Trivinylphosphine Oxide: Synthesis, Characterization, and Polymerization Reactivity Investigated Using Single-Crystal Analysis and Density Functional Theory.

Organophosphorus chemicals are versatile and important in industry. Trivinylphosphine oxide (TVPO), for example, exhibited a promising precursor as a flame-retardant additive for industrial applications. Density functional theory (DFT) simulations were used to explore the kinetic and thermodynamic chemical processes underlying the nucleophilic addition reactions of TVPO in order to better understand their polymerization mechanisms. An experimental X-ray single-crystal study of TVPO supported this work's theory based on its computed findings. TVPO was prepared using POCl3 and VMB in a temperature-dependent reaction. TVPO, the thermodynamically favourable product, is preferentially produced at low temperatures. The endothermic anionic addition polymerization reaction between TVPO and VMB begins when the reaction temperature rises. An implicit solvation model simulated TVPO and piperazine reactions in water, whereas a hybrid model modelled VMB interactions in tetrahydrofuran. The simulations showed a pseudo-Michael addition reaction mechanism with a four-membered ring transition state. The Michael addition reaction is analogous to this process.

Environmental Mercury Chemistry - In Silico.

Mercury (Hg) is a global environmental contaminant. Major anthropogenic sources of Hg emission include gold mining and the burning of fossil fuels. Once deposited in aquatic environments, Hg can undergo redox reactions, form complexes with ligands, and adsorb onto particles. It can also be methylated by microorganisms. Mercury, especially its methylated form methylmercury, can be taken up by organisms, where it bioaccumulates and biomagnifies in the food chain, leading to detrimental effects on ecosystem and human health. In support of the recently enforced Minamata Convention on Mercury, a legally binding international convention aimed at reducing the anthropogenic emission of-and human exposure to-Hg, its global biogeochemical cycle must be understood. Thus, a detailed understanding of the molecular-level interactions of Hg is crucial. The ongoing rapid development of hardware and methods has brought computational chemistry to a point that it can usefully inform environmental science. This is particularly true for Hg, which is difficult to handle experimentally due to its ultratrace concentrations in the environment and its toxicity. The current account provides a synopsis of the application of computational chemistry to filling several major knowledge gaps in environmental Hg chemistry that have not been adequately addressed experimentally. Environmental Hg chemistry requires defining the factors that determine the relative affinities of different ligands for Hg species, as they are critical for understanding its speciation, transformation and bioaccumulation in the environment. Formation constants and the nature of bonding have been determined computationally for environmentally relevant Hg(II) complexes such as chlorides, hydroxides, sulfides and selenides, in various physical phases. Quantum chemistry has been used to determine the driving forces behind the speciation of Hg with hydrochalcogenide and halide ligands. Of particular importance is the detailed characterization of solvation effects. Indeed, the aqueous phase reverses trends in affinities found computationally in the gas phase. Computation has also been used to investigate complexes of methylmercury with (seleno)amino acids, providing a molecular-level understanding of the toxicological antagonism between Hg and selenium (Se). Furthermore, evidence is emerging that ice surfaces play an important role in Hg transport and transformation in polar and alpine regions. Therefore, the diffusion of Hg and its ions through an idealized ice surface has been characterized. Microorganisms are major players in environmental mercury cycling. Some methylate inorganic Hg species, whereas others demethylate methylmercury. Quantum chemistry has been used to investigate catalytic mechanisms of enzymatic Hg methylation and demethylation. The complex interplay between the myriad chemical reactions and transport properties both in and outside microbial cells determines net biogeochemical cycling. Prospects for scaling up molecular work to obtain a mechanistic understanding of Hg cycling with comprehensive multiscale biogeochemical modeling are also discussed.

Fluorescence Enhancement/Quenching Based on Metal Orbital Control: Computational Studies of a 6-Thienyllumazine-Based Mercury Sensor.

To understand the sensing behaviors of molecular fluorescent probes, lumazine (Lm) and 6-thienyllumazine (TLm) and their complexation with metal(II) ions ([(L)nM(H2O)m](2+), M = Cd(2+) and Hg(2+)) were examined by scalar relativistic density functional theory (DFT). A red shifting from L to [(L)nM(H2O)m](2+) was found. This is due to the metal affinity that stabilizes the LUMOs of [(L)nM(H2O)m](2+) greater than the HOMOs. Singlet excited-state structures of L and [(L)nM(H2O)m](2+) (M = Cd(2+) and Hg(2+)) were fully optimized using time-dependent DFT (TDDFT). Their fluorescent emissions in aqueous solution were calculated to be 371 nm (Lm), 439 nm (cis-TLm), and 441 nm (trans-TLm), agreeing with experimental values of 380 nm for Lm and 452 nm for TLm. Theoretical support is presented for a sensing mechanism of photoinduced charge transfer of the L probe. The mechanism of the chelation-enhanced fluorescence (CHEF) and the chelating quenched fluorescence (CHQF) is explained. Fluorescence amplification (for Cd(2+)) is due to blocking of the nitrogen lone pair orbital due to the stabilizing interaction with the vacant s-orbital of the metal ion, while fluorescence quenching (Hg(2+)) results from the energy of the LUMO of the metal ion being between HOMO and LUMO of the sensor. Effects of structure rearrangements on the fluorescence spectra of the sensors are insignificant. This proposed mechanism of metal orbital controlled fluorescence enhancement/quenching suggests a development concept in the future design of fluorescent turn-on/off sensors.

