Norberto Castillo

Atomic contributions to bond dissociation energies in aliphatic hydrocarbons

This paper explores the atomic contributions to the electronic vibrationless bond dissociation enthalpy (BDE) at 0 K of the central C-C bond in straight-chain alkanes (C(n)H(2n+2)) and trans-alkenes (C(n)H(2n)) with an even number of carbon atoms, where n=2, 4, 6, 8. This is achieved using the partitioning of the total molecular energy according to the quantum theory of atoms in molecules by comparing the atomic energies in the intact molecule and its dissociation products. The study is conducted at the MP2(full)6-311++G(d,p) level of theory. It is found that the bulk of the electronic energy necessary to sever a single C-C bond is not supplied by these two carbon atoms (the alpha-carbons) but instead by the atoms directly bonded to them. Thus, the burden of the electronic part of the BDE is primarily carried by the two hydrogens attached to each of the alpha-carbons and by the beta-carbons. The effect drops off rapidly with distance along the hydrocarbon chain. The situation is more complex in the case of the double bond in alkenes, since here the burden is shared between the alpha-carbons as well as the atoms directly bonded to them, namely, again the alpha-hydrogens and the beta-carbons. These observations may lead to a better understanding of the bond dissociation process and should be taken into account when locally dense basis sets are introduced to improve the accuracy of BDE calculations.

Fluorine-fluorine spin-spin coupling constants in aromatic compounds: correlations with the delocalization index and with the internuclear separation.

This paper describes a new empirical approach for the evaluation of fluorine-fluorine spin-spin coupling constants (J(FF)) in aromatic compounds. The correlations between J(FF) and the delocalization index calculated within the framework of the theory of atoms in molecules (AIM) and with the fluorine-fluorine internuclear separation are investigated. Both the internuclear separation and the delocalization index are found to be highly correlated with J(FF). A regression model in which the experimental J(FF) coupling constant is fitted exponentially to the internuclear separation and linearly to the delocalization index yields a squared correlation coefficient as high as 0.96 for a data set consisting of 33 coupling constants spread over a range of 85 Hz.

The effect of multiplicity on the size of iron(II) and the structure of iron(II) porphyrins.

The displacement of the iron(II) atom from the porphyrin plane in iron(II) porphyrin complexes is investigated with respect to the spin state of iron(II) employing density functional theory. In this study the quantum theory of atoms in molecules (QTAIM) is used to show that the atomic volume of iron is smaller in the quintet state of imidazolium ligated iron(II) porphyrin than in the triplet state. This is consistent with what has been found for free atoms and contradicts the original interpretation of structural studies with X-rays, which assumed that the out-of-plane displacement of iron from the porphyrin ring in the quintet state is due to the increased spatial size of the high-spin iron atom. The bonding environment of the iron atom is analyzed with respect to the electron density (ρ) at the bond critical points (BCPs). It is found that in the quintet state, relative to the triplet state, there is a stronger bonding interaction between iron and the nitrogen atoms of the porphyrin despite a longer bond length. It has previously been suggested that the weakening of these bonds is the cause of the out-of-plane displacement of iron. Since this is not the case, this implies that the magnitude of the bonding interaction between the iron atom and the axial ligand has a more significant role in the domed structure of the quintet state.

Conformational Analysis of Arabinofuranosides: Prediction of (3)JH,H Using MD Simulations with DFT-Derived Spin-Spin Coupling Profiles.

A molecular dynamics (MD) investigation on a series of oligo-α-arabinofuranosides (1-8) using the AMBER force field and the GLYCAM carbohydrate parameter set is reported. The validation of the method was carried out by direct comparison of experimental vicinal proton-proton coupling constants ((3)JH,H) with those obtained by using an empirically determined Karplus equation and density functional theory (DFT)-derived relationships specifically tailored for α-arabinofuranosyl systems. A simple code was developed to implement the determination of (3)JH,H by applying these relationships to the probability distributions of rotamers and ring conformations displayed by the simulations. The empirical Karplus relationship and the DFT-derived equations yielded, in most cases, the same trend as experiment for intra-ring (3)JH,H values. This direct comparison circumvents additional sources of errors that may arise from the assumptions introduced by the deconvolution procedures often used to calculate population of rotamers and ring conformations from experimental (3)JH,H.

Bond length and the electron density at the bond critical point: X--X, Z--Z, and C--Z bonds (X = Li-F, Z = Na-Cl).

The aim was to investigate the relationship between the bond length and the electron density at the bond critical point in homonuclear XX and ZZ and heteronuclear CZ bonds (X = Li‐F, Z = Na‐Cl). The d,ρc pairs were obtained from 472 target bonds in DFT‐optimized (B3LYP/6‐311+G(d,p)) small molecular species. These species were selected arbitrarily but with a view to maximize the range widths WR for each atom combination. It was found that (i) with one clear exception, the d(A − A) means (A = X or Z) correlate linearly with the bond lengths d(A2) of the respective diatomic molecules; (ii) the d(A − A) means correlate parabolically with n, the formal number of valence electrons in the atoms of the bond; and (iii) with increasing sample size N the ratio WR(ρc)/WR(d) appears to converge toward a representation f [WR(ρc)/WR(d)]N→∞ characteristic of A. Detailed analysis of the d,ρc relationship has shown that by and large simple power regression accounts best for the DFT data. The regression coefficients of d = aρ  c−b and ρc = αd−β (b, β > 0) vary with n in a seemingly irregular manner but one that is consistent with simple chemical notions. The d(A2) can be approximated in terms of multilinear MO electron occupancies.

An Atoms in Molecules Study of the Halogen Resonance Effect.

We report a detailed study by means of the theory of atoms in molecules (AIM) of the resonance effect exhibited in systems where a halogen is adjacent to a carbon-carbon double bond. Moreover, we have carried out a comparable study of the respective saturated halohydrocarbons and hydrocarbons, as well as the related unsaturated hydrocarbons. The valence shell charge concentration (VSCC) of the atoms in systems that exhibit the halogen resonance effect is considerably different from that of the systems where only the electron withdrawing inductive effect is present. Our analysis of the bonded maximum charge concentration and the electronic properties at the bond critical points clearly indicate that the carbon-carbon double bond is strongly distorted as a result of the halogen resonance effect. Population analyses show that the halogen resonance effect is a donor effect, but the opposing electron-withdrawing inductive effect is stronger. Moreover, the analysis in terms of link points of the VSCCs of the carbons accounts for the observed position-dependence of electrophilic aromatic substitution in α- and β-halonaphthalenes.