Solid Benzene Research Articles

Influences on Infrared Spectra of Benzene Ices for Titan, Comets, and Beyond: Annealings, Artifacts, and Isosbestic Points

Infrared (IR) spectra are needed for both laboratory and observational studies of extraterrestrial ices, both within the solar system, such as for trans-Neptunian objects, comets, and Titan, and in the interstellar medium. Here we present new transmission-IR measurements on solid benzene (C6H6), the simplest common aromatic compound. Spectra are shown with a greater combination of higher resolutions and temperatures than found in the literature. The influences of both warmings and annealings on amorphous benzene are described, and comparisons are made of the IR spectra of warmed and cooled crystalline benzene. Reversible variations with temperature are noted for several IR bands of crystalline benzene, with a suggestion for their use as a possible thermometer for remote sensing of extraterrestrial environments. Comparisons are made to both recent and older literature results, and an artifact produced by high vapor-phase condensation rates is identified. New density measurements are reported for crystalline benzene and used, along with IR-forbidden transitions, to identify amorphous and crystalline ices. It is suggested that differences in spectra of crystalline benzene ices at the same temperature do not necessarily imply partial crystallization.

Temperature Dependence of the Entropy and the Heat Capacity Calculated from the Raman Frequency Shifts for Solid Benzene, Naphthalene and Anthracene

Temperature dependences of the free energy (F), entropy (S) and the heat capacity (C_v) are calculated (P=0) for the organic compounds (solid benzene, naphthalene and anthracene) by using the quasiharmonic approximation. Contributions to those thermodynamic functions due to the Raman frequencies of lattice modes (solid benzene), librational modes (naphthalene), phonons and vibrons (anthracene) are taken into account in our calculations. We obtain that similar linear increase of F and nonlinear increase of S and C_v, occur with the increasing temperature in benzene and naphthalene. This linear (F) and nonlinear (S, C_v) increase is rather different for anthracene as the molecular structure becomes complex (benzene-naphthalene-anthracene), as expected. Our calculations by the quasiharmonic approximation can be compared with the experiments for those organic compounds.

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.

Infrared spectra and optical constants of astronomical ices: IV. Benzene and pyridine

Infrared (IR) spectra of two solid aromatic compounds, benzene (C6H6) and pyridine (C5H5N), have been recorded in their amorphous and crystalline states. Measurements of density and refractive index (λ = 670 nm) are reported for each form of each compound, quantities needed to compute IR intensities and optical constants for use in laboratory experiments and astronomical observations. These are the first such measurements of each compound's density, refractive index, and spectra at temperatures relevant to the outer solar system and interstellar medium, with all measurements being made in a single laboratory. We have used these results to determine both IR band strengths and optical constants for benzene and pyridine ices in amorphous and crystalline forms. Also, the intensity of benzene's IR absorbance near 1477 cm−1 is measured in samples containing H2O-ice and compared to the strength of the same band in anhydrous amorphous benzene, the first comparison of this type for this compound. Suggestions are made for applications and future work related to the chemistry of icy bodies in the Solar System and the interstellar medium.

Equation of State for Solid Benzene Valid for Temperatures up to 470 K and Pressures up to 1800 MPa

The thermodynamic property data for solid phase I of benzene are reviewed and utilized to develop a new fundamental equation of state (EOS) based on Helmholtz energy, following the methodology used for solid phase I of CO2 by Trusler [J. Phys. Chem. Ref. Data 40, 043105 (2011)]. With temperature and molar volume as independent variables, the EOS is able to calculate all thermodynamic properties of solid benzene at temperatures up to 470 K and at pressures up to 1800 MPa. The model is constructed using the quasi-harmonic approximation, incorporating a Debye oscillator distribution for the vibrons, four discrete modes for the librons, and a further 30 distinct modes for the internal vibrations of the benzene molecule. An anharmonic term is used to account for inevitable deviations from the quasi-harmonic model, which are particularly important near the triple point. The new EOS is able to describe the available experimental data to a level comparable with the likely experimental uncertainties. The estimated relative standard uncertainties of the EOS are 0.2% and 1.5% for molar volume on the sublimation curve and in the compressed solid region, respectively; 8%–1% for isobaric heat capacity on the sublimation curve between 4 K and 278 K; 4% for thermal expansivity; 1% for isentropic bulk modulus; 1% for enthalpy of sublimation and melting; and 3% and 4% for the computed sublimation and melting pressures, respectively. The EOS behaves in a physically reasonable manner at temperatures approaching absolute zero and also at very high pressures.

