Conformation, gauche

We have it on good authority (Ege, 1998) that the gauche minimum on the potential energy coordinate is about 0.9 kcal moI higher in energy than the anti conformation. This establishes a two-state energy system for the stable conformers, gauche and anti (Fig. 4-19). 

The thermodynamic analysis of conformational and structural transformations in the melt at high pressures34 showed that the free volume and free energy minimum required for hydrostatic compression is attained as a result of the transition of the molecules in the melt into a more extended conformation (gauche —> trans transitions) since the extended molecules ensure a more compact packing of the chains at compression. Chain uncoiling leads to a decrease in their flexibility parameter f with increasing pressure p ... 

Chain conformation (gauche vs. Irons) interchain coupling Surface topography Chain dynamics... 

Modern methods of vibrational analysis have shown themselves to be unexpectedly powerful tools to study two-dimensional monomolecular films at gas/liquid interfaces. In particular, current work with external reflection-absorbance infrared spectroscopy has been able to derive detailed conformational and orientational information concerning the nature of the monolayer film. The LE-LC first order phase transition as seen by IR involves a conformational gauche-trans isomerization of the hydrocarbon chains a second transition in the acyl chains is seen at low molecular areas that may be related to a solid-solid type hydrocarbon phase change. Orientations and tilt angles of the hydrocarbon chains are able to be calculated from the polarized external reflectance spectra. These calculations find that the lipid acyl chains are relatively unoriented (or possibly randomly oriented) at low-to-intermediate surface pressures, while the orientation at high surface pressures is similar to that of the solid (gel phase) bulk lipid. 

Willow-like cascade construction was facilitated by the preparation of the AB2-type building blocks, e.g., 6-bromo-l-(4-hydroxy-4 -biphenylyl)-2-(4-hydroxyphenyl)hexane (40 Scheme 6.12), 13-bromo-l-(4-hydroxyphenyl)-2-[4-(6-hydroxy-2-naphthalenylyl)-phenyljtridecane (41), and 13-bromo-l-(4-hydroxyphenyl)-2-(4-hydroxy-4"-p-terpheny-lyl)tridecane (42). Scheme 6.12 illustrates the monomer preparative strategy for the incorporation of the monomeric mesogenic moieties predicated on conformational (gauche versus anti) isomerism. Thus, one equivalent of 4-hydroxybiphenyl was allowed to react with 1,4-dibromobutane to yield the monobromide 43, which was converted under Finkelstein conditions to the corresponding iodide 44. Treatment of the latter with ketone 45 under phase transfer conditions afforded the a-substituted ketone 46. Reduc-... 

A potential energy plot for rotation about the C2-C3 bond in butane shows unique maxima and minima. There are two kinds of staggered conformations, gauche (steric strain) and anti, and two distinct eclipsed conformations (torsional and steric strain). 

Butanone and other straight chain homologs were examined, and it was calculated that the carbon skeleton prefers an all anti conformation gauche forms are higher in energy, but as with hydrocarbons, not by very much. For 2-butanone, both the structure and the conformational equilibrium were well reproduced (Abe et al., 1969 Pierce et al., 1969). 

Only the most stable conformer (gauche) was observed in the pulsed molecular beam Fourier transform microwave spectrum. 

Terms used to describe conformations (gauche, synclinal etc.) are defined on p. 365. 

The mode pCFs (Bg,B) is also predicted to be well separated between anti and gauche rotamers but not between gauche and ortho conformers. The two Raman bands at 297 cm and 306 cm" are ascribed to the twisted conformation gauche and ortho) and the anti rotamer, respectively. Again, the relative intensity ratio of the two bands drastically changes with temperature. This line pair could not be used for a van t Hoff plot since the wavenumbers of the gauche and ortho rotamers most likely coincide as predicted by theory. 

---