Thiacrown Ethers Engaged C60 through Charge Transfer: Experimental and Theoretical Study.

UV-Vis spectroscopy is used to study the charge transfer complexes of thiacrown ethers 1-6 with fullerene. The size of TCE1-6 and the nature of the heteroatoms (N, O and S) have been systematically changed to examine the effect of these factors on the HOMO/LUMO energy levels, the optical energy gap and the interactions between TCE's and C60. The negative and positive values of ΔS designate the structural forming method and the randomness of the free solvent molecules, respectively. Thermodynamics and stability data show that the complexes have a 1:1 ratio that has been emphasized by density functional theory calculations. Additionally, they show a synergetic interplay of donor-acceptor, π-π, and n-π interactions, which are the basis for the affinity of our novel receptors toward C60. The proposed system of enzyme model suggests a development concept in the future design of enzyme model organic photovoltaic systems.

Computational study of the unimolecular and bimolecular decomposition mechanisms of propylamine.

A detailed computational study of the dehydrogenation reaction of trans-propylamine (trans-PA) in the gas phase has been performed using density functional method (DFT) and CBS-QB3 calculations. Different mechanistic pathways were studied for the reaction of n-propylamine. Both thermodynamic functions and activation parameters were calculated for all investigated pathways. Most of the dehydrogenation reaction mechanisms occur in a concerted step transition state as an exothermic process. The mechanisms for pathways A and B comprise two key-steps: H2 eliminated from PA leading to the formation of allylamine that undergoes an unimolecular dissociation in the second step of the mechanism. Among these pathways, the formation of ethyl cyanide and H2 is the most significant one (pathway B), both kinetically and thermodynamically, with an energy barrier of 416 kJ mol-1. The individual mechanisms for the pathways from C to N involve the dehydrogenation reaction of PA via hydrogen ion, ammonia ion and methyl cation. The formation of α-propylamine cation and NH3 (pathway E) is the most favorable reaction with an activation barrier of 1 kJ mol-1. This pathway has the lowest activation energy calculated of all proposed pathways.

Theoretical study of the formation of mercury (Hg2+) complexes in solution using an explicit solvation shell in implicit solvent calculations.

The structures and harmonic vibrational frequencies of water clusters (H2O)n, n = 1-10, have been computed using the M06-L/, B3LYP/, and CAM-BLYP/cc-pVTZ levels of theories. On the basis of the literature and our results, we use three hexamer structures of the water molecules to calculate an estimated "experimental" average solvation free energy of [Hg(H2O)6](2+). Aqueous formation constants (log K) for Hg(2+) complexes, [Hg(L)m(H2O)n](2-mq), L = Cl(-), HO(-), HS(-), and S(2-), are calculated using a combination of experimental (solvation free energies of ligands and Hg(2+)) and calculated gas- and liquid-phase free energies. A combined approach has been used that involves attaching n explicit water molecules to the Hg(2+) complexes such that the first coordination sphere is complete, then surrounding the resulting (Hg(2+)-Lm)-(OH2)n cluster by a dielectric continuum, and using suitable thermodynamic cycles. This procedure significantly improves the agreement between the calculated log K values and experiment. Thus, for some neutral and anionic Hg(II) complexes, particularly Hg(II) metal ion surrounded with homo- or heteroatoms, augmenting implicit solvent calculations with sufficient explicit water molecules to complete the first coordination sphere is required-and adequate-to account for strong short-range hydrogen bonding interactions between the anion and the solvent. Calculated values for formation constants of Hg(2+) complexes with S(2-) and SH(-) are proposed. Experimental measurements of these log K values have been lacking or controversial.

Ab Initio calculations of the stabilization energies of the conformational and the structural isomers of C(3)H(7)X where X = F, Cl, and Br.

HF, MP2, and B3LYP calculations with different basis sets have been used in the computation of the stabilization energies of C(3)H(7)X isomers, where X is F, Cl, and Br. The experimental stabilization energies of the structural isomers of C(3)H(7)Cl and C(3)H(7)Br have been reproduced via B3LYP calculations. However, the calculated stabilization energies of fluoropropane isomers from their reported enthalpies of formation have been reproduced in all methods of calculations in present work. The experimental relative stabilities of the gauche conformers of 1-fluoro-, 1-chloro-, and 1-bromopropanes have been also reproduced via some of the used calculations in the present work. The effect of the geminal interactions on X atomic charges and on the C-X and C-C bond lengths in halopropane isomers are also discussed.