Solid Benzene Research Articles

Molecular dynamics study of reorientational motion in solid benzene

The nature of molecular reorientations in a molecular dynamics simulation of crystalline benzene is extensively studied. The model is shown to reproduce well the experimentally determined temperature and pressure dependence of the reorientation rate. The correlations of reorientational events are examined in more detail than is possible experimentally; reorientations are found to occur predominantly as isolated events, both spatially and temporally, contrary to some previous ideas.

The structure and dynamics of solid benzene. I. A neutron powder diffraction study of deuterated benzene from 4 K to the melting point

The crystal structure of deuterated benzene has been investigated over the complete temperature range 4–280 K. Our results are in excellent agreement with previous studies, recorded at a limited number of temperatures, and display a smooth variation of the lattice parameters over the complete temperature range. No evidence of the suggested low-temperature phase transitions, or marked premelting effects have been found. Detailed measurements of the lattice parameters close to the melting point display an anomalous behavior of the b lattice parameter and the orientation of the molecules which is believed to be evidence of a disordering transition at temperatures above the melting point.

Fine structure in the proton magnetic resonance spectrum of solid benzene and solid benzene-cyclohexane mixtures

The PMR spectrum of solid benzene in the vicinity of the melting point consists of broad and narrow lines. New narrow lines in the spectrum of benzene containing some impurity appear at the top of the broad line. Intensities of narrow lines increase with rising of temperature. The temperature dependence of line width and relaxation timesT 1 andT 2 have been measured in pure solid benzene an benzene-cyclohexane mixtures. It is assumed that narrow lines in spectra of pure benzene and its mixtures are caused by appearance of a phase with mobile molecules located in the crystallite joint or on its surface. The spectrum of imperfect monocrystalline benzene have been studied also. The narrow line of this sample is split into several narrow components. This is explained by demagnetization fields the distribution of which has a discrete character,

Heterogeneous nucleation characteristics of mixed vapor of benzene and cyclohexane on a cold substrate.

The heterogeneous phase transition of mixed benzene and cydohexane vapors on a cold copper substrate was observed by slowly cooling the substrate under reduced pressure conditions. In the conversion of the vapor to the condensed phase, vapor-to-liquid or vapor-to-solid phase transitions occurred depending on the mixing ratio of both vapors. By measuring the critical vapor pressure and temperature, corresponding critical supersaturation ratios were determined for various mixing ratios of both vapors. The critical pressure for the formation of solid benzene from mixed vapor agreed closely with that of pure vapor having higher composition. The critical pressure for the formation of solid cyclohexane from mixed vapor decreased from 6 to 9% compared with that from pure vapor. Liquid condensate was formed at critical supersaturation ratios of almost unity. A phase diagram was constructed by solving a non-linear simultaneous equation derived from vapor-liquid equilibrium and critical pressure-temperature relationships. It agreed in a practical sense with the observed results.

Structure of the Lowest Temperature Phase of the Solid Benzene–Hexafluorobenzene Adduct

Similarities with a number of liquid-crystal systems are revealed by the structural characterization of the low-temperature phases II-IV of the solid adduct from benzene and hexafluorobenzene (right). As the temperature is raised, increased rotation leads to a reduction in the interactions between neighboring columns of alternating C6H6 and C6F6 molecules. These results could only be obtained by using a combination of powder diffraction techniques with neutrons and synchrotron radiation.

Aspects of the vibrational spectroscopy of solid benzene and solid benzene: hexafluorobenzene

Using moderately high resolution, inelastic neutron scattering we have investigated the vibrational spectroscopy and dynamics of benzene molecules in solid benzene and in solid C 6H 6:C 6F 6. From our measurements of phonon-molecule vibration coupling we show that it is possible to explain the magnitude of the observed frequency shifts of the phonon side-bands in terms of vibrational amplitudes and the electrical properties of the molecules. We observe anomalous levels of scattering from the hexafluorobenzene molecules which we also rationalise in terms of intermolecular coupling.

