Conformation of the Cycloalkanes C14H28, C16H32, and C22H44 in the Liquid and High-Temperature Crystalline Phases by Vibrational Spectroscopy

Article abstract of DOI:10.1021/ja00188a001

This paper reports a conformational analysis, by means of vibrational red spectroscopy, of the conformationally disorded phases < that is, the liquid and high-temperature crystalline (mesomorphic) phases> of c-C14H28, c-C16H32, and c-C22H44.From the observed spectra, the concentration of the lowest energy conformer in the liquid state has been estimated along with the concentration ratios of certain short sequences of trans and gauche bonds.The experimentally derived values of the concentrations are compared with calculated values derived from various sets of conformers whose calculated strain energies have been previously reported in the literature.Good agreement was found for c-C14H28.However, for c-C16H32, the calculated concentration of the lowest energy conformer is significantly higher than is found experimentally and, in addition, there are some serious differences between the observed and calculated values of the concentrations of the conformational sequences.The most important factor contributing to the discrepanciens between the measured and calculated conformational statistics is probably the force field used in the strain energy calculations.For c-C22H44, a relatively flexible ring, the experimentally measured concentrations are in reasonable agreement with values derived from a set of diamond-lattice conformers.The distribution of conformers in the high-temperature crystalline phase of all three cycloalkanes is similar to, but somewhat narrower than, that found for the liquid.In the case of c-C14H28, for which a measurement is possible, the concentration of the lowest energy (rectangular) conformer in the high-temperature crystalline phase is estimated to be about 70 +/- percent, a value somewhat higher than the 60 +/- 10percent estimated for the liquid.For c-C22H44, the concentration of gauche bonds is used to characterize the liquid and the high-temperature crystalline phases.In terms of the average number of gauche bonds per ring, the high-temperature phase of c-C22H44 is found to lie closer to the liquid than to the low-temperature phase.