Calorimetry of phase transitions in liquid crystal 8CB under shear flow.

A differential scanning calorimeter equipped with a shearing system (shear rate of  < 400 s-1) was developed to elucidate the thermodynamic properties of liquid crystalline phase transitions under shear flow. An analytical method was proposed to accurately estimate the heat flow caused by shear friction to evaluate the transition entropies. The phase transitions of 4'-n-octyl-4-cyano-biphenyl (8CB) under shear flow were investigated using the developed calorimeter. Although several shear-induced transitions for 8CB have been reported in the past using viscosity and small-angle X-ray scattering (SAXS) measurements, only the nematic-isotropic (N-I) and smectic-A-nematic (SA-N) transitions were detected as heat flow peaks. The N-I transition temperature was almost independent of the shear rate. The SA-N transition temperature was also independent of the shear rate, but the transition peak was broadened by applying shear flow. For both transitions, the transition entropies were independent of the shear rate. These results suggest that the thermodynamic properties were not considerably changed by shearing because the molecular alignments in the domains were not substantially changed, whereas shearing changed the LC domain directions, which can be detected by viscosity and SAXS measurements.

Dynamics and magnetic properties of NO molecules encapsulated in open-cage fullerene derivatives evidenced by low temperature heat capacity.

Low-temperature heat capacity analyses for an NO-encapsulated fullerene derivative revealed (i) low-energy motion and (ii) strong magnetic anisotropy of the NO molecule due to its orbital angular momentum. The low-energy motion was attributed to reorientational motions of the NO molecules, in which only a small number (n ∼ 0.04) of NO molecules were found to participate. The NO molecules were confirmed to be paramagnetic even at 1 K. Ab-initio calculation indicated that the magnetic properties of the NO unit strongly depended on its surroundings, allowing the conformation of the fullerene cage to be estimated.

Solid-State Spin Equilibrium of Ni(cyclam)2 Complex: Magnetostructural Correlations in Two Polymorphs.

Two crystal polymorphs of Ni(cyclam)I2 (cyclam = 1,4,8,11-tetraazacyclotetradecane) were synthesized, and their magnetic properties were investigated. Temperature-dependent X-ray structural analysis and magnetic measurements revealed gradual spin transition in molecular-crystal polymorph trans-[Ni(cyclam)I2] (1a), whereas the zigzag-chain polymorph catena-[Ni(cyclam)(µ-I)]I (1b) did not show an obvious spin transition. The entropy difference between high- and low-spin states of 1a estimated by assuming the spin-equilibrium model is much smaller than those in typical iron(II)-based spin-crossover (SCO) complexes, suggesting that the normal mode softening is less remarkable in 1a. In this system, it is clearly evidenced that the interaction mode responsible to the spin equilibrium in octahedral nickel(II) complexes is highly anistropic, i.e., z-elongation and x,y-shortening of the coordination octahedron.

The thermodynamic properties and molecular dynamics of [Li+@C60](PF6-) associated with structural phase transitions.

Calorimetric and terahertz-far-infrared (THz-FIR) spectroscopic and infrared (IR) spectroscopic measurements were conducted for [Li+@C60](PF6-) at temperatures between 1.8 and 395 K. [Li+@C60](PF6-) underwent a structural phase transition at around 360 K accompanied by the orientational order-disorder transition of Li+@C60 and PF6-. The transition occurred in a step-wise manner. The total transition entropy (ΔtrsS) of 40.1 ± 0.4 J K-1 mol-1 was smaller than that of the orientational order-disorder transition in a pristine C60 crystal (ΔtrsS = 45.4 ± 0.5 J K-1 mol-1). Thus, the orientational disorder of Li+@C60 in the high-temperature phase of [Li+@C60](PF6-) was much less excited than that of the pristine C60 owing to the Coulombic interactions, which stabilized the ionic crystal lattice of [Li+@C60](PF6-). At T < 100 K, upon cooling, Li+ ions were trapped in two pockets on the inner surface of C60, and no phase transition was observed. Finally, the Li+ ions achieved a complete order at 24 K through antiferroelectric transition. The ΔtrsS value of 4.6 ± 0.4 J K-1 mol-1 was slightly smaller than R ln 2 = 5.76 J K-1 mol-1 expected for the two-site order-disorder transition. The extent of the Li+ motion in the C60 cage was related to the selection rule in the THz-FIR and IR spectroscopy of the C60 internal vibrations, because a C60 cage should be polarized by the Li+ ion. It is shown that the local symmetry of the caged molecule can be modified by the rotational or hopping motion of the encaged ions.

