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: In this paper, the influence of solvent on the structure and photoinduced proton-transfer processes of 1-methyl-2- (2′-hydroxyphenyl)benzimidazole (MeHBI) was studied by means of UV-VIS absorption and fluorescence spectroscopy.
Abstract: The influence of the solvent on the structure and photoinduced proton-transfer processes of 1-methyl-2- (2′-hydroxyphenyl)benzimidazole (MeHBI) was studied by means of UV–VIS absorption and fluorescence spectroscopy. The steric hindrance of the methyl group determines the non-planar structure of the MeHBI anion and cation in their ground state, but they undergo a rotation towards planarity in their excited state. In hydroxylic solvents, the excited cation loses the hydroxylic proton to the solvent, affording the keto tautomer. The neutral form of MeHBI exhibits conformational equilibrium dependent on the solvent. The cis-enol conformer, with an intramolecular hydrogen bond and a planar structure, is the dominant species in non-hydroxylic solvents and undergoes an excited-state intramolecular proton-transfer (ESIPT) reaction, producing the keto tautomer. The non-planar enol conformer is the only species detected in the ground state in water; in the excited state it loses the hydroxyl proton, leaving the excited anion. Comparable proportions of both enol conformers exist in alcoholic solvents, their relative proportions having been determined. In these solvents the cis-enol form undergoes ESIPT, whereas the non-planar enol rapidly undergoes rotation towards planarity in the excited state, emitting fluorescence from this state. The solvent hydrogen-bond donor acidity determines the ratio of non-planar enol to cis-enol conformers.
51 citations
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Abstract: The compounds ClH2SiONMe2 and ClH2SiONEt2 have been prepared by the reaction of the corresponding O-lithiated hydroxylamines and dichlorosilane. Their identity has been proved by gas-phase IR and solution NMR (1H, 13C, 15N, 17O, and 29Si) spectroscopy. In contrast to ClH2SiONMe2, ClH2SiONEt2 is unstable at ambient temperature and decomposes to give H2SiCl2 and H2Si(ONEt2)2. ClH2SiONEt2 shows dynamic behavior in the solution as shown by low-temperature NMR. In the crystal ClH2SiONMe2 (low-temperature crystallography) is present as anti-conformer (torsional angle Cl−Si−O−N 180°) and shows an exceptionally small Si−O−N angle of 79.7(1)° and a Si···N distance of 2.028(1) A, corresponding to a relatively strong Si−N-β-donor interaction. Ab initio calculations predict a wider Si−O−N angle of 91.6° (MP2/6-311G**). A gas-phase structure determination by electron diffraction shows the presence of two conformers (anti and gauche), which are equal in potential energy. The Si−O−N angle in the anti-conformer is 87.1(9...
51 citations
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TL;DR: In this article, NMR studies of catechin-(4α→8)-epicatechin (1) and (2) provided complete assignment of the proton and carbon resonances for both the more extended and compact conformers in the free phenolic form.
Abstract: NMR studies of catechin-(4α→8)-epicatechin (1) and
catechin-(4α→8)-catechin (2) provided complete assignment of
the proton and carbon resonances for both the more extended and compact
conformers in the free phenolic form. When 1 is in organic solvents, the
more extended rotamer is preferred over the more compact rotamer
(10∶7), but in water, the compact rotamer dominates
(2∶10). When 2 is in organic solvents, the more compact
rotamer is slightly preferred (8∶10), but in water, only
trace amounts of the more extended rotamer are detected. NOE experiments
show rotational conformation exchange despite the fact that two distinct
and sharp sets of signals are seen for each rotamer. The upper unit
heterocyclic ring exists in an approximate half-chair conformation in
each rotamer for both dimers. However, coupling constants of the lower
unit heterocycles show substantial axial orientation of the B-ring.
Lineshape analysis of the terminal unit H-3F excludes E- and
A-conformational interchange. These results and NOE experiments show a
skewed-boat conformation for the terminal unit in 1 and between a
half-chair and skewed-boat conformation for the terminal unit in
2.
51 citations
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TL;DR: Analysis of the solution trajectories reveals that persistent intramolecular hydrogen bonds and intermolecular bridging hydrogen bonds formed by water molecules between the ring oxygen and the hydroxymethyl group further stabilizes the gt conformation making it the preferred rotamer in both hydrated glucose and galactose.
Abstract: Although the conformational preferences in glucose and galactose have been studied since the early 1970s, only recently have the glucose and galactose hydroxymethyl populations been resolved by combining (3)J(HH) and (2)J(HH) NMR coupling data using a modified Karplus equation. A preference for gauche conformations is observed in monosaccharides, but the reasons for this are not understood. We calculated the free energy of rotation profiles for glucose and galactose primary alcohols using a semiempirical description of the monosaccharides in QM/MM simulations. From this we observed excellent agreement between our simulated population distributions for glucose gg/gt/tg = 35:57:3 and galactose gg/gt/tg = 4:86:7 with those measured from NMR. A stereoelectronic analysis of the minimum energy conformations using natural bond orbitals provides a clear description of the stabilizing contribution to the gauche conformers stemming from the C-H bonding and the C-O antibonding orbital interactions, specifically sigma(C6-H) --> sigma*(C5-O5) and sigma(C5-H) --> sigma*(C6-O6). Analysis of the solution trajectories reveals that persistent intramolecular hydrogen bonds and intermolecular bridging hydrogen bonds formed by water molecules between the ring oxygen and the hydroxymethyl group further stabilizes the gt conformation making it the preferred rotamer in both hydrated glucose and galactose. The hydroxymethyl quantum mechanics/molecular mechanics molecular dynamics trajectories and derived rotational free energies for these monosaccharides in water solutions explain that the experimental observations are due to a combination of competing stereoelectronic (gauche), electronic (intramolecular hydrogen bonding), and electrostatic (solvent-saccharide hydrogen bonding) factors.
51 citations
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TL;DR: In this article, a 3D model for cyclopentapeptides (CPP's) using only NMR spectroscopy is presented. But, due to conformer averaging, 3D structure(s) derived directly from NMR data may n...
Abstract: Studies of 3D models for cyclopentapeptides (CPP's) employing only NMR spectroscopy encounter a serious problem. Because of conformer averaging, 3D structure(s) derived directly from NMR data may n...
51 citations