Preference for gauche conformations

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. 

The main difference noted [149] between iV-alkyl- and iV-ether-functionalised imidazolium cations is the preference for gauche conformation at (ring)N—CH2—O—CH2— or... 

Stereoelectronic factors are also important in the conformational dynamics of acyclic acetals [6] (Cosse-Barbi and Dubois, 1986). Here the usual preference for staggered conformations is supplemented by the anomeric effect (Kirby, 1983), which favours the gauche stereochemistry ( = 60°) about both central C-O bonds, mainly because this allows optimal n-rr overlap between an oxygen lone pair and the antibonding (cr ) orbital of the C-0 bond. Thus the pathway for conformational isomerization suggested by... 

Numerical calculations are presented for the PE chains and for the POM chains. The former displays a preference for Trans conformations among its skeletal bonds the latter manifests a strong preference for the gauche conformation. 

The preference for trans conformations in hydrocarbon polymer chains may be affected by the polymer constitution. Gauche conformations become more energetically attractive when atoms with lone electron pairs (like O) are present in the polymer backbone, and polyformaldchyde (or polyoxymethylene), 1-12, crystallizes in the a -gauclte form. 

The above MNDO results disagree however with the earlier MM results which consistently indicate a preference for gauche over trans for the [ SiH2-] model compounds. Values of conformational energies obtained from various theoretical studies are compared in Table Sf which also includes some ab initio results calculated by the author at the 3-21G and 3-21G levels. 

Deviations from the aU-anti conformations occur even when only one C-X bond is present in a 1,2-disubstituted ethane moiety. For example, solutions of dopamine provide mixtures of trans and gauche conformers in the protonated forms whereas the isolated molecules prefer the gauche conformation. The conformational equilibrium in such species often depends on a number of factors but understanding their interplay is important because the shape of neurotransmitters influences their transport properties and plays a key role in the molecular recognition at the receptor site. 

The preference for eclipsed conformations around the sp -sp bond of alkenes is general, but the reasons for the preference are not as yet understood. The rotational barriers are relatively low, that for propene being about 2 kcal/mol. Simple substituent effects appear straightforward. Methyl substitution at C(2), as in 2-methyl-1-butene, introduces a methyl-methyl gauche interaction in the conformation analogous to B, with the result that in 2-methyl-l-butene, the two eclipsed conformations are of approximately equal energy. ... 

The preference for the gauche arrangement is an example of the anomeric effect. An oxygen lone pair is anti to fluorine in the stable conformation but not in the unstable conformation. 

Display electrostatic potential maps for both anti and gauche conformers of 1,2-ethanediol. Do you see any examples of destabilizing interactions (between like charges) or stabilizing interactions (between unlike charges) in either conformer Are you able to explain the observed conformational preference ... 

Similar conclusions were reached for sulfoxides 157. Conformation 158 was preferred for (RS/SR)-157 but with some contribution from conformer 159. The (RR/SS) dias-tereomers preferred the reverse conformer 161 was preferred to 160161. An attractive force between Ph/Ar and Ph/R was thought to be the primary factor in determining the conformational preference of sulfoxides 152 and 157. MM2 calculations were carried out on a series of molecules of general structure PhCHR—X—R with X equal to CHOH, C=0, S and S=0151. The main conformers of these molecules have the Ph (or aryl) and R (alkyl) groups gauche. The calculations supported the existence of CH-tr attractive interactions with minor contributions from other effects. 

---