The hierarchy of localization basins: a tool for the understanding of chemical bonding exemplified by the analysis of the VO x and VO x + ( x =1-4) systems

Dimerization of quercetin, Diels-Alder vs. radical-coupling approach: a joint thermodynamics, kinetics, and topological study

Quercetin is a prototypical antioxidant and prominent member of flavonoids, a large group of natural polyphenols. The oxidation of quercetin may lead to its dimerization, which is a paradigm of the more general polyphenol oligomerization. There exist two opposing mechanisms to describe the dimerization process, namely radical-coupling or Diels-Alder reactions. This work presents a comprehensive rationalization of this dimerization process, acquired from density functional theory (DFT) calculations. It is found that the two-step radical-coupling pathway is thermodynamically and kinetically preferred over the Diels-Alder reaction. This is in agreement with the experimental results showing the formation of only one isomer, whereas the Diels-Alder mechanism would yield two isomers. The evolution in bonding, occurring during these two processes, is investigated using the atoms in molecules (AIM) and electron localization function (ELF) topological approaches. It is shown that some electron density is accumulated between the fragments in the transition state of the radical-coupling reaction, but not in the transition state of the Diels-Alder process. Graphical Abstract Quantum chemistry calculations of the dimerization process of quercetin show that a radical coupling approach is preferred to a Diels-Alder type reaction, in agreement with experimental results. Analysis of the bonding evolution highlights the reaction mechanism.

Theoretical study of the regio- and stereoselectivity of the intramolecular Povarov reactions yielding 5H-chromeno[2,3-c] acridine derivatives

The intramolecular Povarov (IMP) reactions involved in the synthesis of 5H-chromeno[2,3-c] acridine derivatives [Tetrahedron Lett., 2010, 51, 3071–3074] have been studied using density functional theory (DFT) methods.

Computation of Local and Global Properties of the Electron Localization Function Topology in Crystals

We present a novel computational procedure, general, automated, and robust, for the analysis of local and global properties of the electron localization function (ELF) in crystalline solids. Our algorithm successfully faces the two main shortcomings of the ELF analysis in crystals: (i) the automated identification and characterization of the ELF induced topology in periodic systems, which is impeded by the great number and concentration of critical points in crystalline cells, and (ii) the localization of the zero flux surfaces and subsequent integration of basins, whose difficulty is due to the diverse (in many occasions very flat or very steep) ELF profiles connecting the set of critical points. Application of the new code to representative crystals exhibiting different bonding patterns is carried out in order to show the performance of the algorithm and the conceptual possibilities offered by the complete characterization of the ELF topology in solids.

Chemical bonding in crystals: new directions

Analysis of the chemical bonding in the position space, instead of or besides that in the wave function (Hilbert) orbital space, has become increasingly popular for crystalline systems in the past decade. The two most frequently used investigative tools, the Quantum Theory of Atoms in Molecules and Crystal (QTAIMAC) and the Electron Localization Function (ELF) are thoroughly discussed. The treatment is focussed on the topological peculiarities that necessarily arise from the periodicity of the crystal lattice and on those facets of the two tools that have been more debated, especially when these tools are applied to the condensed phase. In particular, in the case of QTAIMAC, the physical and chemical significance of the bond paths for the very weak or the supposedly repulsive interactions, the distinctive features and the appropriateness of the several schemes that have been proposed to classify chemical bonds, and, finally, the relative importance of the local and integrated electron density properties for describing intermolecular interactions. In the case of the ELF, particular attention is devoted to how this function is formulated and to the related physical meaning, and to how can the ELF be chemically interpreted and properly analysed in crystals. Several examples are reported to illustrate all these points and for critically examine the answers obtained and the problems encountered. The discussed examples encompass the case of molecular crystals, Zintl phases, intermetallic compounds, metals, supported and unsupported metal-metal bonds in organometallics, ionic solids, crystal surfaces, crystal defects, etc. Whenever possible joint ELF and QTAIMAC studies are considered, with particular emphasis on the comparison of the bond description afforded by the ELF and the Laplacian of the electron density. Two recently proposed functions, the Localized Orbital Locator (LOL) and the Source Function in its integrated or local form are also presented, in view of their potential interest for stud ies of chemical bonding in crystals. The use of approximated ELF and LOL, as derived from the density functional form of the positive kinetic energy density, is also discussed.

Mechanism and selectivity of cinchona alkaloid catalyzed [1,3]-shifts of allylic trichloroacetimidates

Density functional theory calculations were used to investigate the [3,3]- and [1,3]-shifts of O-allylic trichloroacetimidates in the presence of cinchona alkaloids. Thermal [1,3]- and [3,3]-rearrangements proceed through concerted pseudopericyclic transition states to give the corresponding rearranged products. [1,3]-Rearrangement is catalyzed via a double S(N)2' mechanism in which syn addition of the nucleophile is exclusively preferred in both steps. The catalyzed mechanism is favored by a 6.3 kcal/mol free energy difference compared to the alternative [3,3]-rearrangement pathway. The fast-reacting enantiomer is predicted to be determined by the availability of the H-bonding interaction between the catalyst and the substrate.