Experimental determination of the phase behavior of binary mixtures of methane + benzene

Rijkers M.P.W.M., Malais M., Peters C.J. and de Swaan Arons, J., 1992. Experimental determination of the phase behavior of binary mixtures of methane + benzene. Part I. Vapor + liquid, solid benzene + liquid, solid benzene + vapor and solid benzene + liquid + vapor equilibria. Fluid Phase Equilibria, 77: 327–342. This paper reports experimental results of various types of two-phase boundaries in binary mixtures of methane + benzene. The measurements were performed in the temperature region from about 260 to 350 K and pressures up to 80 MPa were applied, using windowed autoclave equipment. From the experimental results obtained the course of the three-phase equilibrium solid benzene + liquid + vapor was derived.

Core excitation, decay, and fragmentation in solid benzene as studied by x-ray absorption, resonant Auger, and photon stimulated desorption

For condensed benzene ice layers, core photoabsorption near edge structure (x-ray absorption; recorded by Auger electron yield measurements), decay electron spectra for resonant and nonresonant excitation, and fragmentation as evident in yields of hydrogen and other ions, have been measured in the C1s region. The absorption spectrum is better resolved than most previously published spectra, exhibits some new features, and shows a high degree of parallelity to the spectrum of isolated molecules. Interestingly, the hydrogen ion yield indicates a particular dissociativeness of a certain core excitation resonance, X, which in the molecule has previously been assigned to a Rydberg state. This selective dissociation suggests that the responsible excitation is strongly antibonding for the carbon–hydrogen bond, while the degenerate Rydberg states broaden into a conduction band in the solid; and that the bond breaking probably occurs or at least starts in the core-excited state, thus proceeding on an extremely short time scale, similarly to observations for other hydrogen-containing molecules. The decay spectra are analyzed in terms of autoionization vs normal Auger decay and indicate that, apart from the first strong π resonance (which leads to pure autoionization) and the X resonance, the core resonances partly or fully ionize before core decay takes place. For the X resonance, the decay spectrum contains a contribution which cannot be assigned to intact benzene; this is taken as additional evidence for ultrafast dissociation, i.e., competitive with core decay. We use these results for a discussion of the influence of condensation on excitation, decay, and fragmentation.

Ablation of benzene from Van Der Waals films with excimer laser pulses at 248 nm

UV laser-induced resonant ablation at 248 nm from solid benzene films several micrometers thick was studied by the time-of-flight technique. The broadened and structured time-of-flight distributions observed for the first few laser pulses indicate the formation of species with different masses, e.g. benzene clusters. Chemical effects, e.g. the formation of naphthalene, could only be observed in cases where transient substrate heating cannot be excluded. The benzene monomer ablation prevailing after about 50 laser pulses irradiating the same spot could be described by a modified Maxwellian distribution including a stream velocity.

ChemInform Abstract: Hydrophobic Vitamin B12. Part 6. Carbon‐Skeleton Rearrangement via Formation of Host‐Guest Complexes Derived from an “Octopus” Azaparacyclophane and Hydrophobic Vitamin B12 Derivatives: A Novel Holoenzyme Model System.

The alkylation reaction of a hydrophobic vitamin B12 derivative with alkyl bromides in an ‘octopus’ azaparacyclophane having eight hydrocarbon chains have been investigated. Molecular discrimination has been shown to originate from electrostatic interaction between the octopus cyclophane and the alkyl bromides. Alkylation was enhanced by desolvation and proximity effects operating on the reacting species via formation of a ternary complex composed of the octopus cyclophane, the hydrophobic vitamin B12 derivative, and an alkyl halide. Carbon-skeleton rearrangements reactions of alkyl ligands bound to the hydrophobic vitamin B12 were found to be markedly favoured in the hydrophobic cavity provided by the octopus cyclophane, relative to the reactions in methanol and benzene, under anaerobic photolysis conditions at ordinary temperatures. The same reactions took place readily in solid benzene below 4°C under similar conditions. The central cobalt atom of the hydrophobic vitamin B12 participates in the rearrangements reaction via formation of a tight pair with an alkyl radical species. Non-enzymic rearrangement reactions have been shown here to proceed quite efficiently by employing a relevant apoenzyme model.

