Presence Of Heavy Atoms Research Articles

Room-temperature phosphorescence properties of biphenyl and some polychlorinated biphenyls

Room-temperature phosphorescence (RTP) signals on solid substrates depend on certain experimental parameters, including type of solid substrate, drying and cooling times of the sample—solid substrate system, presence of heavy atoms, and presence of quenching concomitants. RTP of 13 solid substrates is examined using 4-chlorobiphenyl as a model compound to determine the best conditions for maximum RTP intensity. It is found that Whatman No. 1 filter paper is the most suitable solid substrate for RTP of 4-chlorobiphenyl. The best method for drying the sample is heating in an oven at 110 °C for 15 min and cooling in a pure nitrogen gas stream inside the sample compartment for 10 min. RTP characteristics of biphenyl, several polychlorinated biphenyls (PCB) congeners, and Aroclors are presented in the absence and presence of thallium ion as an external heavy atom. It is found that it is not possible, under the experimental conditions described, to determine one congener in the presence of another. RTP is, however, shown to be a selective, rapid, inexpensive technique for the determination of individual PCBs with relative standard deviation between 2 and 10 and about 100-pg limits of detection for most compounds.

Kinetics of the excited states of aromatic molecules in the presence of heavy atoms in rigid solutions

The response functions of the decay of the singlet and triplet states of aromatic molecules, in the presence of the heavy-atom perturbers in rigid solutions, are derived using a binomial distribution for averaging and assumption of additivity of the contributions from different perturber molecules. All processes involving the change of multiplicity are taken into account. Direct and indirect excitation modes and different monitoring modes are considered. The response functions and the relations between different luminescence characteristics considered here are valid for all concentrations of the perturber.

Centric, partially bicentric and bicentric intensity statistics in the presence of heavy atoms

Centric, partially bicentric and bicentric intensity statistics in the presence of heavy atoms

PHASE REFINEMENT ON THE CRYSTAL STRUCTURES CONTAINING HEAVY ATOMS

A modified tangent formular taking account of the presence of heavy atoms is given in this paper. It is of the form:The modified tangent formular has the advantage of being able to eliminate the squaring effect. Practical examination shows that it is superior to the conventional tangent formular and can resolve the ambiguity due to the coexistence of both enan-tiomorphs.

The influence of space group symmetry, pseudosymmetry and atomic scattering power on the path of the residual

AbstractThe residual (<Iobs ‐ Icalc2>/ <I2 obs > is a very useful function in structure evaluation procedures. Using simple properties of the Patterson function algebraic expressions for the residual are derived. It is shown that the numerical value depends on various factors such as the presence of heavy atoms or pseudosymmetry in the structure. It appears that coincidence of interatomic vectors increases the value of the residual.The Influence of the origin peak in the Patterson function on the residual is described.

THE PROBLEM OF AMBIGUITY IN PATTERSON ANALYSIS TREATED BY DIRECT METHOD

In crystal structure determination with presence of heavy atoms, Patterson method and methods based on it, i.e. the heary atom method, the isomorphous replacement method and the anomalous scattering method are conventionally employed. When applying these methods ambiguities in results often arise. It had been suggested by one of the present authors in 1965 that the ambiguities can be eliminated by using the direct method. In recent years, similar investigations are continued abroad. These methods not only present their distinctive feathures in the work of structure analysis of crystals of small molecules, but their further developements will be effec-tive in extension the field of application of direct methods in structure analysis of biological macro-molecular crystals, making contribution on improving the state of art of these studies. The application of the method in the case of centric symmetry have been demonstrated in the published work of one of the present authors. Another successful example on the case of non-centric symmetry is given here. Although only the problem of ambiguity of Patterson method is treated in this example, but the same approach can be applied to that of the isomorphous replacement method and the anomalous scattering method.

Radiative and radationless decay of exciplexes of 1,12-benzperylene

Many molecules in their excited states react with other species having suitable electron donor or acceptor properties to form complexes (exciplexes) stable only in the excited state. This letter reports a study of the modes of decay of a series of exciplexes in which the donor molecule (1,2-benzperylene) formed exciplexes with a series of dimethylaniline derivatives. By measuring the flourescence and intersystem crossing quantum yields, together with the fluorescence lifetimes of the exciplexes, it was possible to derive the rate constants for fluorescence, radationless decay to the ground state, and intersystem crossing. The first two decay processes were found to show a marked sensitivity to the exciplex energy, while the intersystem crossing rate constant was affected only by the presence of heavy atoms.

Spin Distribution of Triplet States in Various Environments

Abstract Measurements have been made of the zero-field parameters of both ground state and metastable triplets in a variety of environments. These parameters provide an averaged distance between the unpaired electrons and thus are sensitive to the spin density distribution. The distance is found to increase in the series of matrices saturated fluorocarbons, saturated hydrocarbons, benzene derivatives and naphthalene. A detailed examination indicates that transfer of charge is occurring with electrons being donated into unfilled orbitals. Donation can occur from filled orbitals of the solvent into halffilled orbitals of the triplet or from the triplet to vacant orbitals of the solvent. The effect is essentially independent of local electric moments or the presence of heavy atoms so that polarization or spin-orbit contributions are likely to be of secondary importance.

Remarks on potential peak shapes and on the detection of light atoms in the presence of heavy atoms

Remarks on potential peak shapes and on the detection of light atoms in the presence of heavy atoms

The Born Approximation in Electron Diffraction

The Born approximation for the electron wave scattered by an atom has been universally employed for the interpretation of electron diffraction experiments directed toward the determination of molecular or crystal structure. There is some justification for this. Quantitative measurements of diffraction intensities performed to test the applicability of the approximation for electrons of the energies usually used in structure work have, on the whole, been regarded as satisfactory verifications. In structure work itself—in the work on gas molecules, for example—the considerable success resulting from it use has led to the feeling that even in the presence of heavy atoms there might not be serious error. There has perhaps been the implicit faith that whatever theoretical refinements were lacking would at least not alter substantially the structures to be derived. We have found, however, that there is a phase shift on scattering, not given by the first Born approximation (this is the familiar Born approximation, the first term of a power-series expansion in the parameter α = − Ze^2/hv), which is approximately proportional to atomic number, depends on angle of scattering, and may become large and important. The suggestion of the existence of this phase shift arose from the cases of uranium hexafluoride (UF_6) and some other gas molecules.