Marcos Mandado

QTAIM N-center delocalization indices as descriptors of aromaticity in mono and poly heterocycles.

The implementation of the n‐center electron delocalization indices, n‐DIs, and n‐order electron localization indices, n‐LIs, within the framework of the quantum theory of atoms in molecules, QTAIM, is performed. n‐DIs are shown to be very useful to study the local aromaticity in monocyclic and polycyclic compounds. Total and π n‐DIs from n = 4 to 7 were computed for a series of typical 4, 5, 6, and 7‐center aromatic and antiaromatic rings. For n ≥ 5 the π n‐DI accounts for the 95% of the total n‐DI and can be employed alone to measure the aromaticity. A scaling factor on the n‐DIs is required in order to compare the aromaticity of [5c‐6e] and [6c‐6e] rings, the same correction allows to estimate the relative aromatic stabilization of polycyclic compounds using the sum of its values for individual rings. This is called Effective Scaled Electron Delocalization, ESED. The comparison with other aromaticity indices reflects a good correlation between ESED and both resonance energies, and HOMA indices. The most important differences between scaled π n‐DIs and NICS(0) indices are found for compounds that contain rings with different number of centers or π electrons.

Do 1,2-ethanediol and 1,2-dihydroxybenzene present intramolecular hydrogen bond?

A study of the intramolecular hydrogen bond (IHB) in 1,2-ethanediol and 1,2-dihydroxybenzene (catechol) was carried out using the QTAIM theory. Atomic and bond properties defined within this theory were calculated for different donor–acceptor distances, Hd⋯Oa, in both molecules, and different H–Oa–C–C dihedral angles for 1,2-dihydroxybenzene, optimising the remaining geometry. Though no conformer of both compounds present IHB, it appears when the Hd⋯Oa distance is reduced in both molecules or when the H–Oa–C–C angle is rotated in 1,2-dihydroxybenzene. The evolution of integrated and local properties follows the criteria of Koch and Popelier for hydrogen bond formation when the interatomic distance is modified, but display the opposite trends when the IHB is formed by rotating the dihedral angle.

Chemical graph theory and n-center electron delocalization indices: a study on polycyclic aromatic hydrocarbons.

Relations between aromaticity indices derived from chemical graph theory and those based on 6‐center electron delocalization are investigated for a series of polybenzenoid hydrocarbons. Aromatic stabilization obtained by means of the effective scaled electron delocalization is highly correlated to the resonance energy, RE, obtained both from SCF MO calculations and conjugated ring circuits model. Local aromaticity of benzene rings is discussed using two different criteria, in one of them aromaticity is just given by the cyclic π‐electron conjugation of the ring, whereas terms involving more than one ring are also considered in the other one. Indices derived from chemical graph theory and those obtained from the 6‐center electron delocalization give rise to the same local aromaticity. Moreover, 6‐center electron delocalization provides more quantitative information.

Interplay Between Hydrogen Bond Formation and Multicenter π-Electron Delocalization : Intermolecular Hydrogen Bonds

The interplay between aromatic electron delocalization and intermolecular hydrogen bonding is thoroughly investigated using multicenter delocalization analysis. The effect on the hydrogen bond strength of aromatic electron delocalization within the acceptor and donor molecules is determined by means of the interaction energies between monomers, calculated at the B3LYP/6-311++G(d,p) level of theory. This magnitude is compared to variations of multicenter electron delocalization indices and covalent hydrogen bond indices, which are shown to correlate perfectly with the relative values of the interaction energies for the different complexes studied. The multicenter electron delocalization indices and covalent bond indices have been computed using the quantum theory of atoms in molecules approach. All the hydrogen bonds are formed with oxygen as the acceptor atom; however, the atom bonded to the donor hydrogen has been either oxygen or nitrogen. The water-water complex is taken as reference, where the donor and acceptor molecular environments are modified by substituting the hydrogens and the hydroxyl group by phenol, furan, and pyrrole aromatic rings. The results here shown match perfectly with the qualitative expectations derived from the resonance model.

Transferability of energies of atoms in organic molecules

The variation of virial-corrected energies of atoms in organic molecules (AIMs) with the lengths of the attached alkyl chains and the nature of remote substituents is shown to be largely an artifact of the correction procedure itself. Thus, it is demonstrated that the virial correction for energies of AIMs should be avoided despite the fact that it produces values that sum up to the total molecular energies. Consequently, the assessment of transferability of AIMs should be carried out with either uncorrected total energies (i.e., negative kinetic energies) or atomic properties other than energy.

