Circular dichroism chiroptical properties

On the other hand, telluronium imides 13 were isolated for the first time in 2002 by optical resolution of their racemic samples on an optically active column by medium-pressure column chromatography.27 The relationship between the absolute configurations and the chiroptical properties was clarified on the basis of their specific rotations and circular dichroism spectra. The racemization mechanism of the optically active telluronium imides, which involved the formation of corresponding telluroxides by hydrolysis of the telluronium imides, was proposed (Scheme 6). 

The carotenoid family have chiral centres which enable the use of circular dichroism. However, the chirality of carotenoids is not sufficiently characteristic so that the chiroptical properties do not serve as a good analytical tool. 

The inherent difficulty in analyzing enantiomers arises from the well-known fact that apart from their chiroptical characteristics, optical isomers have identical physical and chemical properties in an achiral environment (assuming ideal conditions). Therefore, methods of distinguishing enantiomers must rely on either their chiroptical properties (optical rotation, optical rotatory dispersion, circular dichroism), or must employ a chiral environment via diastereomer formation or interaction. Recently, it has become increasingly clear that such diastereomeric relationships may already exist in nonracemic mixtures of enantiomers via self-association in the absence of a chiral auxiliary (see Section 3.1.4.7.). 

At least for 14 the usual methods for determining the enantiomeric purity (especially NMR-methods) failed. From 14 and 15 several optically active derivatives were prepared 40 441 and their chiroptical properties [especially the circular dichroism (CD) spectra of derivatives of 14]40) recorded. 

As usual in stereochemical research, four main approaches have been applied to the problem of assigning chiralities to optically active cyclophanes. They are listed in order of their reliabilities i) anomalous X-ray diffraction (Bijvoet method), ii) chemical correlations with compounds of known chiralities (preferably established by the Bijvoet method), iii) kinetic resolutions and/or asymmetric syntheses, iv) interpretation of chiroptical properties (mainly circular dichroism) on the basis of (sector) rules including theoretical methods. 

Polarimetry, circular dichroism (CD) and optical rotatory dispersion (ORD) are the most important tools for the study of properties arising from optical activity. Although many chiral thiophenes have been prepared, there is no secure basis for a systematic discussion of the special effects of thiophene or annelated thiophene rings. For the purpose now at hand it is more expedient to discuss three different areas in which thiophene containing molecules and the related chiroptical techniques are central features. 

The chiroptical properties of 2-hydroxyalkanoic acids have been discussed in relation to their stereochemical features in solution. The potential of circular dichroism techniques was emphasized... 

Measurement of circular dichroism can even permit elucidation of relatively small structural changes. CD spectroscopy is also suitable for the solution of specific application-relevant questions. Studies of the sensor properties of chiral dendrimers make use of the fact that complexation of chiral guest molecules induces changes in the CD bands of the host dendrimers. Thus guest-selective chiroptical effects observed in titration experiments with enantiomeric guest molecules give an indication of the potential of the chiral dendrimer to act as an enantioselective sensor [87]. 

Keywords Chiroptical properties, Electronic circular dichroisms, Planar chirality, Quantum chemical calculations, Cyclophane... 

The chiroptical properties of molecules are of substantial interest in chemistry and biochemistry and become important tools for the determination of the absolute configuration and conformation of molecular systems. In particular, the circular dichroism [1] is a quantitative measure of the difference in absorption coefficient for left and right circular polarized light ... 

Niederalt C, Grimme S, Peyerimhoff SD, Sobanski A, Vogtle A, Lutz M, Spek AL, Eis MJ, Wolf WH, Bickelhaupt F (1999) Chiroptical properties of 12,15-dichloro[3.0]orthometacy-clophane - correlations between molecular structure and circular dichroism spectra of a biphenylophane. Tetrahedron Asymmetry 10 2153-2164... 

