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 most stable forms of 2,4,6,8,10,12-hexanitro-2,4-6, 8, 10, 12-hexaazaisowurtzitane (CL-20) were analyzed at the B3LYP/6-31G+(d,p) level of theory and the activation energies of those transitions were found to be quite low and lie in the range 1.2-4.0 ǫ −1.
78 citations
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TL;DR: This model is in agreement with the observed differences in the IR spectra in nitrogen and argon matrices, in particular, the relative frequencies of the νOH and τCOH modes and the relative intensities of theλOH and νC=O bands.
Abstract: Formic acid (HCOOH, FA) and acetic acid (CH3COOH, AA) are studied in a nitrogen matrix. The infrared (IR) spectra of cis and trans conformers of these carboxylic acids (and also of the HCOOD isotopologue of FA) are reported and analyzed. The higher-energy cis conformer of these molecules is produced by narrowband near-IR excitation of the more stable trans conformer, and the cis-to-trans tunneling decay is evaluated spectroscopically. The tunneling process in both molecules is found to be substantially slower in a nitrogen matrix than in rare-gas matrices, the cis-form decay constants being approximately 55 and 600 times smaller in a nitrogen matrix than in an argon matrix, for FA and AA respectively. The stabilization of the higher-energy cis conformer is discussed in terms of specific interactions with nitrogen molecule binding with the OH group of the carboxylic acid. This model is in agreement with the observed differences in the IR spectra in nitrogen and argon matrices, in particular, the relative f...
78 citations
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TL;DR: In this article, the rotational processes of 2-(2'-hydroxyphenyl)oxazole derivatives in both ground (S0) and first singlet (S1) excited electronic states have been respectively studied from experimental and theoretical points of view.
Abstract: The H-atom transfer and the rotational processes of 2-(2‘-hydroxyphenyl)oxazole derivatives in both ground (S0) and first singlet (S1) excited electronic states have been respectively studied from experimental and theoretical points of view. Experiment and theory support the coexistence of two ground state rotamers, E and ER, with OH···N and OH···O hydrogen bonds, respectively, rotamer E being the most stable and the only one that experiences a photoinduced H-atom motion in the S1 state. The fluorescence of 2-(2‘-hydroxyphenyl)-4-methyloxazole in a rigid polymeric medium suggests that in fluid media the phototautomer of the excited enol rotamer suffers a twisting motion around the C−C bond linking both moieties of the molecule. Ab initio calculations at the Hartree−Fock and CI-all-singles levels reveal (a) the existence of a high-energy barrier to the H-atom transfer in the S0 state, whereas in the S1 state this transfer has a small or null energy barrier, (b) a coupling between a charge transfer and the ...
78 citations
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TL;DR: The high sensitivity of the experiment has allowed us to detect for the first time a conformer uniquely stabilized by an n-pi* hyperconjugative interaction between the nucleophile N: of the amino group and the pi* orbital at the carbonyl group.
Abstract: The combination of Fourier transform microwave spectroscopy in a pulsed supersonic jet with laser ablation has made beta-alanine amenable to a structural study in the gas phase. Two new conformers of beta-alanine have been identified together with the two previously observed by McGlone and Godfrey [J. Am. Chem. Soc. 1995, 117, 1043]. The comparison between the experimental rotational and 14N nuclear quadrupole coupling constants and those calculated ab initio provide a definitive test for molecular structures and confirm unambiguously the identification of all conformers. For the two most abundant conformers, an intramolecular hydrogen bond between the amino group and carbonyl oxygen (N-H...O=C) is established, and the COOH adopts a cis-COOH configuration. The next conformer in order of abundance presents an O-H...N intramolecular hydrogen bond with a trans configuration for the COOH group. The high sensitivity of the experiment has allowed us to detect for the first time a conformer uniquely stabilized by an n-pi* hyperconjugative interaction between the nucleophile N: of the amino group and the pi* orbital at the carbonyl group. Partial conformational relaxation has been observed in the supersonic expansion.
78 citations
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TL;DR: Ionization potentials and O-H bond dissociation energies computed for the most stable conformers of cation, quinonoidal, and anion forms are consistent with an important antioxidant activity.
Abstract: A complete conformational analysis on the isolated and polarizable continuum model (PCM) modeled aqueous solution cation, quinonoidal, and anion forms of pelargonidin, comprising the diverse tautomers of the latter forms, was carried out at the B3LYP/6-31++G(d,p) level. The results indicate that the most stable conformer of cationic and quinonoidal forms of pelargonidin are completely planar in the gas phase, whereas that of the anionic form is not planar. In contrast, PCM calculations show that the plane of the B ring is slightly rotated with regard to the AC bicycle in the most stable conformer of the cation and quinonoidal form. The most stable conformers of the cation, both in gas phase and aqueous solution, display anti and syn orientations for, respectively, C2−C3−O−H and C6−C5−O−H dihedral angles, whereas syn and anti orientation of hydroxyls at 7 and 4‘ positions are nearly isoenergetic. The most stable tautomer of quinonoidal pelargonidin is obtained by deprotonating hydroxyl at C5 in gas phase b...
78 citations