Conformer ratio

Molecular mechanics calculations have become a well established tool in the area of coordination chemistry, including the coordination chemistry of nickel375-379 where they are often applied for the analysis or the prediction of structures,380 the computation of isomer or conformer ratios and metal ion selectivities,381,382 and for simulating spectroscopic properties in combination with AOM calculations or by hybrid quantum mechanics/molecular mechanics (QMMM) methods.383,384 Details of the various approaches, e.g., the incorporation of d-electron stabilization energy... 

Second, the diastereomers ratio in C,C-coupling products of cation (349) with silyl ketene acetal exactly corresponds to the conformers ratio for this cation (349a 349a ), that is, it is evident that the interaction occurs through the distal approach of silyl ketene acetal to the C-6 atom of observed conformations a and a of the above mentioned cation. 

Both the size of the preceding amino acid in the dipeptide X-Pro, and that of groups in the COOR residue are able to influence (730MR547 79MI6) their conformer ratio. The conformational characteristic found for the dipeptide Gly-Pro, with the glycine amino acid bonded to a bulky residue, are close to those of poly (Gly-Pro), suggesting (77MI7) that the conformational behavior of one polymer is related to local properties of dipeptide units. 

The conformer ratio in the 2-alkyl-/V-acylpiperidines depends on the constitution of the acyl group, yet the energy difference between conformers may be small (63JA3728) when R = R = H, = Me in 115, excess of the -form has been reported by different authors (63JA3728 68CB3365), while the N-acetyl derivative is almost completely in the Z-form. For the N-benzoyl derivative 115 (R = Ph, R = Me, R = H), equal amounts of the two con-formers have been reported (68JOC3627). 

Isomer ratios for oligomers Conformer ratios in oligomers... 

A significant factor leading to the somewhat complex photochemistry of 1,3-butadiene 3, is its s-trans s-cis conformer ratio, substantially disfavoring the s-cis conformation that is needed for [4+2] and [4+4] cycloaddition reactions. The position of this equilibrium is dependent, of course, on the diene substitution. 

Stereoisomers in general, and conformational isomers in particular, are characterized not only by the energy barrier separating them, but also by their free energy difference AG°, which is related to the conformational equilibrium constant K (or conformational ratio) by the equation ... 

The epimeric tetrol had a molecular rotation of —12°. The configuration l(125/3) should exist as a mixture of the aaee conformations (147 and 148) in about equal proportions. The rotation predicted is +45° for (148), and —90° for (147). The apparent conformational ratio for (148)/(147) is about 3 2. In any event, the more levorotatory epimer must have the (12/35) configuration (146). The observed conformational ratio of 3 2 for (148)7(147) probably results from competition between 1,3-diaxial repulsion and 1,3-diaxial hydrogen-bonding. ... 

Molecular Mechanics calculations can be used (1) in the analysis of disordered structures, (2) in the prediction of known structures, (3) in the prediction of isomer ratios, and (4) in the prediction of conformer ratios. The majority of studies have dealt with Co hexaamine systems,but now force fields are available for amine complexes of Co , Ni , Cu , Zn , Cri , Fe i, and Rh F... 

So ihe conformational ratio of 2-chloroethanol should be more like that of 1,2-ethanediol than 1,2-dichIoroethane, in which hydrogen bonding is absent. ... 

In comparison to the fully saturated 1,3-dioxepane system, the number of possible conformations for 4,7-dihydro-1,3-dioxepins is reduced because of the sp hybridization at carbons 5 and 6 <76BSF307). There are two chair, two twist and two boat conformations. NMR spectroscopic data indicate a rapid equilibrium between the chair and the twist conformations. It has also been shown that the conformational ratio strongly depends on the substitution pattern in positions 2, 4, and 7 <65MI 911-01,75JOC450,86ZOB144,95HCA2I48). The most Stable Conformation of benzodioxepins was determined by low-temperature NMR measurements <81CJC2283). Evidence for a twist-chair con-... 

The potential of micro-Fourier transform Raman spectroscopy for examining specific localised regions in polymeric materials with some degree of fluorescence when analysed by conventional Raman spectroscopy was examined. Analysis of characteristic bands of the vibrational spectra obtained in a small area damaged by a visible and near-IR laser beam in commercial PETP showed a different conformer ratio than that observed in a non-irradiated zone. 32 refs. 

Table 3.2 Conformer ratio in RO-substituted cis- and fran -thiane-l-oxide (12) ...

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