Density functional theory computations at the B3LYP/6-311+G(2d,2p) and BPW91/6-311G(d,p) levels were carried out for a series of 15 monosubstituted benzene derivatives to study dependencies between electronic structure parameters and experimental reactivity constants. An efficient and accurate computational approach for the evaluation of sigma(0) substituent constants for substituted benzene systems is outlined. It is based on the excellent linear correlation between the experimental reactivity constants and the theoretical electrostatic potential values (EPN) at the carbon atoms in the para and meta positions. The results underline the usefulness of the EPN as a local reactivity descriptor. Theoretical computations to assess the influence of water solvent using the SCIPCM method showed that the solvent enhances the overall effect of polar substituents by about 30%. The results obtained indicate also that the relative values of the sigma(0) constants are predominantly determined by intramolecular influences.

The aminolysis of esters is a basic organic reaction considered as a model for the interaction of carbonyl group with nucleophiles. In the present computational study the different possible mechanistic pathways of the reaction are reinvestigated by applying higher level electronic structure theory, examining the general base catalysis by the nucleophile, and a more comprehensive study the solvent effect. Both the ab initio QCISD/6-31(d,p) method and density functional theory at the B3LYP/6-31G(d) level were employed to calculate the reaction pathways for the simplest model aminolysis reaction between methylformate and ammonia. Solvent effects were assessed by the PCM method. The results show that in the case of noncatalyzed aminolysis the addition/elimination stepwise mechanism involving two transition states and the concerted mechanism have very similar activation energies. However, in the case of catalyzed aminolysis by a second ammonia molecule the stepwise mechanism has a distinctly lower activation energy. All transition states in the catalyzed aminolysis are 10-17 kcal/mol lower than those for the uncatalyzed process.

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The applicability of molecular electrostatic potential values at atomic sites as a reactivity descriptor for the process of hydrogen bonding is assessed for a series of complexes involving acetylene and diacetylene derivatives as proton donors and ammonia as a model proton acceptor. The acetylenic compounds studied were of the type R-CtC-H, where R represents H, F, Cl, CH 3 , CH 2 F, CHF 2 , CF 3 , CH 2 Cl, CHCl 2 , CCl 3 , CN, H-CtC, F-CtC, Cl-CtC. Density functional theory computations at the B3LYP/6-31G(d,p) level were employed. An excellent linear relation between the molecular electrostatic potential at the acetylenic hydrogen atom in the isolated acetylenes with the energy of hydrogen-bond formation is found. It is concluded that the value of the electrostatic potential at the acidic hydrogen atomic site can be used as a reactivity descriptor for the hydrogen bonding ability of the molecules studied.