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A high-level gas and aqueous phase theoretical thermodynamic study was conducted on the primary and related chemical reactions which occur during chloramination for water treatment using the G4MP2, G4, and W1BD composite methods with the SMD, PCM, and CPCM solvation models. The standard state (298.15 K, 1 atm or 1M) formation of mono-, di-, and tri-chloramines from their precursors via hypochlorous acid chlorination is substantially exothermic and exergonic in both the gas and aqueous phases. The excellent agreement between experimental and theoretical values for a range of structural and thermodynamic calculations on a suite of calibration compounds suggests that the G4MP2, G4, and W1BD calculations meet or exceed criteria for thermochemical accuracy. The temperature influence on the thermodynamics of chloramine formation is projected to be negligible regardless of phase between 0 and 100°C. Additional thermodynamic calculations were undertaken on associated chloramination reactions involving the disproportionation of monochloramine, the decomposition of di- and tri-chloramine, and the reactions of trichloramine with ammonia and dichloramine. The results from these investigations not only provide a better understanding of the reaction thermodynamics, they also allow for a more rigorous interpretation of proposed chloramination mechanisms. [ABSTRACT FROM PUBLISHER]

Highlights: [•] Examined thermodynamic properties of ozonation reactions in presence of bromide. [•] High level G4MP2 and G4 theoretical methods employed. [•] Aqueous phase behavior modeled with SMD, PCM, and CPCM solvation models. [•] Results will help improve mechanistic understandings of water treatment. [ABSTRACT FROM AUTHOR]

Abstract: Singlet–triplet excitation energies (ES–T) were calculated for the phenyl, 1-naphthyl, and 2-naphthyl cations using a broad range of model chemistries, including semiempirical, Hartree–Fock, density functional, Moller–Plesset perturbation, composite, coupled cluster, and quadratic configuration interaction methods and various basis sets. Substantial model chemistry dependent ES–T results were obtained for all three cations with correspondingly minimal basis set size effects. G4/G4MP2 composite method well-to-well (and adiabatic) ES–T for the phenyl, 1-naphthyl, and 2-naphthyl cations are 101.9/102.3 (101.7/102.0), 20.4/18.8 (19.3/17.8), and 21.6/21.4 (20.8/20.7)kJ/mol, respectively. All composite methods predict a substantially positive ES–T for the 1- and 2-naphthyl cations, and are in both quantitative and qualitative disagreement with many other model chemistries (particularly density functionals such as B3LYP) in estimating both the magnitude and sign of the singlet–triplet excitation energy for the 1- and 2-naphthyl cations. Composite method approaches suggest both the 1- and 2-naphthyl cations are ground state singlets with sufficiently large ES–T such that the population of the corresponding triplet state should be negligible, and thereby non-observable, where experimental conditions operate under thermodynamic control. [Copyright &y& Elsevier]

Abstract: Gas phase adiabatic ionization energies (AIEs) and standard state (298.15K, 1atm) enthalpies of formation were calculated at the Gaussian-4 (G4) level of theory for a suite of 319 and 398 main group compounds, respectively, containing the elements hydrogen through bromine. National Institute of Standards and Technology (NIST) evaluated AIEs were available for comparison with 44 compounds, yielding good agreement when the significant variability among the individual experimental reports was taken into consideration. AIEs were estimated at the G4 level of theory for a further 126 main group compounds having at least one experimental datapoint in the NIST database, but not a NIST evaluated AIE, as well as 149 main group compounds without any experimental data in the NIST database. Experimental were available for comparison with 144 compounds, yielding good agreement, particularly against compounds with evaluated . were estimated at the G4 level of theory for a further 254 main group compounds without any experimental data. The datasets comprise a benchmark set of theoretical data against which to assess future experimental and theoretical results. [ABSTRACT FROM AUTHOR]

