Volume 40, Issue 11

Analytical expressions for the components of the vector potential and magnetic field of a current-carrying elliptic arc filament of arbitrary length are derived. All the expressions developed consist of only known functions such as Jacobian elliptic functions, complete and incomplete elliptic integrals of the first, second and third kind, and thus permit a compact timesaving efficient computation algorithm.

The frequency dependence of the spin-lattice relaxation time T 1 was measured at three temperatures near one of the Zeeman-tunneling level matching resonances for pentamethylbenzene. These measurements are correlated with 71 temperature dependence data from the literature. It is shown that the frequency dependence of the Zeeman-torsion coupling time cannot be explained in terms of the semiclassical perturbation theory using time correlation functions. A three bath polarization transfer model is also employed and the applicability of both models discussed. Zeeman-torsion coupling is further investigated using a saturation sequence and the results are compared with the predictions of the three bath polarization transfer model.

The deuterium spin-lattice relaxation time, T 1 , of H 2 O/D 2 O mixtures is measured at 298.2 K. The relaxation rate, T 1 -1 , is found to increase with increasing deuterium atom fraction, n, the plot of T 1 -1 vs. n exhibiting a small depature from linearity. A general equation T 1 -1 (n) for the H 2 O/D 2 O system is formulated. The temperature dependence of T 1 is investigated in the temperature range 278.2 K to 298.2 K for n = 6.8 x 10 -3 , 6.8 x 10 -2 , 0.244, 0.500, and 0.997. On the assumption that the electric field gradient parameters (e 2 q Q/h and δ) are independent of n and temperature, an effective correlation time, τ c,eff , is derived from the T 1 data. Relatively large isotope effects on τ c,eff are found; possible reasons for the existence of such isotope effects are discussed in terms of a simple Debye model. The mean activation enthalpy (Δ ≠ H) and entropy (Δ ≠ S) for the relaxation process within the temperature range studied are derived on the basis of Eyring's absolute rate theory and the temperature dependence of τ c,eff . Both activation parameters are found to increase linearly with n: Δ ≠ H/kJ mol -1 = 18.2 + 2.46 n, Δ ≠ S/J K -1 mol -1 = 37.5 + 6.77 n.

In this paper two new experiments in microwave Fourier transform spectroscopy are presented: The first one gives an experimental evidence of the polarization inhomogeneity that occurs when a microwave pulse passes through a waveguide which is filled with a molecular sample at low pressure. In the second experiment a similar technique is applied to a double resonance experiment in order to show polarization inhomogeneity effects caused by the pump signal. The theoretical treatment is given in terms of the density matrix formalism.

The temperature dependence of the ratio of formation of the two isomeric products (head-to-tail- and head-to-head-photodimer) of the photodimerization of 9-methylanthracene was measured in chloroform, ethanol, diethyl ether, and methylcyclohexane as solvents. The ratio increases or decreases with temperature depending on the solvent. One of the isomeric photoproducts was converted photochemically into the other one, indicating an equilibrium of excimer precursors of the products.

The paper presents and illustrates a method which uses numerical integration of the van der Waals envelope(s) to calculate with desired accuracy the molecular van der Waals volume and the three-dimensional molecular shape descriptor defined as the twin-number [OV(α, β); NOV(β, α), where OV and NOV represent the overlapping and, respectively, the nonoverlapping van der Waals volumes of the molecules α and ß superimposed according to appropriate criteria.

A methodology to incorporate the three-dimensional molecular shape descriptor (3 D-MSD) into a quantitative structure-activity relationship is discussed in detail. The 3 D-MSD is calculated and correlated with K iapp values for a set of 2,4-diamino-5-benzylpyrimidines which inhibit E. coli DHFR. The correlation (n = 22, r = 0.95, s = 0.214, F = 55.10) indicates that the polarization interaction dominates the enzyme-inhibitor interactional pattern.

The paper presents and illustrates and improved version of the SIBIS algorithm. SIBIS is a self-consistent type algorithm which is well suited to delineate the main steric features of biological receptor, identify missing relevant atoms and/or presence of superfluous atoms in the molecules studied, and to provide an approximate estimate for the receptor bulk tolerance. SIBIS calculations for a set of substituted benzoates in the anti-p-(p'-azophenylazo)benzoate system yield an excellent QSAR (n = 18, r = 0.989, s = 0.192, F= 20S.16) which is further supported by the information correlation coefficients, R (/?'s express quantitatively the pairwise degree of the molecular shape relatedness); the optimal receptor steric map is in agreement with experimental evidences.

