Volume 38, Issue 8

The foundations of the inverse mean field method (Imefim) and its relation to traditional approaches are discussed. Imefim predicts the energy dependence of the real central (nuclear) part of the optical model potential to have the functional form (1 + eE p ) -3 . Treating for the time being the constant e as an adjustable parameter, this prediction is shown to compare nicely with well-established formal and heuristic results.

The change in phase factor of the wavefunction does not affect the absolute value of the matrix element, but does change the phase factor of the off-diagonal matrix element. This phase dependence causes a serious confusion in the sign of some parameters in the molecular Hamiltonian, which appear only in the off-diagonal matrix element; for example, the sign of the l-type doubling constant q of a linear or a symmetric-top molecule. In the present paper, the energy eigenvalues, symmetry species, and labeling of the eigenfunctions are discussed for the K-type doubling of asymmetric-top molecules and for the l-type doubling of linear or symmetric-top molecules in relation to the choices of phases in the basis wavefunctions.

On the basis of a generalized diffusion equation for the rotational motion of an asymmetric rigid rotor a general expression for steady-state fluorescence depolarization has been obtained.

The pressure broadening and shift of the 535.0 nm Tl line resulting from the photodissociation of thallium iodide mixed with sulfur hexafluoride were studied at low densities using a Fabry-Perot interferometer. A linear variation of both the width and shift of the line with the SF 6 density was found and the cross sections for the broadening and shift were determined.

Arguments and representative examples are given that suggest that exact energy conservation and Galilei invariance are incompatible in non-relativistic guiding-center mechanics. Provided that this is true in general it also follows that exact energy conservation and Lorentz invariance are incompatible in relativistic guiding-center mechanics. It would furthermore follow that every guiding-center mechanics with exact energy conservation is a non-unique theory owing to the principle of relativity. The paper also presents a Galilei invariant guiding-center mechanics that does not conserve energy.

Polycrystalline samples of stoichiometric YCrO 3 and LuCrO 3 were investigated by ESR in the temperature range between 140 K and 520 K. Although the orthochromites have a complicated spin structure, the line width shows the temperature dependence of simple antiferromagnetic systems. Up to a high degree the spectra have a Lorentzian line shape. The ESR intensity confirms the predictions of a cluster model. The coupling energy describing the gap between the bounded and the resonance state was found to be ⊿E = 250 cm -1 for YCrO 3 and ⊿E = 210 cm -1 for LuCrO 3 . There is a distinct relation between the excitation energy ⊿E and the volume V of the elementary cell. Further calculations concerning the isotropic exchange interaction result in a coupling constant of J = 5.55 cm -1 for YCrO 3 and J = 4.67 cm -1 for LuCrO 3 .

The clustering energies and geometries of the H 2 CN + · nN 2 (n = 1, 2 and 3) species have been determined by ab initio SCF calculations with the 4-31G basis set. The calculated clustering energies are in good agreement with the experimentally estimated heats of formation of the cor-responding clusters. The stability of various conformers has been studied in terms of localized orbitals and charge distributions.

For the metallic glass Ni 81 B 19 the partial pair-distribution functions are determind up to 14Å by a molecular dynamics simulation using effective Lennard-Jones potentials. The obtained pair-distribution functions are in good agreement with the measured ones. As the simulation was carried out at the thermodynamic conditions for a liquid, we conclude that the characteristic features of the structure of the metallic glass are essentially not different from those of the fluid system Ni 81 B 19 at high temperature and pressure. Both the high reduced density of the system and the large differences in the atomic radii of the two species dominantly determine the form of the partial structure factors of the glass. These Findings have been confirmed by our molecular dynamics simulation of a further metallic glass, the Cu 57 Zr 43 system.

The solid state binding energies of 8-azaguanine, 8-azaxanthine, hypoxanthine and xanthine were calculated by using semiempirical self-consistent charge intramolecular electrostatic potentials expressed in the point-charge approximation. The results are essentially in good agree-ment with the experimental data giving in the same time a very simple and transparent inter-pretation of ESCA spectra which is close to the chemical intuition. Splitting of some unresolved N(1s) peaks is proposed and separate binding energies are attributed to particular nitrogen atoms. Since the point-charge model describes a number of molecular properties, it is concluded that formal atomic charges are meaningful within the adopted theoretical framework despite the fact that they can not be defined in a unique fashion. The relaxation energy and work functions of the studied molecular crystals are briefly discussed.

Pb 2 P 2 S 6 crystallizes in the monoclinic system, space group Pn with the lattice constants a = 940.2(4) pm, b= 746.6(3) pm, c = 661.2(3) pm, β = 91.53(5)°. The compound is isotypic to the monoclinic modification II of Sn 2 P 2 S 6 . In the structure there are discrete P2S 4- 6 -anions. Far infrared, infrared and Raman spectra of this compound have been recorded. The observed frequencies are assigned on the basis of P2S 4- 6 units with C 2h symmetry in the crystal. DTA-data have been determined and interpreted.