Rotational Motion and Nuclear Spin Interconversion of H2O Encapsulated in C60 Appearing in the Low-Temperature Heat Capacity.

The heat capacity of H2O encapsulated in fullerene C60 is determined for the first time at temperatures between 0.6 and 200 K. The water molecule in H2O@C60 undergoes quantum rotation at low temperature, and the ortho-H2O and para-H2O isomers are identified by labeling the rotational energy levels with the nuclear spin states. A rounded heat capacity maximum is observed at ∼2 K after rapid cooling due to splitting of the rotational J KaKc = 101 ground state of ortho-H2O. This anomalous feature decreases in magnitude over time, reflecting the conversion of ortho-H2O to para-H2O. Time-dependent heat capacity measurements at constant temperature reveal three nuclear spin conversion processes: a thermally activated transition with Ea ≈ 3.2 meV and two temperature-independent tunneling processes with time constants of τ1 ≈ 1.5 h and τ2 ≈ 11 h.

Isotopic localization of the partially deuterated methyl group in solid methanol and methyl iodide.

Heat capacity measurements were made down to 0.35 K for the isotopic modifications of methanol, CH3-nDnOH, and methyl iodide, CH3-nDnI, (n = 0, 1, 2, 3) to determine the orientation of the partially deuterated methyl group in the solid phase. The mono-deuterated modifications favor the symmetric conformation, whereas the di-deuterated ones favor the asymmetric conformation. Infrared spectroscopy demonstrates that some vibrational modes change in intensity depending on temperature, which supports the energy scheme obtained by calorimetry. Zero-point kinetic energies were obtained by single molecule density functional theory calculations. Although the favorable conformations of CH2DOH and CHD2OH were confirmed, the energy difference between symmetric and asymmetric conformations was twice as large as that determined experimentally, which indicates that intermolecular forces significantly decrease the energy difference. For CHD2OH, the conversion between the two asymmetric conformations becomes very slow at low temperature and results in a residual entropy of R ln 2.

Rotational dynamics of Li+ ions encapsulated in C60 cages at low temperatures.

Li+ ions encapsulated in fullerene C60 cages (Li+@C60) are expected to be suitable as molecular switches that respond to local electric fields. In this study, the rotational dynamics of Li+ ions in C60 cages at low temperatures are experimentally revealed for the first time using terahertz absorption spectroscopy. In crystalline [Li+@C60](PF6-), the Li+ ion rotates in the carbon cage even at 150 K. The rotational mode gradually changes into a librational mode below 120 K, which is associated with the localization of Li+ ions due to the electrostatic interactions with its screening image charge on the C60 cage as well as with the neighboring Li+@C60 and PF6- ions. A simple rotational/librational energy scheme for the Li+ ions successfully explains the spectroscopic results, and the potential of Li+@C60 as a molecular switch is discussed based on the energy scheme.

Polymer Morphological Change Induced by Terahertz Irradiation.

As terahertz (THz) frequencies correspond to those of the intermolecular vibrational modes in a polymer, intense THz wave irradiation affects the macromolecular polymorph, which determines the polymer properties and functions. THz photon energy is quite low compared to the covalent bond energy; therefore, conformational changes can be induced "softly," without damaging the chemical structures. Here, we irradiate a poly(3-hydroxybutylate) (PHB) / chloroform solution during solvent casting crystallization using a THz wave generated by a free electron laser (FEL). Morphological observation shows the formation of micrometer-sized crystals in response to the THz wave irradiation. Further, a 10-20% increase in crystallinity is observed through analysis of the infrared (IR) absorption spectra. The peak power density of the irradiating THz wave is 40 MW/cm(2), which is significantly lower than the typical laser intensities used for material manipulation. We demonstrate for the first time that the THz wave effectively induces the intermolecular rearrangement of polymer macromolecules.