Far infrared spectra of liquid and solid benzene

The far infrared spectra of liquid and solid benzene are compared. The benzene was maintained at a temperature of 330 K and the applied pressure varied to alter the state. Liquid benzene is observed to possess a structure resembling that of the solid, close to the melting point.

Hydrophobic vitamin B12. Part 6. Carbon-skeleton rearrangement via formation of host–guest complexes derived from an ‘octopus’ azaparacyclophane and hydrophobic vitamin B12derivatives: a novel holoenzyme model system

The alkylation reaction of a hydrophobic vitamin B12 derivative with alkyl bromides in an ‘octopus’ azaparacyclophane having eight hydrocarbon chains have been investigated. Molecular discrimination has been shown to originate from electrostatic interaction between the octopus cyclophane and the alkyl bromides. Alkylation was enhanced by desolvation and proximity effects operating on the reacting species via formation of a ternary complex composed of the octopus cyclophane, the hydrophobic vitamin B12 derivative, and an alkyl halide. Carbon-skeleton rearrangements reactions of alkyl ligands bound to the hydrophobic vitamin B12 were found to be markedly favoured in the hydrophobic cavity provided by the octopus cyclophane, relative to the reactions in methanol and benzene, under anaerobic photolysis conditions at ordinary temperatures. The same reactions took place readily in solid benzene below 4°C under similar conditions. The central cobalt atom of the hydrophobic vitamin B12 participates in the rearrangements reaction via formation of a tight pair with an alkyl radical species. Non-enzymic rearrangement reactions have been shown here to proceed quite efficiently by employing a relevant apoenzyme model.

Deuterium nuclear magnetic resonance studies of the molecular dynamics of benzene in zeolites

Deuterium n.m.r. spectroscopy has been used to investigate the microdynamics of benzene molecules sorbed on HZSM-5, NaX and NaY zeolites. As in the liquid and solid benzene phases, the sorbed benzene molecules perform fast reorientations about their hexad axes. Superimposed on this C6 reorientation are jumps which the benzene molecules perform between the sorption sites. The mean residence time between two succeeding jumps depends significantly on the zeolite framework, the temperature and the loading. From this finding, diffusivities of sorbed benzene molecules can be estimated.

SEXAFS studies of CC bond lengths in condensed and chemisorbed molecules

Extended X-ray absorption fine structure (EXAFS) studies are reported above the carbon K-edge for solid benzene and cyclohexane. Both molecules exhibit pronounced EXAFS extending more than 500 eV above the edge. The measured EXAFS is in good agreement with single scattering simulations for the isolated molecules using a phase shift obtained from graphite data. We also explored the possibility of measuring the intra-molecular CC EXAFS for a chemisorbed molecule on a metal surface. Model calculations for a di-carbon species chemisorbed on Cu(100) reveals the feasibility of such studies for certain chemisorption geometries by use of a suitable sample orientation in the X-ray beam.

Molecular dynamics simulation of hydrocarbon molecules in condensed phases. II. Benzene

Molecular dynamics simulations of liquid (305 K) and solid (10 K) benzene have been carried out to investigate the vibrational properties of benzene in the condensed phase. The benzene molecule is modeled as a fully flexible system of 12 atoms with intramolecular interactions based on ab initio harmonic potentials and intermolecular interactions given by semiempirical atom–atom potentials. Vibrational frequencies are analyzed in terms of the frequency spectrum of the velocity autocorrelation function and the results are found to correlate well with optical absorption and scattering data. Single particle density fluctuations are calculated over a range of wave numbers and results are obtained that can be directly compared to the intensity distributions of inelastically scattered thermal neutrons. The different comparisons indicate that the existing potential energy function provides an essentially quantitative description of molecular vibrations of benzene.

