Bonding and stability of donor ligand-supported heavier analogues of cyanogen halides (L′)PSi(X)(L)

The bonding and stability of elusive heavier cyanogen halide analogues (L′)PSi(X)(L) have been theoretically investigated using EDA-NOCV method. Variation of ligands and halogen on Si atom had a significant effect on the stability of these species.

Solubility and solution-phase chemistry of isocyanic acid, methyl isocyanate, and cyanogen halides

Condensed-phase uptake and reaction are important atmospheric removal processes for reduced nitrogen species, isocyanic acid (HNCO), methyl isocyanate (CH3NCO), and cyanogen halides (XCN, X = Cl, Br, I); yet many of the fundamental quantities that govern this chemistry have not been measured or are not well studied. These nitrogen species are of emerging interest in the atmosphere as they have either biomass burning sources, i.e., HNCO and CH3NCO, or, like the XCN species, have the potential to be a significant condensed-phase source of NCO− and therefore HNCO. Solubilities and the first-order reaction rate of these species were measured for a variety of solutions using a bubble flow reactor method with total reactive nitrogen (Nr) detection. The aqueous solubility of HNCO was measured as a function of pH and had an intrinsic Henry's law solubility of 20 (±2) M atm−1 and a Ka of 2.0 (±0.3) × 10−4 M (pKa = 3.7±0.1) at 298 K. The temperature dependence of HNCO solubility was very similar to other small nitrogen-containing compounds, such as HCN, acetonitrile (CH3CN), and nitromethane, and the dependence on salt concentration exhibited the “salting out” phenomenon. The rate constant of reaction of HNCO with 0.45 M NH4+, as NH4Cl, was measured at pH = 3 and found to be 1.2 (±0.1) × 10−3 M−1 s−1, faster than the rate that would be estimated from rate measurements at much higher pHs. The solubilities of HNCO in the non-polar solvents n-octanol (n-C8H17OH) and tridecane (C13H28) were found to be higher than aqueous solution for n-octanol (87±9 M atm−1 at 298 K) and much lower than aqueous solution for tridecane (1.7±0.17 M atm−1 at 298 K), features that have implications for multi-phase and membrane transport of HNCO. The first-order loss rate of HNCO in n-octanol was determined to be relatively slow, 5.7 (±1.4) × 10−5 s−1. The aqueous solubility of CH3NCO was found to be 1.3 (±0.13) M atm−1 independent of pH, and CH3NCO solubility in n-octanol was also determined at several temperatures and ranged from 4.0 (±0.5) M atm−1 at 298 K to 2.8 (±0.3) M atm−1 at 310 K. The aqueous hydrolysis of CH3NCO was observed to be slightly acid-catalyzed, in agreement with literature values, and reactions with n-octanol ranged from 2.5 (±0.5) to 5.3 (±0.7) × 10−3 s−1 from 298 to 310 K. The aqueous solubilities of XCN, determined at room temperature and neutral pH, were found to increase with halogen atom polarizability from 1.4 (±0.2) M atm−1 for ClCN and 8.2 (±0.8) M atm−1 for BrCN to 270 (±54) M atm−1 for ICN. Hydrolysis rates, where measurable, were in agreement with literature values. The atmospheric loss rates of HNCO, CH3NCO, and XCN due to heterogeneous processes are estimated from solubilities and reaction rates. Lifetimes of HNCO range from about 1 day against deposition to neutral pH surfaces in the boundary layer, but otherwise can be as long as several months in the middle troposphere. The loss of CH3NCO due to aqueous-phase processes is estimated to be slower than, or comparable to, the lifetime against OH reaction (3 months). The loss of XCNs due to aqueous uptake is estimated to range from being quite slow, with a lifetime of 2–6 months or more for ClCN and 1 week to 6 months for BrCN to 1 to 10 days for ICN. These characteristic times are shorter than photolysis lifetimes for ClCN and BrCN, implying that heterogeneous chemistry will be the controlling factor in their atmospheric removal. In contrast, the photolysis of ICN is estimated to be faster than heterogeneous loss for average midlatitude conditions.

