Experimental investigation of double ionization to ground and electronically excited states of chlorobenzene, 1,2-, 1,3- and 1,4-dichlorobenzene, 1,2,3-, 1,2,4- and 1,3,5-trichlorobenzene, 1,2,3,4-and 1,2,4,5-tetrachlorobenzene and pentachlorobenzene dications

Double-charge-transfer spectrometry was used to measure double-ionization energies to ground and electronically excited states of various chlorobenzenes. Since OH⁺ was the projectile ion used in these experiments, it is probable that triplet states of the dications were populated because of spin conservation in the double-electron-capture reactions which are the basis of this type of spectrometry. The lowest double-ionization energies for all the molecules studied are within ±0.3 eV of 25.9 eV, except that for 1,3-dichlorobenzene, which is at 26.6 eV. In general, double-ionization energies to three higher lying states (or groups of states) were measured for each molecule. The energies of these states are the same, within experimental uncertainties, for the three trichlorobenzenes, two tetrachlorobenzenes and pentachlorobenzene dications, suggesting that they have the same or very similar distributions of triplet-state energies.     

Polymerization of 1,2,4,5-tetramethylbenzene (durene) in direct-current discharge

Polymer films possessing dielectric properties were prepared from 1,2,4,5-tetramethylbenzene (durene) at the cathode in a dc discharge. The chemical structure of the films was studied by IR and UV spectroscopy. It was shown that the polymer synthesized from durene in a dc discharge contains basically polyene structures produced via the opening of aromatic rings.     

Total luminescence of organic molecules of petrochemical interest : Naphthalene,phenanthrene and 1,2,4,5-tetramethylbenzene

Total emission, phosphoroscopically resolved emission, excitation and mean lifetime in the excited triplet state were measured for three organic molecules of petrochemical interest in EPA rigid glass at 77° K. Fluorescence was also measured at room temperature in n-heptane. Methods for the determination of these three molecules in individual solutions are presented and compared with known methods by absorption spectroscopy. An attempt was made to quantify a blend of the three components by luminescence techniques.     

Synthesis of liquid crystalline materials based on 1,3,5-triphenylbenzene and 2,4,6-triphenyl-1,3,5-s-triazine

C₃-symmetric alkyloxy/aryloxy polyether dendrimers have been synthesized from 1,3,5-triphenylbenzene and 2,4,6-triphenyl-1,3,5-s-triazine and their liquid crystalline properties have been studied. C₃-symmetric derivatives with n-hexyl and n-dodecyl chains at the periphery show mesophase properties when they are complexed with trinitrofluorenone at different temperatures.     

Magnetic anisotropy parameters of matrix-isolated septet 1,3,5-trinitreno-2,4,6-trichlorobenzene

An ESR spectrum of high-symmetry septet 1,3,5-trinitreno-2,4,6-trichlorobenzene generated under photolysis of 1,3,5-triazido-2,4,6-trichlorobenzene in solid argon at 15 K was recorded. Computer simulation revealed that the spectrum corresponds to the septet spin state with the fine structure parameters D _S = ?0.0957±0.0006 cm^?1 and E _S = 0±0.0004 cm^?1. These values of the magnetic anisotropy parameters D _S and E _S are in good agreement with the results of UDFT calculations. The spin-spin (D _SS) and spin-orbit (D _SO) coupling parameters of septet molecules with D _3h symmetry are negative and mutually enhance the magnetic anisotropy of these molecules. The contribution of the spin-orbit coupling to the magnetic anisotropy of 1,3,5-trinitreno-2,4,6-trichlorobenzene is higher than 11% due to the presence of three chlorine atoms in the molecule. This suggests the possibility of further strengthening the magnetic properties of septet 1,3,5-trinitrenobenzenes by introducing bromine and iodine atoms into positions 2, 4, and 6 of their benzene rings.     