When chirality is involved, information on solid-state structures and supra-molecular properties must be obtained by solid-state circular dichroism (CDf spectroscopy, as certain characteristics may be lost upon dissolution. However extreme care is required to obtain artifact-free solid-state CD spectra. This is because CD spectra in the solid state (except for special homogeneous cases [9,10]) are inevitably accompanied by parasitic signals that originate from thd macroscopic anisotropies of a sample such as LD (linear dichroism) and LB (linear birefringence) [11-16]. We have been working in the field of solid-state chirality for the last 30 years and recently developed a novel universal chiroptical spectrophotometer, UCS J-800KCM, for the measurement of true CD and circular birefringence (CB) spectra in the solid state [17]. 

The fundamental requirement for the existence of molecular dissymmetry is that the molecule cannot possess any improper axes of rofation, the minimal interpretation of which implies additional interaction with light whose electric vectors are circularly polarized. This property manifests itself in an apparent rotation of the plane of linearly polarized light (polarimetry and optical rotatory dispersion) [1-5], or in a preferential absorption of either left- or right-circularly polarized light (circular dichroism) that can be observed in spectroscopy associated with either transitions among electronic [3-7] or vibrational states [6-8]. Optical activity has also been studied in the excited state of chiral compounds [9,10]. An overview of the instrumentation associated with these various chiroptical techniques is available [11]. 

Discussions of chiroptical properties of corresponding compounds in this series of The Chemistry of Functional Groups generally refer to the spectral range 589-190 nm, i.e. the emphasis is on optical (molar) rotations [O] J in the transparent region (optical rotatory dispersion, ORD) and rotations [0]J measured at the wavelength of the sodium-D-line (X = 589 nm) as well as electronic circular dichroism A e (CD) in the near ultraviolet region (X 190 nm) ... 

Current instruments allow CD measurements not only to be performed in the vacuum-ultraviolet (vacuum-UV) region X < 190 nm), but also in the infrared (IR) spectral region. This means that not only chiral absorption effects related to excitations of molecular electronic subsystems are amenable to experimental observations, but also effects involving excitations of the nuclear subsystems of molecules ( vibrational circular dichroism VCD) Recently, results of VCD experiments with cyclopropanes were published. Therefore, in the present chapter the discussion of chiroptical properties of cyclopropanes can include vibrational circular dichroism. Hence, the discussions of chiroptical properties of cyclopropanes will cover the spectral range extending from the vacuum-ultraviolet to the infrared region. 

Chiroptical properties related to molecular vibrations can be studied not only by vibrational circular dichroism, but also by using the chiral variant of the Raman spectroscopy - Raman optical activity (ROA) [14-16]. This method has been developed into practical use only recently, but it is very promising and similarly to the... 

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. 

Chiroptical Properties O.R.D., C.D.). A French language review surveys progress in circular dichroism between 1961 and 1969. 

Chiroptic Referring to the optical properties of chiral substances, such as optical rotation, circular dichroism, and optical rotatory dispersion. 

The u.v. and c.d. spectra of (364 R = Me, Et, Pr, Bu , or PhCH2) in EtOH and in iso-octane have been reported.The chiroptical properties of the oxaziridine chromophore were generally characterized by a positive pattern at 190—350 nm. The notable exception to this was a clear negative Cotton effect for (364 R = Pr ). The seemingly anomalous behaviour of this compound was interpreted in terms of the possible existence of solvation equilibria and conformational rotamers about the N—CHMe2 bond. The relatively new technique of Liquid-Crystal-Induced Circular Dichroism (LCICD) has been used in the... 

The term chiroptical refers to spectroscopic properties (primarily the techniques of optical rotatory dispersion (o.r.d.) and circular dichroism (c.d.)) which are dependent upon the chirality of the compound under investigation. A few relevant studies have appeared in this area and will be briefly discussed. The area has been extensively reviewed for organoselenium compounds ... 

A recent circular dichroism study reported the chiroptical properties of four selenoamino acids all of which are selenides The compounds studied were optically active selenocystathionine and its alio diaStereomer, selenomethione and selenolan-thionine, Se(CH2CH(NH2)COOH)2- In the 190-250 nm region, positive Cotton effects of the carboxyl and selenide chromophores were found to correlate with L(=S) or L,L(=S,S) absolute configurations. 

---