Organic molecules sublimation enthalpy

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. 

Figure 7.5 shows a plot of experimental [ 10] molar sublimation enthalpies, showing a weak correlation with molecular weight. Points above average are for crystals of very polar compounds or for hydrogen-bonded crystals, points below average pertain to non-polar compounds or molecules that, for several reasons, pack less efficiently in the crystal. A typical value for the sublimation enthalpy of a medium-size organic... 

On the basis of published data for enthalpies of formation, sublimation, and vaporization, the dissociation enthalpies of terminal N—O bonds, DH°(N—O), in various organic compounds including nitrile oxides, were calculated and critically evaluated (18). The derived DH°(N—O) values can be used to estimate enthalpies of formation of other molecules, in particular nitrile oxides. N—O Bond energy in alkyl nitrile oxides was evaluated using known and new data concerning kinetics of recyclization of dimethylfurazan and dimethylfuroxan (19). 

In this chapter, intermolecular forces are viewed as complications and nuisances it is the molecule per se that is of interest. Therefore, unless explicitly noted to the contrary, any species of interest in this chapter is to be assumed in the (ideal) gas phase. Most organic compounds are naturally liquids or solids under the thermochemically desired conditions, much less as found by the synthetically or mechanistically inclined chemist. Corrections are naturally made by using enthalpies of vaporization (v) and of sublimation ), defined by equations la and lb ... 

Organic thermochemistry usually deals with molecules in the gaseous state in order to study their intrinsic stabilities in the absence of a crystal lattice, intermolecular bindings in the liquid state, or solvation forces. Therefore the determination of the enthalpy of vaporization or the enfrialpy of sublimation is an essential step in obtaining the enthalpy of formation in die gas phase. The enthalpy of sublimation can be obtained by combination of the enthalpy of vaporization and the enthalpy of fusion (equation (12)). The entibialpy of sion is easily and reliably obtained by DSC. 

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