Low frequency internal modes of 1,2,4,5-tetramethylbenzene, tetramethylpyrazine and tetramethyl-1,4-benzoquinone INS, Raman, infrared and theoretical DFT studies

The results of inelastic neutron scattering (INS), Raman and infrared (IR) studies on 1,2,4,5-tetramethylbenzene (durene), tetramethylpyrazine (TMP) and tetramethyl-1,4-benzoquinone (TMBQ) in the solid state are reported. The observed frequencies are analyzed on the basis of DFT calculations. The low frequency region, below 400 cm(-1), related to the torsional and bending out-of-plane vibrations of the CH(3) groups, is of particular interest. The detailed analysis is possible due to the simulation of the INS spectra by using the auntie-CLIMAX program. It is shown that the observed low frequency INS bands are dramatically shifted, compared to the calculated ones, towards higher frequencies. Although one cannot exclude deficiencies of theoretical methods as applied to low frequency modes, it seems more probable the interpretation based on an existence of non-conventional CH(...)pi, CH(...)N, CH(...)O hydrogen bonds formed by the methyl groups in crystalline phases.     

Raman spectrum of a single crystal of 1,2,4,5-tetrachlorobenzene. Calculation of lattice vibrations.

Polarized Raman spectra of a single crystal of 1,2,4,5-C₆H₂Cl₄ at room temperature (β phase) in the lattice mode region have been obtained. The assignment of the bands to their proper factor group species allows a comparison with frequencies calculated using the atom-atom model. The agreement is good, supporting the transferability of the potential parameters. The lattice vibrations of the low-temperature α phase have also been obtained. The appearance of only five bands favours P₁ as the correct space group for this phase.     

A supramolecular and liquid crystalline water-based alignment medium based on azobenzene-substituted 1,3,5-benzenetricarboxamides

A supramolecular, lyotropic liquid crystalline alignment medium based on an azobenzene-containing 1,3,5-benzenetricarboxamide (BTA) building block is described and investigated. As we demonstrate, this water-based system is suitable for the investigation of various water-soluble analytes and allows for a scaling of alignment strength through variation of temperature. Additionally, alignment is shown to reversibly collapse above a certain temperature, yielding an isotropic solution. This collapse allows for isotropic reference measurements, which are typically needed in addition to those in an anisotropic environment, to be performed using the same sample just by varying the temperature. The medium described thus provides easy access to anisotropic NMR observables and simplifies structure elucidation techniques based thereon.     

Vapour pressures and sublimation enthalpies of thymine and cytosine

The vapour pressures of cytosine and thymine were measured using the torsion-effusion technique. The sublimation processes of cytosine and thymine were investigated over the temperature ranges 480–553 K and 420–503 K, respectively. The following pressure—temperature equations were derived by least-squares treatment of the vapour pressure data

The standard sublimation enthalpies were obtained by second-and third-law treatment of the experimental data and the values ΔH⁰₂₉₈ = 167 ± 10 kJ mole^?1 and ΔH⁰₂₉₈ = 138 ± 10 kJ mole^?1 were derived for cytosine and thymine, respectively. IR and Raman spectra were recorded in the gas phase in order to evaluate the thermodynamic functions of gaseous cytosine and thymine.     

Cross relaxation between the spin levels of phosphorescent 1,2,4,5-tetrachlorobenzene and photochemical products of the durene host

At certain magnetic fields, intensity changes have been observed in the phosphorescence of 1,2,4,5-tetrachlorobenzene (TCB) doped in a single crystal oFollowing the application of a magnetic field of the proper magnitude and direction to induce CR, the phosphorescence intensity of TCB displays an obse     

Vapour pressures of n-pentane determined by comparative ebulliometry

The vapour pressures of n-pentane have been measured using comparative ebulliometry with water as the reference substance. The measurements cover the temperature and pressure ranges 309 K and 102 kPa to 456 K and 2728 kPa. When combined with selected literature results, the range was extended downwards to a temperature and pressure of 268.8 K and 19.9 kPa and the combined data sets were correlated by a Wagner-type equation with a standard deviation of 18 Pa in the vapour pressure. The critical pressure was treated as an adjustable parameter and the value p_c = 3367.4 kPa was obtained using a selected critical temperature, T_c = 469.7 K. The calculated normal boiling temperature was T_b = 309.207 K and an extrapolation to the triple point temperature T_tp = 143.48 K predicted a pressure of p_tp = 0.078 Pa.     

