Theoretical Investigation of the Atmospheric Photochemistry of Glyoxylic Acid in the Gas Phase.

The photochemistry of glyoxylic acid (HC(O)C(O)OH) is explored in the near UV in both the singlet (S1/S0) and triplet (T1) manifolds using density functional theory (M06-2X/aug-cc-pVTZ) to reach an overall mechanistic picture of the atmospherically relevant photochemistry in the gas phase. The calculated energies and structures are also used in RRKM kinetics calculations to compare the relative reaction rates on each of these electronic states. The major photolysis pathways are two possible photodecarboxylation reactions: direct C-C bond cleavage (Norrish Type I reaction) and ß-hydrogen transfer followed by CO2 loss. These results indicate that from λ = 350-380 nm both photodecarboxylation pathways can occur following intersystem crossing to the T1 surface. However, hydrogen transfer-decarboxylation initiated on S1 becomes increasingly important at λ < 350 nm. At the lower energy UV wavelengths available in the atmosphere (λ = 380-400 nm), reactions can only occur in S0 where concerted hydrogen transfer-decarboxylation is the dominant dissociation pathway with some minor contributions from CO loss/decarbonylation reactions.

Dynamics and quantum yields of H2 + CH2CO as a primary photolysis channel in CH3CHO.

The first experimental observation of the primary photochemical channel of acetaldehyde leading to the formation of ketene (CH2CO) and hydrogen (H2) molecular products is reported. Acetaldehyde (CH3CHO) was photolysed in a molecular beam at 305.6 nm and the resulting H2 product characterized using velocity-map ion (VMI) imaging. Resonance-enhanced multiphoton ionization (REMPI), via two-photon excitation to the double-well EF 1Σ state, was used to state-selectively ionize the H2 and determine angular momentum distributions for H2 (ν = 0) and H2 (ν = 1). Velocity-map ion images were obtained for H2 (ν = 0 and 1, J = 5), allowing the total translational energy release of the photodissociation process to be determined. Following photolysis of CH3CHO in a gas cell, the CH2CO co-fragment was identified, using Fourier transform infrared spectroscopy, by its characteristic infrared absorption at 2150 cm-1. The measured quantum yield of the CH2CO + H2 product channel at 305.0 nm is φ = 0.0075 ± 0.0025 for both 15 Torr of neat CH3CHO and a mixture with 745 Torr of N2. Although small, this result has implications for the atmospheric photochemistry of carbonyls and this reaction represents a new tropospheric source of H2. Quasi-classical trajectory (QCT) simulations on a zero-point energy corrected reaction-path potential are also performed. The experimental REMPI and VMI image distributions are not consistent with the QCT simulations, indicating a non reaction-path mechanism should be considered.

Photo-tautomerization of acetaldehyde as a photochemical source of formic acid in the troposphere.

Organic acids play a key role in the troposphere, contributing to atmospheric aqueous-phase chemistry, aerosol formation, and precipitation acidity. Atmospheric models currently account for less than half the observed, globally averaged formic acid loading. Here we report that acetaldehyde photo-tautomerizes to vinyl alcohol under atmospherically relevant pressures of nitrogen, in the actinic wavelength range, λ = 300-330 nm, with measured quantum yields of 2-25%. Recent theoretical kinetics studies show hydroxyl-initiated oxidation of vinyl alcohol produces formic acid. Adding these pathways to an atmospheric chemistry box model (Master Chemical Mechanism) demonstrates increased formic acid concentrations by a factor of ~1.7 in the polluted troposphere and a factor of ~3 under pristine conditions. Incorporating this mechanism into the GEOS-Chem 3D global chemical transport model reveals an estimated 7% contribution to worldwide formic acid production, with up to 60% of the total modeled formic acid production over oceans arising from photo-tautomerization.

Infrared Spectra of Gas-Phase 1- and 2-Propenol Isomers.

Fourier transform infrared spectra of isolated 1-propenol and 2-propenol in the gas-phase have been collected in the range of 900-3800 cm-1, and the absolute infrared absorption cross sections reported for the first time. Both cis and trans isomers of 1-propenol were observed with the trans isomer in greater abundance. Syn and anti conformers of both 1- and 2-propenol were also observed, with abundance consistent with thermal population. The FTIR spectrum of the smaller ethenol (vinyl alcohol) was used as a benchmark for our computational results. As a consequence, its spectrum has been partially reassigned resulting in the first report of the anti-ethenol conformer. Electronic structure calculations were used to support our experimental results and assign vibrational modes for the most abundant isomers, syn-trans-1-propenol and syn-2-propenol.

