Alkaline Hydrolysis of Organophosphorus Pesticides: The Dependence of the Reaction Mechanism on the Incoming Group Conformation

The fundamental mechanism of organophosphate hydrolysis is the subject of a growing interest resulting from the need for safe disposal of phosphoroorganic pesticides. Herein, we present a detailed ab initio study of the gas-phase mechanisms of alkaline hydrolysis of P–O and P–S bonds in a number of organophosphorus pesticides, including paraoxon, methyl parathion, fenitrothion, demeton-S, acephate, phosalone, azinophos-ethyl, and malathion. Our main finding is that the incoming group conformation influences the mechanism of decomposition of organophosphate and organothiophosphate compounds. Depending on the orientation of the attacking nucleophile, hydrolysis reaction might follow either a multistep pathway characterized by the presence of a pentavalent intermediate or a one-step mechanism proceeding through a single transition state. Despite a widely accepted view of the phosphotriester P–O bonds being decomposed exclusively via a direct-displacement mechanism, the occurrence of alternative, qualitatively distinct reaction pathways was confirmed for alkaline hydrolysis of both P–O and P–S bonds. As the pesticides included in our quantum chemical analysis involve organophosphate, phosphorothioate, and phosphorodithioate compounds, the influence of oxygen to sulfur substitution on the structural and energetic characteristics of the hydrolysis pathway is also discussed

Exploring Relative Thermodynamic Stabilities of Formic Acid and Formamide Dimers – Role of Low-Frequency Hydrogen-Bond Vibrations

The low-frequency fundamentals together with the high-frequency modes, responsible for hydrogen bonding (OH/NH stretching modes), were analyzed to correlate the intensities with the hydrogen-bond strengths/binding energies of the formic acid and formamide dimers using Møller–Plesset second-order perturbation (MP2) and coupled cluster computations with explicit anharmonicity corrections. Linear correlations were observed for both the formic acid and formamide dimers, and as consequence of such correlation an additive properties of binding energies with respect to the local hydrogen-bond energies of fragments involved (for these dimers) has been proposed. It has been further observed that (i) the nature of their six low-frequency fundamentals are very similar, and (ii) the in-plane bending and stretch–bend fundamentals of different dimers of these two species (depending on the dimer structure), in this low-frequency region, modulate their strength of hydrogen-bond/binding hence their relative stability order. These results were further verified against the results from Gaussian-G4-MP2 (G4MP2), Gaussian-G2-MP2 (G2MP2), and complete basis set (CBS-QB3) methods of high accuracy energy calculations

Unique Bonding Nature of Carbon-Substituted Be₂ Dimer inside the Carbon (sp²) Network

Controlled doping of active carbon materials (viz., graphenes, carbon nanotubes etc.) may lead to the enhancement of their desired properties. The least studied case of C/Be substitution offers an attractive possibility in this respect. The interactions of Be₂ with Be or C atoms are dominated by the large repulsive Pauli exchange contributions, which in turn offsets the attractive interactions leading to relatively small binding energies. The Be₂ dimer, e.g., after being doped inside a planar carbon network, undergoes orbital adjustments due to charge transfer and unusual intermolecular interactions and is oriented perpendicular to the plane of the carbon network with the Be–Be bond center located inside the plane. The present theoretical investigation on the nature of bonding in C/Be₂ exchange complexes, using state of the art quantum chemical techniques, reveals a sp² carbon-like bonding scheme in Be₂ arising due to the molecular hybridization of σ and two π orbitals. The perturbations imposed by doped Be₂ dimers exhibit a local character of the structural and electronic properties of the complexes, and the separation by two carbon atoms between beryllium active centers is sufficient to consider these centers as independent sites

Role of the Multipolar Electrostatic Interaction Energy Components in Strong and Weak Cation−π Interactions

Density functional and Møller–Plesset second-order perturbation (MP2) calculations have been carried out on various model cation−π complexes formed through the interactions of Mg²⁺, Ca²⁺, and NH₄⁺ cations with benzene, <i>p</i>-methylphenol, and 3-methylindole. Partial hydration of the metal cations was also considered in these model studies to monitor the effect of hydration of cations in cation−π interactions. The binding energies of these complexes were computed from the fully optimized structures using coupled cluster calculations including triple excitations (CCSD­(T)) and Gaussian-G4-MP2 (G4MP2) techniques. An analysis of the charge sharing between the donor (the π-systems) and the acceptors (the cations) together with the partitioning of total interaction energies revealed that the strong and weak cation−π interactions have similar electrostatic interaction properties. Further decomposition of such electrostatic terms into their multipolar components showed the importance of the charge–dipole, charge–quadrupole, and charge–octopole terms in shaping the electrostatic forces in such interactions. The computed vibrational spectra of the complexes were analyzed for the specific cation−π interaction modes and have been shown to contain the signature of higher order electrostatic interaction energy components (quadrupole and octopole) in such interactions