Miguel Castro

Bonding of benzene molecules on the surface of neutral and charged Fe₇ clusters, which have pentagonal bipyramids (PBP), was studied by means of all–electrons density functional calculations. Dispersion corrections were done with the BPW91-D2 method using the 6-311++G(2d,2p) basis sets. With two less coordinated equatorial sites (bonded to four iron atoms) and one axial site (bonded to five atoms), a triangular face of Fe₇ emerges as the basic unit for the absorption of benzene moieties. Bonding of benzene (Bz) on such triangle yields the ground state (GS) for Fe₇Bz, Fe₇Bz^–, and Fe₇Bz⁺. Without dispersion, in the GS of Fe₇Bz^–, the ligand is η⁶ coordinated with a single equatorial iron site, and in the GSs of Fe₇Bz₂ and Fe₇Bz₂^–, each benzene moiety is η⁶ bonded on opposite equatorial sites. However, BPW91-D2 yields GS structures for Fe₇Bz₂, Fe₇Bz₂^–, and Fe₇Bz₂⁺, where the absorption is done on opposite triangles. Therefore, dispersion corrections are crucial for a proper study of Fe₇Bz₂. The multiplicities (M = 2S + 1, where S is the total spin) of these species, 17, 16, and 18, respectively, are smaller than those of Fe₇(23), Fe₇^–(22) and Fe₇⁺(24) showing important quenching of the magnetic moment of Fe₇. Bond dissociation energies (BDE), in kcal/mol, for Fe₇Bz (32.7), Fe₇Bz⁺ (47.3), and Fe₇Bz^– (27.2) show bigger (smaller) values for the cation (anion). A similar picture was found for the BDEs of Fe₇Bz₂. Ionization energies, 5.37 and 4.94 eV, for m = 1 and 2 are smaller than that of Fe₇, 6.00 eV; which is due to delocalization of the electrons through the network of 3d-π bonds. Electron affinities of Fe₇Bz_1,2 are also smaller than that of Fe₇, being mainly due to the increased repulsion.

The ligand versus solvent behavior of Ni⁺(C₆H₆)_3,4 complexes was studied using density functional theory all-electron calculations. Dispersion corrections were included with the BPW91-D2 method using the 6-311++G(2d,2p) basis set. The ground state (GS) for Ni⁺(C₆H₆)₃ has three benzene rings 3d−π bonded to the metal. A two-layer isomer with two moieties coordinated η³–η² with Ni⁺, and the other one adsorbed by van der Waals interactions to the Ni⁺(C₆H₆)₂ subcluster, i.e., a 2 + 1 structure, is within about 8.4 kJ/mol of the GS. Structures with 3 + 1 and 2 + 2 ligand coordination were found for Ni⁺(C₆H₆)₄. The binding energies (D₀) of 28.9 and 26.0 kJ/mol for the external moieties of Ni⁺(C₆H₆)_3,4 are much smaller than that for Ni⁺(C₆H₆)₂, 193.0 kJ/mol, obtained also with BPW91-D2. This last D₀ overestimates somehow the experimental value, of 146.7 ± 11.6 kJ/mol, for Ni⁺(C₆H₆)₂. The abrupt fall for D₀(Ni⁺(C₆H₆)_3,4) shows that such molecules are bound externally as solvent species. These results agree with the D₀(Ni⁺(C₆H₆)₃) < 37.1 kJ/mol limit found experimentally for this kind of two-layer clusters. The ionization energies also decrease for m = 2, 3, and 4 (580.8, 573.1, and 558.6 kJ/mol). For Ni⁺(C₆H₆)_3,4, each solvent moiety bridges the benzenes of Ni⁺(C₆H₆)₂; their position and that of one internal ring mimics the tilted T-shape geometry of the benzene dimer (Bz₂). The distances from the center of the external to the center of the internal rings for m = 3 (4.686 Å) and m = 4 (4.523 Å) are shorter than that for Bz₂ (4.850 Å). This and charge transfer effects promote the (C^δ−–H^δ+)_int dipole−π_ext interactions in Ni⁺(C₆H₆)_3,4; π–π interactions also occur. The predicted IR spectra, having multiplet structure in the C–H region, provide insight into the experimental spectra of these ions.