Sodium hydroxide reacts with α -(4-nitrobenzylthio)-acetic acid in aqueous-dioxane media to give 4,4'-diformylazoxybenzene as the main product besides 4,4'-dicarboxyazoxybenzene and a nitrone acid. This reaction was kinetically studied in presence of excess of alkali in different dioxane-water media at different temperatures. It started by a fast reversible a-proton abstraction step followed by two consecutive irreversible first-order steps forming two intermediates (α -hydroxy, 4-nitrosobenzylthio)-acetic acid and 4-nitrosobenzaldehyde. The latter underwent a Cannizzaro's reaction, the products of which changed in the reaction medium into 4,4'-diformylazoxybenzene and 4,4'-dicarboxyazoxybenzene. The rate constants and the thermodynamic parameters of the two consecutive steps were calculated and discussed. A mechanism was put forward for the formation of the nitrone acid. Other six 4-nitrobenzyl, aryl sulphides were qualitatively studied and they gave mainly 4,4'-diformylazoxybenzene beside 4,4'-dicarboxyazoxybenzene or its corresponding azo acid.

In the presence of spin-orbit interaction, six coupling schemes are possible for cubic complexes. Five of these coupling schemes have been worked out although only three, called strong-field, weak-field and (j, j) coupling schemes, are well-known. We show the derivation of the sixth coupling scheme and review briefly the significance and utility of the six coupling schemes.

The polarizability, α, and second hyperpolarizability, γ, of the phenyl cation are computed by employing the CHF-PT-EB-CNDO method. Variations of the properties induced by changes in the structure of the cation are discussed.

Our investigations concerning the protonaffmity of aliphatic amines have been confined so far to primary unbranched species. In order to gain further information about the applicability and the limitations of the statements derived from the results of the previous calculations, we extended our investigations to some tertiary, secondary and branched primary amines. Correlations between the experimental protonaffinities and the Mulliken charges of the nitrogen atom and the NH x -group (x = 0, 1, 2) of the amines, as well as those of the NH x + 1 ,-group of the corresponding ammonium cations have been found. However, these correlations only hold within the different groups of amines (primary, secondary and tertiary) and not between them. A somewhat better correlation throughout all groups is found if one compares the charges of the protons in the NH x + 1 -groups of the ammonium cations with the experimental protonaffinities of the parent amines but deviations from the expected order are found in this case, too. This shows the limited value of charges as calculated by the methods used here, in predicting the relative order of protonaffinities of amines if they belong to different groups.

The density, the electrolytic conductivity, and the viscosity of molten silver chloride doped with LiCl, KCl, CsCl, and BaCl 2 have been measured in the concentration range 0 ≦x dopant ≦0.1 and temperature range 740 K ≧ Tm ≦900 K. The molar conductivity, Λ, decreases linearly with x dopant , the curve being independent of the charge of the added cation. Only in the case of LiCl is a very small increase of Λ observed. The viscosity of the melt remains virtually unchanged when doped with alkali halides. Addition of BaCl 2 leads to a strong increase of the viscosity. The findings show that for these systems no correlation exists between the conductivity and the viscosity. It is further suggested that there is a strong influence of the long range Coulomb interaction on the viscosity of molten salts.

Formyl-ring conjugation has been examined in benzaldehyde, 2,4,6-trimethyl- and 2,4,6-tri t- butyl-benzaldehyde, and the ab initio STO-3G π-electron distribution in benzaldehyde has been analysed in terms of the mesomeric and π-inductive effect of the formyl group. The electric dipole moment determined in benzene solution at 30.0 °C, and formyl-carbon n.m.r. chemical shift, recorded in deuterochloroform, of 2,4,6-tri-t-butylbenzaldehyde indicate a nonplanar conformation with a formyl-group rotational angle (ϕ) of about 65°. The dipole moment of 2,4,6-tri- t-butylthiobenzaldehyde, also in benzene solution at 30.0 °C, is consistent with a nearly orthogonal model [ϕ ~ 90°), in agreement with the X-ray structure in the crystalline phase giving ϕ = 89.1 ° .

Structure calculations are reported for a quasilattice associated with the icosahedral group. The diffraction pattern is computed in a one-center and long-range approximation and compared to experiments on Al + 14% Mn. Edge and vertex positions of the quasilattice are computed in a projection onto a plane with a layer structure.

Results of the optical absorption study of vanadyl ion doped in magnesium ammonium sulphate hexahydrate, rubidium sulphate and potassium dihydrogen phosphate single crystals at RT are reported. The nature of optical bands suggests a C 4v symmetry of the Vanadyl complexes in conformity with the EPR results. Powder EPR data and optical data are correlated to obtain the MO coefficients.

A novel upper bound for the π-electron energy (E π ) is described. The formula, valid for benzenoid hydrocarbons yields values closer to the true E π than the McClelland's or even Gutman's estimates.

The application of the Gauss-Chebyshev quadrature to the integral type formula for the total π-electron energy (E π ) leads to the well-known relation between the Hosoya modified topological index Z̃ and E π . Possible alternative formulae are discussed and tested on some benzenoid hydrocarbons.