Paweł Skowronek

Chiral iminospherand of a tetrahedral symmetry spontaneously assembled in a [6 + 4] cyclocondensation.

Thermodynamically driven cyclocondensation of (R,R)-1,2-diaminocyclohexane and 1,3,5-triformylbenzene yields solely a [4 + 6] spherand type product. This is a unique example of spontaneous formation of an organic compound of permanent chiral tetrahedral (T) symmetry.

Chirality of aromatic bis-imides from their circular dichroism spectra

It is demonstrated that chirality of molecules composed of 1,2,4, 5-benzenetetracarboxydiimide (pyromellitic diimide) or 1,4,5, 8-naphthalenetetracarboxydiimide units is reflected by their exciton Cotton effects. The analysis is based on the calculated (ZINDO/S) excited states of the model diimide chromophores 1a and 2a. Rotation of the diimide chromophores around the C-N bond in diimides 3-5 is evaluated from the dynamic (1)H NMR data. A comparison of chiroptical properties of bis-diimides 3-5 with the CD spectra of bis-imides 6-8 is also presented. Copyright 2000 Wiley-Liss, Inc.

Factors Affecting Conformation of (R,R)-Tartaric Acid Ester, Amide and Nitrile Derivatives. X-Ray Diffraction, Circular Dichroism, Nuclear Magnetic Resonance and Ab Initio Studies

Abstract Derivatives 2a–15a of (R,R)-tartaric acid (1a) with all combinations of methyl ester, amide, N-methylamide and N,N-dimethylamide groups, as well as the corresponding O,O′-dibenzoyl derivatives 1b–15b and nitriles 16–18 have been synthesized. Their conformations have been studied by the NMR and CD methods in solution as well as by X-ray diffraction in the crystalline state. The preference for planar. T conformation of the four carbon chain is observed under conditions restricting the α-hydroxyacid, ester or amide group to be nearly planar, this conformation being stabilized by intramolecular hydrogen bonds of the S(5) motif and the electrostatic CO/C(β)H and CN/C(β)H coplanar bond interactions. The C=O/C(α)-O bond system tends to be either synplanar (ester, acid), or antiplanar (ester, primary and secondary amide). Ab initio calculations allowed to demonstrate that for the isolated molecules of diamides 10a and 15a there is strong preference for gauche G+(a,a) conformers, the driving force being the formation of the hydrogen bonded six-membered cycles of the S(6) motif joining the OH and C=O groups from two different halves of the molecule. The results compare favourably with the experimental values derived from NMR spectra of 15a in nonpolar solvent. In the absence of intramolecular hydrogen bonding the N,N-dimethylamide group is better accomodated in a gauche G− conformer. This releases the nonbonded interaction due to the amide methyl group anti to the carbonyl group.

Novel chiral pyromellitdiimide (1,2,4,5-benzenetetracarboxydiimide) dimers and trimers: Exploring their structure, electronic transitions, and exciton coupling

The chiral but highly symmetrical acyclic and cyclic pyromellitic diimide dimers and trimers 2-5 have been obtained and characterized for the first time. The pyromellitdiimide chromophores in these molecules are linked by a rigid diequatorially 1,2-disubstituted cyclohexane skeleton. The structures of the compounds have been determined in detail by molecular modeling and, in the case of cyclic dimer 4 and trimer 5, by means of X-ray diffraction analysis. The electronically excited states of the pyromellitdiimide chromophore (1a) have been studied in these and other model compounds by means of linear dichroism (LD), magnetic circular dichroism (MCD), and circular dichroism (CD) spectroscopy. CD spectra of the rigid cyclic trimer 5 have provided the most detailed information on the excited states of the pyromellitdiimide chromophore. The low-energy tail (340-360 nm) of the absorption envelope can be assigned to out-of-plane polarized n-pi* transitions (I, II). The higher energy bands are due to contributions from up to six pi-pi* transitions, these being polarized either along the long (IV-VI, VIII) or short axis (III, VII). The results of ab initio CIS/cc-pVDZ and semiempirical INDO/S-CI calculations have been compared with the experimental data. CD Cotton effects in the region 200-260 nm, which result from exciton interactions between electric dipole allowed transitions of two pyromellitdiimide chromophores in compounds 2-5, provide reliable and useful information concerning the conformation and absolute configuration of these molecules, which may be extrapolated to other oligoimide systems.

