Molecular Motion of Solid Cycloalkanes

Abstract

Cycloalkanes with 12 to 96 methylene groups show solid-solid transitions of high cooperativity. Although the molecules are highly flexible and do not contain mesogenic units as it is typical for liquid crystals, they form a mesomorphic phase. In this mesomorphic state, fast molecular motion with correlation times smaller than s is observed by 2H-NMR. The molecular structure is characterized by dynamic conformational disorder. In addition slow conformational interconversion is evident below the phase transition. Thus, jump type interconversion processes which do not result in disordering occur in the fully ordered crystal. At the phase transition, the increase in the specific volume and the onset of additional molecular mobility results in the formation of a partially disordered, fluid structure.

Conformational Distinguishability of Medium Cycloalkanes in Crystals via Inelastic Neutron Scattering

“…The crystal undergoes a phase transition in the 173-153 K range, 53 the crystallographic details of which are not known. The solid-state 2 H NMR 25 and the 13 C NMR 24 spectra as a function of temperature indicate that the same conformation remains dominant in the melt, in a mesophase, and in the crystal at 150 K, with temperature-dependent conformational interconversion dynamics observed.…”

“…This is the conformation predicted to be lowest in energy in this (see Table 12) and previous studies. 55 Solid-state 2 H NMR 25 and the 13 C NMR 24 spectra have been obtained over a temperature range that includes the melt, a mesophase, and the crystal at 175 K. These studies conclude that the same conformation persists through these phase transitions with dynamics observed due to temperature-dependent conformational reorientation. This a For this set of spectra, j ) 77 with F(60,60) ) 1.5 required to determine the presence of one conformer over another with 95% confidence.…”

“…Vibrational spectroscopy is a useful technique in this area because it is able to report on the presence of each of the multiple conformations as distinct signals. [19][20][21][22] Solid-state NMR [23][24][25][26][27] has played an important role in these studies because of its ability to detect phase transitions, to distinguish between static and dynamic disorder, and to detect the presence of minor species.…”

“…The medium ring cycloalkanes have a long history as a testing ground for empirical treatments of conformational energetics and interconversions. − The cyclo C 7 −C 10 compounds have recently received extensive treatment by post-Hartree−Fock ab initio methods. , The major interest and challenge of these studies stem from the presence of multiple low-energy minima on the conformational energy surface. Vibrational spectroscopy is a useful technique in this area because it is able to report on the presence of each of the multiple conformations as distinct signals. − Solid-state NMR − has played an important role in these studies because of its ability to detect phase transitions, to distinguish between static and dynamic disorder, and to detect the presence of minor species.…”

Learning Polymer Crystallization with the Aid of Linear, Branched and Cyclic Model Compounds

“…Chemical shifts for carbons at the center of sequences gtgg , gttg , ggtt , tggt , gttt , and tttt were identified. The assignments were based on low temperature spectra, since at higher temperatures the cycloalkanes investigated transform into highly conformationally disordered crystals with a pseudohexagonal structure. , In terms of enthalpy and entropy, as well as number of nonplanar conformers, this “mesophase” is closer to the melt than to the ordered crystal. It has therefore been compared to the high-pressure hexagonal mesophase in polyethylene, rather than to the “rotator” phase in linear alkanes with n ≤ 40 …”