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.

CONSPECTUS: 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 continue...

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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.

The absorption spectra modeled as the vertical excitation energies of 13 dye sensitizers used in dye-sensitized solar cells (DSSCs) are benchmarked by means of time-dependent (TD)-DFT, using 36 functionals from...

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Density functional theory p)] was used in combination with the conductor-like polarizable continuum model (CPCM) solvation model to investigate the relative stability and site-specific pK ij a values of neutral and ionized tautomers of lumazine (LM) and 6-thienylLM (TLM). Two types of populations should be taken into consideration when calculating the pK ij a , tautomers, and conformers. The major tautomer of neutral LM in aqueous solution is 13-LM (the 13 notation refers to the acidic protons being in positions 1 and 3 of LM) TLM has decreased acidity at N 8 relative to LM. Further, the trans conformer of TLM is more acidic than cis. Similar to the case of LM, for TLM, N 1 is more acidic than N 3 in the uracil part. However, N 8 is predicted to be a stronger acid than N 1 for TLM. This acidity enhancement is essentially because of a specific stabilization of the anion when the thienyl group replaces H. Two factors are responsible for the acidity strength of N 8 : The thienyl ring upon deprotonation acts inductively as an electron-withdrawing group, and the excess electron density is dispersed better when the system is trans and contains second-row atoms. Accurate pK a calculation requires that all conformers/tautomers be included into the calculation.

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: H 2 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 H 2 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 nH 3 (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. Propylamine is of significant importance in chemistry, as it constitutes a central structure block for aliphatic amines 1. It is widely utilized as a solvent in organic synthesis, and as a finishing agent for drugs, rubber, fiber, paints, pesticides, textile and resin 2,3 , and in the generation of fungicides 4-6. Furthermore, it may very well be used as a petroleum additive and preservative. The disintegration of protonated of propylamine has attracted a noteworthy arrangement of fascination in the previous decade 7-10. This is mainly due to the way that the proton affinity and the structural difference in propylamine through protonation influences the separation items through the arrangement of protonated amines, methane, propene and hydrogen gas 11,12. Additionally, this reaction prompts the generation of various poisonous synthetic substances such as, alkyl cyanide, propylene, ethylene, nitrogen and hydrogen gases 13,14. Protonation (B + H + → BH +) and deprotonation (dehydrogenation) (HA − H + → A −) reactions assume a significant role in natural science and organic chemistry, where A and B are the acidic and the basic centers, respectively. They are considered as the first step in several fundamental chemical mechanisms elucidated in the cited reference 15. The ability of an atom or molecule in the gas phase to accept or to lose a proton can be described by calculating the proton affinity (PA), deprotonation (dehydrogenation) enthalpy, and molecular gasphase basicity, which offer a profound understanding of the connections between the reactivity of the organic molecules, their molecular structures, and molecular stability 16. The negative of the enthalpy change related to the gas-phase protonation reaction is referred to as proton affinity, while dehydrogenation energy is defined as the en...