A Semiquantitative Description of Electrostatics and Polarization Substituent Effects: Gas-Phase Acid−Base Equilibria as Test Cases

Condensed-to-atoms electronic Fukui functions within the framework of spin-polarized density-functional theory

A simple formalism devised to calculate the condensed-to-atoms Fukui function [R. R. Contreras, P. Fuentealba, M. Galván, and P. Pérez, Chem. Phys. Lett. 304, 405 (1999)] has been further analyzed within a spin-polarized density-functional theory framework. The model is based on a frozen-core approximation to these local reactivity indices [M. Galván, A. Vela, and J. L. Gázquez, J. Phys. Chem. 92, 6470 (1988)], giving us an extended local reactivity description of systems based on the frontier spin-up and spin-down molecular orbitals. Degenerate molecular spin orbitals have been explicitly included in our model equations. Computational results for the nitric oxide (NO) and some simple carbene systems are presented in order to test the model. These quantities have been discussed in the context of changes both in charge density and spin density within the context of electron charge transfer or spin-polarization processes.

Methyl and Phenyl Substitution Effects on the Proton Affinities of Hydrides of First and Second Row Elements and Substituent Effects on the Proton Affinities of Ring Carbons in Benzene: A DFT Study

Theoretical calculations using B3LYP density functional theory (DFT) with the 6-311++G(d,p) basis set have been performed to determine proton affinities (PAs) of a series of H-X compounds and the corresponding methyl- (H(3)C-X) and phenyl- (Ph-X) substituted derivatives with a variety of proton acceptor atoms, such as C, O, N, F, Si, P, S, Cl, etc. Our results illustrate an interesting substituent effect on PAs. The PAs of ring carbon atoms for a series of monosubstituted benzene molecules (Y-C(6)H(5); Y = F, Cl, CH(3), OCH(3), NH(2), PH(2), OH, SH, SiH(3), CN, CF(3), and NO(2)) have also been estimated. Correlations between proton affinities of H-X, H(3)C-X, and Ph-X and substituent effects on the PAs of the ring carbon atoms for a series of monosubstituted benzene molecules have been studied. It has been observed that substituent effects on the PAs of the ring carbon atoms follow a good Hammett-type correlation.

Empirical Energy−Density Relationships Applied to the Analysis of the Basicity of Strong Organic Superbases

Site selectivity and reactivity in some strong organic super bases RN=C(NR(2))R, containing several possible sites for protonation, have been analyzed qualitatively in terms of global and local chemical reactivity descriptors defined in the context of conceptual density functional theory. On the basis of the condensed-to-site Fukui function values, the protonation site is predicted to be located at the imino nitrogen. Linear relationships between global energy-dependent quantities and the variation of the Fukui function (or the local softness) at the protonation site have been also found. These linear relationships can be interpreted within a local HSAB rule framework, as observed for instance for the gas-phase basicity of alkylamines and gas-phase acidity of alcohols and thioalcohols. This work extends the range of applicability of reactivity models developed for the treatment of substituent effects based on empirical energy-density Hammett-like relationships.