Crystal environment

It is concluded that the cooperative effect observed is of long-range nature and therefore of elastic rather than of electronic origin. Recently, the additional suggestion has been made [138] that, due to intermolecular interactions in the crystal environment of [Fe(ptz)g](BF4)2, domains of iron(II) complexes interconvert together. The observed kinetics would then correspond to a first- or higher-order phase transition rather than to the kinetics which are characteristic for the conversion of isolated molecules. 

It is finally assumed that with all force constants and potential functions correctly specified in terms of the electronic configuration of the molecule, the nuclear arrangement that minimizes the steric strain corresponds to the observed structure of the isolated (gas phase) molecule. In practice however, the adjustable parameters, in virtually all cases, are chosen to reproduce molecular structures observed by solid-state diffraction methods. The parameters are therefore conditioned by the crystal environment and the minimized structure corresponds to neither gas phase nor isolated molecule [109],... 

For a recent discussion of the effects of the crystal environment on molecular structure see Bernstein (1992). 

The term center should be understood here as an optically active ion in a specific crystal environment. 

Other studies, such as infrared and Raman spectra of gaseous benzene, neutron diffraction studies of crystalline benzene, and electron diffraction and microwave spectral studies, are equally incapable, according to critical analysis [87AG(E)782], of resolving unanimously the Dih—Deh structural dilemma of the benzene molecule. Furthermore, no decisive conclusion could be drawn from photoelectron spectra or H—NMR spectrum measurements of benzene molecules in a liquid crystal environment. The latter experiments merely indicate that the average lifetime of a Dih structure (if it appears on the PES) is less than 10 4 sec corresponding to the energy barrier of the Dih- >D6h-+D h interconversion of approximately 12 kcal/mol. 

Many molecules contain chemically equivalent atoms, which, though in a different crystal environment, have, to a good approximation, the same electron distribution. Such atoms may be linked, provided equivalent local coordinate systems are used in defining the multipoles. In particular, for the weakly scattering hydrogen atoms, abundant in most organic molecules, this procedure can lead to more precisely determined population parameters. 

II should be noted lhat the proton affimlies of aD of the irinegatrve and dinegative anions are calculated by means of a Bom-Haber cycle. They are not experimentally accessAle a nee these ions have no existence outside of a stabilizing crystal environment—they would exothermcally expel an electron (see Chapter 2). 

EXAFS studies are few in this area an interesting one on solutions of ZnBr2 in EtOAc has indicated that quasi-solid clusters, resembling the crystal environment, persist even in dilute (0.05 M) solution.109 Biological applications are important. l463 14668jl... 

Not accounting for secondary bonds arising in crystals of compounds 94 due to shortened contacts between tellurium and halogen centers, the configuration of four-coordinated tellurium may be described as trigonal bipyramidal, the lone pair at tellurium being viewed as the phantom-ligand. In a crystal environment the coordination number of tellurium in... 

Excited states of Cr + complexes were explored by single crystal spectroscopy at low temperatures. In the dimeric [a Cr(0H)2Cra ] + the sharp 2E single excitations were used to determine orbital exchange parameters. Out-of-plane interactions are dominant. The complex CrCljt was studied in two exactly octahedral crystal environments. 

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