Volume 42, Issue 7

The article is aimed at an intuitive understanding of the recently explored deep connections between therm al physics, quantum field theory and general relativity. The physical effects involved in particle creation by a black hole are viewed in terms of more familiar quantum -field effects in flat spacetime. Black hole evaporation is investigated in terms of temperature correction to the Casimir effect. T he application of the Casimir effect results and those for accelerated mirrors reveals that a black hole should produce the blackbody radiation at a temperature that exactly coincides with Hawking’s result. Its blackbody nature is due to the interaction of virtual positive-energy particles with the surface of a “cavity” formed by the Schwarzchild gravitational field potential barrier. The virtual particles are “squeezed out” by the contraction of the potential barrier and appear to an observer at J + as the real blackbody ones.

CODATA has recently published its readjustment of the fundamental physical constants and assigns a relative precision of 128 x 10 -6 to G, the Newtonian constant of gravitation. Given that most of the other constants in physics have relative precisions of ~10 -6 or better, we examine the reasons why the value for G remains so imprecise: The role of G in physics in general is considered and the most recent experimental determinations are examined. Constraints are given for perturbing effects in G measurements and a key result is that horizontal ground movements must be taken more carefully into account in future more precise terrestrial experiments.

The electronic structures of the phases LiBC and CaAl 2 Si 2 have been investigated by two dimensional crystal orbital calculations. The corresponding solid-state ensembles have been separated into anionic (e.g. [BC] - , [AlSi] - , [Al 2 Si 2 ] 2- ) and cationic substructures. The employed fragmentation is topologically related to the electron counting schemes used in the classical Zintl approach. The band structures of the anionic fragments have been determined by a semiempirical Hartree-Fock crystal orbital (CO ) Hamiltonian on the basis of the tight-binding approximation. The adopted self-consistent-field variant is an intermediate neglect of differential overlap (INDO ) scheme. The influence of the cationic substructure has been simulated by an electrostatic point-charge model. The computational formalism allows for a suitable explanation of the conformations of the anionic subunits of the studied solids. In the CaAl 2 Si 2 model the site symmetry in the anionic substructure has been studied in larger detail. The Al atoms show a tetrahedral coordination while an inverted tetrahedron (i.e. umbrella-like structure) is realized at the more electronegative Si centers. This local geometry is stabilized by the Coulomb interaction between the anionic [Al 2 Si 2 ] 2- layer and the cationic substructure. The energetic sources leading to the different conformations in LiBC and CaAl 2 Si 2 are also quantified. The observed geometric preferences are rationalized by means of an energy fragmentation into covalent resonance (kinetic energy of the electrons) and classical Coulomb elements. This energy decomposition in the planar and chair-like conformations of LiBC and CaAl 2 Si 2 shows that the kinetic energy favours the former, the electrostatic part the latter conformation. The dimerization of two anionic substructures (CaAl 2 Si 2 structure) is also studied by the tight-binding formalism . The observed geometric differences between intra- and interlayer bonds are satisfactorily reproduced by the semiempirical CO model.

With the Klemm method internal mobility ratios of the two cations have been measured for melts of the binary system (Li, K ) NO 3 over a wide range of temperature and composition, and their isotope effects have been determined near the eutectic composition. The Chemla effect has been observed. With increasing temperature the Chemla crossing point shifts, as expected, toward higher concentrations of the smaller cation. At very high concentrations of LiNO 3 the mobility of K + tends to decrease, which is attributed to the free space effect. For both Li + and K + the isotope effect increases with temperature.

Molar volumes and surface tensions of molten LaCl 3 -KCl mixtures were measured by dilatometry and the maxim um bubble pressure method, respectively. The molar volume isotherms were found to deviate positively from additivity over the whole composition range, with the maxim um deviation at ca. 30 mol% LaCl 3 . At the same concentration the isotherms of surface tension at temperatures below 950 °C show a minimum . Surface energy of mixing per unit area calculated from the surface tensions shows a large negative excess at the corresponding composition. These observations are related to the existence o f LaCl 6 3- in the KCl-rich melt.

The relative difference in internal mobilities o f 6 Li and 7 Li, ε, in molten LiNO 3 has been remeasured in the temperature range from 559 K to 726 K by countercurrent electromigration (Klemm 's method). As temperature increases, the isotope effect increases, although the increasing rate decreases: it is practically constant above 650 K. The ε values are lower than those measured by Lundén and Ekhed [5] by 15-25% . The effective diffusion coefficients have been estimated from the distribution of the isotopes in the separation tube by numerical simulation.

The isotope separation coefficient of lithium electromigration through a cation exchange membrane is determined at 6, 11, 20 and 40 °C . It is found that this coefficient increases with temperature while the slope of the isotope distribution in the band decreases with increasing temperature. These tendencies are opposite to those usually observed in ion exchange chromatography.

