Atmospheric Gas-Phase Formation of Methanesulfonic Acid.

Despite its impact on the climate, the mechanism of methanesulfonic acid (MSA) formation in the oxidation of dimethyl sulfide (DMS) remains unclear. The DMS + OH reaction is known to form methanesulfinic acid (MSIA), methane sulfenic acid (MSEA), the methylthio radical (CH3S), and hydroperoxymethyl thioformate (HPMTF). Among them, HPMTF reacts further to form SO2 and OCS, while the other three form the CH3SO2 radical. Based on theoretical calculations, we find that the CH3SO2 radical can add O2 to form CH3S(O)2OO, which can react further to form MSA. The branching ratio is highly temperature sensitive, and the MSA yield increases with decreasing temperature. In warmer regions, SO2 is the dominant product of DMS oxidation, while in colder regions, large amounts of MSA can form. Global modeling indicates that the proposed temperature-sensitive MSA formation mechanism leads to a substantial increase in the simulated global atmospheric MSA formation and burden.

Reaction of Atmospherically Relevant Sulfur-Centered Radicals with RO2 and HO2.

The atmospheric oxidation of dimethyl sulfide and other emitted sulfur species leads to the formation of the methylthio radical, CH3S, which can be further oxidized to the CH3SO and CH3SO2 radicals. We investigated computationally the reactions of these three sulfur-centered radicals with the peroxy radicals ROO and HOO. Our results demonstrate that CH3S and CH3SO react with these peroxy radicals to form short-lived peroxide intermediates, which then decompose via a concerted O-O bond scission and S═O double bond formation that results in an increased valence of the sulfur atom. In contrast, CH3SO2 reacts to form stable CH3S(O)2OOR and CH3S(O)2OOH peroxide products, as sulfur is already at its highest valence. Multireference methods were used to describe these reactions in which the valence of the sulfur atom changes.

Against the Norm: Non Irving-Williams Transmetalation in Transition Metal Dimers.

We report the synthesis and characterization of three dinuclear 3d3d' complexes, CuCu ([Cu2IIL(NO3)2]), MnMn ([Mn2IIL(MeOH)2(NO3)2]), and CuMn ([CuIIMnIIL(NO3)2]), that utilize the ligand, H2L (6,6'-dimethoxy-2,2'-[(1,3-propylene)dioxybis(nitrilomethylidyne)]diphenol). The relative stabilities of these complexes were investigated using experimental and computational techniques, revealing a non-Irving-Williams transmetalation, whereby a MnII ion can displace a CuII ion from its binding pocket in CuCu to yield the more stable CuMn complex. Magnetic characterization of the reported complexes revealed an unexpected ferromagnetic coupling between the two CuII ions of CuCu with J = +63.0 cm-1.

Atmospheric Fate of the CH3SOO Radical from the CH3S + O2 Equilibrium.

The atmospheric oxidation mechanisms of reduced sulfur compounds are of great importance in the biogeochemical sulfur cycle. The CH3S radical represents an important intermediate in these oxidation processes. Under atmospheric conditions, CH3S will predominantly react with O2 to form the peroxy radical CH3SOO. The formed CH3SOO has two competing unimolecular reaction pathways: isomerization to CH3SO2, which further decomposes into CH3 and SO2, or a hydrogen shift followed by HO2 loss, leading to CH2S. Previous theoretical calculations have suggested that CH2S formation should be the dominant pathway, in disagreement with existing experimental results. Our large active space multireference configuration interaction calculations agree with the experimental results that the formation of CH3 and SO2 is the dominant route and the formation of CH2S and HO2 can, at most, be a minor pathway. We support the calculations with new experiments starting from the OH + CH3SH reaction for CH3S formation under low NOx conditions and find a SO2 yield of 0.86 ± 0.18 within our reaction time of 7.9 s. Model simulations of our experiments show that the SO2 yield converges to 0.98. This combined theoretical and experimental study thus furthers the understanding of the general oxidation mechanisms of sulfur compounds in the atmosphere.

Lactic Acid Spectroscopy: Intra- and Intermolecular Interactions.

