Gauche, definition

Extensive discussions have focused on the conformation of the alkyl chains in the interior ". It has been has demonstrated that the alkyl chains of micellised surfactant are not fully extended. Starting from the headgroup, the first two or three carbon-carbon bonds are usually trans, whereas gauche conformations are likely to be encountered near the centre of tlie chain ". As a result, the methyl termini of the surfactant molecules can be located near the surface of the micelle, and have even been suggested to be able to protrude into the aqueous phase "". They are definitely not all gathered in the centre of tire micelle as is often suggested in pictorial representations. NMR studies have indicated that the hydrocarbon chains in a micelle are highly mobile, comparable to the mobility of a liquid alkane ... 

Although the definitions of isotactic, syndiotactic, and atactic polymers according to International Union of Pure and Applied Chemistry (IUPAC) rules are well established in terms of succession of mesa (m) or racemic (r) dyads,12 the symbolism of (+) and (—) bonds allows the easy treatments of possible configurations in cases of any complexity.1 Moreover, the (+) or (—) character of the bonds in a polymer chain is strictly related to the accessibility of gauche+ or gauche conformations of the bonds and, therefore, to the formation of right-handed or left-handed helical conformations.1... 

R3 R2 and R2 Ri gauche interactions however, for the same set of substituents, an increase in the steric requirements of either Rj or R3 will influence only one set of vicinal steric interactions (Rj R2 or R3 R2). Some support for these conclusions has been cited (eqs. [6] and [7]). These qualitative arguments may also be relevant to the observed populations of hydrogen- and nonhydrogen-bonded populations of the aldol adducts as well (see Table 1, entries K, L). Unfortunately, little detailed information exists on the solution geometries of these metal chelates. Furthermore, in many studies it is impossible to ascertain whether the aldol condensations between metal enolates and aldehydes were carried out under kinetic or thermodynamic conditions. Consequently, the importance of metal structure and enolate geometry in the definition of product stereochemistry remains ill defined. This is particularly true in the numerous studies reported on the Reformatsky reaction (20) and related variants (21). 

Wolfe23 defines the gauche effect as the tendency of molecules to adopt a structure maximizing the number of gauche interactions between adjacent lone pairs and/or polar bonds. Heteroatoms are assumed to be sp3 hybridized, thus having equivalent lone pairs (22). This phenomenological definition is unsatisfactory because it groups three different situations under the same title. 

This is the common definition of conformers. The IUPAC definition also requires that a conformer correspond to a distinct potential energy minimum, such as the anti and gauche conformations of butane. 

Definition 9.1. Etant donnA un monomorphisme u G —> G de S-groupes, on note G/G (resp. G G) et on appelle faisceau quotient A droite (resp. A gauche) de G par G le faisceau (pour la topologie fpqc) quotient de G par la... 

The most extensive studies of phenylethylamines have been done by Pullman and coworkers (59,65,67/68). Their PCILO calculations indicated that gauche and trans conformers have nearly identical energies in the majority of phenylethylamines and phenylethanolamines studied. AI studied were carried out using three different basis sets (67) these all indicated a slight but definite preference for gauche conformers. These authors noted the preponderance of extended conformers in crystal structures and in solution they attributed this to environmental forces. To test this hypothesis, computations were repeated with the inclusion of water molecules, and a tendency toward the extended conformation was indeed observed. 

For a monosubstituted oxane, the excess free enthalpy of the conformation with an axial substituent over the conformation with an equatorial substituent is, as we know by definition, the conformational free energy (CFE) of the substituent in oxane at this position. These values, possibly measured indirectly by utilizing intennediate compounds, are shown in Table 2.2. Equatorial conformations correspond to anti conformations in butane and methoxyethane, and the axial conformations (not represented) to gauche conformations. We can observe that the environment of derivative 2.20 is closest to that of the cyclohexane and that the CFE is of the same order. On the other hand, the presence of the cyclic oxygen lowers notably the CFE of derivative 2.19. The important point is the noteworthy increase in the CFE of compound 2.18, where the methyl group is close to the cyclic oxygen and possesses, on one side, an environment similar to that of methoxyethane. Let us look at the equilibrium (2.4) of the dimethylated derivative 2.21. 

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