Structure sublimation

A. Warshel and S. Lifson, ]. Chem. Phys., 53, 582 (1970). Consistent Force Field Calculations. II. Crystal Structures Sublimation Energies, Molecular and Lattice Vibrations, Molecular Conformations and Enthalpies of Alkanes. 

Yellow powder or bronze-yellow micalike particles. M.p. 963° (in sealed tube) d 5.018. Heat of formation —51.8 kcal. per mole (25°). Solubility in HgO (g. NiBrg/lOO g. solution) 56.6 (19°C) 61.0 (100°C). Soluble in methyl and ethyl alcohols, acetone and quinoline insoluble in toluene. Crystal structure sublimed product, C 19 type unsublimed product, variable between C 6 and V19 types. 

CONSISTENT FORCE FIELD CALCULATIONS. II. CRYSTAL STRUCTURES, SUBLIMATION ENERGIES, MOLECULAR AND LATTICE VIBRATIONS, MOLECULAR CONFORMATIONS, AND ENTHALPIES OF ALKANES. 

Electrostatic interactions can be most simply modeled as the Coulomb interaction between partial atomic charges, while the repulsion-dispersion part is usually described by a Lennard-Jones or, more accurately, an exp-6 form, each of which contains parameters that must be fixed. High-quality empirically fitted parameter sets have been developed, where the atom-atom interactions are parameterized to reproduce the structures, sublimation enthalpies and, sometimes, further observable properties of organic molecular crystals [73,74]. Their use has been very effective in CSP. Nonempirical approaches to fitting intermolecular force fields, where the parameters are derived from quantum mechanical calculations, have occasionally been applied for CSP [75-78], but these are currently limited to small molecules, so currently lack relevance for typical pharmaceutical molecules. 

Warshel, A., Lifson, S. Consistent force field calculations. II. Crystal structures, sublimation energies, molecular and lattice vibrations, molecular cmiRmnations, and enthalpies of alkanes. J. Chem. Phys. 53, 582-594 (1970)... 

Charlton, M. H., Docherty, R., and Hutchings, M. G., Quantitative structure-sublimation enthalpy relationships studied by neural networks, theoretical crystal packing calculations and multilinear regression analysis, J. Chem. Soc. Perkin Trans. 2, 2203, 1995. 

A three-step procedure was used to derive the CFF, resulting in a force field suitable for the calculation of properties of molecules in vacuo and in condensed phases. " First, non-bonded energy function parameters were derived from fits of crystal structures, sublimation energies, and gas phase dipole moment data. Then, a quantum mechanical force field was derived from fitting Hartree-Fock energies and energy derivatives of equilibrium and systematically distorted molecules... 

The melting and boiling points of the aluminium halides, in contrast to the boron compounds, are irregular. It might reasonably be expected that aluminium, being a more metallic element than boron, would form an ionic fluoride and indeed the fact that it remains solid until 1564 K. when it sublimes, would tend to confirm this, although it should not be concluded that the fluoride is, therefore, wholly ionic. The crystal structure is such that each aluminium has a coordination number of six, being surrounded by six fluoride ions. 

The advantages of this method are twofold (i) It is possible to observe minute changes in colour and structure before and during the process of melting. (2) It is possible to use a single crystal which, e.., is often obtained from a semi-micro sublimation. 

The approximate calculation of the surface energies of metals as a function of crystal structure described earlier uses the enthalpy of sublimation, s, and the co-ordination number to calculate the energy as a function of the atomic concentration on the surface. The atomic areas of the principal configurations are as follows ... 

As an example of a multilayer system we reproduce, in Fig. 3, experimental TPD spectra of Cs/Ru(0001) [34,35] and theoretical spectra [36] calculated from Eq. (4) with 6, T) calculated by the transfer matrix method with M = 6 on a hexagonal lattice. In the lattice gas Hamiltonian we have short-ranged repulsions in the first layer to reproduce the (V X a/3) and p 2 x 2) structures in addition to a long-ranged mean field repulsion. Second and third layers have attractive interactions to account for condensation in layer-by-layer growth. The calculations not only successfully account for the gross features of the TPD spectra but also explain a subtle feature of delayed desorption between third and second layers. As well, the lattice gas parameters obtained by this fit reproduce the bulk sublimation energy of cesium in the third layer. 

Tin(IV) halides are more straightforward. Snp4 (prepared by the action of anhydrous HF on SnCU) is an extremely hygroscopic, white crystalline compound which sublimes above 700°. The structure (unlike that of CF4, SiF4 and GeF4) is polymeric with octahedral coordination... 

In the vapour phase As is known to exist as tetrahedral Asa molecules with (As-As 243.5 pm) and when the element is sublimed, a yellow, cubic modification is obtained which probably also contains Asa units though the structure has not yet been determined because the crystals decompose in the X-ray beam. The mineral arsenolamprite is another polymorph, e-As it is possibly isostructural with metallic orthorhombic P. 

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