Gauche torsion angles

A pentane violation arises when there are successive gauche(+) and gauche(-) torsion angles in an alkane tin. 

The conformation of the chains of isotactic polymers in the crystalline state is generally helical and corresponds to a succession of nearly trans and gauche torsion angles, the exact values depending on the bulkiness of the side groups. Molecular mechanics calculations have been extensively used for the prediction of the chain conformation of polymers in the crystal.29... 

Anti and gauche forms are present with the anti form 81(6)%. Computed differences between the parameters of the anti and gauche forms were assumed from the ab initio results. The sttuctural parameters of the anti form and the gauche torsional angle (pigauche) are listed. The CONO2 moiety was assumed to be planar. The nozzle temperature was 37 °C. 

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. 

Eclipsed bonds are characterized by a torsion angle of 0° When the torsion angle is approximately 60° we say that the spatial relationship is gauche, and when it is 180° we say that it is anti Staggered conformations have only gauche or anti relationships between bonds on adjacent atoms... 

Note that the otCH-pCH torsion angle is trans in all cases (D. W. Urry and N. G. Kumar, Biochemistry, 13, 1829-1831, (1974). A gauche orientation gives so much steric crowding that the complex will not form... 

Figure 3 plots the torsional angle potential for dihedral d in PCH5, It shows behaviour which is generic for alkyl chain dihedrals. The gauche conformers... 

Examination of the backbone torsion angles in a number of crystal stractures of /9-alanine-containing peptides reveals that the conformation around the C(a)-C(/9) bond of /9-alanine residues is essentially gauche or trans (anti) with values close to 60° or 180°, respectively [158]. Populating the gauche conformation of /9-ami-... 

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. 

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