Conformational interconversion effect

Medium-sized and large ring systems often show complicated conformational interconversions involving pseudorotations in one or even more conformational families. This makes stereochemical assignments in diastereomers rather difficult. Thus, very few systematic studies have been published. The situation is improved if such rings are embedded in polycyclic systems, or if they contain double bonds, which leads to restricted conformational mobility. An example is the differentiation of diastereomeric 2,3-dihydro-lf/-benzo[6]azepines 1 on the basis of y-gauche effects and on d(13C) and 3/H H values640. 

The effect of temperature upon the conformational equilibrium was examined by using / -D-xylopyranose tetraacetate (Table IX), which at room temperature has about 72% of the all-equatorial conformer in equilibrium with about 28% of the all-axial form. As the temperature is lowered, the conformation tends more exclusively towards the Cl (d) form, so that at temperatures where conformational freeze-out would be expected the single Cl (d) conformation becomes so preponderant that any contribution from the minor conformer would be lost in the background noise of the spectrum (36). This result indicates that the entropy change in the conformational interconversion is not zero and explains why it is difficult to find many examples where a conformational freeze-out can be shown directly with such derivatives (25). 

The gas-phase thermodynamic (van t Hoff) parameters AH°, A5°, and AG029g for the syn-anti conformer interconversion of these gaseous alkyl nitrites are presented in Table 2. For MeONO this data compared well with those obtained by direct integration of slow exchange spectra. [27,22] The agreement of these gas-phase NMR thermodynamic parameters with microwave and theoretical data reinforce the validity of this technique applied to the syn-anti equilibria. Comparison of these gas-phase data with solution values yields information on the effects solvents have on the conformer equilibria, which in the particular case of these alkyl nitrites reflect a balance of steric and dielectric effects. 

Brunschwig, B. S., and Sutin, N., 1989, Directional electron transfer Conformational interconversions and their effects on observed electron-transfer rate constants. J. Am. Chem. Soc. 110 745497465. 

NMR spectroscopy has been widely used in particular to study the conformations and conformational interconversions of reduced pyridazines. The conformations of tetra- and hexahydropyridazines have been extensively studied by several groups in order to examine the influence of steric and electronic effects upon the geometry of the hydrazine part of the molecule. The temperature dependence of their NMR spectra was used to examine ring inversion and nitrogen inversion processes. 

The physical effects of introducing constraints into a molecular model have been discussed by several authors. > This chapter is concerned mainly with the methods of constraint dynamics. In addition to descriptions of bond-stretch and angle-bend constraints, dihedral (or torsional) constraints are explicitly considered. Torsional modes generally have frequencies comparable to those of other modes, and the weak coupling condition is not satisfied in this case. Hence the constraint approximation is not justified for torsion. This fact is particularly important because torsional motions play a major role in conformation interconversion in small molecules as well as polymers, and constraining them can seriously alter the dynamics of the original, unconstrained system. 

---