Gibbs Free Energy Equation of State for Phase I of Solid Benzene from 15 to 488 K and up to 1165 MPa

Gibbs Free Energy Equation of State for Phase I of Solid Benzene from 15 to 488 K and up to 1165 MPa

A microwave sensor for detecting impurity freeze out in liquefied natural gas production

Direct measurements of impurity freeze-out conditions in real time are needed to improve the reliability of liquefied natural gas (LNG) production. Benzene and carbon dioxide are two impurities in the feed mixtures processed by LNG plants that can freeze, deposit and block cryogenic heat exchangers, even at trace concentrations. Existing technologies for monitoring the risk of impurity freeze out, such as compositional analysis, are inherently indirect and of limited resolution and accuracy. Here, we present a window-free, microwave sensor capable of measuring the freeze-out conditions of low-concentration impurities in high-pressure natural gas mixtures at cryogenic temperatures. The sensor is designed so that solids freezing out from the high-pressure LNG mixture are located in a region of high electric field strength, causing resonance frequency shifts of order 0.03 MHz per nanolitre of solid benzene. Finite-element models of the sensor performance were validated by measuring the freeze-out of benzene and carbon dioxide solutes at (50 to 100) parts-per-million in liquid methane at pressures from (7 to 9.5) MPa and temperatures from (90 to 150) K. The operational use of such a sensor has significant potential for improving the reliability of LNG production and identifying blockage remediation options that avoid complete plant shut downs.

Phase transitions in benzene under dynamic and static compression

AbstractPhase transitions in liquid benzene under laser shocked conditions are studied using high‐resolution time‐resolved Raman spectroscopic technique. The C‐C ring breathing mode (υ1) of benzene at 993 cm−1 is analyzed to monitor the phase transitions occurring in the Raman spectra during shock wave propagation in the pressure range 1.3–5 GPa. Phase transition from liquid benzene to solid benzene (Phase I) is observed below 1.3 GPa, and above this pressure, another phase transition from Phases I and II occurs. We also performed Raman spectroscopic measurements under static compression employing diamond anvil cell (DAC) to compare the effect of shock wave on benzene with isothermal compression using DAC. In static pressure measurements, the first phase transition from liquid to solid benzene (orthorhombic) occurs at 0.3 GPa, and the second phase transition from benzene I (orthorhombic) to benzene Phase II (monoclinic) occurs at 2.3 GPa, which is higher than that observed in case of shock compression experiments. The observed high‐resolution spectrum enables us to determine the effect of pressure gradient upon the Raman spectrum of benzene in shocked condition. One‐dimensional radiation‐hydrodynamic simulations were performed to corroborate the Raman spectroscopic results under dynamic compression. Simulated spatial profiles of the shock wave propagation in benzene at different delay times are used to explain the observation of multiple Raman modes in the spectral region 990–1020 cm−1. It is understood that different regions of the sample experience distinct pressure due to shock pressure gradient across the sample. The new Raman modes appearing in this spectral region are attributed to the Raman signals due to phase transition and from differently shocked regions of benzene under dynamic compression. The effect of reflected shock wave from benzene‐glass interface manifested in terms of red shifting and intensity enhancement of Raman modes at higher pressure is explained in this study.

