Topic
Conformational isomerism
About: Conformational isomerism is a research topic. Over the lifetime, 11563 publications have been published within this topic receiving 199312 citations.
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TL;DR: The three smallest symmetrical paracyclophanes, having tethers with two, three, or four methylene groups, have been examined with four density functional methods and are nearly strain-free, reflecting the small out-of-plane distortion of its phenyl rings.
Abstract: The three smallest symmetrical paracyclophanes, having tethers with two, three, or four methylene groups, have been examined with four density functional methods (B3LYP, M06-2x, B97-D, ωB97X-D). The geometries predicted by functionals accounting for medium-range correlation or long-range exchange and/or dispersion are in close agreement with experiment. In addition, these methods provide similar estimates of the strain energy of the paracylcophanes, which decrease with increasing tether length. [4.4]Paracyclophane is nearly strain-free, reflecting the small out-of-plane distortion of its phenyl rings. Lastly, the barrier for interconversion of the conformers of [3.3]paracylcophane is computed in close agreement with experiment, and an estimate for phenyl rotation in [4.4]paracyclophane of about 19 kcal mol(-1) is predicted by the DFT methods employed.
54 citations
TL;DR: Selective collisional relaxation processes in the jet (associated with low interconversion barriers between different alanine conformations) that depopulate the high-energy conformers were experimentally demonstrated.
Abstract: The jet-cooled spontaneous Raman spectrum of an amino acid—alanine (Ala, 2-aminopropanoic acid; H2NCH(CH3)COOH)—is reported. The low-frequency vibrational spectrum (below 500 cm−1) was recorded and assigned using quantum chemical data: ab initio (MP2) and DFT (BLYP, B3LYP, and PBE0). Band polarization measurements were used to confirm the vibrational assignments. The acquired medium resolution spectra (HWHM of ∼4 cm−1) allow the different alanine conformations to be distinguished. Four alanine conformers were observed and identified: two previously reported by microwave spectroscopy studies and two that were previously unreported. A set of reasons for why these conformers eluded previous studies are discussed. Selective collisional relaxation processes in the jet (associated with low interconversion barriers between different alanine conformations) that depopulate the high-energy conformers were experimentally demonstrated.
54 citations
TL;DR: IR-UV double resonance spectroscopy and ab initio calculations were employed to investigate the structures and vibrations of the aromatic amino acid, L-phenylalanine-(H(2)O)(n) clusters formed in a supersonic free jet, and the D conformer was found to be the most stable conformer.
Abstract: IR-UV double resonance spectroscopy and ab initio calculations were employed to investigate the structures and vibrations of the aromatic amino acid, L-phenylalanine–(H2O)n clusters formed in a supersonic free jet. Our results indicate that up to three water molecules are preferentially bound to both the carbonyl oxygen and the carboxyl hydrogen of L-phenylalanine (L-Phe) in a bridged hydrogen-bonded conformation. As the number of water molecules is increased, the bridge becomes longer. Two isomers are found for L-Phe–(H2O)1, and both of them form a cyclic hydrogen-bond between the carboxyl group and the water molecule. In L-Phe–(H2O)2, only one isomer was identified, in which two water molecules form extended cyclic hydrogen bonds with the carboxyl group. In the calculated structure of L-Phe–(H2O)3 the bridge of water molecules becomes larger and exhibits an extended hydrogen-bond to the π-system. Finally, in isolated L-Phe, the D conformer was found to be the most stable conformer by the experiment and by the ab initio calculation.
54 citations
TL;DR: The analysis of the 13C and 1H nuclear magnetic resonance data of 2-substituted methylenecyclohexanes and 3-substantituted cyclohexenes demonstrates that a double bond stabilizes the axial conformer for an electronegative substituent as discussed by the authors.
Abstract: The analysis of the 13C and 1H nuclear magnetic resonance data of 2-substituted methylenecyclohexanes and 3-substituted cyclohexenes demonstrates that a double bond stabilizes the axial conformer for an electronegative substituent. Introduction of a methoxy group on the double bond further increases the relative stability of the axial conformer. These results are interpreted in terms of the 'double bond – no bond' resonance.
54 citations
TL;DR: The conformational preference of the tertiary amide in the starting material leads to intramolecular migration of a range of aryl rings, even those lacking electron-withdrawing activating groups, and provides a method for n→n+4 ring expansion.
Abstract: Analogues of dibenzodiazepines, in which the seven-membered nitrogen heterocycle is replaced by a 9–12-membered ring, were made by an unactivated Smiles rearrangement of five- to eight-membered heterocyclic anthranilamides. The conformational preference of the tertiary amide in the starting material leads to intramolecular migration of a range of aryl rings, even those lacking electron-withdrawing activating groups, and provides a method for n→n+4 ring expansion. The medium-ring products adopt a chiral ground state with an intramolecular, transannular hydrogen bond. The rate of interconversion of their enantiomeric conformers depends on solvent polarity. Ring size and adjacent steric hindrance modulate this hidden hydrophilicity, thus making this scaffold a good candidate for drug development.
54 citations