Conformational isomerism

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

13C−1H and 13C−13C Spin-Coupling Constants in Methyl β-d-Ribofuranoside and Methyl 2-Deoxy-β-d-erythro- pentofuranoside: Correlations with Molecular Structure and Conformation

Abstract: Methyl β-d-ribofuranoside 5 and methyl 2-deoxy-β-d-erythro-pentofuranoside (methyl 2-deoxy-β-d-ribofuranoside) 6 were synthesized with single sites of 13C-enrichment at each carbon, and a complete set of 13C−1H and 13C−13C spin-coupling constants (37 couplings in 5, 41 couplings in 6) in these molecules were obtained by 1D and 2D NMR spectroscopy. 2JCH coupling signs were determined from the observation of relative cross-peak displacements in 2D TOCSY data. The 13C−1H couplings (one-, two-, and three-bond) were interpreted in structural and conformational terms with assistance from conformational models of 5 and 6 based on conventional two-state pseudorotational analysis of 3JHH values (PSEUROT) and on theoretical predictions of conformational energies and JCH values obtained from ab initio molecular orbital calculations on the ten envelope and planar conformers of β-d-ribofuranose 4 and 2-deoxy-β-d-erythro-pentofuranose (2-deoxy-β-d-ribofuranose) 3. A comparison of theoretical JCH values in 3 and 4 allow...

Internal rotation in propionic acid: near-infrared-induced isomerization in solid argon.

Abstract: The conformational system of propionic acid (CH3CH2COOH) is studied in solid argon. It is predicted by the ab initio calculations that this molecule has four stable conformers. These four structures are denoted Tt, Tg±, Ct, and Cg±, and they differ by the arrangement around the C−O and Cα−C bonds. The ground-state Tt conformer is the only form present at 8 K after deposition of an argon matrix containing propionic acid. For the CH3CH2COOH and CH3CH2COOD isotopologues, narrow-band excitation of the first hydroxyl stretching overtone of the conformational ground state promotes the Cα−C and C−O internal rotations producing the Tg± and Ct conformers, respectively. A subsequent vibrational excitation of the produced Tg± form induces its conversion to the Cg± conformer by rotation around the C−O bond. In the dark, all of the produced conformers decay to the conformational ground state at different rates. The decay kinetics and its temperature dependence allow the identification of the conformers by IR absorptio...

Conformation of L-Tyrosine Studied by Fluorescence-Detected UV-UV and IR-UV Double-Resonance Spectroscopy

Abstract: The laser-induced fluorescence spectrum of jet-cooled l-tyrosine exhibits more than 20 vibronic bands in the 35450−35750 cm-1 region. We attribute these bands to eight conformers by using results of UV−UV hole-burning spectroscopy. These isomers are classified into four groups; each group consists of two rotational isomers that have a similar side-chain conformation but different orientations of the phenolic OH. The splitting of band origins of rotational isomers is 31, 21, 5, and 0 cm-1 for these groups. IR−UV spectra suggest that conformers belonging to two of the four groups have an intramolecular OH···N hydrogen bond between the COOH and NH2 groups. By comparing experimental and theoretical results of l-tyrosine with those of l-phenylalanine, we propose probable conformers of l-tyrosine.

Fundamental and overtone vibrational spectra of gas-phase pyruvic acid.

Abstract: Pyruvic acid (CH(3)COCOOH) is an important keto acid present in the atmosphere. In this study, the vibrational spectroscopy of gas-phase pyruvic acid has been investigated with special emphasis on the overtone transitions of the OH-stretch, with Delta v(OH) = 2, 4, 5. Assignments were made to fundamental and combination bands in the mid-IR. The two lowest energy rotational conformers of pyruvic acid are clearly observed in the spectrum. The lowest energy conformer possesses an intramolecular hydrogen bond, while the next lowest rotational conformer does not. This difference is clearly seen in the spectra of the OH vibrational overtone transitions, and it is reflected in the anharmonicities of the OH-stretching modes for each conformer. The spectra of the OH-stretching vibration for both conformers were investigated to establish the effect of the hydrogen bond on frequency, intensity, and line width.