Crystalline Cyclohexane Research Articles

Benzene Vapor Pressures at Titan Temperatures: First Microbalance Results

The vapor pressure of crystalline benzene (C6H6) has been measured for the first time with a quartz-crystal microbalance under ultrahigh vacuum conditions from about 135 to 160 K, temperatures relevant for Titan’s atmosphere. Good agreement is found with the most recent thermodynamic analysis of higher-temperature literature results, but there is less satisfactory agreement with recently published low-temperature data obtained with a different laboratory method. An enthalpy of sublimation of 49.7 ± 0.1 kJ mol−1 for benzene is reported, and a few brief comments are made concerning the connections between our new data and Titan’s atmosphere. Supporting our benzene results are new vapor-pressure measurements for crystalline H2O ice and crystalline cyclohexane (C6H12). The latter are the first of their type and enable the two sets of recently published vapor pressures for solid benzene to be brought into close agreement.

The synthesis and liquid crystal transition temperatures of some weakly polar nematic methyl (E)-[trans-4-substituted-cyclohexyl]-allyl ethers

Abstract We have introduced an oxygen atom and a carbon-carbon double bond with a trans-configuration (E) into the terminal alkyl chain of a wide variety of liquid crystalline cyclohexane derivatives to produce a variety of new methyl (E)-allyl ethers. The melting points and tendency to form smectic mesophases are often low, while nearly all of the compounds prepared exhibit a nematic phase. Thus, even two-ring derivatives can exhibit nematic phases over a wide temperature range (≤80°C), sometimes starting below room temperature (T m≈10°C). Comparisons with the corresponding derivatives incorporating either just an oxygen atom or just a carbon-carbon double bond in the same position indicate that synergetic effects lead to broader nematic phases than would otherwise have been expected. Thus many of the new methyl (E)-allyl ethers exhibit nematic phases over a wider temperature range than the corresponding materials with an unsubstituted alkyl chain attached to the cyclohexyl ring. The new compounds are easily prepared from known starting materials. Many intermediates are themselves liquid crystalline. This allows investigation of the relationship between liquid crystal transition temperatures and the nature of the terminally substituted alkyl chain (for example, incorporating C[dbnd]C, OH, CO2C2H5 and OCH3 groups).

High pressure phase studies of some liquid crystalline cyclohexane derivatives by differential thermal analysis

Abstract T(p) phase diagrams for the mesomorphic compounds trans-4-n-butylcyclohexane-1-carboxylic acid 4-cyanophenyl ester (D4N), trans4-n-pentylcyclohexane-1-carboxylic acid 4-n-pentylphenyl ester (D55), trans-4,4′-di-n-propyl-1,1-bicyclohexyl-cis-4-carbonitrile (33CCN), trans-4-methoxy-4′-propyl-1,1′-bicyclohexane (3O1CCH) and trans-4-methoxy-4′-n-butyl-1,1′-bicyclohexane (4O1CCH) are presented. The experiments were performed using high pressure microcomputer-assisted differential thermal analysis equipment in the temperature range 300 to 500 K up to a maximum pressure of 8 kbar. Some special high pressure effects for liquid crystals, such as pressure-induced or pressure-limited phases and changes from monotropic to enantiotropic polymorphism were observed. For the correlation of the experimental results, an extension of the Simon equation was used; one of the regression parameters of the clearing curve equation was found to be correlated with the molecular structure of the liquid crystal substances un...

Environmental effects on the absorption and luminescence properties of aromatic amines: Aniline

Abstract Emission and absorption spectra of aniline have been studied in different solvents at 300 K and 77 K. This molecule exhibits broad and structureless fluorescence and poorly resolved phosphorescence in glassy polar ethanol and non-polar methylcyclohexane and n -hexane matrices at 77 K. Unlike the behaviour in a polar medium, fluorescence is found to be more intense than phosphorescence in non-polar media. In contrast to the case of glassy matrices, aniline shows highly stuctured emission and excitation spectra in crystalline cyclohexane and benzene matrices at 77 K. Excited state dipole moment calculation shows that both the singlet states contain charge transfer character, which is responsible for the strong phosphorescence in polar glassy medium. Spectral analysis indicates that the aniline molecule is slightly distorted in both the singlet and triplet states. The marked reduction in phosphorescence intensity of this molecule observed in non-polar matrices, particularly in a benzene crystalline matrix, relative to that in an ethanol matrix, is attributable to the decreased intersystem crossing rate and increased non-radiative rate from the emitting triplet state.

Effects of surface and physical confinement on the phase transitions of cyclohexane in porous silica.

We have measured the specific heat (${\mathit{C}}_{\mathit{p}}$) of cyclohexane at 120T300 K when cyclohexane was physically restricted in porous Spherosil (silica) samples of pore radii 4, 7.5, 15, 30, and 62.5 nm. The behaviors of the monoclinic-to-cubic structural transition and of the melting transition of cyclohexane were determined. As expected, both transition temperatures, i.e., solid-solid and melting, inversely scaled with the pore radius (${\mathit{R}}_{\mathit{p}}$). It is argued that the surface heterogeneity, the presence of hydroxyl groups, and the radius of curvature (especially for smaller pores) induce considerable disorder in the adsorbed layers of cyclohexane, thus, resulting in the nucleation of crystalline grains of various sizes rather than in a single crystalline plug of cyclohexane in the porous silica samples. The distribution of the grain boundaries of crystalline cyclohexane produces a specific-heat peak for the melting transition, which almost resembles the \ensuremath{\lambda} shape for ${\mathit{R}}_{\mathit{p}}$\ensuremath{\le}30 nm. Unlike the monoclinic-to-cubic transition, the \ensuremath{\lambda} anomaly of the melting transition does not reflect a logarithmic dependence near the transition temperature. It is also argued, from the comparative observed ${\mathit{C}}_{\mathit{p}}$ behavior of the bulk cyclohexane with the physically restricted cyclohexane, that the cyclohexane liquid is more viscous than the bulk when it is confined in pores of 4 and 7.5 nm.