Vibrationally resolved circular dichroism spectra of a molecule with isotopically engendered chirality

We present a theoretical study of vibrationally resolved circular dichroism spectra, both in the adiabatic and non-adiabatic frameworks, with a full account of Franck-Condon and Herzberg-Teller vibrational contributions for the former. Model calculations have been performed on 2(R)-deuteriocyclopentanone, whose chirality is due solely to isotopic substitution. This molecule has two distinct, nearly isoenergetic, half-chair conformations in equilibrium, and its demanding nature in terms of computational accuracy makes it a perfect candidate for performing a detailed comparison between different vibronic models. Comparisons are made with experimental spectra, and we also consider temperature effects. In order to reproduce the experimental spectrum, it is necessary to consider the geometry relaxation occurring during the n→π* transition.

Exciton coupling in various substituted aryl-phthalimide bichromophoric systems☆

Abstract As demonstrated in this paper, exciton Cotton effects reflecting the absolute stereochemistry of the molecule can result from coupling between the allowed transitions of phthalimide. 3,6-dichlorophthalimide, 4,5-dichlorophthalimide or 3,4,5,6-tetrachlorophthalimide and p-substituted benzene, naphthalene, benzoate or cinnamate chromophores. This significantly extends the scope of application of phthalimide chromophoric derivatives to stereochemical assignments by CD spectroscopy. The direction of the transition moments in chlorinated phthalimide chromophores are determined from linear dichroism measurements of N -butyl derivatives oriented in polyethylene film.

Absolute configuration of α-phthalimido carboxylic acid derivatives from circular dichroism spectra

Abstract It is demonstrated that the Cotton effects due to the 220 nm phthalimide π–π* transition observed for a series of derivatives of α-phthalimidocarboxylic acids unequivocally reflect the amino acid absolute configuration. This method is based on the exciton coupling of the allowed transitions of the phthalimide and the carboxylic acid derivative chromophores.

Electronic circular dichroism for chiral analysis

Publisher Summary This chapter discusses circular dichroism (CD) for chiral analysis. CD is a spectroscopic technique based on differential absorption of left- and right-handed circularly polarized light. It is ideally disposed to analyze molecular structure, composition, and interactions of chiral systems. Apart from X-ray diffraction analysis, it is the only method capable of absolute stereostructure assignment. An advantage of the CD method is its independence of the physical form of the substance analyzed. While the high-resolution X-ray diffraction experiment heavily relies on the availability of good quality monocrystal, CD measurements can be done with samples in solution, gas phase, solid dispersions (CD of randomly oriented molecules), films, gels, liquid crystals, and even on monocrystals (CD of oriented systems). CD measurements in the vacuum UV region are greatly enhanced with the use of synchrotron radiation as the light source (SRCD) that provides higher intensity radiation at low wavelengths (160 nm). Time-resolved CD spectroscopy (TRCD) offers possibilities to measure with nano- and pico-second time resolution, the conformational changes of proteins upon light excitation.

Chirality transfer through sulfur or selenium to chiral propellers

The mechanism of chirality transfer from a chiral alkyl substituent to a trityl moiety through sulfur or selenium atoms is analysed and discussed on the basis of ECD measurements, DFT structure and ECD spectra calculations. It is shown that the presence of a chalcogen atom is manifested by elongation of the distance between the chiral and the trityl moieties as well as by the change of electronic properties of the trityl chromophore while maintaining its chiroptical response to the chirality of the molecule.

Chiral dialkyl ditellurides: Conformation of chiral chromophore by circular dichroism and DFT calculation

The twisted structure of ditellurides, in a similar way as in other dichalcogenes, leads to different absorption of circularly polarized light by quasi-enantiomeric chiral orbitals. Chiral optically active ditellurides are not common compounds and this phenomenon is not widely reported. As chiral ditellurides found an application in asymmetric synthesis, their molecular structure, understood as their conformation, became an important factor for understanding their reactivity. Until now there are few examples of chiral ditellurides known and their structure was not analyzed in details. This article presents the results of our most recent research on the structure of chiral ditellurides investigated by electronic circular spectroscopy (ECD) supported by quantum-chemical calculation. This enables us to suggest a relationship between chirality of alkyl substituent and chirality (conformation) of ditelluride.