 

Fluorescence-detected circular dichroism (FDCD) is a chiroptical technique in which the spectrum is obtained by measuring the difference in total luminescence obtained after the sample is excited by left- and right-circularly polarized light. For the FDCD spectrum of a given molecular species to match its CD spectrum, the luminescence excitation spectrum must be identical to the absorption spectrum. 

In chiroptical spectroscopy, an important part of structural information is provided by the intensity parameters of the spectrum and also by its overall shape. Primarily... 

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