Solubility and Solution-phase Chemistry of Isocyanic Acid, Methyl Isocyanate, and Cyanogen Halides

Condensed phase uptake and reaction are import atmospheric removal processes for reduced nitrogen species, isocyanic acid (HNCO), methyl isocyanate (CH3NCO) and cyanogen halides (XCN, X =Cl, Br, I), yet many of the fundamental quantities that govern this chemistry have not been measured or are understudied. Solubilities and first-order reaction rate of these species were measured for a variety of solutions using a bubble flow reactor method with total reactive nitrogen (Nr) detection. The aqueous solubility of HNCO was measured as a function of pH, and exhibited the classic behavior of a weak acid, with an intrinsic Henry's law solubility of 20 (±2) M/atm, and a Ka of 2.0 (±0.28) × 10−4 M (which corresponds to pKa = 3.7 ± 0.06) at 298 K. The temperature dependence of HNCO solubility was very similar to other small nitrogen-containing compounds and the dependence on salt concentration exhibited the “salting out” phenomenon that was also similar to small polar molecules. The rate constant of reaction of HNCO with 0.45 M NH4+ was measured at pH = 3, and found to be 1.2 (±0.1) × 10−3 M−1 sec−1, which is much faster than the rate that would be estimated from rate measurements at much higher pHs, and the assumption that the mechanism is solely by reaction of the un-dissociated acid with NH3. The solubilities of HNCO in the non-polar solvents n-octanol (n-C8H17OH) and tridecane (C13H28) were found to be higher than aqueous solution for n-octanol (87 ± 3 M/atm at 298 K) and much lower than aqueous solution for tridecane (1.7 ± 0.17 M/atm at 298 K), but the first-order loss rate of HNCO in n-octanol was determined to be relatively slow 5.7 (±1.4) × 10−5sec−1. The aqueous solubility of CH3NCO was measured at several pHs and found to be 1.3 (±0.13) M/atm independent of pH, and CH3NCO solubility in n-octanol was also determined at several temperatures and ranged from 4.0 (±0.5) to 2.8 (±0.3) M/atm. The aqueous hydrolysis of CH3NCO was observed to be slightly acid-catalyzed, in agreement with literature values, and reactions with n-octanol ranged from 2.5 (±0.5) to 5.3 (±0.7) × 10−3 sec−1 from 298 to 310 K. The aqueous solubilities of XCN was determined at room temperature and neutral pH were found to increase with halogen atom polarizability from 1.4 (±0.2) M/atm for ClCN, 8.2 (±0.8) M/atm for BrCN, to 270 (±41) M/atm for ICN. Hydrolysis rates where measurable, were in agreement with literature values. The atmospheric loss rates of HNCO, CH3NCO, and XCN due to heterogeneous processes are estimated from solubilities and reaction rates. Lifetimes of HNCO range from about 1 day against deposition to neutral pH surfaces in the boundary layer, but otherwise can be as long as several months in the mid troposphere. The loss of CH3NCO due to aqueous phase processes is estimated to be slower than, or comparable to, the lifetime against OH reaction (3 months). The loss of XCNs due to aqueous uptake are estimated to range from quite slow, lifetime of 2–6 months or more for ClCN, 1 week to 6 months for BrCN, to 1 to 10 days for ICN. These characteristic times are shorter than photolysis lifetimes for ClCN, and BrCN, implying that heterogeneous chemistry will be the controlling factor in their atmospheric removal. In contrast, the photolysis of ICN is estimated to be faster than heterogeneous loss for average mid-latitude conditions.

Modeling cooperative effects in halogen-bonded infinite linear chains

It is shown that the nature of halogen bonds in cyanogen halides and 4-halopyridines differs, still cooperativity in both systems results from interacting point-dipoles.

Optimization of methods for the determination of DBPs

Chloroform and other bromochlorotrihalomethanes were first identified as disinfection byproducts (DBPs) in chlorinated water in 1970s. Since then, many other DBPs have been identified such as haloacetonitriles, haloacetaldehydes, cyanogen halides, aldehydes, ketoacids, chlorite, bromate and other organic and inorganic compounds. Due to their occurrence and potential health risks, the U.S.EPA promulgated the Stage I Disinfectants and Disinfection Byproducts (D-DBP) Rule in 1998. To assist water utilities monitoring DBPs in their finished water, the U.S. EPA published a list of approved analytical methods under the D-DBP Rule. In 1996, the U.S. EPA also promulgated the Information Collection Rule (ICR) to collect brackground information on DBPs and pathogens for the Stage II D-DBP-Rule. Actually 500 DBPs are known but few have been investigated for their quantitative occurrence and health effects. Due to the fact that their identification and quantitation have become extremely important to drinking water companies in order to reduce or remove their presence, other analytical methods different from those proposed by U.S. EPA have been optimized and are now commented in this article.