Lactic acid, a relevant molecule in biology and the environment, is an α-hydroxy acid with a high propensity to form hydrogen bonds, both internally and to other hydrogen-bond-accepting molecules. This work includes the novel recording of infrared spectra of gas-phase lactic acid using Fourier transform infrared spectroscopy, and the vibrational absorption features of lactic acid are assigned with the aid of computationally simulated vibrational spectra with anharmonic corrections. Theoretical chemistry methods are used to relate intramolecular hydrogen-bond strengths to the relative stability of lactic acid conformers. The formation of hydrogen-bonded lactic acid dimers and 1:1 water complexes is investigated by simulated vibrational spectra and calculated thermodynamic parameters for the lactic acid monomer and dimer and its water complex in the gas phase. The results of this study are discussed in the context of environmental chemistry with an emphasis on indoor environments.

Spectroscopy of OSSO and Other Sulfur Compounds Thought to be Present in the Venus Atmosphere.

The spectroscopy of cis-OSSO and trans-OSSO is explored and put into the context of the Venusian atmosphere, along with other sulfur compounds potentially present there, namely, S2O, C1-S2O2, trigonal-S2O2, and S3. UV-vis spectra were calculated using the nuclear ensemble approach. The calculated OSSO spectra are shown to match well with the 320-400 nm near-UV absorption previously measured on Venus, and we discuss the challenges of assigning OSSO as the Venusian near-UV absorber. The largest source of uncertainty is getting accurate concentrations of sulfur monoxide (3SO) in the upper cloud layer of Venus (60-70 km altitude) since the 3SO self-reaction is what causes cis- and trans-OSSO to form. Additionally, we employed the matrix-isolation technique to trap OSSO formed by microwave discharging a gas mixture of argon and SO2 and then depositing the mixture onto a cold window (6-12 K). Anharmonic vibrational transition frequencies and intensities were calculated at the coupled cluster level to corroborate the matrix-isolation FTIR spectra. The computationally calculated UV-vis and experimentally recorded IR spectra presented in this work aid future attempts at detecting these sulfur compounds in the Venusian atmosphere.

Theoretical Study of the Structures of 4-(2,3,5,6-Tetrafluoropyridyl)Diphenylphosphine Oxide and Tris(Pentafluorophenyl)Phosphine Oxide: Why Does the Crystal Structure of (Tetrafluoropyridyl)Diphenylphosphine Oxide Have Two Different P=O Bond Lengths?

The crystal structure of 4-(2,3,5,6-tetrafluoropyridyl)diphenylphosphine oxide (1) contains two independent molecules in the asymmetric unit. Although the molecules are virtually identical in all other aspects, the P=O bond distances differ by ca. 0.02 Å. In contrast, although tris(pentafluorophenyl)phosphine oxide (2) has a similar crystal structure, the P=O bond distances of the two independent molecules are identical. To investigate the reason for the difference, a density functional theory study was undertaken. Both structures comprise chains of molecules. The attraction between molecules of 1, which comprises lone pair-π, weak hydrogen bonding and C-H∙∙∙arene interactions, has energies of 70 and 71 kJ mol-1. The attraction between molecules of 2 comprises two lone pair-π interactions, and has energies of 99 and 100 kJ mol-1. There is weak hydrogen bonding between molecules of adjacent chains involving the oxygen atom of 1. For one molecule, this interaction is with a symmetry independent molecule, whereas for the other, it also occurs with a symmetry related molecule. This provides a reason for the difference in P=O distance. This interaction is not possible for 2, and so there is no difference between the P=O distances of 2.

Ab Initio Molecular Dynamics Investigation of Beryllium Complexes.

Structures of aqueous [Be(H2O)4]2+, its outer-sphere and inner-sphere complexes with F-, Cl-, and SO42-, and dinuclear complexes with a [Be2(κ-OH)(κ-SO4)]+ core have been studied through Car-Parrinello molecular dynamics (CPMD) simulations with the BLYP functional. According to constrained CPMD/BLYP simulations and pointwise thermodynamic integration, the free energy of deprotonation of [Be(H2O)4]2+ and its binding free energy with F- are 9.6 and -6.2 kcal/mol, respectively, in good accord with available experimental data. The computed activation barriers for replacing a water ligand in [Be(H2O)4]2+ with F- and SO42-, 10.9 and 13.6 kcal/mol, respectively, are also in good qualitative agreement with available experimental data. These ligand-substitution reactions are indicated to follow associative interchange mechanisms with backside (SN2-like) attack of the anion relative to the aquo ligand it is displacing. Outperforming static density functional theory computations of the salient kinetic and thermodynamic quantities involving simple polarizable continuum solvent models, CPMD simulations are validated as a promising tool for studying the structures and speciation of beryllium complexes in aqueous solution.

Room Temperature Gibbs Energies of Hydrogen-Bonded Alcohol Dimethylselenide Complexes.

A number of hydrogen-bonded complexes, formed between an alcohol donor and dimethylselenide, have been detected experimentally, at room temperature in the gas phase using FTIR spectroscopy. The Gibbs energy of complex formation has been determined from the measured integrated absorbance of the hydrogen-bonded OH stretching band and the calculated oscillator strength of the associated transition. The OH stretching frequency and Gibbs energy of the selenium hydrogen-bonded complexes are compared to those found in complexes with the same donor molecule and either dimethylether (O) or dimethylsulfide (S) as the acceptor molecule. For a given donor, we found a similar OH stretching frequency in the complexes for each of the three acceptors O, S, and Se. However, the Gibbs energies were found to be less positive (i.e., stronger bound) for the dimethylether complexes (OH·O), as compared to the dimethylsulfide (OH·S) and dimethylselenide (OH·Se) complexes, with the latter two having comparable Gibbs energies.

Simulated Electronic Absorption Spectra of Sulfur-Containing Molecules Present in Earth's Atmosphere.

We have calculated, ab initio, the electronic absorption spectrum of sulfuric acid (H2SO4) under atmospherically relevant conditions using a nuclear ensemble approach. The experimental electronic spectrum of H2SO4 is unknown so we benchmark our theoretical results by also considering other related sulfur-containing molecules, namely, sulfur dioxide (SO2), sulfur trioxide (SO3), hydrogen sulfide (H2S), carbonyl sulfide (OCS), and carbon disulfide (CS2), where experimental spectra are available. In general, we find very good agreement between our calculated spectra, which are based on underlying EOM-CCSD electronic structure calculations, and the available experimental spectra. We show that the computational cost of these calculated spectra can be substantively reduced with negligible loss of accuracy by using a combination of results obtained with the aug-cc-pV(D+d)Z+3 and aug-cc-pV(T+d)Z+3 basis sets. Our calculated cross-section for H2SO4 in the UV/VUV region is larger than previous theoretical estimates and greater than the experimentally measured upper limits. We suggest that further experimental attempts to measure the electronic absorption spectrum of H2SO4 in the actinic region (4.0-7.5 eV, 313-167 nm) region are warranted.

Electrospray Ionization Mass Spectrometric Study of the Gas-Phase Coordination Chemistry of Be2+ Ions with 1,2- and 1,3-Diketone Ligands.

Electrospray ionization mass spectrometry (ESI MS) is a powerful technique for the study of coordination complexes because of its ability to analyze solution systems involving very low concentrations of metal complexes. In this work, the coordination chemistry of Be ions with a selection of well-known 1,3-diketone and related 1,2-diketone ligands has been investigated using ESI MS. With acetylacetone (Hacac), a range of acac-containing ions is observed, including [Be(acac)2H]+, [Be(acac)(MeOH) n]+ ( n = 1, 2), and polynuclear species such as the dinuclear [Be2(acac)3]+ and trinuclear [Be3O(acac)3]+. Density functional theory calculations indicate that the latter species has a central Be3(µ3-O) core, with each Be chelated (as opposed to being bridged) by an acac ligand. The effect of changing the substituents on 1,3-diketone was explored by an investigation of mixtures of Be2+ with other 1,3-diketones such as dibenzoylmethane (Hdbm), where the [Be(dbm)2H]+ ion showed a lesser tendency to undergo fragmentation and aggregation processes. Comparisons with the corresponding aluminum acetylacetone system were also made. In contrast, mixtures of Be2+ and the 1,2-diketones diacetyl and phenanthrenequinone showed poor metal-ligand interactions. Be2+ interacted with the 1,2-diketone benzil [PhC(O)C(O)Ph], forming the [Be(benzil) n]2+ ( n = 2-4) ions. The synthesis (from BeCl2) and X-ray structures of the dibenzoylmethanato (dbm) complex Be(dbm)2 and the benzil complex [BeCl2(benzil)] are also reported.

