Gauche bond/conformation

Figure 7.6 Illustration of the SHH, SHH and SCH conformers around a -CHCl-unit, where the circled atoms (H or C) are trans to Cl. t, g+, g- denote trans, gauche+ and gauche- bond conformations. Source Author s own files.)...

This particular sequence of conformations-trans bonds that advance the helix along the axis, alternating with gauche bonds which provide the twist-takes the chain through a series of relatively low potential energy states and generates a structure with minimum steric hindrance between substituents. If the polymer series is extended to include bulkier substituents, for example. 

In a class of reahstic lattice models, hydrocarbon chains are placed on a diamond lattice in order to imitate the zigzag structure of the carbon backbones and the trans and gauche bonds. Such models have been used early on to study micelle structures [104], monolayers [105], and bilayers [106]. Levine and coworkers have introduced an even more sophisticated model, which allows one to consider unsaturated C=C bonds and stiffer molecules such as cholesterol a monomer occupies several lattice sites on a cubic lattice, the saturated bonds between monomers are taken from a given set of allowed bonds with length /5, and torsional potentials are introduced to distinguish between trans and "gauche conformations [107,108]. 

For a particular conformation c of a molecule, the positions of all (united) atoms in space as well as the chain conformers are known. The potential energy of this conformation is therefore just the sum of the contributions, as given by equation (9) for all the united atoms and a particular energy quantity per gauche bond in the chain. The statistical weight for this conformation is proportional to the Boltzmann factor containing this segment potential ... 

Figure 7. Crystal structure of bmimBr viewed in the direction of the a axis. Carbon atoms, nitrogen atoms, and bromide anions are shown. The gauche-tram conformation of the butyl group of the bmim cation is shown in the inset with the relevant C-O bonds marked by thick bars.

The diastereomeric ethyl a-hydroxy-2-oxocyclohexaneacetates were assigned after an inspection of their C-6 chemical shifts342. When a stabilization of cyclic hydrogen bonded conformations is assumed, there is a gauche interaction of C-6 with the carboxyl function in the /-diastereomer (5 = 27.0), whereas the ester group is far away from this atom in the u-form (d = 30.1). In addition, smaller S values of the a-protons in the u- compared to the /-isomer have been observed in these and related compounds342. 

Obviously Meng et al. [5] have reached an anti-gauche (AG) conformation that probably just is local but not a global minimum. The hydrogen bonding has previously been detected by NMR relaxahon studies on [C4Cilm] [PFJ and related systems in the liquid state [5,6,75,76]. 

A general theory of dichroism induced by strain in polymeric networks Is developed by adaptation of methods developed earlier for treating strain birefringence. It is generally applicable to dichrolc bands associated with any specified conformation involving sequences of one or more consecutive bonds. The transition dipole moment is introduced in the local framework of the skeletal bonds associated therewith. Possible differences in transition moments for various conformations and repeat units are taken into account. Numerical calculations for PE chains show gauche bonds, rather than trans, to be more favorably oriented with respect to the chain vector r. 

Some comments on the liquid state conformations of n-alkanes are made. Calculated are the average probability

and number of gauche bond rotations assumed by internal bonds, and the probability Pg(central) that the central bond in each of the even members (Cg, C3,...,C22) is in a gauche rotational state. 

Conformational characteristics of PTFE chains are studied in detail, based upon ab initio electronic structure calculations on perfluorobutane, perfluoropentane, and perfluorohexane. The found conformational characteristics are fully represented by a six-state RIS model. This six-state model, with no adjustment of the geometric or energy parameters as determined from the ab initio calculations, predicts the unperturbed chain dimensions, and the fraction of gauche bonds as a function of temperature, in good agreement with available experimental values. 

In spite of the fact that a six-state model is a natural choice for describing the conformational distribution in PIB chains, a simpler model can be obtained by labeling all the bond conformations in the domain [60,160°] and all those in the domain [-60, - 180°I with the symbols g+ and g , respectively. This leads to a four state model formally analogous to the older ones but leaves the exact location of the gauche states undefined. 

The model of Robertson (Robertson, 1966 Escaig and G Sell, 1982 Cook et al., 1998) is based on a statistical distribution of stiff and flexible bonds (gauche —> trans conformation). The applied stress improves the density of flexible bonds up to a critical value where the material becomes equivalent to a liquid. 

The notation of Maroncelli et al. (20,21) will be followed in discussing conformation, so that, in our designation of the conformational state of a CD2 group, the CC bonds that are directly bonded to the CD2 group will be underlined. Thus t means the CD2 moiety connects a trans and a gauche bond. End-gauche conformations... 

Quantitative determination of conformational disorder, in addition to assignments of the spectral features, requires accurate calibrations of relative absorptivities (extinction coefficients) for the various bands. These were calculated with the aid of the rotational isomeric state model of Flory (22), a statistical mechanical determination of conformational disorder in alkanes. The model requires an estimate for the energies of gauche bonds, chosen from the best available experiments to be 508 +/- 50 cal/mole, and an energy for the bond pairs E, chosen to be 3000 cal/mole. The... 

Molecular orbital calculations have also provided theoretical justification for these stereoelectronic effects in tetracovalent and pentacovalent phosphorus species (2-7). As has been shown in molecular orbital calculations on the X -P-X2 (X = 0,N) structural fragments, the X.-P bond is strengthened (as indicated by an increase in the Mulliken overlap population) while the P-X3 bond is weakened when the X atom lone pair is app to the P-X3 bond. Thus, in the g,t conformation of dimethyl phosphate (Structure ll the overlap population for the trans P-0 bond is. 017 electron lower than the overlap population for the gauche P-0 bond. As shown for g,t dimethyl phosphate one lone pair (shaded in 1) on the gauche bond oxygen is app to the trans bond, while no lone pairs on the trans bond oxygen are app to the gauche bond. Thus, the weakest X.-P bond has one app lone pair and no lone pairs on X. app to the P=X2 bond. 1... 

The higher alkanes resemble butane in their preference for anti and gauche conforma-Conformations tions about the carbon-carbon bonds. The lowest-energy conformation for any straight-Of Hi Cl her Alkanes c la n alkane is the one with all the internal carbon-carbon bonds in their anti a conformations. These anti conformations give the chain a zigzag shape. At room... 

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