Temperature conformational properties

The ESR spectra of a large variety of sulfonyl radicals have been obtained photolytically in liquid phase over a wide range of temperature. Some selected data are summarized in Table 2. The magnitudes of hyperfine splittings and the observations of line broadening resulting from restricted rotation about the C—S bond have been used successfully in conjunction with INDO SCF MO calculations to elucidate both structure and conformational properties. Thus the spin distribution in these species is typical of (T-radicals with a pyramidal center at sulfur and in accord with the solid-state ESR data. 

The conformational properties of mono-substituted cyclohexanes, C V, 1111X, in their thiourea inclusion compounds have been studied102. Variable-temperature MAS spectra demonstrate that a chair-chair ring inversion process occurs in the thiourea tunnel, in which the axial and equatorial conformers are interconverted. Predominance of the equatorial conformer is found when X = NH2. 

Rabinovich and Ripatti (1990) have shown that docosohexaenoic acid has conformational properties which keep its physico-chemical and, possibly, functional characteristics effective over a wide temperature range. This ensures the adaptation of cell membranes to changes of metabolic activity. Fluctuation in locomotory activity is one factor responsible for these changes. From their studies of the sea cucumber, Cucumaria frondatrix, Kostetsky et al. (1992) concluded that polyenoic acids of linolenic affinity did not exhibit a direct relatonship with temperature adaptation. In contrast to this, Zabelinsky et al. (1995) claim that C20 5o>3 (not C22 6a>3) and Cl8 1 are the fatty acids of key importance for temperature adaptation in marine fish. 

Molecular mechanics (MM) and dynamics (MD) are useful tools to investigate the conformational properties of organic molecules. (75) In particular, the combined use of MM and MD can be very effective in sampling the potential energy hypersurface (PES) when structurally constrained molecules are considered. In the present work, the PES has been described using the MM+ forcefield (16) and MM optimizations were followed by short MD runs (10 ps) carried out at different temperature (from 300 to 700 K) in order to sample the PES efficiently. Usually, due to the steric properties of the molecules investigated, no more than 10 MM/MD cycles were necessary to localize all the relevant energy minimum structures. 

In an effort to evaluate conformational properties, pyridinophane 3 was synthesized 7-9) and compared to cyclophanes 2 and 4. Perusal of the variable temperature NMR spectra of 2-4 demonstrates that 2 is the only cyclophane in this series which shows a temperature dependence the AG for 2 has been found to be >27 kcal/ mole. The H NMR spectra of 3 and 4 remain unchanged to 200 °C the bridging methylenes in 3 exhibit and ABCD pattern whereas in 4 and A2B2 pattern is observed. Comparison of these data with results reported by Vogtle10) indicates the relative order of steric interactions (hydrogen-hydrogen, hydrogen-A-lone pair, and jV-lone pair-JV-lone pair) to be 2 < 3 < 4 3i 7). 

Many properties of polyacetylenes with bulky substituents are substantially different from those of polyacetylene. For example, the substituted polyacetylenes are soluble because of the interaction between the substituents and solvent. Furthermore, such polymers are usually only lightly colored and are stable in air at room temperature. These properties arise from twisted conformations assumed by the main chain because of the presence of substituents. The electrically insulating and nonparamagnetic properties of substituted polyacetylenes are attributable also to the same cause. 

The increase of n(A] with decreasing A, at constant temperature, is the two-dimensional analogue of am osmotic pressure-concentration Isotherm. Such surface pressure isotherms are the prime source of information about the orientational and/or conformational properties of the molecules in the monolayer they reflect their dimensional properties as well as interactions between them. In this respect, x(A) isotherms have about the same function as adsorption Isotherms. This matter will be discussed in more detaill in secs. 3.4 and 5. 

Summary An investigation of the conformational properties of the title compounds is presented. Energy profiles for rotation around the central bonds were computed at HF level. Frequency calculations for the optimized geometries were carried out. Temperature-dependent Raman spectroscopy was utilized to revise the cmnputed results. While the results of the ab initio calculations predict up to four minima on the potential energy surfaces (PES), Raman spectroscopy gives unambiguous evidence for more than one rotamer for tetrakis(trimethylsilyl)disilane only. 

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