Comparing Energies of Structurally Different Molecules

These heat of formation parameters may be considered as shifting the zero point of Fpp to a common origin. Since corrections from larger moieties are small, it follows that energy differences between systems having the same groups (for example methyl-cyclohexane and ethyl-cyclopentane) can be calculated directly from differences in steric energy. 

Deriving such heat of formation parameters requires a large body of experimental Ai/f values. For many classes of compound there are not sufficient data available. Only a few force fields, notably MM2 and MM3, attempt to parameterize also heats of formation. Most force fields are only concerned with reproducing geometries and possibly conformational relative energies, for which the steric energy is sufficient. 

To convert the steric energy to heat of formation, terms can be added depending on the number and types of bond present in the molecule. This again rests on the assumption of transferability, e.g. all C-H bonds have a dissociation energy close to lOOkcal/mol. A heat of formation parameter can be assigned to each bond type, and the numerical 

Relative values, however, should ideally reflect conformational energies. If all atom and bond types are the same, as in cyclohexane and methyl-cyclopentane, the energy functions have the same zero point, and relative stabilities can be directly compared. This is a rather special situation, however, and stabilities of different molecules can normally not be calculated by force fleld techniques. For comparing relative stabilities of chemically different molecules such as dimethyl ether and ethyl alcohol, or for comparing with experimental heat of formations, the zero point of the energy scale must be the same. 

In electronic structure calculations, the zero point for the energy function has all particles (electrons and nuclei) infinitely removed from each other, and this common reference state allows energies for systems with different numbers of particles to be directly compared. If the same reference is used in force field methods, the energy function becomes an absolute measure of molecular stability. The difference relative to the normal reference state for force field functions is the sum of all bond dissociation energies, at least for a simple diagonal force field. If correction terms are added to the 

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From the several continuum solvation methods available in the literature, the PCM model and its derivatives seem to be most used for pK calculations. In fact, the best pK results reported so far have been obtained with one of the PCM-based methods. Nevertheless, the fact that these calculations differ in many respects precludes any comparative evaluation of the different solvation models employed. More systematic studies are needed, taking into consideration the level of theory (method and basis sets) employed, the various continuum solvation models, and different classes of compounds. The introduction of solvent molecules into the solute cavity, in order to better represent the short-range solute-solvent interactions, must also be carefully examined. There is no apparent relationship between the structure of the solute and the number of intracavity solvent molecules that best reproduces the solvation energy. Hence, this best number is generally established on a trial-and-error basis. Also, the use of a such hybrid (discrete -I- continuum) description of the solvent molecules seems to be inconsistent with the fact that for some of the solvation models employed, the effects of the first solvation shell have been already incorporated when parameterizing the cavity. 

Is the second step of the overall reaction for R=Me (N-methylphthalimide + hydrazine —> phthalimide hydrazide + methylamine) exothermic or endothermic Will higher temperatures accelerate or inhibit the reaction Is the structure drawn above for phthalimide hydrazide its lowest-energy form or are either the imine or diimine tautomers preferred Compare energies for the hydrazide and imine and diimine tautomers. Examine the geometry of phthalimide hydrazide and any low energy tautomer, and draw the Lewis structure(s) that best describes it. Can your Lewis structures account for the energy differences Examine electrostatic potential maps for all three molecules. Which molecule(s) are stablized by favorable electrostatic interactions Which are destabilized Can this help explain the energy differences Elaborate. 

In this figure, the activation energies of N2 dissociation are compared for the different reaction centers the (111) surface structure ofan fee crystal and a stepped surface. Activation energies with respect to the energy of the gas-phase molecule are related to the adsorption energies of the N atoms. As often found for bond activating surface reactions, a value of a close to 1 is obtained. It implies that the electronic interactions between the surface and the reactant in the transition state and product state are similar. The bond strength of the chemical bond... 

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