Studies of the Excited State Dynamics of N2O2 by Dissociative Photodetachment of N2O2-

A density-matrix adaptation of the Hückel method to weak covalent networks

The coupled-monomers model is built as an adaptation of the Hückel MO theory based on a self-consistent density-matrix formalism. The distinguishing feature of the model is its reliance on variable bond and Coulomb integrals that depend on the elements of the density matrix: the bond orders and partial charges, respectively. Here the model is used to describe electron reactivity in weak covalent networks Xn±, where X is a closed-shell monomer. Viewing the electron as the simplest chemical reagent, the model provides insight into charge sharing and localisation in chains of such identical monomers. Data-driven modelling improves the results by training the model to experimental or ab initio data. Among key outcomes is the prediction that the charge in Xn± clusters tends to localise on a few (2-3) monomers. This is confirmed by the properties of several known cluster families, including Hen+, Arn+, (glyoxal)n-, and (biacetyl)n-. Since this prediction is obtained in a purely coherent covalent regime without any thermal excitation, it implies that charge localisation does not require non-covalent perturbations (such as solvation), decoherence, or free-energy effects. Instead, charge localisation is an intrinsic feature of weak covalent networks arising from their geometry relaxation and is ultimately attributed to the correlation between covalent bond orders and equilibrium bond integrals.

Mechanisms for sonochemical oxidation of nitrogen

N₂O, and mixtures of N₂ and O₂, dissolved in water-both in the presence and absence of added noble gases-have been subjected to ultrasonication with quantification of nitrite and nitrate products. Significant increase in product formation upon adding noble gas for both reactant systems is observed, with the reactivity order Ne < Ar < Kr < Xe. These observations lend support to the idea that extraordinarily high electronic and vibrational temperatures arise under these conditions. This is based on recent observations of sonoluminescence in the presence of noble gases and is inconsistent with the classical picture of adiabatic bubble collapse upon acoustic cavitation. The reaction mechanisms of the first few reaction steps necessary for the critical formation of NO are discussed, illustrated by quantum chemical calculations. The role of intermediate N₂O in this series of elementary steps is also discussed to better understand the difference between the two reactant sources (N₂O and 2 : 1 N₂ : O₂; same stoichiometry).

Weak covalent interactions and anionic charge-sharing polymerisation in cluster environments

We discuss the formation of weak covalent bonds leading to anionic charge-sharing dimerisation or polymerisation in microscopic cluster environments. The covalent bonding between cluster building blocks is described in terms of coherent charge sharing, conceptualised using a coupled-monomers molecular-orbital model. The model assumes first-order separability of the inter- and intra-monomer bonding structures. Combined with a Hückel-style formalism adapted to weak covalent and solvation interactions, it offers insight into the competition between the two types of forces and illuminates the properties of the inter-monomer orbitals responsible for charge-sharing dimerisation and polymerisation. Under typical conditions, the cumulative effect of solvation obstructs the polymerisation, limiting the size of covalently bound core anions.

Not Limited to Iron: A Cobalt Heme-NO Model Facilitates N-N Coupling with External NO in the Presence of a Lewis Acid to Generate N2 O

Some bacterial heme proteins catalyze the coupling of two NO molecules to generate N2 O. We previously reported that a heme Fe-NO model engages in this N-N bond-forming reaction with NO. We now demonstrate that (OEP)CoII (NO) similarly reacts with 1 equiv of NO in the presence of the Lewis acids BX3 (X=F, C6 F5 ) to generate N2 O. DFT calculations support retention of the CoII oxidation state for the experimentally observed adduct (OEP)CoII (NO⋅BF3 ), the presumed hyponitrite intermediate (P.+ )CoII (ONNO⋅BF3 ), and the porphyrin π-radical cation by-product of this reaction, and that the π-radical cation formation likely occurs at the hyponitrite stage. In contrast, the Fe analogue undergoes a ferrous-to-ferric oxidation state conversion during this reaction. Our work shows that cobalt hemes are chemically competent to engage in the NO-to-N2 O conversion reaction.

Disproportionation pathways of aqueous hyponitrite radicals (HN2O2(*)/N2O2(*-))

Pulse radiolysis and flash photolysis are used to generate the hyponitrite radicals (HN2O2(*)/N2O2(*-)) by one-electron oxidation of the hyponitrite in aqueous solution. Although the radical decay conforms to simple second-order kinetics, its mechanism is complex, comprising a short chain of NO release-consumption steps. In the first, rate-determining step, two N2O2(*-) radicals disproportionate with the rate constant 2k = (8.2 +/- 0.5) x 10(7) M(-1) s(-1) (at zero ionic strength) effectively in a redox reaction regenerating N2O2(2-) and releasing two NO. This occurs either by electron transfer or, more likely, through radical recombination-dissociation. Each NO so-produced rapidly adds to another N2O2(*-), yielding the N3O3(-) ion, which slowly decomposes at 300 s(-1) to the final N2O + NO2(-) products. The N2O2(*-) radical protonates with pKa = 5.6 +/- 0.3. The neutral HN2O2(*) radical decays by an analogous mechanism but much more rapidly with the apparent second-order rate constant 2k = (1.1 +/- 0.1) x 10(9) M(-1) s(-1). The N2O2(*-) radical shows surprisingly low reactivity toward O2 and O2(*-), with the corresponding rate constants below 1 x 10(6) and 5 x 10(7) M(-1) s(-1). The previously reported rapid dissociation of N2O2(*-) into N2O and O(*-) does not occur. The thermochemistry of HN2O2(*)/N2O2(*-) is discussed in the context of these new kinetic and mechanistic results.