The determination of mercury in soils and related materials by colc-vapour atomic absorption spectrometry

A method for the determination of mercury in soils and related materials, by the cold-vapour, reduction-aeration absorption technique, is described. The mercury in the reduced sample solution is partitioned, by agitation, between the liquid phase and a fixed volume of air which is then blown through an absorption cell for measurement. Three preparative methods were developed, two of which use acid digestion and wet oxidation and the other direct oxidation in an oxygenflask combustion technique. Comparative analysis of natural samples by the three methods was used to validate the techniques developed, since no authenticated reference samples were available. For 1-g samples, the analytical range was 0.01–0.5 p.p.m.     

Dispersion energy between closed-shell and half-closed-shell molecules.an ab inito calculation from a perturbation procedure on LiHe

A perturbation procedure previously used for closed-shell molecules is generalized to the case of the interaction between closed-shell and half-closed-shell molecules In particular, the use of different denominators is considered and special attention is paid to the effect of small basis sets for LiHe.     

New experimental evidence for the energy corrected sudden scaling law

New measurements of rate constants for the rotationally inelastic process Na₂^*(j_o) + Xe → Na₂^*(j_f) + Xe show the superiority of the energy corrected sudden scaling law of DePristo and Rabitz over other such laws, including the statistical power gap law which we proposed earlier, particularly at high j_o. All rates decrease as an inverse power of the energy transfer.     

Automated nephelometric determination of polyvinyl-pyrrolidone in salazopyrin.

An automatic nephelometric method is described for the determination of polyvinyl-pyrrolidone (PVP) in Salazopyrin. PVP is separated from the other constituents of Salazopyrin on a two-layer anion and cation exchanger in 96—99 % yield. The PVP is determined nephelometrically after precipitation with perchloric acid, in an AutoAnalyzer II system at 30 samples per hour with a relative standard deviation of 0.5 %. The nephelometric response of the precipitation reaction is strongly dependent upon reaction time and, because of the precipitation reagent used, on the molecular weight of the sample. An investigation of the influence of these factors is presented.     

Laser induced fluorescence in NO2: absolute fluorescence cross sections and quantum yields

Spectrally resolved collision-free absolute fluorescence cross sections have been measured, for NO₂ excited, by the second harmonic of the Nd: YAG laser. The total cross section into the 523.0–650.0 nm range was found to be 2.3 × 10^?20 cm². The measured absorption cross section of 1.46 × 10^?19 cm² implies a quantum yield of 16% over this range.     

Properties of the X 1Σ state of BF

A perturbation theoretical approach for treating electron correlation has been used to calculate the potential energy curve and dipole moment function of BF near its equilibrium bond length. A dipole moment of 0.89 D (B^?F⁺) is predicted at R_e. When the bond is stretched by ≈0.2 Å the dipole reverses sign.     

Effects of geometric distortions and substituents on rates of intersystem crossing; diazomethane and pyrazoline

We report a qualititive perturbation-theoretic analysis of the effects of substituents on the spin-orbit coupling and on the density of states in diazomethane and pyrazoline systems. The importance of the form of the distortion which accompanies approach to the singlet-triplet crossing zone is also discussed with the aim of determining when reactions involving a spin multiplicity change can compete in rate with spin-conserving reactions.     

Molecular conformation of cyclenes: V. Cyclodecadienes

The strain energy of 1,3-, 1,4-, 1,5- and 1,6-cyclodecadienes has been calculated as a function of various geometric parameters. The relative stability of the minimum-energy forms is discussed and, where possible, compared with thermodynamic and electron diffraction data. In the 1,5- and 1,6-isomers agreement with experimental data is obtained if the transannular interaction between the double bonds is taken into account.     

Effect of collision and magnetic field on quantum beat in biacetyl

Molecular quantum beats are shown for the case of biacetyl, to vary strongly under the influence of weak magnetic fields. Values for dephasing constants for a gas-phase molecular system were determined. Information on sub-Doppler level splittings in the intersystem-crossing process is obtained.     

Determination of N-nitrosamines by high-performance liquid chromatographic separation with voltammetric detection

The highly polar, nonvolatile N-nitrosamines, N-nitrosoproline and N-nitrosodiethanolamine, are determined by high-performance liquid Chromatographic separation with electrochemical detection using d.c. voltammetry, normal pulse voltammetry, and differential pulse voltammetry. The influence of flow rate and pulse time are determined, and detection limits on the order of 10^-7 M are obtained.     

Franck-condon factors for I2 lines arising from Ar, K and He-Ne laser excitation

We report Franck-Condon factors for the I₂ B → X transitions excited by several lines of Ar⁺, Kr⁺ and He-Ne lasers and observed in fluorescence. The most recent and accurate molecular constants of Luc have been used in the calculation.     

Charge-transfer properties of the hydrogen bond: Part 6. Charge-transfer theory and dipole moments of hydrogen-bonded complexes

The enhancement of the dipole moments of hydrogen-bonded complexes are discussed using Mulliken's charge-transfer theory.A linear relation is found between the ratio a/b and the ionization potential of the donor, I^V_D. This behaviour is similar to that previously found for halogen charge-transfer complexes [6].     

Ab inttio structural analysis of some saturated 3- and 4-membered rings

Complete geometry optimizations of the ground states of ten 3- and 4-membered ring compounds have been carried out by ab initio SCF calculations using the gradient method at the 4–21 level. For sulfur, an optimized 3-3-21 basis has been constructed, and the importance of including d-functions is demonstrated. The main purpose of the study was to determine local orientations and geometries of the molecular CH₂-groups, information that is very difficult to obtain from experiment. A simple interaction model seems to account for the CH₂-group distortions in the 3-membered rings considered here, and also for similar structural deformations previously reported in a series of cyclopropyl derivatives.     

Resonance Raman scattering near a threshold for dissociation

The threshold behavior of Raman amplitudes for photon energies near dissociation of the intermediate potential surface of a diatomic molecule is determined. Structure in the Raman amplitudes is calculated and found to be due to resonance of the intermediate potential surface.     

Intermolecular coupling of the CO stretching vibrations in liquid acetone: an isotopic dilution study

The separation Δv between the isotropic and anisotropic component of the CO stretching Raman band was studied in mixtures of acetone-¹⁶O and acetone-¹⁸O. An explanation is proposed for the observed invariance of the aniso tropic frequency. The isotropic bandwidth and the concentration threshold for the appearance of Δv are discussed.     

The 9-barbaralyl cation, 1,4-bishomotropylium ion and some related C9H9 carbocations : Electronic structures, stability and reactivity

The electronic structures of the 9-barbaralyl cation(1), the 1,4-bishomotropylium ion (5) and some related C₉H₉⁺ isomers have been studied byab initio STO-3G calculations. The stability of the cyclopropylcarbinyl cation 1 and the homoaromatic ion 5 as compared with their allylic counterparts is explained in terms of delocalization and MO interactions between molecular fragments. Both the symmetry of, and the distance between, molecular fragments are of importance for the electronic structure, and even symmetry-allowed interactions are negligible in the absence of favourable geometrical distortions. The non-classical ion 3 with D_3h symmetry presents special problems where other factors, such as strain, are important for the total energy. A choice between proposed mechanisms for the degenerate rearrangements of 1 can now be made with confidence.