Neutron spectroscopic study of polycrystalline benzene and of benzene adsorbed in Na-Y zeolite

The neutron inelastic spectra of solid benzene and of benzene adsorbed in Na-Y zeolite have been obtained between 200 and 2000 cm–1. The intensities of the vibrational transitions of solid benzene are calculated from a force field. Multiphonon processes and an isotropic Debye–Waller factor are also included. After adsorption in Na- Y, two benzene species are observed. The stretching modes of benzene relative to the SII sodium ions are assigned at 230 and 270 cm–1. Small frequency shifts indicating a weak perturbation of the benzene ring are observed.

High pressure effects on the Raman spectra of solid C 6H 6

Raman spectra of solid benzene have been investigated under high pressures up to 15 GPa at room temperature using a diamond anvil cell. The pressure dependences of the intramolecular and lattice vibration modes were used to identify different solid phases of C 6H 6. It is found that discontinuities and changes in slopes occur in the ν( P) curves at about 4 GPa. This probably corresponds to the II–III phase transition which has been reported previously. Moreover, a new band appears in the lattice vibration spectra, above 9 GPa, indicative of a new phase (IV). The controversy over the location of triple points previously observed is discussed with respect to our results. A schematic P-T phase diagram, where speculative phase boundaries II–III and III–IV are drawn, is suggested.

Theoretical study of the transmission of low-energy (0–10 eV) electrons through thin-film organic molecular solids: Benzene

A theoretical study of the transmission of low-energy (0–10 eV) electrons incident from vacuum through thin-film organic molecular solids deposited on a cold metal substrate is presented and developed for the specific case of solid benzene. In essence, using a semiclassical description of electron transport in solids with an energy-independent scattering mean free path and assuming an isotropic electron scattering, the behavior of a penetrating electron in the film is simulated when a large number of scattering events are present. The good agreement between the calculated electron transmission spectra and those obtained experimentally indicates that our study provides a realistic description of the electron transport in the film, and accounts for the influence of the various electron-molecule scattering processes upon the energy dependence of the transmitted current. In particular, we show that the excitonic subionization energy losses are at the origin of the main structures of the observed electron transmission spectra. It is also shown that our study can successfully be used to estimate the probabilities of the various electron scattering processes which occur in the film, as well as the electron mean free path l. For solid benzene, l is about 8 Å in the considered electron energy range.

Inelastic neuron scattering by benzene: Direct spectral analysis using molecular dynamics simulation

Incoherent inelastic scattering cross sections for solid and liquid benzene are calculated from molecular dynamics simulation data. The benzene molecule is modeled as a flexible twelve particle system with intramolecular forces based on an ab initio potential and the intermolecular interactions given by empirical atom-atom Buckingham (6-exp) potentials. Hydrogen atom trajectories from simulations at 10 K and 298 K are used to calculate directly the intermediate scattering function, F s (q, t) , and the proton velocity autocorrelation function. For liquid benzene, the cross section is calculated from the associated S s (q, w) . For solid benzene, the cross section is calculated from the proton velocity autocorrelation spectrum. The calculated cross sections are compared with recent experimental measurements carried out at the IPNS-ANL and are foundto show allthe observed peaks in the measured spectra (ca. 20-1600cm -1 ) with their relative intensities in reasonable agreement.

Thermal transport behavior of van der Waals solids and liquids in the neighborhood of the solid-liquid phase transition

The thermal conductivity, λ, is one of the few transport coefficients which shows a relatively small change at the solid-liquid phase transition, and hence it is a property that can be used in comparing dynamic properties of both ordered and disordered systems. Although the discontinuity in λ can be accounted for largely by the difference in density, ρ, of solid and liquid at the phase transition, its volume dependence is examined more closely. The thermal diffusivity, which is known to dominate the dynamic structure factor of liquid argon, has been determined around the phase transition also; the sound velocity has been considered in addition. The results are discussed and a comparison is made with these properties in solid and liquid benzene and cyclohexane.