Kinetics of conversion of dihydroxyacetone to methylglyoxal in New Zealand manuka honey: Part V - The rate determining step.

Monomer formation from dimeric DHA has previously been suggested as the rate-determining step in formation of methylglyoxal, the bioactive component in manuka honey. This step was studied by 1H NMR in DMSO­d6. First order reaction rate was 3.31 × 10-3 ±â€¯9.1 × 10-4 min-1. Upon titration with D2O, little change was observed until ∼15 mass% whereupon an exponential increase in rate occurred until indistinguishable from the rate observed in water. Acid or base caused rate accelerations. Theoretical modelling confirmed the existence of acid and base-catalysed mechanisms for dimer decomposition and the structures of two intermediates observed. In honey it is likely the base-catalysed decomposition predominates with water as catalyst but there is little rate acceleration at the levels of water present normally in honey however a small increase in the mass% of water in the honey could cause significant rate acceleration of dimer decomposition and hence formation of methylglyoxal.

Kinetics of conversion of dihydroxyacetone to methylglyoxal in New Zealand manuka honey: Part IV - Formation of HMF.

During a study of the conversion of dihydroxyacetone (DHA) to methylglyoxal (MGO) in maturing New Zealand manuka honey, the kinetics of formation of 5-(hydroxymethyl)furfural (HMF) was studied at temperatures from 4 to 37°C. Formation of HMF was first-order during an induction period and zero-order thereafter indicating that the mechanism includes the formation of certain critical intermediates and that these require time to build up; the duration of the induction period depended primarily upon temperature. The zero-order rate constant at 37°C was the same for manuka honey and clover honey doped with 2000 or 10,000mg/kg DHA and for artificial honey with 2000mg/kg of DHA and either alanine or proline and alanine added. Zero-order rate constants for artificial honey with added amino acids were less than for a control without amino acids. A simulation was created to predict the formation of HMF over time at 37°C in manuka honey.

Kinetic Energy Density as a Predictor of Hydrogen-Bonded OH-Stretching Frequencies.

This work considers the nature of the intermolecular hydrogen bond in a series of 15 different complexes with OH donor groups and N, O, P, or S acceptor atoms. To complement the existing literature, room-temperature gas-phase vibrational spectra of the methanol-pyridine, ethanol-pyridine, and 2,2,2-trifluoroethanol-pyridine complexes were recorded. These complexes were chosen, as they exhibit hydrogen bonds of intermediate strength as compared to previous investigations that involved strong or weak hydrogen bonds. Non Covalent Interactions (NCI) theory was used to calculate various properties of the intermolecular hydrogen bonds, which were compared to the experimental OH-stretching vibrational red shifts. We find that the experimental OH-stretching red shifts correlate strongly with the kinetic energy density integrated within the reduced density gradient volume that describes a hydrogen bond [G(s0.5)]. Given that vibrational red shifts are commonly used as a metric of the strength of a hydrogen bond, this suggests that G(s0.5) could be used as a predictor of hydrogen bonding strength.

Intramolecular Hydrogen Bonding in Substituted Aminoalcohols.

The qualifying features of a hydrogen bond can be contentious, particularly where the hydrogen bond is due to a constrained intramolecular interaction. Indeed there is disagreement within the literature whether it is even possible for an intramolecular hydrogen bond to form between functional groups on adjacent carbon atoms. This work considers the nature of the intramolecular interaction between the OH (donor) and NH2 (acceptor) groups of 2-aminoethanol, with varying substitution at the OH carbon. Gas-phase vibrational spectra of 1-amino-2-methyl-2-propanol (BMAE) and 1-amino-2,2-bis(trifluoromethyl)-2-ethanol (BFMAE) were recorded using Fourier transform infrared spectroscopy and compared to literature spectra of 2-aminoethanol (AE). Based on the experimental OH-stretching frequencies, the strength of the intramolecular hydrogen bond appears to increase from AE < BMAE ≪ BFMAE. Non-covalent interaction analysis shows evidence of an intramolecular hydrogen bond in all three molecules, with the order of the strength of interaction matching that of experiment. The experimental OH-stretching vibrational frequencies were found to correlate well with the calculated kinetic energy density, suggesting that this approach can be used to estimate the strength of an intramolecular hydrogen bond.

