Band 46, Heft 10

The density matrix formalism is used to simulate motional averaging in the 2 H-NMR spectra of reorienting ND + 4 ions. The development of the spectra under increasing jump frequency about a single C 3 or C 2 axis is followed. Next we assume a hierarchy of axes in terms of activation energies sufficient to reach extreme narrowing conditions for some axes before activating jumps about a next one. Primary reorientations about the fastest C 3 or C 2 symmetry axes define the shape of spectra, the width of which is then stepwise reduced by fast reorientations about the subsequent axes in a postulated sequence of statistically uncorrelated jumps.

The study by Fourier transform (FT) infrared (IR) spectroscopy of the fundamental vibrational bands v12 and v5 of furazan yields the origins of these bands with a statistical uncertainty of 10 -6 cm -1 , which leads to an estimated absolute uncertainty of 10 -4 cm -1 . The values are v° 12 = 952.6123 cm -1 and v° 5 = 1.005.3536 cm -1 . They confirm the values previously deduced from laser/microwave double resonance (LMDR) experiments. Previous results for the molecular constants of the vibrational ground state and of the two vibrationally excited states, as obtained by double resonance modulation (DRM) microwave spectroscopy alone, are confirmed and refined. Advantages brought about through the combination of the DRM microwave and the FT-IR technique are outlined.

We consider inelastic backscattering of electrons with initial energy of tens and hundreds of keV by plane-parallel homogeneous and sandwiched targets. Basing on the invariance principle, we find expressions that describe the dynamics of the changes in the energy spectra of electrons reflected into a given solid angle that occur with increase of the thickness of films of different materials on substrates of finite and infinite thickness. We substantiate a procedure of linearizing the equations for the reflection function obtained by the method of invariant imbedding. We obtain an analytical solution of linearized equations in the form of a series in Legendre polynomials. A comparison with experimental data shows that the theory developed gives an adequate description of the process of electron backscattering.

The refractive indices (n o , n e ) and densities of two mesogens have been measured in their liquid crystalline and liquid phases. The molecular polarizabilities (x o , x e ) were evaluated by Vuks' and Neugebauer's relations. The polarizabilities thus obtained are compared with those estimated from the bond polarizabilities, and the orientational order parameters, <P 2 >, are compared with the mean field theory of Maier and Saupe, the modified mean field theory of Humphries, James and Luckhurst. and the continuum theory suggested by T. E. Faber. We have also calculated the three order parameters (<P 2 ), τ, σ) describing the smectic A phase following McMillan's model. Possible causes of the discrepancy are discussed.

Sharp lower and upper bounds for the Wiener index (W) of a connected (n, m)-graph are reported; n = number of vertices, m = number of edges. The mean isomer degeneracy of W is estimated and is shown to unboundedly increase with increasing n. Thus the isomer-discriminating power of W is confirmed to be very low in the case of large molecules.

After a discussion of the one-component Schrödinger (1926) and the four-component Dirac (1928) representation of hydrogen it is shown that the six-component electrodynamic picture turns out to be considerably simpler and clearer. The computational effort is reduced to a fraction.

Two versions of the effective field method have been applied to calculate lower bounds to the energy eigenvalues of the small systems He, Li + , and Li. Although our modified method leads to an improvement compared with the originally proposed scheme, the results are still far from being satisfying.

Two Molecular Dynamics simulations have been performed where a Pt(100) surface is covered with three layers of water molecules and a lithium or an iodide ion is placed additionally in the boundary layer. The flexible BJH model of water is employed in the simulations and the ion-water, platinum-water and platinum-ion potentials are derived from molecular orbital calculations. The simulations extended over 7.5 ps at an average temperature of 298 K. The effect of the Pt(100) surface on the ionic hydration is demonstrated by the comparison of the radial distribution functions, the orientation of the water molecules and their geometrical arrangement in the first hydration shells of the ions in the boundary layer with those in a 2.2 molal bulk Lil solution.

A recently developed flexible three-site model for methanol was employed to perform a Molecular Dynamics simulation of a 0.6 molal NaCl solution. The ion-methanol and ion-ion potential functions were derived from ab initio calculations. The structural properties of the solution are discussed on the basis of radial and angular distribution functions, the orientation of the methanol molecules, and their geometrical arrangement in the solvation shells of the ions. The dynamical properties of the solution - like self-diffusion coefficients, hindered translations, librations, and internal vibrations of the methanol molecules - are calculated from various autocorrelation functions.

The structures of α-X-cyclopropyl and α-X-isopropyl radicals (X = H, O - , OH, NH 2 , CH 3 , NO 2 , CHO, CN, F and CF 3 ) are reported using MINDO-Forces MO method. In both radicals the planar structure is prefered over the pyramidal. All the substituents are stabilizing; the O - group is the most stabilizing substituent, while F group has little effect. O - , OH, F substituents are electron releasing groups on cyclopropyl ring, while NO 2 , CHO and CF 3 are electron widthdrawing. For isopropanes and x-X-isopropyl radicals, O - , OH, NH 2 and F act as electron releasing, while NO, and CF 3 act as electron withdrawing. The strain energies of the cyclopropyl radicals (43.8-23.7 kcal/mol) are compared with those of similarly substituted cyclopropanes. Dipole moments and spin densities are reported.

The boron and nitrogen hyperfine structure in the rotational spectra of aminodifluoroborane has been investigated and the quadrupole coupling constants of 11 B and nitrogen have been determined. We get the following results for the nuclear quadrupole coupling constants: Χ aa ( 11 B) = - 1.971 (6) MHz, X bb ( 11 B) = 0.500(11) MHz, X cc ( 11 B) - 2.471 (11) MHz, and X aa ( 14 N) = 0.890 (5) MHz, X bb ( 14 N) = 2.303 (7) MHz, X cc ( 14 N) = - 3.193 (8) MHz. Additionally we determined rotational and centrifugal distortion constants according to Watson's A reduction.

We report on first experiments with our pulsed molecular beam microwave Fourier transform (MB-MWFT) spectrometer using a special nozzle which allows high voltage discharges within the nozzle orifice. Under these conditions we observed low J rotational transitions in highly excited vibrational states of carbonyl sulfide and sulfur dioxide, and also rotational lines of the SO radical in the 3 Σ - electronic ground state and both the vibrational ground and first excited state.

Bartis recently proposed a theory which should allow to determine whether impurities or defects are responsible for the premelting phenomena in crystalline solids. We have tested this model on several orientationally disordered molecular crystals studied by proton NMR spectroscopy. This showed that extended defects, and more likely grain boundaries are at the origin of the premelting phenomena in these crystals.

The complex permittivity of the title solution system (c≤1.5 mol/1) was measured at frequencies ranging up to 72 GHz at 20 °C. Apart from the conductivity contribution there are two distinguishable relaxation regions, which are ascribed to the solvent (at higher frequencies) and to solvated ionic species (at lower frequencies).

In contrast to arenes, the fluorescence of protonated azaarenes in fluid solution is more readily quenched by methyl iodide than by silver Perchlorate. This is probably due to the Coulomb repulsion between the fluorescent ion and the silver ion, thus inhibiting the formation of a non-fluorescent complex.