Gauche conformation calculations

The observed nonequivalence (1.0-l.lppm) is in close agreement with the value expected from the y-gauche conformational calculation. The temperature dependence of the observed magnetic nonequivalence is also successfully reproduced by the y-gauche effect calculations, leaving little doubt that its origin is the conformationally sensitive y-gauche effect. 

With all-atom simulations the locations of the hydrogen atoms are known and so the order parameters can be calculated directly. Another structural property of interest is the ratio of trans conformations to gauche conformations for the CH2—CH2 bonds in the hydrocarbon tail. The trans gauche ratio can be estimated using a variety of experimental techniques such as Raman, infrared and NMR spectroscopy. 

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. 

This type of calculation was carried one stage further by Cunningham, Manuel and Ward n). On the assumption that the contributions to the overall second moment from structural units with trans and gauche conformations can be regarded as additive, we have... 

Ab initio calculations show that the relative energies of the cis, trans and gauche conformers of HOOH are as follows313 ... 

Clearly, the preferred order of relative stability is gauche > anti > syn, since the n-o interaction is much stronger than the a—a interaction. Furthermore, anti 0nh-°nh is more favorable than syn aNH—aNH interaction. Ab initio calculations show that the preferred conformation of hydrazine is the gauche conformer with a dihedral angle of IOO0314. ... 

Ethylamine. All force fields (save Tripos 5.2) predict the wrong (gauche) global minimum for this molecule. However, both the experimental and calculated energy differences between the anti and gauche conformers are small. 

Since the structure of tetra-tert-butyltetrahedrane (19) has not been experimentally determined (128), one needs some courage to make predictions about it. Nevertheless, it seems likely that C—C, bond will be twisted from a gauche value (0(CCC,jMe) = 60°, point group) because of severe crowding among the tert-butyl groups at the tetrahedrane nucleus. MM calculations predict a structure with a 0 value near 45° T symmetry) to be 4 kcal/mol more stable than the gauche conformation (130). 

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