摘要:

Three distinct theoretical approaches, of varying sophistication, to our understanding of the equilibrium structure of molecular crystals may be discerned as a result of several publications during the past few years. In the first, which is largely due to Kitajgorodskij, the intermolecular potential is obtained by summing individual, empirically based, atom-atom potential functions which take the form

ISSN:0369-1152    

DOI:10.1080/15421406908082729

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The accuracy of present-day X-ray and neutron diffraction studies is such that root-mean-square vibrational amplitudes of atoms in complex structures can be determined to an accuracy of 0.01 Å or better. These amplitudes are usually in agreement with spectroscopically derived frequencies where these are available. With the aid of suitable models, amplitudes of rigid body rotations, translations, screw motions, and of internal rotations may be obtained. Such information is invaluable in the interpretation of spectra from complex crystals and in the understanding of intermolecular forces. Several examples from recent work at Brookhaven will be given.

ISSN:0369-1152    

DOI:10.1080/15421406908082730

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

A number of accurate diffraction studies are discussed which have yielded information of theoretical chemical interest. The information is two-fold: accurate atomic parameters can be obtained and compared with theoretical calculations through bond length-bond order curves and through analysis of molecular geometry. In addition, more direct information on chemical bonding can be obtained from combined X-ray and neutron diffraction analyses. The experimental charge density obtained in this way can be compared with theory without use of empirical correlation curves. It is concluded that accurate diffraction data have a potential for the study of chemical bonding which is as yet largely unexplored.

ISSN:0369-1152    

DOI:10.1080/15421406908082731

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

Recent studies of the physical and chemical properties of organic solids have revealed that crystalline imperfections may play a major role in determining these properties. As a result, efforts have been made to examine and characterize defects in organic crystals. The experimental techniques used for this purpose are outlined and the present state of knowledge summarized.

ISSN:0369-1152    

DOI:10.1080/15421406908082732

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

Efforts have been made to detect and characterize individual dislocations, small-angle boundaries and extended (nonequilibrium) lattice vacancies, formed by the annihilation of dislocations, in single crystals of anthracene. Only those dislocations emergent at the basal (OOl) faces may be identified using the dislocation-etch-pit technique; but some information regarding dislocations which glide in the basal plane may be deduced from deformation and cleavage studies. Three distinct kinds of experiments have been carried out on a variety of crystals, the dislocation content of which ranges from 102cm−2(for vapor-grown) to 108cm−2(for deformed melt-grown crystals).

ISSN:0369-1152    

DOI:10.1080/15421406908082733

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

Nuclear magnetic relaxation in organic crystals is strongly influenced by processes of motion, such as the rotation of molecular substituents, the reorientation of molecules in the lattice, and the diffusion of molecules through the lattice. In company with studies of dielectric relaxation, heat capacity, and structure, nuclear magnetic resonance (NMR) experiments are valuable in examining these processes. In this paper, we review some advantages and limitations of the NMR method, and show correlations with other methods of studying molecular motions. NMR studies over a range of temperatures and moderate pressures are described for a variety of organic solids, including long-chainn-alkanes, planar molecules such as benzene, and nearly spherical molecules such as camphor and hexamethylbenzene.

ISSN:0369-1152    

DOI:10.1080/15421406908082734

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The ultraslow-motion NMR technique has been used to measure translational diffusion coefficients in theplasticcrystal adamantane;

ISSN:0369-1152    

DOI:10.1080/15421406908082735

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

This paper reviews: (1) methods by which bond rupture, and ionization, can be produced in organic solids; (2) methods of detecting trapped reaction intermediates; (3) illustrative experiments dealing with the yields and the decay of trapped electrons and free radicals. Factors to be considered in deciding whether trapped electrons and radicals decay by random reaction with potential reacting partners, by combination with sibling partners, or by combination with other predestined partners are discussed. It is shown that radical decay rates do not show consistent correlation with viscosity.

ISSN:0369-1152    

DOI:10.1080/15421406908082736

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

High-energy bombardment is well-known to produce luminescence in condensed systems which contain suitable scintillators. Such light emission has been observed for weak scintillators like benzene in sufficiently dilute solutions in aliphatic hydrocarbons and, more recently, even in pure liquid benzene. It is not observed in pure liquid alkanes.

ISSN:0369-1152    

DOI:10.1080/15421406908082737

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

Paramagnetic relaxation characteristics of trapped electrons in γ-irradiated, glassy methanol, ethanol and 2-methyltetrahydrofuran (MTHF) at 73 °K were studied by power saturation techniques. The spin-spin relaxation time is constant with radiation dose at low doses and decreases at higher doses. Together with data on the total spin concentration the results are interpreted in terms of a “spur” model in which the initial spatial distribution of radiation-produced, trapped electrons and radicals is non-uniform. According to this model the spur radius is about 59 Å in methanol and 72 Å in MTHF. These radii compare with 42 Å for trapped electrons in alkaline ice. The radius within which electrons are trapped appears to increase with decreasing polarity of the matrix. The ratio of local spin concentration to the sample average spin concentration is greater than one at low doses in all matrices.

ISSN:0369-1152    

DOI:10.1080/15421406908082738

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor