Conformational isomerism

About: Conformational isomerism is a research topic. Over the lifetime, 11563 publications have been published within this topic receiving 199312 citations.

Poling: Promoting conformational variation

Abstract: This article introduces several methods of assessing the extent to which a collection of conformations represents or covers conformational space. It also describes poling: a novel technique for promoting conformational variation that can be applied to any method of conformational analysis that locally minimizes a penalty or energy function. The function being minimized is modified to force similar conformers away from each other. The method is independent of the origin of the initial conformers and of the particular minimization method used. It is found that, with the modification of the penalty function, clustering of the resulting conformers is generally unnecessary because the conformers are forced to be dissimilar. The functional form of the poling function is presented, and the merits are discussed with reference to (1) efficacy at promoting variation and (2) perturbation of the unmodified function. Results will be presented using conformers obtained from distance geometry with and without poling. It will be shown that the addition of poling eliminates much redundancy in conformer generation and improves the coverage of the conformational space. © 1995 by John Wiley & Sons, Inc.

Barriers to rotation adjacent to double bonds. 3. The carbon-oxygen barrier in formic acid, methyl formate, acetic acid, and methyl acetate. The origin of ester and amide resonance

Abstract: The structures of the rotamers about the C-O bonds of formic acid, methyl formate, acetic acid, and methyl acetate were calculated by using the 6-31G* basis set and complete geometrical relaxation Large basis sets (6-311+G**) and correction for electron correlation were needed in order to obtain calculated barriers that were in good agreement with the available experimental data The factors that control the geometry at a carbonyl group are considered, and it is shown that an analysis in terms of bond path angles leads to a direct connection with electronegativity The nature of the interaction between an amino group and a carbonyl, as in an amide, is examined and shown not to involve charge transfer from the nitrogen to the carbonyl oxygen, but rather it involves charge transfer between carbon and nitrogen The origin of the rotational barrier in esters and of the difference in energy between the E and Z conformers is discussed

Linear Polyene Electronic Structure and Spectroscopy

Abstract: A linear polyene is a chain of conjugated carbon-carbon double bonds with an unbranched n-electron system. The conventional representation of alternating double and single bonds corresponds to the resonance structure of major impor­ tance in the ground state. Polyenes are distinguished from symmetrical cyanines and aromatic compounds by the fact that these other compounds have at least two resonance structures which contribute significantly to their ground state. A more quantitative distinction between linear polyenes and other classes of compounds concerns the presence of bond length