The benzenoid classes of multiple zigzag chains, A (n, m ), and incomplete multiple zigzag chains. A (n, m, l), are considered. Numerical numbers of Kekulé structures (K ) are given for n up to 5 and m up to 10. A recurrence relation is developed for the K numbers of A(n, m) with fixed values of n; a general formulation for arbitrary m values was achieved. Six new benzenoid classes are studied in connection with the application of the John-Sachs theorem to double zigzag chains. A new approach to the K enumeration is introduced by nonlinearly dependent recurrence relations. Finally a new type of combinatorial K formulas in terms of determinants and based on the John-Sachs theorem , is introduced. The new enumeration techniques are applied to multiple zigzag chains.

We have computed rotational energy transfer differential and state-to-state integral quasiclassical cross sections for the He - N 2 system at 27.3 meV. By comparing these differential cross sections to close coupling ones, the accuracy of the quasiclassical approximation at such a low collision energy has been assessed as satisfactory.

Empirical quantitative models have been studied that relate the phosphorescence transition energies of thiophene benzologs and corresponding aromatic hydrocarbon systems. Phosphorescence 0,0 bands, phosphorescence/fluorescence quantum yield ratios, and phosphorescence lifetimes of 12 thiophene benzologs of which the phosphorescence properties were unknown are reported.

A phase transition has been observed in bis(pyridinium) hexachlorometallates (C 5 H 5 NH) 2 [M IV Cl 6 ]. M = Sn. Te. Pb. Pt. The crystal structure of the low temperature phase II of the salt with M = Sn was determined, space group C 1 ḷ - P 1̅, Z = 1 (a = 734.1pm, b = 799.0 pm, c = 799.7 pm,α= 83.229°. β = 65.377°, γ= 84.387°, T = 297 K). The four compounds are isotypic in phase II as well as in the high temperature phase I (C 2H 2 -B2 /m, Z = 2) for which the crystal structure is known for M = Te . The lattice constants of all compounds (both phases) are given. The temperature dependence of the 35 Cl NQR spectrum was investigated. The three line 35 Cl NQR spectrum is in agreement with the crystal structure. The dynamics of the pyridinium ring shows up in a fade out of part of the 35 Cl NQR spectrum . The influence o f H ↔ D exchange on 35 Cl NQR is studied and an assignment of ν ( 35 Cl) ↔ Cl (i) is proposed. The nature of the phase transition P1̅ (Z = 1) ↔ B2 /m (Z = 2) is discussed.

The polarisability and second hyperpolarisability of ferrocene have been calculated using the CHF -PT -EB -CNDO method, which has been extended for the study of compounds containing transition metal elements. The computed polarisability value is in good agreement with experiment.

MINDO-Forces calculations are reported, after complete geometry optimization, for 2-X-allyl cations and anions, where X is O - , OH, NH 2 , CH 3 , NO 2 , CN, CF 3 , F, CHO.In the case of the allyl cation, it was found that the substituents O - , OH, NH 2 are stabilizing, CH 3 is slightly stabilizing and all other substituents are destabilizing.In the case of the allyl anions, all the substituents are stabilizing. It was found that the substituents CH 3 , O - , CN show the amphielectronic behaviour. Calculations predict no (1,3) π interaction in 2-substituted ally cations and anions.

The energy of the charge transfer interaction is expressed using absolute electronegativities and hardness parameters. The master equation appears to bind together Pauling and Mulliken scales by the factor (7/4 π ε 0 ) 1/2 = 3.174 V 1/2 nm 1/2 electron -1/2 . An index of charge transfer affinity (CTA) is proposed as possible measure of the tendency to ionic bonding between atoms.

The liquid crystalline phases of the title compounds have been investigated by differential scanning calorimetry, optical microscopy and X -ray diffraction methods. Transition temperatures and transition enthalpies of these compounds have been determined. The diffraction photographs in the nematic phase of the first compound showed diffraction patterns corresponding to two coexisting mass density fluctuations with commensurate wavelengths, whereas a single diffraction pattern was observed in the nematic phase of the second compound. The thermal and X -ray data showed that the first compound has a quasi-smectic layer structure.

Molar volumes in the liquid state and melting points of several rare earth chlorides RCl 3 (R = La, Pr, Nd, Gd, Dy, and Y) have been measured by dilatometry and DTA. respectively. The volume changes on melting of these chlorides were evaluated on the basis of these result and available crystal structure data. The volume increase on melting of the hexagonal chlorides from LaCl 3 to GdCl 3 was found to be more than 20%. On the other hand, the volume changes of the monoclinic DyCl 3 and YCl 3 were less than 1%.

Mean amplitudes of vibration of CrO 2 Br 2 in the temperature range between 0 and 1000 K have been calculated from recently reported spectroscopic data. The results are briefly discussed and compared with those of CrO 2 F 2 and CrO 2 Cl 2 .