Measurements of solidification kinetics for benzene in methane at high pressures and cryogenic temperatures

A stirred, high-pressure, visual microscopy-based apparatus was used to investigate solid-formation kinetics in LNG-relevant binary mixtures. The apparatus can detect solid crystals as small as 20 µm in a 3.5 mm field of view and is capable of measurements at temperatures as low as 90 K and pressures up to 20 MPa. The apparatus is described in detail and is presented alongside experimental results for two benzene + methane mixtures. Measurements of the solid–fluid equilibrium temperatures for the benzene + methane mixtures at 10 MPa were determined by raising the temperature of the mixture in (0.5 to 1)K steps and observing whether the crystals were still present after 2 h. The equilibrium temperatures were consistent with those predicted using the software package ThermoFAST within the combined experimental and model uncertainties. Formation measurements were carried out using a 100 ppm benzene-in-methane sample. Isobaric constant-cooling experiments at pressures of 8 and 10 MPa were used to construct subcooling formation-probability distributions for this mixture by identifying the temperature at which solid crystals were first observed to form on a copper substrate. The subcooling values at formation ranged from (4.4 to 11.0) K and were used to generate a cumulative probability of formation. The measured formation-probability distribution was fit to a model based on Classical Nucleation Theory to yield an estimate of 5 mJ/m2 for the effective surface free energy of solid benzene in liquid methane on the copper substrate. These results pave the way to probabilistic estimates of risk for solids formation in cryogenic heat exchangers.

Calculation of the heat capacity Cp from the temperature-induced and pressure-induced Raman frequency shifts for solid benzene, naphthalene and anthracene

The heat capacity Cp is calculated from the temperature-induced and pressure-induced Raman frequency shifts by means of the mode Grüneisen parameter for the I-II transition of solid benzene, naphthalene and anthracene. For this calculation, contributions to Cp are considered due to the six lattice modes of solid benzene, six librational modes of naphthalene and, six phonons and nine vibrons of anthracene with their mode Grüneisen parameters at various temperature (P=0) and pressures (T=300 K). Our calculated Cp explains the observed behavior of the I–II transition in solid benzene and also of naphthalene and anthracene. Temperature dependence of the entropy difference is calculated from the heat capacity for those hydrocarbons, which can be compared with the observed data.Our results indicate that the heat capacity Cp can be calculated as functions of temperature and pressure from the spectroscopic parameters, in particular, the Raman frequency shifts as we have studied here for hydrocarbons.

Constraining Carbon Nanothread Structures by Experimental and Calculated Nuclear Magnetic Resonance Spectra.

A one-dimensional (1D) sp3 carbon nanomaterial with high lateral packing order, known as carbon nanothreads, has recently been synthesized by slowly compressing and decompressing crystalline solid benzene at high pressure. The atomic structure of an individual nanothread has not yet been determined experimentally. We have calculated the 13C nuclear magnetic resonance (NMR) chemical shifts, chemical shielding tensors, and anisotropies of several axially ordered and disordered partially saturated and fully saturated nanothreads within density functional theory and systematically compared the results with experimental solid-state NMR data to assist in identifying the structures of the synthesized nanothreads. In the fully saturated threads, every carbon atom in each progenitor benzene molecule has bonded to a neighboring molecule (i.e., 6 bonds per molecule, a so-called "degree-6" nanothread), while the partially saturated threads examined retain a single double bond per benzene ring ("degree-4"). The most-parsimonious theoretical fit to the experimental 1D solid-state NMR spectrum, constrained by the measured chemical shift anisotropies and key features of two-dimensional NMR spectra, suggests a certain combination of degree-4 and degree-6 nanothreads as plausible components of this 1D sp3 carbon nanomaterial, with intriguing hints of a [4 + 2] cycloaddition pathway toward nanothread formation from benzene columns in the progenitor molecular crystal, based on the presence of nanothreads IV-7, IV-8, and square polymer in the minimal fit.

Benzene ice clouds in Titan's atmosphere

Planetary Science Titan, the largest moon of Saturn, has a thick atmosphere composed mostly of nitrogen, with smaller amounts of organic molecules such as methane and benzene. Vinatier et al. have analyzed infrared spectra of Titan obtained by the Cassini spacecraft during flybys of the moon. In addition to the expected gas-phase species, colder regions show a spectral signature consistent with solid benzene. After eliminating alternative explanations, the authors conclude that stratospheric clouds of benzene ice are present, particularly over Titan's south pole. The clouds are analogous to high-altitude cirrus clouds on Earth, which are composed of solid water-ice crystals. Icarus 310 , 89 (2018).

