Intramolecular interactions of trityl groups.

Trityl group, Tr, is a molecular dynamic rotor of which the conformation and helicity depend on other groups in the close vicinity. Interactions with another covalently linked Tr group and with other substituents are analyzed in terms of transfer of chirality to the trityl group. Two trityl groups in a molecule can mutually interact at a distance of two, three, or five bonds. Despite its size, a Tr group attached to a cyclohexane or cyclopentane ring through an oxygen or nitrogen atom adopts either an axial or equatorial position, depending on additional stabilizing interactions, such as hydrogen bonding.

From single molecule to crystal: mapping out the conformations of tartaric acids and their derivatives.

Stereoisomers of one of the most important organic compounds, tartaric acid, optically active and meso as well as the ester or amide derivatives, can show diverse structures related to the rotation around the three carbon-carbon bonds. This study determines the controlling factors for conformational changes of these molecules in vacuo, in solution, and in the crystalline state using DFT calculations, spectroscopic measurements, and X-ray diffraction. All structural variations can be logically accounted for by the possibility of formation and breaking of hydrogen bonds between the hydroxy or amide donors and oxygen acceptors, among these the hydrogen bonds that close five-membered rings being the most stable. These findings are useful in designing molecular and crystal structures of highly polar, polyfunctional, chiral compounds.

Probing the shapes of chiral bis-(o-naphthalimidobenzoyl) systems using X-ray and circular dichroism methods.

CD (circular dichroism) and X-ray investigations have been carried out in order to identify the prevalent conformations and define the forces that determine the molecular and supramolecular organization of the alkyl-bridged bichromophoric [NAB, ortho-(1,8-naphthalimido)benzoyl] units, each consisting of the benzoyl substituted in the ortho position with the 1,8-naphthalimide group. The results reveal that NAB bichromophores incorporated into the same molecule exist in a variety of conformation/helicity combinations. The molecular structures are largely stabilized by local 1,3-CH/CO dipole-dipole interactions, while the crystal packing besides dispersive H...H interactions is mostly governed by multiple C-H...O(=C) and C-H...pi interactions. The relatively small contribution of pi...pi interactions comes from a pairwise off-face stacking between naphthalimide rings or from pairwise carbonyl...pi interactions. All these types of intermolecular interactions have been summarized quantitatively by means of a Hirshfeld surface analysis.

Trianglamines--readily prepared, conformationally flexible inclusion-forming chiral hexamines.

Trianglamines, macrocyclic heteraphanes, were readily synthesised through a [3+3] cyclocondensation of (R,R)-1,2-diaminocyclohexane with terephthalaldehyde, followed by NaBH4 reduction and N-alkylation. The macrocyclic ring shows a remarkable ability to change its conformation, as a consequence of rotation about the C-N bonds or nitrogen inversion due to protonation or N-alkylation, as revealed by circular dichroism spectra, computational modelling and X-ray diffraction analysis. The flexible natures of the trianglamine macrocycles allow ready accommodation of a variety of guest molecules to form crystalline inclusion complexes of highly diversified interpenetrating structures.

Structure and unexpected chiroptical properties of chiral 4-pyrrolidinyl substituted 2(5H)-furanones.

Planar 2(5H)-furanones substituted at C4 with a chiral pyrrolidinyl group show CD spectra which are apparently due to the distortion of the C4-N1 bond of sp2 character from the plane defined by the 2(5H)-furanone ring atoms and/or due to the presence of substituents in the pyrrolidine ring. This is a new, previously not encountered structural factor determining the chiroptical properties of 2(5H)-furanones and emerging from the analysis of X-ray diffraction data and quantum mechanical DFT computations. In the presence of a C5 pseudoaxial substituent in the furanone ring, the sign of the furanone n-pi* and pi-pi* transition Cotton effects is determined primarily by the previously postulated allylic helicity rule.

Helicity induction in a bichromophore: a sensitive and practical chiroptical method for absolute configuration determination of aliphatic alcohols.

[reaction: see text] A practical method for the determination of the absolute configuration of aliphatic secondary alcohols, based on the circular dichroism of the readily available N-(1,8-naphthaloyl)-2-aminobenzoyl (NAB) derivative, is presented. The origin of the induced Cotton effects is traced by ab initio calculations to the dominant helicity of the NAB pi-electron system.

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.

Folding of aromatic oligoimides of trans-1,2-diaminocyclohexane.

Chiral oligomeric diimides prepared from pyromellitic dianhydride, (R,R)-1,2-diaminocyclohexane and phthalic anhydride fold into M or P helical conformers; trimer 1 folds into the P conformer in the crystal but the M conformer dominates in solution; longer chain oligomers 2 and 3 form preferentially P conformers in solution, as a result of intermolecular interactions.