Axial interaction

Equatorial-axial interactions in complexes as a manifestation of mutual influence of ligands. 

Hall et al.1 s estimated the conformational equilibrium for the structural units in the polymer of 2 using the numerical parameters determined for carbohydrates16. For a frans-l,3-tetrahydropyranoside, conformer 8 is calculated to be more stable than 7 by 9.2 kJmol-1 and would therefore occur almost exclusively (ca. 98%) at equilibrium. For a m-1,3-tetrahydropyranoside unit, the anomeric effect favors con-former 9, but its severe syn-axial interaction between alkoxy and alkyl groups would highly favor 10 (ca. 99%). 

Like the polymethylene hydrocarbons, acyclic carbohydrates and alditols tend to adopt the planar, zigzag conformation wherein all of the carbon atoms lie in the same plane. " However, non-bonded interactions between parallel hydroxyl groups in the 1,3 positions (sy/i-axial interaction) are the most highly destabilizing, and, in xylitol and ribitol, this... 

Other studies have also established the preference of the chair conformation with the oxygen in the axial position the rationale for this preference is different from the attractive interaction between the sulfoxide oxygen and the syn-axial hydrogens proposed previously . Rather, a repulsion effect is advocated the equatorial oxygen is squeezed between four vicinal hydrogens, while there are only two corresponding repulsions if it is in the axial position. The correlation between the predicted and observed conformational/orientational preferences in 3,3-dimethylthiane oxide (e.g., equatorial preference in the chair conformation) corroborates this interpretation. The axial preferences of the sulfur-oxygen bond in the thiane oxide is reversed in 3,3-dimethylthiane oxide because of the syn-axial interaction. 4,4-Dimethylthiane oxide, however, maintains a predominance of the axial isomers as deduced from the analysis of NMR data . ... 

An important addition compared to previous models was the parameterization of the internucleosomal interaction potential in the form of an anisotropic attractive potential of the Lennard-Jones form, the so-called Gay-Berne potential [90]. Here, the depth and location of the potential minimum can be set independently for radial and axial interactions, effectively allowing the use of an ellipsoid as a good first-order approximation of the shape of the nucleosome. The potential had to be calibrated from independent experimental data, which exists, e.g., from the studies of mononucleosome liquid crystals by the Livolant group [44,46] (see above). The position of the potential minima in axial and radial direction were obtained from the periodicity of the liquid crystal in these directions, and the depth of the potential minimum was estimated from a simulation of liquid crystals using the same potential. 

It is investigated whether the stereochemical 13C NMR chemical shifts in the resonance peaks can be ascribed to differences in the conformations in the various stereoisomers. The authors follow Boyd and Breitling (A 022) in thair statistical treatment of the PP chain, with the exception that here conformational sequences are not excluded of the type XG/G Y for two adjacent diads unless XG or G Y imply another c (syn-axiall interaction within either diad. Therefore this treatment, which is more rigorous but consistent with Boyd and Breitling s energy calculations, requires statistical weights which are functions of three adjacent torsional angles. 

The oxidation of Cr(MPDME) in solution by 02 or H202 can be reversed by sodium dithionite. Since /ie = 2.84 BM in the solid and 5.19 BM in CHC13, there are axial interactions in the solid state which are removed on dissolution. 

Thus the conformational free energy of the N-methyl group may be accepted as 2.7kcal mol-1 and the difference from that (1.7 kcal mol 1) in methylcyclohexane is primarily due to the changes in bond lengths, causing an increase in the syn-axial interactions in axial N-methylpiperidine. 

In the conformational equilibrium (Fig. 22) for 5-methyldihydro-l,3,5,-dioxazine (454) the generalized anomeric effect favors the N-Meax conformer 455 and, in addition (relative to jV-Meax piperidine), 455 does not contain the two unfavorable syn-axial interactions involving the axial N-methyl... 

The successive change in position of the N-Mem N-Meeq equilibrium from that in 5-methyldihydro-l,3,5-dioxazine (AG° > 1 kcal mol-1) to 3-methyltetrahydro-l,3-oxazine (AG° 0.10 + 0.05 at — 120°C) to that (AG° —2.7 kcal mol-1) in 1-methylpiperidine results from successive differences of syn-axial interactions involving the axial methyl group and a favoring of the axial methyl conformer by the generalized anomeric effect. 

Usually 2,2-disubstitnted 1,3-dioxanes (for instance, acetonides) are hydrolyzed more easily than corresponding 1,3-dioxolanes (essentially owing to the strong ryn-axial interaction operative in the six-membered ring [79,98]. 

The crystal structure of Ag(meso-Me6[14]ane)(N03)2 was found to contain a square planar arrangement of N atoms from the macrocycle at Ag—N distances of approximately 216 pm. Axial interactions with nitrate oxygens resulted in distorted octahedral coordination in which the Ag—O distance was 280.7 pm. 

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