Vapour pressures of n-hexane determined by comparative ebulliometry

The vapour pressures of n-hexane have been measured using comparative ebulliometry with water as the reference fluid. The measurements cover the temperature and pressure range (315.7 K, 41.1 kPa) to (504.0 K, 2876.8 kPa) and join smoothly with results selected from the literature to provide consistent results down to (289.7 K, 13.8 kPa). The combined data set have been described by a Wagner style equation with a fractional standard deviation of 4.2 · 10⁻⁵ in the vapour pressure. The critical pressure p_c was treated as an adjustable parameter and the value of p_c = 3027 kPa was calculated from the smoothing equation using a selected critical temperature of T_c = 507.49 K. The calculated normal boiling temperature is T_b = 341.866 K and an extrapolation to the triple-point temperature T_tp = 177.87 K predicts a triple-point pressure of p_tp = 1.23 Pa.     

Vapour—liquid equilibrium at elevated pressures from the perturbation theory. II. Binary systems

Vapour—liquid equilibria in binary systems of non-polar non-spherical molecule compounds were studied theoretically by combining the perturbation theory of convex molecule fluids with a new variant of the conformal solution theory. The recently proposed equation of state of hard convex body mixtures and the corresponding expressions for the contact values of distribution functions were employed to determine the reference thermodynamic functions and the perturbation terms. Ten binary systems, i.e. ArCH₄, N₂CH₄, N₂C₂H₄, N₂C₂H₆, CH₄C₂H₄, CH₄C₂H₆, C₂H₄C₂H₆, CO₂C₂H₄, CO₂C₂H₆, and ArCO₂ were studied at constant temperatures. Comparison of theoretical predictions with experimental data is given.     

Vapour pressures and excess Gibbs energy of ethylbenzene-cyclooctane binary mixtures

Isothermal vapour-liquid equilibrium data at 343.15, 353.15 and 373.15 K for ethylbenzene + cyclooctane mixtures are reported. Use has been made of an ebullioneter which allowed sampling from both phases in equilibrium.Different G^E expressions suitable for correlation of these data were tested. The results evidenced a positive deviation from ideality of the system.     

Emission spectra of the radical cations of 1,3-dichlorobenzene, 1,4-dichlorobenzene and 1,3,5-trichlorobenzene in the gas phase

Emission spectra of the radical cations of 1,3-dichlorobenzene, 1,4-dichlorobenzene-h₄ (and -d₄), and 1,3,5-trichlorobenzene, excited in the gas phase by controlled electron impact, are presented. The band systems, which lie in the 500–750 nm wavelength region, are attributed to the B?(π^?1) → X?(π^?) electronic transition of the cations on the basis of photoelectron spectroscopic data. The NeI excited photoelectron spectra and the ionisation energies of chloro-,o-,m-,p-dichloro- and 1,3,5-trichlorobenzene have been obtained. The information acquired from the emission and photoelectron spectra is discussed and compiled to deduce the symmetry of the B? states. Emission, with quantum yield > 10^?5, could not be detected with electronically excited radical cations of chloro-,o-dichloro-, 1,2,4- and 1,2,3-trichloro- and tetrachloro-benzenes. This is attributed to the nature of the B? states, which arise by σ^?1 ionisation processes. The lifetimes of the zeroth and some vibrationally excited levels of the B?(π^?1) states were also measured and found to be 22 ± 2 ns for 1,3,5-trichlorobenzene cation and < 6 ns for 1,3- and 1,4-dichlorobenzene cations. The lifetimes of the latter two electronically excited cations are estimated to be two orders of magnitude shorter than 6 ns from the measurement of the relative emission intensities of the B? → B? band systems of the three title cations.     