Approximate transferability in alkanols

Abstract The approximate transferability of OH, CH2, and CH3 groups in unbranched primary alkanols has been studied by comparing several atomic and bond properties of the 12 smallest members of this series. These properties were obtained by employing the Atoms in Molecules theory on HF/6-31++G∗∗//HF/6-31G∗ and QCISD/6-31++G∗∗//QCISD/6-31G∗ wave functions. The properties computed at both levels follow parallel evolutions along the series, which allow to conclude that the OH group can be considered approximately transferable along the set of 1-alkanols larger than ethanol, whereas ethanol and methanol present specific hydroxyls. The electron population of the oxygen atom is lower than in ethers, aldehydes, and ketones. The hydroxyl affects significantly the CH3 and CH2 groups that are in positions α, β, γ, or δ, whereas those groups separated from the oxygen by more than 4 bonds can be considered similar to those included in a n-alkane. CH2 groups in the series can be classified into 6 quasi-transferable fragments taking into account their position with regard to the OH (α, β, γ, δ, or beyond (ν)), and respect to the terminal CH3 (attached to it or not). The simultaneous occurrence of both facts gives rise to four specific CH2 fragments: α-CH2 in ethanol, β-CH2 in 1-propanol, γ-CH2 in 1-butanol, and δ-CH2 in 1-pentanol. It has been found that all the CH2 and CH3 fragments that are γ or δ to the OH group do not differ significantly from the corresponding fragments of a dialkyl ether. The energy of oxygen, CH2 and CH3 fragments depend on the molecular size. The effect of the basis set size error on this quantity has been investigated, concluding that the molecular-size dependence is not an artifact due to it. The destabilization experienced by the oxygen atom for a common increase in the molecular size in alcohols is equivalent to that of ethers and smaller than the one displayed by aldehydes and ketones. It was also concluded that the effect due to the variation of the molecular size is independent on the number of alkyl chains that are increased.

Theoretical chemical characterization of phosphate‐metal–humic complexes and relationships with their effects on both phosphorus soil fixation and phosphorus availability for plants

BACKGROUND Previous studies showed that phosphate can be complexed by humic acids (HA) through stable metal (M) bridges (PMHA). We studied the thermodynamic properties of PMHA and their relationships with the ability of PMHA to both decrease soil P fixation and increase P availability for plants. With this aim, we studied the theoretical stability of PFeHA, PAlHA and PCaHA by molecular modelling methods in relation to the degree and intensity of P absorption in soils and the ability of plants to take up complexed P. RESULTS A density functional theory (DFT) quantum chemical study enabled us to obtain stable structures for the three PMHA complexes in water solution. The theoretical stabilities (ΔG⁰) were consistent with that for apparent stability obtained by Scatchard method, PFeHA ≥ PAlHA > PCaHA, though the differences were clearer by the DFT method. Also the reduction of soil P fixation and the release of P from PMHA in the presence of an anionic resin confirmed the stability order of the different PMHA. Plant studies confirmed the ability of diverse plant species to take up both P and metal complexed in PMHA. CONCLUSION The results indicated the potential efficiency of PMHA-based fertilizers to optimize P fertilization for crops cultivated in soils with high P fixation ability.

How does aromaticity rule the thermodynamic stability of hydroporphyrins

Several measures of aromaticity including energetic, magnetic, and electron density criteria are employed to show how aromatic stabilization can explain the stability sequence of hydroporphyrins, ranging from porphin to octahydroporphin, and their preferred hydrogenation paths. The methods employed involve topological resonance energies and their circuit energy effects, bond resonance energies, multicenter delocalization indices, ring current maps, magnetic susceptibilities, and nuclear-independent chemical shifts. To compare the information obtained by the different methods, the results have been put in the same scale by using recently proposed approaches. It is found that all of them provide essentially the same information and lead to similar conclusions. Also, hydrogenation energies along different hydrogenation paths connecting porphin with octahydroporphin have been calculated with density functional theory. It is shown by using the methods mentioned above that the relative stability of different hydroporphyrin isomers and the observed inaccessibility of octahydroporphin both synthetically and in nature can be perfectly rationalized in terms of aromaticity.

Aromaticity in spin-polarized systems: Can rings be simultaneously alpha aromatic and beta antiaromatic?

The partition of the multicenter electron delocalization indices and the nucleus independent chemical shift indices into alpha and beta contributions in open-shell systems has been performed. In general it is shown that a full understanding of aromaticity in these systems cannot be achieved by restricting the calculations to the global properties but by dissecting these properties into alpha and beta terms. The 4n+2- and 4n-aromaticity rules for singlet and triplet annulenes, respectively, reduce to a general aromaticity rule when the alpha and beta terms are studied separately. This new rule allows us to extend the concept of conflicting aromaticities to radical systems that are simultaneously alpha-aromatic and beta-antiaromatic or vice versa. The existence of such systems is demonstrated here by means of multicenter electron delocalization indices and nucleus independent chemical shifts. Finally, the global aromatic/antiaromatic character of these radical systems is estimated by means of aromatic stabilization energy, which is shown to be either slightly positive or slightly negative, thus reflecting the small aromatic/antiaromatic character of these radicals and reinforcing the conclusions obtained with aromaticity indices.

Revisiting the calculation of condensed Fukui functions using the quantum theory of atoms in molecules

The analysis of previously reported shortcomings of the condensed Fukui functions obtained making use of the quantum theory of atoms in molecules indicates these drawbacks are due to the inadequacy of the definition employed to compute them and not to the partitioning. A new procedure, which respects the mathematical definition and solves these problems, is presented for the calculation of condensed Fukui functions for atomic basins defined according to the quantum theory of atoms in molecules. It is tested in a set of 18 molecules, which includes the most controversial reported cases.