Superconductivity in solid benzene molecular crystal

Light-element compounds hold great promise of high critical temperature superconductivity judging from the theoretical perspective. A hydrogen-rich material, benzene, is such a kind of candidate but also an organic compound. A series of first-principles calculations are performed on the electronic structures, dynamics properties, and electron–phonon interactions of solid benzene at high pressures. Benzene is found to be dynamically stable in the pressure range of 180–200 GPa and to exhibit superconductivity with a maximum transition temperature of 20 K at 195 GPa. The phonon modes of carbon atoms are identified to mainly contribute to the electron–phonon interactions driving this superconductivity. The predicted superconductivity in this simplest pristine hydrocarbon shows a common feature in aromatic hydrocarbons and also makes it a bridge to organic and hydrogen-rich superconductors.

Using reweighting and free energy surface interpolation to predict solid-solid phase diagrams.

Many physical properties of small organic molecules are dependent on the current crystal packing, or polymorph, of the material, including bioavailability of pharmaceuticals, optical properties of dyes, and charge transport properties of semiconductors. Predicting the most stable crystalline form at a given temperature and pressure requires determining the crystalline form with the lowest relative Gibbs free energy. Effective computational prediction of the most stable polymorph could save significant time and effort in the design of novel molecular crystalline solids or predict their behavior under new conditions. In this study, we introduce a new approach using multistate reweighting to address the problem of determining solid-solid phase diagrams and apply this approach to the phase diagram of solid benzene. For this approach, we perform sampling at a selection of temperature and pressure states in the region of interest. We use multistate reweighting methods to determine the reduced free energy differences between T and P states within a given polymorph and validate this phase diagram using several measures. The relative stability of the polymorphs at the sampled states can be successively interpolated from these points to create the phase diagram by combining these reduced free energy differences with a reference Gibbs free energy difference between polymorphs. The method also allows for straightforward estimation of uncertainties in the phase boundary. We also find that when properly implemented, multistate reweighting for phase diagram determination scales better with the size of the system than previously estimated.

Carbon Nitride Nanothread Crystals Derived from Pyridine.

Carbon nanothreads are a new one-dimensional sp3 carbon nanomaterial. They assemble into hexagonal crystals in a room temperature, nontopochemical solid-state reaction induced by slow compression of benzene to 23 GPa. Here we show that pyridine also reacts under compression to form a well-ordered sp3 product: C5NH5 carbon nitride nanothreads. Solid pyridine has a different crystal structure from solid benzene, so the nontopochemical formation of low-dimensional crystalline solids by slow compression of small aromatics may be a general phenomenon that enables chemical design of properties. The nitrogen in the carbon nitride nanothreads may improve processability, alters photoluminescence, and is predicted to reduce the bandgap.

High-Pressure–High-Temperature Study of Benzene: Refined Crystal Structure and New Phase Diagram up to 8 GPa and 923 K

The high-temperature structural properties of solid benzene were studied at 1.5–8.2 GPa up to melting or decomposition using multianvil apparatus and in situ neutron and X-ray diffraction. The crystal structure of deuterated benzene phase II (P21/c unit cell) was refined at 3.6–8.2 GPa and 473–873 K. Our data show a minor temperature effect on the change in the unit cell parameters of deuterated benzene at 7.8–8.2 GPa. At 3.6–4.0 GPa, we observed the deviation of deuterium atoms from the benzene ring plane and minor zigzag deformation of the benzene ring, enhancing with the temperature increase caused by the displacement of benzene molecules and decrease of van der Waals bond length between the π-conjuncted carbon skeleton and the deuterium atom of adjacent molecule. Deformation of benzene molecule at 723–773 K and 3.9–4.0 GPa could be related to the benzene oligomerization at the same conditions. In the pressure range of 1.5–8.2 GPa, benzene decomposition was defined between 773–923 K. Melting was identi...