The transport numbers of the molten systems (Li.Ag)Cl, (Na.Ag)I, (K,Ag)I, and (Rb,Ag)I have been determined by e.m.f. measurements on concentration cells with transference. With the exception of (Na,Ag)I, for these systems the existence of inversion points (the mobility ratio reverses at a certain mixing ratio) is proven within experimental accuracy. For (Na,Ag)I an inversion point might exist at the high Agl concentration side.

To measure the composition dependence of the interdiffusion coefficients in molten mixtures of alkali nitrates and silver nitrate, a wave-front-shearing interferometer, together with an initial “plane source” was used. With this technique, the diffusion coefficients were measured in nearly differential steps in the three mixtures (Na,Ag)NO 3 , (K,Ag)NO 3 , and (Rb,Ag)NO 3 at 300°C. The composition dependence of the diffusion coefficients D of all three systems could be described by parabolas of the form D = a + b x 2 + c x 2 2, where x 2 denotes the mole fraction of the silver nitrate. The three coefficients a,b, and c all show a clear proportionality to the radii of the akali ions. Thus an extrapolation to the not measured systems (Li, Ag)NO 3 and (Cs, Ag)NO 3 was possible.

Measurements of the densities (and with these of the molar volumes and the excess volumes) of the molten salt systems LiCl + AgCl, NaCl + AgCl, KCl + AgCl, RbCl + AgCl, and CsCl + AgCl as functions of composition and temperature from the liquidus curve up to 1200 K are reported. It is shown that a linear relation with the volume fraction fits the measured densities of the alkali chloride + silver chloride mixtures better than one with the mole fraction.

Photodecomposition of gaseous ClONO 2 was investigated in a static system at wavelengths 265 nm and 313 nm. From the quantitative determination of the products O 2 , N 2 O 5 , and ClO 15 NO 2 formed, and from Φ(-ClONO 2 ) it is concluded that the predominant primary step is ClONO 2 + h v → Cl + NO 3 (3).

The reactions of hydrogen atoms with silane and the methylated silanes - with the exception of tetramethylsilane -have been investigated in a fast flow reactor. Under our experimental conditions hydrogen abstraction from the Si-H bond is followed by combination of hydrogen atoms with the corresponding silyl radicals. The molecules formed in this way are activated by about 375 kJ/mol of vibrational energy. Two decomposition channels have been unequivocally identified, namely the elimination of molecular hydrogen and of methane, both with concomittant formation of the respective silylenes. In a subsequent step, silylene inserts into the substrate under formation of disilanes. With increasing degree of methylation. stabilization of the activated molecule competes with decomposition and dominates the kinetics in the case of trimethylsilane. With methyl -and dimethyl-silane, methyl radicals are observed as an additional reaction product. On the basis of RRKM calculations it is unlikely that they originate from a direct decomposition of the activated molecules. Absolute values for the room temperature rate constants of the abstraction reactions are given; for H+CH 3 SiH 3 , Arrhenius parameters have been determined.

The π-band structure of Möbius polymers (M) is compared with that of the open strip (F) and the cylindrical form (R) of the polymer considered. Under certain conditions, the bands of all these forms coincide. This is to be expected also for the σ-bands within the framework of extended Hückel theory.

The approach of the S and T isomers is applied to eight pairs of topologically related hetero-cyclic molecules with non-isomorphic partial structures. The CNDO/2 calculations as well as the experimental data clearly support the TEMO predictions.

The fluorescence properties of trans-4-cyano-4′-dimethylaminostilbene (trans-1) and a rigid analogue, in which the dimethylaminophenyl group is replaced by an indoline ring, trans-l-(4-cyanophenyl)-2-(5-[2,3-dihydro]N-methylindolyl)ethylene (trans-2) were studied in 20 solvents. The quantum yields of fluorescence (Φ f ) and of trans→cis photoisomerization (Φ t→c ) as well as the fluorescence maximum (ν̃ f ) and the bandwidth (⊿ν̃ f ) were examined. The activation energies (range: 3.5-7.8 kcal/mol) obtained from <2>f and <£t_c measurements are nearly the same for 1 and 2 in a given solvent. Φ f , ν̃ f and ⊿ν̃ f as a function of solvent polarity and temperature show similar trends for 1 and 2. No dual emission was observed for either of the cyanostilbenes. These findings indicate that a twisted intramolecular charge transfer (TICT) state is not involved in the relaxation process of the first excited trans singlet state ( 1 t*). The main deactivation pathway of 1 t* competing with fluorescence is trans→cis photoisomerization by twisting about the C = C double bond in the first excited singlet state for both.

The influence of the normal mode rotation (i.e. the Duschinsky mixing) on the molecular electronic spectra in polyatomic molecules is treated by means of multidimensional intramolecular distributions (MID). It is shown that symmetry properties of the two-dimensional MID which relate emission and absorption spectra or pertain to the exchange of modes do not exist if the number of non-separable modes exceeds 2. Specific examples of emission band shapes are calculated for weakly (linear) coupled electronic states for both, zero and finite temperatures. The strength of the mixed quadratic interaction parameter is shown to influence the shape considerably.

Spin-lattice relaxation times T 1 of protons in aqueous solutions of gadolinium chloride have been measured at 2.11 T for a larger range of concentrations down to 0.035 millimolal. Very small amounts of GdCl 3 decrease strongly T 1 of the water protons.