Structure and Abundance of Nitrous Oxide Complexes in Earth's Atmosphere.

We have investigated the lowest energy structures and binding energies of a series of atmospherically relevant nitrous oxide (N2O) complexes using explicitly correlated coupled cluster theory. Specifically, we have considered complexes with nitrogen (N2-N2O), oxygen (O2-N2O), argon (Ar-N2O), and water (H2O-N2O). We have calculated rotational constants and harmonic vibrational frequencies for the complexes and the constituent monomers. Statistical mechanics was used to determine the thermodynamic parameters for complex formation as a function of temperature and pressure. These results, in combination with relevant atmospheric data, were used to estimate the abundance of N2O complexes in Earth's atmosphere as a function of altitude. We find that the abundance of N2O complexes in Earth's atmosphere is small but non-negligible, and we suggest that N2O complexes may contribute to absorption of terrestrial radiation and be relevant for understanding the atmospheric fate of N2O.

Kinetics of conversion of dihydroxyacetone to methylglyoxal in New Zealand manuka honey: Part I--Honey systems.

The kinetics of conversion of dihydroxyacetone (DHA) to methylglyoxal (MGO) were investigated in manuka honeys and DHA-doped clover honeys stored between 4 and 37°C. Both the disappearance of DHA and appearance of MGO were confirmed as overall, first order reactions, albeit probably composites of multiple reactions. Increasing the storage temperature accelerated the rate of DHA loss and the initial rate of formation of MGO, but better conversion efficiency was observed at lower temperature. At 37°C, more MGO was lost at later times in manuka honey compared to DHA-doped-clover honey. Thirty-seven New Zealand manuka honeys and four clover honeys were analysed for various chemical and physical properties; comparison of rate constants and these parameters identified some positive correlations.

Kinetics of the conversion of dihydroxyacetone to methylglyoxal in New Zealand manuka honey: Part II--Model systems.

The irreversible dehydration reaction of dihydroxyacetone (DHA) to methylglyoxal (MGO) in a honey model system has been examined to investigate the influence of added perturbant species on the reaction rate. The secondary amino acid proline, primary amino acids (alanine, lysine and serine), and iron, or combinations of these perturbants, were added to artificial honey with either DHA or MGO and stored at 20, 27 and 37°C. These systems were monitored over time. A 1:1 conversion of DHA to MGO was not observed in any system studied, including the control system with no added perturbants. Addition of proline to the matrix increased consumption of DHA but did not produce any more MGO than the control sample. Lysine and serine behaved similarly. Alanine enhanced the conversion of DHA to MGO and had the best efficiency of conversion of DHA to MGO for the amino acids studied. An iron II salt enhanced the conversion of DHA to MGO, even in the presence of proline.

Kinetics of conversion of dihydroxyacetone to methylglyoxal in New Zealand manuka honey: Part III--A model to simulate the conversion.

A kinetic model for the conversion of dihydroxyacetone (DHA) to methylglyoxal (MGO) in honey is proposed; a building block approach was used to create the model. Artificial honeys doped with DHA and individual perturbants were fitted first, then multiple perturbants (alanine, proline and iron, and combinations of these) were fitted before comparing the simulation to real honey samples (doped clover and manuka honey). The main responses in the prediction model were DHA, MGO, proline, primary amino acids, acidity, 3-phenyllactic acid and 4-methoxyphenyllactic acid. Three temperatures (20, 27 and 37°C) were studied and the conversion of DHA to MGO was monitored over at least 1year. Differences in the conversion between clover doped with DHA and manuka honey were observed. The simulation fitted well for the honeys tested.

The structure of the O2-N2O complex.

We have investigated the lowest energy structures and interaction energies of the oxygen nitrous oxide complex (O2-N2O) using explicitly correlated coupled cluster theory. We find that the intermolecular potential energy surface of O2-N2O is very flat, with two minima of comparable energy separated by a low energy first order saddle point. Our results are able to conclusively distinguish between the two sets of experimental geometric parameters for O2-N2O that were previously determined from rotationally resolved infrared spectra. The global minimum structure of O2-N2O is therefore found to be planar with a distorted slipped parallel structure. Finally, we show that the very flat potential energy surface of O2-N2O is problematic when evaluating vibrational frequencies with a numerical Hessian and that consideration should be given as to whether results might change if the step-size is varied.