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Hindered Rotation (hindered + rotation)
Selected AbstractsDissecting the Hindered Rotation of EthaneCHEMPHYSCHEM, Issue 12 2009David Asturiol With a twist: The rotational barrier of ethane (see picture) is analyzed in terms of atomic and diatomic contributions of the energy. No charge-localized reference state is necessary. Hyperconjugation effects give rise to different individual energy components that cancel almost completely. The picture of attributing the barrier to the repulsion between the hydrogen atoms is legitimate. [source] A Pragmatic Recipe for the Treatment of Hindered Rotations in the Vibrational Averaging of Molecular PropertiesCHEMPHYSCHEM, Issue 1 2008Brendan C. Mort Dr. Abstract A computational protocol for the treatment of hindered rotations in the vibrational averaging of molecular properties is described. As examples, the specific rotations of (R) -methyloxirane, (1S) -methylnorbornanone, and (S) -epichlorohydrin and the spin-spin coupling constant for are investigated. For each of the four molecules, the relaxed and unrelaxed potential energy surfaces along the rotational coordinate of interest are used to solve the nuclear Schrödinger equation using a pseudospectral method. Analysis of the results demonstrate that the vibrational averaging for low-frequency modes that represent hindered rotations can lead to unphysical behavior at high temperatures while simply neglecting the rotational contributions is also not desirable. The method described in this work connects the (an)harmonic oscillator behavior at low temperatures with the free rotor-like behavior at high temperatures. [source] Synthesis and stereochemistry of 3-acetyl-5-aminospiro[1,3,4-thiadiazoline-2,4,-thioflavans]JOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 2 2008László Somogyi Under acetylating conditions racemic thioflavanone thiosemicarbazones cyclize into racemic 3-acetyl-spiro[1,3,4-thiadiazoline-2,4,-thioflavans] and a racemic 3-acetylspiro[1,3,4-oxadiazoline-2,4,-thioflavan] with trans O(1) or S(1) and Ph(2,eq). Hindered rotation of the endocyclic N(3) acetyl group spirothia-diazolines caused the formation of isomers separable by HPLC. X-ray diffraction analyses, 1H-, 13C-, and 15N NMR measurements as well as MOPAC QM calculations were performed to reveal the structures of these isomers. [source] N,N,-Bis(2-pyridyl)benzene-1,2-diamineACTA CRYSTALLOGRAPHICA SECTION C, Issue 3 2004Maria Gdaniec Hindered rotation about the partial double C,N bonds between the amine and pyridine moieties in the title molecule, C16H14N4, results in two different conformations of the N -aryl-2-aminopyridine units. One, assuming an E conformation, is involved in a pair of N,H,N hydrogen bonds that generate a centrosymmetric (8) motif. The second, adopting a Z conformation, is not engaged in any hydrogen bonding and is flattened, the dihedral angle between the benzene and pyridine rings being 12.07,(7)°. This conformation is stabilized by an intramolecular C,H,N interaction [C,N,= 2.9126,(19),Å, H,N,=,2.31,Å and C,H,N,=,120°]. [source] Conformational spaces of the gastrointestinal antisecretory chiral drug omeprazole: Stereochemistry and tautomerismCHIRALITY, Issue 1 2006Hava Caner Abstract A study of the conformational spaces of the chiral proton pump inhibitor (PPI) drug omeprazole by semiempirical, ab-initio, and DFT methods is described. In addition to the chiral center at the sulfinyl sulfur atom, the chiral axis at the pyridine ring (due to the hindered rotation of the 4-methoxy substituents) was considered. The results were analyzed in terms of the 5-methoxy and 6-methoxy tautomers and the two pairs of enantiomers (R,P)/(S,M) and (R,M)/(S,P). Five torsion angles were systematically explored: the backbone rotations defined by D1 (N3,C2,S10,O11), D2 (C2,S10,C12,C13), and D3 (S10,C12,C13,N14) and two methoxy rotations defined by D4 (C6,C5,O8,C9) and D5 (C16,C17,O19,C20). Significant energy differences were revealed between the 5- and 6-methoxy tautomers, the extended and folded conformations, and the (S,M) and (S,P) diastereomers. The "extended M" conformation of the 6-methoxy tautomer of (S)-omeprazole was found to be the most stable conformer. © 2005 Wiley-Liss, Inc. Chirality [source] A Pragmatic Recipe for the Treatment of Hindered Rotations in the Vibrational Averaging of Molecular PropertiesCHEMPHYSCHEM, Issue 1 2008Brendan C. Mort Dr. Abstract A computational protocol for the treatment of hindered rotations in the vibrational averaging of molecular properties is described. As examples, the specific rotations of (R) -methyloxirane, (1S) -methylnorbornanone, and (S) -epichlorohydrin and the spin-spin coupling constant for are investigated. For each of the four molecules, the relaxed and unrelaxed potential energy surfaces along the rotational coordinate of interest are used to solve the nuclear Schrödinger equation using a pseudospectral method. Analysis of the results demonstrate that the vibrational averaging for low-frequency modes that represent hindered rotations can lead to unphysical behavior at high temperatures while simply neglecting the rotational contributions is also not desirable. The method described in this work connects the (an)harmonic oscillator behavior at low temperatures with the free rotor-like behavior at high temperatures. [source] | ||||||||||