Abstract: Gas phase enthalpies of formation at 298.15K and 1atm () and adiabatic ionization energies (AIEs) were calculated at the Gaussian-4 (G4) level of theory for a suite of mono- and disubstituted acetylenes and polyynes. were estimated for 167 acetylene derivatives, of which only 16 had previously reported experimental values. Of the 122 acetylenes with G4 estimated AIEs, 30 compounds had available experimental characterization for comparison. Excellent agreement with thermochemical accuracy was obtained between G4 and AIEs and corresponding high quality experimental data and prior high-level theoretical estimates. The findings extend the existing thermochemical database on this important class of compounds to a range of derivatives without prior high level theoretical calculations or experimental reports. [ABSTRACT FROM AUTHOR]

Enthalpies (Δ H), Gibbs free energies (Δ G), and equilibrium constants (log K) for the trans → cis isomerization of various 3,3′-, 4,4′-, and 5,5′- disubstituted 2,2′-diphenoquinones with a range of electron withdrawing and releasing moieties (methyl, fluoro, chloro, bromo, trifluoromethyl, and amino) were calculated in the gas phase and in the solvent phase ( n-hexane, benzene, n-octanol, acetonitrile, and water). In the gas phase, the trans isomer of the parent and all substituted 2,2′-diphenoquinones is predicted to be more thermodynamically stable than the cis configuration, with log K ranging from −2.8 to −7.0. For all compounds, increasing solvent polarity/proticity progressively favors shifting the cis/ trans equilibrium towards greater contributions of the cis configuration and substantially increases the log K by up to 5.1 units relative to the gas phase. In polar protic and polar aprotic solvents, the estimated log K ranges as low as −0.4, indicating significant populations of the cis isomers should be present. The findings support the polar solvent phase mechanistic predictions for a cis configuration of 2,2′-diphenoquinones participating in the thermal transformation of trans-2,2′-diphenoquinones to oxepino[2,3-b]benzofurans. With limited exceptions for some amino derivatives, the cis-2,2′-diphenoquinone to oxepino[2,3-b]benzofuran isomerization is expected to be thermodynamically favorable for all substituents/phases under consideration. The cis-2,2′-diphenoquinone to oxepino[2,3-b]benzofuran rearrangement is predicted to become less thermodynamically favored with increasing solvent polarity/proticity. [ABSTRACT FROM AUTHOR]

Gas-phase standard state (298.15 K, 1 atm) enthalpies of formation (ΔH) and enthalpies (ΔH) and free energies (ΔG) of acid dissociation were calculated for the [ m, n]polyprismanes ( m ≥ 2; n = 3-8; m × n ≤ 16) using the composite method CBS-Q//B3, G4MP2, and G4 levels of theory. Excellent agreement was obtained between the theoretical estimates and experimental values for [2, 4]polyprismane (cubane), the only member with reliable experimental data. Gas-phase acidities of the [2, 3], [2, 4], and [2, 5]polyprismanes correlate well with percent s-character of the C-H bonds as determined by nuclear magnetic resonance spectroscopy. Based on this correlation, and calculations on the [2, n] ( n = 3-8) series, no substantial change in the gas-phase acidity or C-H bond s-character is expected with increasing [2, n]polyprismane ring size at n = 6-8. Where three or more stacked [ m, n]polyprismane rings are present ( m > 2), geometry optimizations for the deprotonated anions converged on cage-opened non-prismatic geometries, suggesting these compounds may be structurally unstable to gas-phase deprotonation/protonation cycles and potentially preventing reliable experimental measurement or calculation of their acidities. Total strain energies ( E) increase with larger [ m, n]polyprismane size, and strain energies per one C-C bond ( E) increase with increasing stack height within a given ring size series. Within a given stack height, E reaches a minimum with a ring size of five, increasing with either decreasing or increasing ring size away from this minima. E and E do not correlate with gas-phase acidity, likely because the deprotonated anionic [2, n]polyprismane geometry retains approximately equal strain as the undissociated species despite substantial geometry changes due to carbon acid dissociation. [ABSTRACT FROM AUTHOR]

