First principles computational chemistry predictions of equilibrium and kinetic constants for electrophilic reactions in water

Aqueous electrophilic reactions are broadly important in environmental chemistry, but the thermodynamic equilibrium constants, rate constants, and mechanisms describing these reactions are often difficult to determine by experiment. Here we report on our efforts to establish thermodynamic and kinetic constants of aqueous electrophilic reactions by way of ab initio computational chemistry. To accomplish this, we employ a modular suite of specialized computational techniques. We first estimate reaction free energies (for equilibrium constants) or activation free energies (for rate constants) in the gas phase, employing a combination of high-quality Coupled Cluster computations and Density Functional Theory methods. To account for the influence of aqueous solvent, we conduct additional simulations with implicit solvation models and/or microsolvated clusters of solute and water molecules, with careful attention to the handling of standard states. The resulting model predictions are relatively independent of experimental parameterization and can be applied to a very broad range of reactions, in principle. We report on comparisons of our computational results with available experimental data for several electrophilic reactions in water. These include: the equilibrium constants describing the reaction of NH3 with HOCl and HOBr to form NH2Cl and NH2Br, respectively; the equilibrium constant for the dehydration of HOCl to form Cl2O; the kinetic constant of O3 reaction with Br- to form HOBr; and the kinetic constants for the reaction of HOBr with NH3 and the reaction of HOBr with dimethylamine. These preliminary results suggest that the tested prediction approaches have order-of-magnitude uncertainty for equilibrium constants involving neutral solutes, and 2-3 orders-of-magnitude uncertainty for rate constants. As we illustrate with selected case studies, this level of accuracy is sufficient to provide useful estimates of thermodynamic and kinetic properties in cases where they have not been successfully elucidated by experiment. For example, our recent computational estimates of thermodynamic properties enabled us to ascertain the relative stabilities of bromamines, chloramines, and bromochloramines during disinfection of drinking and swimming pool water.

Computational Quantum Chemical Studies on Radicals

Peter Rudolf Tentscher

Radicals play an important role in many areas of chemistry, such as atmospheric, aquatic, polymer, and biological, to name a few. Radicals are often highly reactive species which are short-lived and therefore harder to study by experimental techniques. In principle, computational quantum chemistry enables the study of arbitrary chemical species, including elusive radicals. However, the approximate quantum chemical methods which are used for production simulations need to be validated for the type of problem in question. First, it is necessary to validate that highly accurate reference methods commonly applied to closed-shell systems are equally applicable to radicals. I show that the “gold standard” for closed-shell systems (coupled cluster with singles, doubles, and perturbative triples excitations (CCSD(T))) can produce molecular geometries and vibrational frequencies suitable for (sub-)kJ/mol thermochemistry for radical species as well. However, additional diagnosis is necessary, especially for vibrational frequency calculations. Coupled cluster methods are then used to compute benchmark binding energies between small radicals and water or hydrogen fluoride as a proxy to the aqueous condensed phase. I find that many common density functional theory (DFT) methods cannot reproduce these binding energies to within chemical accuracy (1 kcal/mol). However, some functionals, for example MPW1K and BHandHLYP, perform better than others. If more accurate ab initio methods are computationally too demanding for the system of interest, these DFT methods should be used for (micro-)solvated radicals. Complexes of radicals with polar solvent are found to be more strongly bound than expected. This can not be explained by dispersive and electrostatic interactions alone. Using natural bond orbitals, I show that additional donor-acceptor interactions may be responsible for the strong intermolecular binding observed. One of the most fundamental processes involving aqueous organic radicals is the one-electron oxidation of closed-shell organics. I study the vertical ionization of neutral organics. That is, the ionization is faster than the reorganization of the solvent (and solute), and the conformation is assumed to be unchanged during the ionization process. I find that the difference between the gas-phase and aqueous-phase vertical ionization energy, ∆VIE = VIEaq − VIEgas, ranges from ≈ 0 to -1.0 eV for small aromatic compounds. The differences in ∆VIE between different compounds seem to be caused by the solvent structures of the ≈ 70 water molecules closest to the solute. In natural surface waters, oxidized organics arise through the reaction of photochemically created radicals with organic molecules already present in the water column. One example for such a reaction is the oxidation of sulfonamide antibiotics. I show that upon oxidation of sulfadiazine and related compounds, an aniline radical cation is formed. The reactivity of this oxidized species is strikingly different from its reduced counterpart: the barrier for a nucleophilic attack on the aniline ring is drastically lowered, and a fast rearrangement reaction can take place. I demonstrate that current quantum chemical protocols are suitable for mechanistic studies of solvated radicals, and I successfully apply them to such problems. To achieve chemical accuracy for quantum computational protocols, further developments are necessary. To validate novel approaches, the benchmark data presented in this thesis may be used.