On the origin of some anomalous luminescence behavior of isomeric cyanopyridines

The results of a comparative study of the absorption and emission properties and the triplet yields of the isomeric cyanopyridines (CNP) and benzonitrile under various environmental conditions are reported. All the three CNP isomers are found to be nonfluorescent but exhibit a fairly strong phosphorescence emission of π, π character in rigid polar media at 77 K. ΦP values of the CNP are found to be sensitively dependent on the rigidity (or viscosity) and crystalline structure of the solvent medium. Whereas the 3-CNP molecule phosphoresces with moderately strong intensity in hydrocarbon solvents, the 2 and 4 isomers are completely nonluminescent in glassy or highly disordered crystalline phases of hydrocarbons, e.g., n-alkanes, cyclohexane (CH), and methylcyclohexane (MCH). A dramatic reappearance of the phosphorescence emission from these two isomers (with anomalously large red-shifted spectral positions) has, however, been observed in crystalline CH and MCH matrices. Unlike the 3 isomer, both the 2- and 4-CNP have been found to exhibit phosphorescence in pure crystalline form at 77 K. The n, π electronic states associated with the pyridinic nitrogen atom have been identified in the absorption (or excitation) spectra and shown to be responsible for the nonfluorescent nature and the high triplet yields (∼0.8) of CNP molecules. A tentative interpretation of all the diverse emissive characteristics of cyanopyridines has been offered in terms of the predominance of the effect of pseudo-Jahn–Teller distortion and the matrix potential in governing the radiative and radiationless deactivation of the lowest triplet state of these molecules. The distinctive differences observed in the emissive characteristics of the 3-CNP and other two isomers have been attributed to the differences in the distortability of the conformational mobility of the CNP isomers arising primarily as a result of inductive effects of the pyridinic nitrogen atom and the nitrile group.

Far-infrared spectrum and phase transitions of solid cyclohexane

Far-infrared of crystalline cyclohexane shows a discontinuity at the temperature of a solid-phase transition. Two lattice modes are observed in the low-temperature stable phase II. Annealing causes an increase in the intensity of the absorption due to an irreversible phase transition. Band frequencies, which differ from those of the Raman active modes, support a centrosymmetric structure.

Determination of the Symmetry of the First Excited Electronic State of Benzyl by Rotational Contour Analysis of Vibronic Bands of the Emission Spectra of C6H5CH2, C6H5CD2, and C6D5CD2

Theoretical calculations have differed in their predictions as to the symmetry of the first excited state of benzyl. Furthermore, conflicting interpretations have been given to previous experimental results. Rotational band contour analysis of high resolution gas phase emission spectra of C6H5CH2, C6D5CD2, and C6H5CD2 are here used to determine this electronic symmetry. A discussion is given of the treatment of new problems which arise in the band contour analysis of emission spectra and of free radicals as compared with previous contour work on absorption spectra of stable molecules, in particular, as regards the determination of molecular geometry, temperature effects and contour program modifications. Qualitative and quantitative criteria for matching experimental and calculated contours are made explicit. Nonmatching features are assigned to hot bands, using supporting assignments. The effect on calculated contours of the interpolation procedure used in Parkin's program is discussed in the Appendix. Good fit of calculated to experimental contours for type A and type B vibronic bands of the three isotopic benzyl radicals, including the vibronic origin band of the C6H5CH2 species, leads to the conclusion that the first doublet-doublet electronic transition in benzyl is 1 2A2–1 2B2. This agrees with an earlier experimental assignment, based on the observation of a2 vibrations in the fluorescence spectrum of benzyl in crystalline cyclohexane solution but is in disagreement with the 2 2B2–1 2B2 assignment of Johnson and Albrecht based on three-step photoselection studies in glassy solutions.

Excited Dimer (Excimer) Luminescence from Aromatic Molecules in Crystalline Cyclohexane

Excited Dimer (Excimer) Luminescence from Aromatic Molecules in Crystalline Cyclohexane

Normal vibrations of cyclohexane. Infrared spectrum of the crystal

Study of the infrared spectrum of crystalline cyclohexane has established values for the normal frequencies which were previously in doubt. In addition to the allowed a 2 u and e u transitions, the a 2 g and most e g modes appear in the crystal spectrum, and the a 1 u modes are observed in unannealed samples. All frequencies newly observed are close to the values calculated by Takahashi et al., using Urey-Bradley force constants transferred from other hydrocarbons. Spectral changes consistent with some disordering were observed at the (186°K) phase transition, where NMR and x-ray studies have already indicated the onset of hindered rotation. Below 186°K the spectra suggest a cubic crystal structure.

Resonance Absorption in Solid Cyclohexane

According to the resonance absorption theory recently proposed by L. N. Liebermann, large acoustic attenuations can occur in molecular crystals as the result of weak coupling among lattice and intramolecular oscillators near resonance. Theoretical and experimental investigations have been undertaken for the purpose of testing this theory. A refinement of the original theory is presented and employed to calculate the absorption coefficient for crystalline cyclohexane. The close agreement between calculated and experimentally observed attenuations in cyclohexane together with Liebermann's results for benzene is interpreted as strong evidence of the resonance absorption mechanism.

Environmental effects on the longest benzene absorption in a crystalline cyclohexane matrix

Environmental effects on the longest benzene absorption in a crystalline cyclohexane matrix