Thermophysical properties of azomethine ether main-chain liquid crystalline polymers at pressures to 200 MPa

This article reports on an experimental investigation of the equation of state and the transition behavior of main-chain thermotropic liquid crystalline polymers over a wide temperature range, and at pressures to 200 MPa. The materials studied were a series of azomethine ether polymers. A varying number n (= 4, 7, 8, 9, 10 and 11) of methylene spacer units in the backbone provided systematic variation of the structure. Experimental techniques used included high-pressure dilatometry (PVT measurements) to 200 MPa, high-pressure differential thermal analysis, also to 200 MPa, and conventional (atmospheric-pressure) differential scanning calorimetry (DSC). The equation of state of the materials can be well represented by the Tait equation in distinct regions, separated by a glass transition, T_g(P), a first-order transition to a nematic state, T_k-n(P), and a first-order transition to an isotropic melt state T_c(P). The atmospheric pressure values of T_k-n and T_c decreased with increasing number of spacer units and showed a clear odd-even effect. T_g and T_k-n both increased with pressure. The pressure dependence of T_c could not be observed due to the onset of degradation in the same temperature region. On isobaric cooling at 3°C/min, the crystallization from the nematic state occurred a few tens of degrees below T_k-n. This supercooling was independent of pressure for some materials, while for others it increased with increasing pressure. The values of the enthalpy and entropy associated with the first-order transition into the nematic state were lower than those of typical isotropic polymers at their melting transitions. The transition enthalpy did not have any systematic variation with increasing number of spacer units. Values of the transition enthalpy calculated from the Ciapeyron equation did not always agree with the values measured by DSC. This may be due to the two-phase nature of the low-temperature state. At the transition to the isotropic state, the transition enthalpy at P = 0 decreased with n and showed an odd-even effect. © 1994 John Wiley & Sons, Inc.     

Molecular dynamics simulations of melting of perfect crystalline hexahydro-1,3,5-trinitro-1,3,5-s-triazine

The melting mechanism of superheated perfect crystalline hexahydro-1,3,5-trinitro-1,3,5-s-triazine (alpha-RDX) has been investigated using molecular dynamics simulations with the fully flexible force field developed by Smith and Bharadwaj [J. Phys. Chem. B 103, 3570 (1999)]. Sequential 50 ps equilibration simulations of the constant stress-constant temperature ensemble were performed at 10 K intervals over the range of 300-650 K, corresponding to a heating rate of 2.0 x 10(11) Ks. A solid-solid phase transition is observed between 480 and 490 K, followed by melting, which occurs between 500 and 510 K. The solid-solid phase transition, both displacive and rotational, is characterized by an abrupt decrease in the lengths of the unit cell edges a and b and an increase of the length of edge c. The molecular conformation in the new phase is AAE, although the axial nitro groups have different changes: one shift is more axial and the other is more equatorial. Phases other than alpha-RDX have been observed experimentally, however, there are insufficient data for comparisons to ascertain that the new phase observed here corresponds to a real phase. At the high heating rate (2.0 x 10(11) Ks) used in the simulations, the melted RDX reaches full orientational disorder at about 540 K and translational freedom at around 580 K. If the simulation at the melting temperature (510 K) is run sufficiently long complete rotational freedom is achieved in a few hundreds of picoseconds, while complete translational freedom requires much longer. These results show that given a sufficiently high heating rate, the system can exist for significant periods of time in a near-liquid state in which the molecules are not as free to rotate and diffuse as in the true liquid state. The bond lengths and bond angles undergo little change upon melting, while there are significant changes in the dihedral angles. The molecular conformation of RDX changes from AAE to EEE upon melting. The ramification of this for formulating force fields that accurately describe melting is that it is important that the torsional motions are accurately described.     

Discotic liquid crystalline materials for potential nonlinear optical applications: synthesis and liquid crystalline behavior of 1,3,5-triphenyl-2,4,6-triazine derivatives containing achiral and chiral alkyl chains at the periphery

As a novel approach to nonlinear optical materials, new octupolar discotic liquid crystalline materials 1,3,5-triphenyl-2,4,6-triazine derivatives containing achiral alkyl chains (5a) and chiral alkyl chains (5b) at the periphery were synthesized. The former exhibits an ordered hexagonal columnar mesophase, whereas the latter displays a rectangular columnar mesophase. The negative exciton splitting observed in the CD spectrum of a thin film of 5b suggests that it has a left-handed helical structure within the column.