Molecular heats of formation

First-principle computation of gas-phase molecular heats of formation by definition requires the gas-phase heats of formation of the elements ... 

For chemical purposes, substitution of total energy hypersurfaces by those based on the heat of formation seems more reasonable, with the difference given by the zero point energy corrections. However, their calculations from first principles can be rather cumbersome (12) and, moreover, for a given variation of some nuclear coordinates it usually can be assumed that the change in zero point energy is small compared to that of the total energy. On the other hand, se eral semiempirical quantum chemical procedures which are appropriately parametrized often yield satisfactory approximations for molecular heats of formation (10) and, therefore, AH hypersurfaces have become rather common. 

Once the electronic energy has been calculated, the molecular heat of formation AW is calculated as follows ... 

What is necessary to compute a heat of formation, then, is to define the heat of formation of each hypothetical, unstrained atom type. The molecular heat of formation can then be computed as the sum of the heats of formation of all of the atom types plus the strain energy. Assigning atom-type heats of formation can be accomplished using additivity methods originally developed for organic functional groups (Cohen and Benson 1993). The process is typically iterative in conjunction with parameter determination. 

Eor some atom types, thermodynamic data may be lacking to assign a reference heat of formation. When a molecule contains one or more of these atom types, the force field cannot compute a molecular heat of formation, and energetic comparisons are necessarily limited to conformers, or other isomers that can be formed without any change in atom types. 

Table XVII.-—The Molecular Heats of Formation of the Halides (Cals, per...

Assuming that nitroguanidine explodes to produce carbon dioxide, water, carbon monoxide, hydrogen, and nitrogen,21 assuming that the equilibrium constant for the reaction, CO + H20 C02 + H2, is 6, and that the molecular heat of formation at con-... 

A knowledge of molecular heats of formation and chemical bond dissociation energies has always been regarded as fundamental to the understanding of chemical bonding and reactivity. This chapter deals with the extent and reliability of our knowledge of these... 

J. C. Grossman and L. Mitas, Phys. Rev. Lett., 79, 4353 (1997). FFigh Accuracy Molecular Heats of Formation and Reaction Barriers Essential Role of Electron Correlation. 

For neutral organic compounds there are only some 3000 good quality data for molecular heats of formation, AfH°[M]. These have been carefully selected by Pedley and co-workers over an extended period. The standard heat of formation of an entity, Af/r, is defined as the heat absorbed or released when 1 mole of the species is formed from its constituent elements at 298 K and at 1 atm pressure. This is represented by an equation, e.g. 

Erev = ri rgy barrier of reverse reaction n = number of atoms in a molecule AfH[M] = molecular heat of formation. 

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