Interactions of a singly negatively charged iron atom with water molecules, Fe^––(H₂O)_n≤6, in the gas phase were studied by means of density functional theory. All-electron calculations were performed using the B3LYP functional and the 6-311++G(2d,2p) basis set for the Fe, O, and H atoms. In the lowest total energy states of Fe^––(H₂O)_n, the metal–hydrogen bonding is stronger than the metal–oxygen one, producing low-symmetry structures because the water molecules are directly attached to the metal by basically one of their hydrogen atoms, whereas the other ones are involved in a network of hydrogen bonds, which together with the Fe^δ−–H^δ+ bonding accounts for the nascent hydration of the Fe^– anion. For Fe^––(H₂O)_3≤n, three-, four-, five-, and six-membered rings of water molecules are bonded to the metal, which is located at the surface of the cluster in such a way as to reduce the repulsion with the oxygen atoms. Nevertheless, internal isomers appear also, lying less than 3 or 5 kcal/mol for n = 2–3 or n = 4–6. These results are in contrast with those of classical TM⁺–(H₂O)_n complexes, where the direct TM⁺–O bonding usually produces high symmetry structures with the metal defining the center of the complex. They show also that the Fe^– anions, as the TM⁺ ions, have great capability for the adsorption of water molecules, forming Fe^––(H₂O)_n structures stabilized by Fe^δ−–H^δ+ and H–bond interactions.

Linear oligomerization of 3,5-dimethyl benzyl alcohol is induced by a montmorillonite clay (Tonsil Optimum Extra), producing 1,3,5,7-tetramethyl-9,10-dihydro-anthracene, which, by loss of protons results in the product 1,3,5,7-tetramethylanthracene. It was also found that the compounds 4-(3´,5´-dimethylbenzyl)-1,3,5,7-tetramethyl-9,10-dihydroanthracece and 4-(3´,5´-dimethylbenzyl)-1,3,5,7-tetra-methylanthracene were formed from 1,3,5,7-tetramethyl-9,10-dihydroanthracene. 1,3,5,7-Tetramethylanthryl radical cation was formed from 1,3,5,7-tetramethyl-9,10-dihydroanthracene; it was characterized by Electronic Paramagnetic Resonance (EPR). On the other hand, a theoretical analysis was performed, allowing the rationalization of the observed products and some of the key reaction steps.

The ground state, GS, geometries for Fe_1,2(benzene)_1,2 clusters were determined by means of all-electron calculations done with the density functional BPW91/6311++G(2d,2p) method. The stability of Fe(C₆H₆)_1,2 is accomplished by the formation of Fe–C η⁶ coordinations in the half-sandwich and sandwich GS structures, which are of lower spin, 2S = 2 (S is the total spin) than the Fe atom, 2S = 4. Departures from η⁶ bonding occur on [Fe(C₆H₆)₂]⁻, since the GS of this anion, of less symmetric sandwich geometry, presents η⁶ and η² coordination, which is mainly due to the enhanced repulsion of the adsorbed benzene units. On Fe₂(C₆H₆)_1,2 the stronger Fe₂ bond, compared to the Fe–C ones, produce rice-ball geometries, where the Fe₂ molecule, although with a longer bond length, is preserved. For example, in Fe₂(C₆H₆), Fe₂ lies perpendicular or parallel to the benzene ring depending on the charge of the complex, and in [Fe₂(C₆H₆)₂]^±0, ±1 the benzene ligands are placed above and beneath the molecular axis of Fe₂ , producing highly compact structures. Multiple decker sandwich states, where Fe₂ is not retained, are located more than 20 kcal mol⁻¹ above the GS levels. Electron affinities, agreeing well with experimental results, ionization and binding energies, and vibrational frequencies were also determined, providing insight on the complexes.

This study involved quantum mechanical calculations to explain the chemical behavior of the lactone ring of aflatoxin B1, which is a carcinogenic hazardous compound. The aflatoxin B1 compound, produced by the fungi Aspergillum flavus, was studied with the B3LYP/6-311+G(d,p) method; its reactivity properties were accounted for by means of the calculated geometrical and electronic parameters. The results obtained indicate that the fused A, B, C, and D rings of aflatoxin adopt a continuous planar conformation. The carbon atom of the lactone group presents a highly electrophilic character, since the population analysis yields a high positive charge for this atom, whereas high negative charges were recorded for both oxygen sites of that group. Thus, in an acidic aqueous medium, the oxygen atoms could be protonated and the carbon site may suffer a nucleophilic attack by water. Accordingly, the OC−O bond length has been lengthened substantially. So it was demonstrated that the lactonic ring of aflatoxin B1 is hydrolyzed under acidic conditions by an acid-acyl bimolecular mechanisms, A_AC2, suggesting the deletion of its carcinogenic properties.

The ephedrine derivative, (H₂ceph), yields [Cu(Hceph)₂], showing a CH⋯Cu(II) agostic interaction; while in the analogous compound [Cu(Hcpse)₂], with pseudoephedrine (H₂cpse), that interaction is absent, despite the fact that these two diasteromers differ only in the orientation of the methyl and phenyl groups: erythro in H₂ceph and threo in H₂cpse. The X-ray crystal structure of [Cu(Hceph)₂], indicates a Cu⋯HC length of 2.454 Å and the theoretical study reveals the formation of a Cu⋯HC bond since the associated electronic density shows both a bond critical point and a bond ring critical point.