Abstract: A systematic and comprehensive semiempirical, Hartree–Fock (HF) ab initio, and B3LYP density functional theory (DFT) study was conducted on the relative thermodynamic properties of various linear and branched perfluorinated and perhydrogenated alkyl compounds. The semiempirical AM1, PM3, and PM6 methods all consistently and accurately predict that branched alkyl compounds will generally be more thermodynamically stable than their linear counterparts. In contrast, HF and B3LYP calculations with the 6-31G(d,p), 6-31++G(d,p), and 6-311++G(d,p) basis sets predict that linear isomers will be more stable than branched analogs. These different linear versus branched perfluoroalkyl/perhydroalkyl thermodynamic property trends between semiempirical and ab initio/DFT methods were evident in both gas and aqueous phase calculations. Comparison of experimentally determined thermodynamic properties for several classes of linear and branched alkanes and alcohols with values calculated at the PM6 and B3LYP/6-311++G(d,p) levels of theory supported the well known findings that such DFT and HF approaches incorrectly predict branched alkyl compounds will be less thermodynamically stable than linear isomers. Calculations at the MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p) and M05-2X/6-311++G(d,p) levels of theory on a representative subset of the linear and branched perfluorinated compounds supported the thermodynamic conclusions from the PM6 method. Strong agreement between PM6 estimated thermodynamic properties and available experimental data supports use of this computational method for accurately calculating the well established higher thermodynamic stability of branched alkyl compounds. Branched perfluoroalkyl compounds are thus expected to be more thermodynamically stable than their linear analogs. [Copyright &y& Elsevier]

Water temperature was continuously recorded during the ice-free season between June/July and October/November at 90 sites with lentic and lotic stream sources distributed throughout the Nicola River watershed (British Columbia, Canada) in 1999, 2000, and 2001. The eight lentic-sourced stream temperature monitoring sites were located in two adjacent watersheds. The headwaters and riparian areas around the wetland outlet of the treatment watershed were harvested during the overwinter period between 1999 and 2000. Areas around and downstream of the headwater wetland outlet in the control watershed were not harvested. Reducing riparian shade by harvesting activities increased maximum stream temperatures in the treatment watershed by up to 1-2°C relative to the control watershed. Because of the general downstream cooling trends in lentic-sourced headwater streams, riparian harvesting activities in these regions have a reduced thermal impact relative to similar harvesting alongside lotic-sourced headwater streams, whose maximum stream temperatures may warm by up to 8°C following harvesting. The downstream influence of elevated maximum stream temperatures from riparian harvesting of lentic-sourced headwater streams appears to be localized, but persists for at least 2 years following harvesting. Both lentic-sourced treatment and control streams in the current study relaxed towards baseline equilibrium temperature estimated by the lotic-sourced watershed trend within several hundred meters of downstream travel distance, with cooling rates proportional to the distance from expected thermal equilibrium. Due to the heating in wetland-sourced stream reaches adjacent to riparian harvesting, the regions downstream of treatment areas cool more rapidly than similar regions in control watersheds as the stream attempts to achieve thermal equilibrium. [ABSTRACT FROM AUTHOR]

Abstract: Gas-phase optimized geometries, enthalpies, entropies, and free energies of formation at 298K were calculated for the 159 C3 through C8 perfluoroalkyl sulfonic acid (PFSA) congeners using the PM6 semi-empirical method. Within each homologue group, linear PFSA congeners are generally less thermodynamically stable than their branched counterparts. The results contrast with the known predominance of linear isomers in PFSA technical mixtures, suggesting that during the electrochemical synthesis of PFSAs from straight-chain precursors, full fluorination of the hydrocarbon chain is more rapid than the kinetics for intramolecular rearrangements of the linear isomer into the more stable branched congeners. As a result, thermodynamic computational methods will be of little value in predicting likely source input profiles of PFSAs into environmental systems. [Copyright &y& Elsevier]