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On the basis of energy decomposition analysis, the rotational energy profile of ethane is explained by using two models: rigid rotation with instantaneous geometry relaxations of the eclipsed and staggered conformations and relaxed rotation with continuous geometry relaxations. Both models can be applied to the real system. A distinction between the cause of an initial energy rise and energetic consequences of structural changes accompanying the rotation is made. It is concluded that the increased Pauli repulsion is the main cause for the initial energy rise and geometry changes. However, after the structural changes take place, the Pauli repulsion is not responsible for the higher energy of the eclipsed state. It then originates from energetic consequences of geometry changes, which include decrease in electrostatic and orbital stabilization energies, mainly due to the C-C bond lengthening, and an energy rise due tomethyl groups bending.

On the basis of energy decomposition analysis, the rotational energy profile of ethane is explained by using two models: rigid rotation with instantaneous geometry relaxations of the eclipsed and staggered conformations and relaxed rotation with continuous geometry relaxations. Both models can be applied to the real system. A distinction between the cause of an initial energy rise and energetic consequences of structural changes accompanying the rotation is made. It is concluded that the increased Pauli repulsion is the main cause for the initial energy rise and geometry changes. However, after the structural changes take place, the Pauli repulsion is not responsible for the higher energy of the eclipsed state. It then originates from energetic consequences of geometry changes, which include decrease in electrostatic and orbital stabilization energies, mainly due to the C-C bond lengthening, and an energy rise due tomethyl groups bending.

The barriers to interconversion of the two enantiomeric atropisomers of 6-methoxy-6,7-dihydro-5H-dibenzo[c,e]phosphepine and that of the diastereomeric forms of 6-(-)menthoxy-6,7-dihydro-3H-dibenzo[c,e]phosphepine were determined by NMR spectroscopical methods to be 19.3 and 18.5 kcalmol(-1), respectively, at 298 K. The ratio of the atropisomers was shown to depend on the group bound to phosphorus. Only complexes with two homochiral ligands bound to the each metal center were obtained upon reaction with [Rh(COD)(2)](+) BF4-. The Rh complexes catalyzed the hydrogenation of alpha-acetamidocinnamate. The major isomer of 6-(-)-menthoxy-6,7-dihydro-5H-dibenzo[c,e]phosphepine was found to exhibit higher activity but to afford a product with lower ee than its diastereomer., QC20100630

The rotational barriers between the configurational isomers of two structurally related push-pull 4-oxothiazolidines, differing in the number of exocyclic C=C bonds, have been determined by dynamic H-1 NMR spectroscopy. The equilibrium mixture of (5-ethoxy-carbonylmethyl-4-oxothiazolidin-2-ylidene)-1-phenylethanone (1a) in CDCl3 at room temperature to 333 K consists of the E- and Z-isomers which are separated by an energy barrier Delta G(#) 98.5 kJ/mol (at 298 K). The variable-temperature H-1 NMR data for the isomerization of ethyl (5-ethoxycarbonylmethylidene-4-oxothiazolidin-2-ylidene)ethanoate (2b) in DMSO-d(6), possessing the two exocyclic C=C bonds at the C(2)- and C(5)-positions, indicate that the rotational barrier Delta G(#) separating the (2E,5Z)-2b and (2Z,5Z)-2b isomers is 100.2 kJ/mol (at 298 K). In a polar solvent-dependent equilibrium the major (2Z,5Z)-form ( gt 90%) is stabilized by the intermolecular resonance-assisted hydrogen bonding and strong 1,5-type (SO)-O-... interactions within the S-C=C-C=O entity. The C-13 NMR Delta delta(C(2)C(2')) values, ranging from 58 to 69 ppm in 1a-d and 49-58 ppm in 2a-d, correlate with the degree of the push-pull character of the exocyclic C(2)=C(2') bond, which increases with the electron withdrawing ability of the substituents at the vinylic C(2') position in the following order: COPh similar to COEt gt CONHPh gt CONHCH2CH2Ph. The decrease of the Delta delta(C(2)C(2')) values in 2a-d has been discussed for the first time in terms of an estimation of the electron donor capacity of the -S- fragment on the polarization of the C=C bonds. (c) 2006 Elsevier B.V. All rights reserved.

The rotational barriers between the configurational isomers of two structurally related push-pull 4-oxothiazolidines, differing in the number of exocyclic C=C bonds, have been determined by dynamic H-1 NMR spectroscopy. The equilibrium mixture of (5-ethoxy-carbonylmethyl-4-oxothiazolidin-2-ylidene)-1-phenylethanone (1a) in CDCl3 at room temperature to 333 K consists of the E- and Z-isomers which are separated by an energy barrier Delta G(#) 98.5 kJ/mol (at 298 K). The variable-temperature H-1 NMR data for the isomerization of ethyl (5-ethoxycarbonylmethylidene-4-oxothiazolidin-2-ylidene)ethanoate (2b) in DMSO-d(6), possessing the two exocyclic C=C bonds at the C(2)- and C(5)-positions, indicate that the rotational barrier Delta G(#) separating the (2E,5Z)-2b and (2Z,5Z)-2b isomers is 100.2 kJ/mol (at 298 K). In a polar solvent-dependent equilibrium the major (2Z,5Z)-form ( gt 90%) is stabilized by the intermolecular resonance-assisted hydrogen bonding and strong 1,5-type (SO)-O-... interactions within the S-C=C-C=O entity. The C-13 NMR Delta delta(C(2)C(2')) values, ranging from 58 to 69 ppm in 1a-d and 49-58 ppm in 2a-d, correlate with the degree of the push-pull character of the exocyclic C(2)=C(2') bond, which increases with the electron withdrawing ability of the substituents at the vinylic C(2') position in the following order: COPh similar to COEt gt CONHPh gt CONHCH2CH2Ph. The decrease of the Delta delta(C(2)C(2')) values in 2a-d has been discussed for the first time in terms of an estimation of the electron donor capacity of the -S- fragment on the polarization of the C=C bonds. (c) 2006 Elsevier B.V. All rights reserved.

The internal barrier of rotation (Erot) was calculated for all 209 polychlorinated biphenyls (PCBs) by using a semi-empirical method, viz. the Austin Model 1 (AM1) Hamiltonian. The difference in total energy between a forced planar state and an optimised twisted structure was defined as Erot. The Erot values were in the range of 8.33 to 483 kJ/mol, and were significantly influenced by the number of chlorine atoms in ortho position. An additional structural characteristic of the PCBs influencing Erot of ortho substituted congeners was substitution by chlorine atoms in vicinal meta positions, which is assumed to prevent outward bending of ortho substituents. This so-called buttressing effect contributed with 4 to 31 kJ/mol per added chlorine atom. In conclusion, the internal barrier of rotation, calculated for all 209 PCBs, provides an important structure dependent physico-chemical parameter for multivariate modelling of future quantitative structure-activity and structure-property relationships (QSARs/QSPRs).