Theoretical study of the mechanism for the sequential N-O and N-N bond cleavage within N2O adducts of N-heterocyclic carbenes by a vanadium(iii) complex.

A theoretical study into the reactions of the N2O adducts of N-heterocyclic carbenes (NHCs) and a V((III)) complex was carried out using DFT calculations. Unlike most transition metal reactions with N2O that simply release N2 following O-atom transfer onto the metal centre, this NHC-based system traps the entire N2O molecule and then cleaves both the N-O and N-N bond in two consecutive reactions. The NHC presence increases the reactivity of N2O by altering the distribution of electron density away from the O-atom towards the two N-atoms. This electronic redistribution enables V-N binding interactions to form a reactive N,O-donor intermediate species. Our results show that bond breaking with concomitant ligand migration occurs via a concerted process for both the N-O and N-N cleavage reactions.

NO2 bond cleavage by MoL3 complexes.

The cleavage of one N-O bond in NO2 by two equivalents of Mo(NRAr)3 has been shown to occur to form molybdenum oxide and nitrosyl complexes. The mechanism and electronic rearrangement of this reaction was investigated using density functional theory, using both a model Mo(NH2)3 system and the full [N((t)Bu)(3,5-dimethylphenyl)] experimental ligand. For the model ligand, several possible modes of coordination for the resulting complex were observed, along with isomerisation and bond breaking pathways. The lowest barrier for direct bond cleavage was found to be via the singlet η(2)-N,O complex (7 kJ mol(-1)). Formation of a bimetallic species was also possible, giving an overall decrease in energy and a lower barrier for reaction (3 kJ mol(-1)). Results for the full ligand showed similar trends in energies for both isomerisation between the different isomers, and for the mononuclear bond cleavage. The lowest calculated barrier for cleavage was only 21 kJ mol(-1)via the triplet η(1)-O isomer, with a strong thermodynamic driving force to the final products of the doublet metal oxide and a molecule of NO. Formation of the full ligand dinuclear complex was not accompanied by an equivalent decrease in energy seen with the model ligand. Direct bond cleavage via an η(1)-O complex is thus the likely mechanism for the experimental reaction that occurs at ambient temperature and pressure. Unlike the other known reactions between MoL3 complexes and small molecules, the second equivalent of the metal does not appear to be necessary, but instead irreversibly binds to the released nitric oxide.

Synthesis of Ti(IV) complexes of donor-functionalised phenoxy-imine tridentates and their evaluation in ethylene oligomerisation and polymerisation.

A number of analogues of the Mitsui Chemicals ethylene trimerisation system (IV) have been explored, in which one of the donor atoms have been modified. Thus, a series of mono-anionic tridentate phenoxy-imine (3-(t-butyl)-2-(OH)-C6H4C=N(C(CH3)2CH2OMe) 1, 3-(adamantyl)-2-(OH)-C6H4C=N(2'-(2''-(SMe)C6H4)-C6H4) 2, 3-(t-butyl)-2-(OSiMe3)-C6H4C=N(C(CH3)2CH2OMe) 3) or phenoxy-amine (3,5-di(t-butyl)-2-(OH)-C6H4CH2-N(2'-(2''-(OMe)C6H4)-C6H4) 4) ligands have been prepared and reacted with TiCl4 or TiCl4(thf)2 to give the mono-ligand complexes 5-7. The solid state structures of compounds 4-6 have been determined. Complexes 5-7 have been tested for their potential as ethylene oligomerisation/polymerisation systems in conjunction with MAO activator and benchmarked against the Mitsui phenoxy-imine trimerisation system IV. While the phenoxy-amine complex 6 shows a propensity for polymer formation, the phenoxy-imine complexes 5 and 7 show somewhat increased formation of short chain LAOs. Complex 5 is selective for 1-butene in the oligomeric fraction, while 7 displays liquid phase selectivity to 1-hexene. As such 7, which is a sulfur substituted analogue of the Mitsui system IV, displays similar characteristics to the parent catalyst. However, its utility is limited by the lower activity and predominant formation of polyethylene.