Butane torsional angle

Figure 2-102. Dependence of the potential energy ctirve of n-butane on the torsion angle r between carbon atoins C2 and C3.

The trans conformation corresponds to a torsion angle of 180°, the gauche(+) conformation to oni + 60° and the gauche -) conformation to -60°. These approximately correspond to the torsion anj of the three minimum energy conformations of butane. 

Muller et al. focused on polybead molecules in the united atom approximation as a test system these are chains formed by spherical methylene beads connected by rigid bonds of length 1.53 A. The angle between successive bonds of a chain is also fixed at 112°. The torsion angles around the chain backbone are restricted to three rotational isomeric states, the trans (t) and gauche states (g+ and g ). The three-fold torsional potential energy function introduced [142] in a study of butane was used to calculate the RIS correlation matrix. Second order interactions , reflected in the so-called pentane effect, which almost excludes the consecutive combination of g+g- states (and vice-versa) are taken into account. In analogy to the polyethylene molecule, a standard RIS-model [143] was used to account for the pentane effect. 

The structure of a second polymorph of 4,5-diphenyl- lH-imidazole has been discussed, with the new form exhibiting significantly different phenyl/imidazole dihedral angles and mode of crystal packing relative to the known form [53], A new triclinic polymorph of 1,4-dibenzoyl-butane was found, differing from the monoclinic form in the torsional angles of the central chain [54], Two polymorphs of diphenyl-(4-pyridyl)methyl methacrylate have been found, where the molecules in the two forms contain weak C—H— n and C—H O/N contacts that lead to the existence of different conformations [55]. 

Aliphatic dienes undergo three main photochemical pericyclic processes, whose individual efficiencies depend largely on the torsional angle about the central bond in the specific diene conformer which is excited. These are (a) cyclobutene formation, (b) bicyclof 1.1.0] butane formation and (c) [l,5]-hydrogen migration. A fourth process, methylcyclopropene formation, has also been observed in minor amounts in several cases. 

Somewhat more complicated but also familiar is a plot of energy vs. the torsion angle involving the central carbon-carbon bond in -butane. 

For rotation around single bonds in substimted systems other terms may be necessary. the butane molecule, for example, there are still three minima, but the two gauche Ttorsional angle 60°) and anti (torsional angle = 180°) conformations now hav ... 

Validation of the biasing potential on pentane in vacuum Pentane has only two main configurational degrees of freedom, 0, and 02, the main chain torsional angles. Thus, it is the simplest model which can be used to test and validate the ID and 2D biasing techniques described above. Also the computed PMFs for rotation of butane and pentane in vacuum (data not shown)... 

Fig. lA Theoretical quantitative data on the confonnational equilibrium of butane (1) and methoxyethane (II). Abscissae MeC-CMe or MeC-OMe torsion angles ordinates (a) 10 molar fractions by angle degree (b) kcal mol (from Jorgensen et al. 1981 Jorgensen and Ibrahim 1981) (reproduced with kind permission from the American Chemical Society). 

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