An Overlooked Series of Gas Phase Diatomic Metal Oxide Ions that Are Long-lived

Although the "Golden" years of spectroscopy and the major studies on ionization processes now are behind us, as with many branches of science, much yet remains to be gleaned from such topics that is both full of interest and of significance to present day research. Presented here is one such overlooked example, an observation that relates to both these fields. An analysis is presented for the periodic table concerning the gas-phase thermochemical nature of MO+ and MO2+ ions. Unexpectedly, a pattern of 18 elements has been identified that exhibit the potential for having long-lived MO+ ions. Normally such molecular ions are expected to decay extremely rapidly by dissociative recombination with electrons, but in particular, 12 of this group behave not like molecules but rather as atomic ions. These are the diatomic oxide ions of Sc, Y, La, Zr, Hf, Ce, Pr, Nd, Pm, Gd, Tb, and Th. In the gas phase, they decay by much slower three-body recombination channels. As may be noted, these elements are located in the first two columns of the transition elements, among the earlier rare earths and an actinide. From all the elements, UO2+ is the only dioxide ion that behaves similarly. These findings now elevate the potential importance of these ions and should facilitate their spectral characterization. Moreover, subsequent comparisons with spectra of well-known isoelectronic and isovalent neutral monoxides and other diatomics will help in the stimulation of further theoretical advances. In addition, once characterized, an ease of spectrally monitoring such ionic states will provide a useful analytical tool.

Gas-Phase Oxidation of Isomeric Butenes and Small Alkanes by Vanadium-Oxide and -Hydroxide Cluster Cations

Bare vanadium-oxide and -hydroxide cluster cations (V(m)O(n)H(o)+, m = 2-4, n = 1-10, o = 0, 1) were generated by electrospray ionization in order to examine their intrinsic reactivity toward isomeric butenes and small alkanes using mass spectrometric techniques. Two of the major reactions described here concern the activation of C-H bonds of the alkene/alkane substrates resulting in the transfer of two hydrogen atoms and/or attachment of the dehydrogenated hydrocarbon to the cluster cations; these processes are classified as oxidative dehydrogenation (ODH) and dehydrogenation, respectively. For the dehydrogenation of butene, it evolved as a general trend that high-valent clusters prefer ODH resulting in the addition of two hydrogen atoms to the cluster concomitant with elimination of neutral butadiene, whereas low-valent clusters tend to add the diene with parallel loss of molecular hydrogen. Deuterium labeling experiments suggest the operation of a different reaction mechanism for V2O2(+) and V4O10(+) compared to the other cluster cations investigated, and these two cluster cations also are the only ones of the vanadium-oxide ions examined here that are able to dehydrogenate small alkanes. The kinetic isotope effects observed experimentally imply an electron transfer mechanism for the ion-molecule reactions of the alkanes with V4O10(+).

Comparative Study of the Bonding in the First Series of Transition Metal 1:1 Complexes M−L (M = Sc, ..., Cu; L = CO, N2, C2H2, CN-, NH3, H2O, and F-)

The nature of the chemical bonding in the 1:1 complexes formed by the fourth period transition metals (Sc, ..., Cu) with 14 electrons (N(2), CN(-), C(2)H(2)) and 10 electrons (NH(3), H(2)O, F(-)) ligands has been investigated at the ROB3LYP/6-311+G(2d) level by the ELF topological approach. The bonding is ruled by the nature of the ligand. The 10 electrons and anionic ligands are very poor electron acceptors and therefore the interaction with the metal is mostly electrostatic and for all metal except Cr the multiplicity is given by the [Ar]c(n)() configuration of the metallic core (n = Z - 20). The electron acceptor ligands which have at least a lone pair form linear or bent complexes involving a dative bond with the metal and the rules proposed previously for monocarbonyls hold. In the case of ethyne, it is not possible to form a linear complex and the cyclic C(2)(v)() structure imposed by symmetry possesses two covalent M-C bonds, therefore the multiplicity is given by the local core configuration [Ar]c(n)() for all metals except Mn and Ni.

Energetic and topological analyses of the oxidation reaction between Mon (n = 1, 2) and N2O

The interaction between molybdenum, atom, and dimer, with nitrous oxide has been investigated using density functional theory. The analysis of the potential energy surfaces for both reactions has revealed that a single molybdenum atom can activate the N--O bond of N2O requiring a small activation energy. However, the presence of several intersystem crossings between three different spin states, namely, septet, quintet and triplet states, seems to be the major constraint to the Mo + N2O reaction. Contrarily, the low-lying excited states (triplet and quintet) do not participate in the reaction between the molybdenum dimer and N2O. The latter reaction fully evolves on the singlet spin surface. Three different regions have been distinguished along the pathway: formation of an adduct complex, formation of an inserted compound, and the N2 detachment. The connection between the two first regions has been characterized by the formation of a special complex in which the N--O bond is so weakened that it could be considered as a first step in the insertion process. It has been shown that the topological changes along the pathways provide a clear explanation for the geometrical changes that occur along the reaction pathway. In summary, the detachment of the N2 molecule is found to be kinetically an effective process for both reactions, owing to the high exothermicity and consequently to the high internal energy of the insertion intermediates. However, in the case of Mo atom, the reaction should be a slow process due to the presence of spin-forbidden transitions. These results fully agree with previous experimental works.