EPFL2013

Benchmark thermochemistry of chloramines, bromamines, and bromochloramines: Halogen oxidants stabilized by electron correlation

Jeremy Samuel Arey, Daniela Trogolo

Chloramines, bromamines, and bromochloramines are halogen-containing oxidants that arise from the reaction of hypohalous acids with ammonia in water. Although relevant to both water disinfection chemistry and biochemistry, these molecules are difficult to study in the laboratory, and their thermochemical properties remain poorly established. We developed a benchmark level ab initio calculation protocol, termed TA14, adapted from the Weizmann theory and Feller-Peterson-Dixon approaches to determine the molecular structures and thermochemical properties of these compounds. We find that the halamine molecules are bound largely, and in some cases entirely, by electron correlation forces. This presumably explains their high reactivity as electrophilic oxidants. We provide computed heats of formation at 0 K (Delta H-f(0)K(0)) and at 298 K (Delta H-f(298)K(0)) and Gibbs free energies of formation at 298 K (Delta(f)G(298)(K)(0)) for the 9 inorganic chloramines, bromamines, bromochloramines in gas phase. Based on comparisons to previous theoretical and experimental data for a set of 11 small molecules containing N, O, H, Cl, and Br, we propose uncertainties ranging from 1 to 3 kJ mol(-1) for computed thermodynamic properties of the halamines. Reported thermochemical data enable the determination of equilibrium constants for reactions involving halamines, opening possibilities for more quantitative studies of the chemistry of these poorly understood compounds.

Royal Soc Chemistry2015

Predicting aqueous speciation of environmentally relevant halogen oxidants: chloramines, bromamines, and bromochloramines

Jeremy Samuel Arey, Daniela Trogolo

Chloramines, bromamines, and bromochloramines are halogen-containing oxidants that arise from the reaction between ammonia and hypohalous acids during the disinfection of drinking and wastewaters. Although involved in the formation of toxic disinfection byproducts, these molecules are difficult to study by experimental means, and their speciation remains poorly defined. To bridge this gap, we successfully designed a benchmark level computational method, called TA14, adapted from the Weizmann theory and the Feller-Peterson-Dixon procedures, able to establish the thermodynamics of these species in gas phase. Our composite method combines a systematic sequence of wavefunction theory calculations (up to CCSDTQ), relativistic effects, diagonal Born-Oppenheimer corrections, and anharmonic vibrational modes. Free energies of aqueous solvation were modeled with the SMD implicit solvation model together with the cluster-continuum approach developed by Bryantsev in 2008. This combined solvation model allowed us to evaluate the effects of aqueous solvation on halamine formation and decomposition. These gas phase and solution phase thermodynamic data are used to predict aqueous equilibrium constants describing the formation reactions of chloramines, bromamines, and bromochloramines. Based on the comparisons with the available experimental data, we propose uncertainties ranging from 1 to 2 logarithm units for computed aqueous equilibrium constants. These newly reported estimates of reaction thermodynamics will enable us to assess the formation rate constants of these reactive halogenated species, leading to more accurate predictions of speciation of halamines during water treatment.

2015

Computational Quantum Chemical Studies on Radicals

Peter Rudolf Tentscher

EPFL2013