Probing the interaction between solid benzene and water using vacuum ultraviolet and infrared spectroscopy

We present results of a combined vacuum ultraviolet (VUV) and infrared (IR) photoabsorption study of amorphous benzene : water mixtures and layers to investigate the benzene-water interaction in the solid phase. VUV spectra of 1 : 1, 1 : 10 and 1 : 100 benzene : water mixtures at 24 K reveal a concentration dependent shift in the energies of the 1B2u, 1B1u and 1E1u electronic states of benzene. All the electronic bands blueshift from pure amorphous benzene towards gas phase energies with increasing water concentration. IR results reveal a strong dOH-π benzene-water interaction via the dangling OH stretch of water with the delocalised π system of the benzene molecule. Although this interaction influences the electronic states of benzene with the benzene-water interaction causing a redshift in the electronic states from that of the free benzene molecule, the benzene-benzene interaction has a more significant effect on the electronic states of benzene. VUV spectra of benzene and water layers show evidence of non-wetting between benzene and water, characterised by Rayleigh scattering tails at wavelengths greater than 220 nm. Our results also show evidence of benzene-water interaction at the benzene-water interface affecting both the benzene and the water electronic states. Annealing the mixtures and layers of benzene and water show that benzene remains trapped in/under water ice until water desorption near 160 K. These first systematic studies of binary amorphous mixtures in the VUV, supported with complementary IR studies, provide a deeper insight into the influence of intermolecular interactions on intramolecular electronic states with significant implications for our understanding of photochemical processes in more realistic astrochemical environments.

Calculation of the CP-CV as a function of temperature close to the melting point in benzene

ABSTRACTThe temperature dependence of the is calculated at constant pressures using the observed volume data from the literature for solid and liquid benzene near the melting point. Our calculated values are compared with some earlier treatments and we find that they disagree at constant pressures for the solid benzene, whereas for the liquid benzene agreement is reasonable in terms of the order of magnitude between our calculated and those predicted from the Peng–Robinson equation of state. Our calculated also agrees with the experimental data at normal pressure and has lower values at higher pressures in liquid benzene. Our treatment given here indicates that the and the heat capacity can be calculated from the molar volume as a function of temperature at constant pressures using the thermodynamic relations for the solid and liquid benzene near the melting point.

Effects of pyrolysis conditions on Miscanthus and corncob chars: Characterization by IR, solid state NMR and BPCA analysis

Infrared and 13C solid state nuclear magnetic resonance spectroscopies and benzene polycarboxylic acids (BPCA) analysis were used to characterize the structural changes occurring during slow pyrolysis of corncob and Miscanthus at different temperatures from 235°C to 800°C. In the case of corncob, a char sample obtained from flash carbonization was also investigated. Spectroscopic techniques gave detailed information on the transformations of the different biomass components, whereas BPCA analysis allowed the amount of aromatic structures present in the different chars and the degree of aromatic condensation to be determined. The results showed that above 500°C both corncob and Miscanthus give polyaromatic solid residues with similar degree of aromatic condensation but with differences in the structure. On the other hand, at lower temperatures, char composition was observed to depend on the different cellulose/hemicellulose/lignin ratios in the feedstocks. Flash carbonization was found to mainly affect the degree of aromatic condensation.

Rewriting the Story of Excimer Formation in Liquid Benzene.

Formation of benzene excimer following UV excitation of the neat liquid is monitored with femtosecond spectroscopy. A prompt rise component in excimer transient absorption, which contradicts the classical scenario of gradual reorientation and pairing of the excited monomers, is observed. Three-pulse experiments in which the population of evolving excimers is depleted by a secondary dump pulse demonstrate that the excimer absorption band is polarized along the interfragment axis. The experiments furthermore prove that the subsequent 4-fold increase in excimer absorption over ∼50 ps is primarily due to an increase in the transition dipole of pairs which are formed early on, and not to excited monomers forming excimers in a delayed fashion due to unfavorable initial geometry. Results are analyzed in light of recent studies of local structure in the liquid benzene combined with advanced electronic structure calculations. The prompt absorption rise is ascribed to excited states delocalized over nearby benzene molecules, which are sufficiently close and nearly parallel in the pure liquid. Such low-symmetry structures, which differ considerably from the optimized structures of isolated benzene dimer and solid benzene, are sufficiently abundant in liquid benzene. Electronic structure calculations confirm the orientation of transition dipoles of the excimers along the interparticle axis and demonstrate how slow refinement of the intermolecular geometry leads to a significant increase in the excimer absorption strength.