Gas Phase Spectroscopy and Kinetics of Atmospheric Radicals

An important goal for atmospheric and combustion chemists is continued improvement in our understanding of the gas phase reactivity of free radical intermediates formed during hydrocarbon oxidation. The primary focus of this thesis was to measure gas phase kinetics of prototypical free radicals relevant to atmospheric and combustion chemistry, a goal that requires spectroscopy, quantitative product detection, and computational chemistry in order to address these complex chemical systems. Near-infrared cavity ringdown spectroscopy was used to study the peroxy radicals (RO2) formed from chlorine-initiated oxidation of isoprene and other unsaturated hydrocarbons. Isoprene is one of the most important hydrocarbons in the atmosphere; detection of RO2 formed directly from isoprene oxidation will aid in understanding the initial steps of its fate in the atmosphere. As expected, the near-infrared chloro-isoprenyl peroxy radical spectrum has many features; each spectral feature corresponds to a different isomer and conformer, indicating that several RO2 structures are formed. In small RO2, it was possible to identify the molecular structure of the absorber by comparing the experimental spectrum with the vibrationally-resolved electronic spectrum generated by computational chemistry. Identification of each feature then enabled preliminary isomer-specific kinetics measurements. Photoionization mass spectrometry is another useful method for selective detection of radicals, with the added bonus of detecting many of the other species of interest, leading to a comprehensive understanding of the reaction mechanism. The yields of radical chain-propagating product channels of prototypical RO2 reactions (self- and cross-reactions) are important in understanding radical chemistry in gas phase hydrocarbon oxidation. We obtained branching ratio information for reactions of acetyl peroxy radicals with HO2, with particular focus on OH-regenerating reactions. Along the way, we observed unexpected product formation from low-pressure reactions of acetyl radicals and oxygen. Using the same techniques, we also looked at the self-reaction of ethyl peroxy radicals, confirming past measurements of the radical-propagating channel for this reaction, and investigated interesting product formation, like what may be the dialkyl peroxide. These studies were supported by measurements of VUV photoionization cross sections for several radical species. The utility of this instrumentation was also extended by the development of a low-temperature (200–300 K) flow reactor. Finally, using time-resolved broadband cavity-enhanced absorption spectroscopy, we measured the rate coefficient of reactions of the smallest Criegee intermediate, CH2OO with ozone. We observed that this reaction is rather fast, which could have significant implications for experimental ozonolysis studies that are carried out under high initial reactant concentrations. </p

INFRARED PHOTODISSOCIATION CLUSTER STUDIES ON CO2 INTERACTION WITH TITANIUM OXIDE CATALYST MODELS

Titanium oxide catalysts are some of the most promising photocatalyst candidates for renewable energy storage applications via production of solar fuels. To contribute to a molecular-level understanding of the interaction of CO$_{2}$ with titanium oxide, we turn to cluster models in order to circumvent the challenges posed by speciation in the condensed phase. In this work, we use infrared photodissociation spectroscopy ($950-2400$ cm$^{-1}$) in concert with density functional theory calculations to identify and characterize $[text{TiO}_{x}(text{CO}_{2})_{y}]^{-}$ ($x$ = $1-3$, $y$ = $3-7$) clusters. We use these model systems to study the interaction of CO$_{2}$ with TiO, TiO$_{2}$, and TiO$_{3}$, and we find that each species exhibits unique infrared signatures and binding motifs. We will discuss the structures of these cluster ions, and how the coordination of the titanium atom plays a role in reduction of CO$_{2}$

OXALATE FORMATION IN TITANIUM-CARBON DIOXIDE ANIONIC CLUSTERS STUDIED BY INFRARED PHOTODISSOCIATION SPECTROSCOPY

Carbon-carbon bond formation during carbon dioxide fixation would enable bulk synthesis of hydrocarbon chains, generally through formation of an oxalate intermediate. In this talk, we demonstrate the formation of $[text{Ti}(text{CO}_{2})_{y}]^{-}$ ($y$ = $4-6$) gas phase clusters with an oxalate ligand bearing significant ($> 1$ e$^{-}$) negative charge. Gas phase anionic clusters were generated using laser ablation of a titanium metal target in the presence of a CO$_{2}$ expansion, and the infrared photodissociation spectra were measured from $950-2400$ cm$^{-1}$, revealing vibrations characteristic of the oxalate anion. The molecular structure of these clusters was identified by comparing the experimental vibrational spectra with density functional theory calculations

BOND INSERTION IN METAL–CARBON DIOXIDE ANIONIC CLUSTERS STUDIED BY INFRARED PHOTODISSOCIATION SPECTROSCOPY

C–O bond breaking is an important process in the activation of \chem{CO_2} that can be catalyzed by the presence of a metal. In this talk, we investigate the factors that lead to bond insertion in [M(\chem{CO_2})$_{y}$]$^{-}$ gas phase clusters, specifically addressing differences amongst the metals M = Ni, Fe, and Ti. Gas phase anionic clusters were generated using laser ablation of a metal target in the presence of a \chem{CO_2} expansion, and the infrared photodissociation spectra were measured from 950–2400 \wn. Metal carbonyl vibrational signatures were used to infer bond insertion, and computational chemistry simulations were used to assess the feasibility of bond breaking in these systems

Vacuum ultraviolet photoionization cross section of the hydroxyl radical

The absolute photoionization spectrum of the hydroxyl (OH) radical from 12.513 to 14.213 eV was measured by multiplexed photoionization mass spectrometry with time-resolved radical kinetics. Tunable vacuum ultraviolet (VUV) synchrotron radiation was generated at the Advanced Light Source. OH radicals were generated from the reaction of O(^1D) + H_2O in a flow reactor in He at 8 Torr. The initial O(^1D) concentration, where the atom was formed by pulsed laser photolysis of ozone, was determined from the measured depletion of a known concentration of ozone. Concentrations of OH and O(^3P) were obtained by fitting observed time traces with a kinetics model constructed with literature rate coefficients. The absolute cross section of OH was determined to be σ(13.436 eV) = 3.2 ± 1.0 Mb and σ(14.193 eV) = 4.7 ± 1.6 Mb relative to the known cross section for O(^3P) at 14.193 eV. The absolute photoionization spectrum was obtained by recording a spectrum at a resolution of 8 meV (50 meV steps) and scaling to the single-energy cross sections. We computed the absolute VUV photoionization spectrum of OH and O(^3P) using equation-of-motion coupled-cluster Dyson orbitals and a Coulomb photoelectron wave function and found good agreement with the observed absolute photoionization spectra

VUV Photoionization Cross Sections of HO_2, H_2O_2, and H_2CO

The absolute vacuum ultraviolet (VUV) photoionization spectra of the hydroperoxyl radical (HO_2), hydrogen peroxide (H_2O_2), and formaldehyde (H_2CO) have been measured from their first ionization thresholds to 12.008 eV. HO_2, H_2O_2, and H_2CO were generated from the oxidation of methanol initiated by pulsed-laser-photolysis of Cl_2 in a low-pressure slow flow reactor. Reactants, intermediates, and products were detected by time-resolved multiplexed synchrotron photoionization mass spectrometry. Absolute concentrations were obtained from the time-dependent photoion signals by modeling the kinetics of the methanol oxidation chemistry. Photoionization cross sections were determined at several photon energies relative to the cross section of methanol, which was in turn determined relative to that of propene. These measurements were used to place relative photoionization spectra of HO_2, H_2O_2, and H_2CO on an absolute scale, resulting in absolute photoionization spectra

Vacuum ultraviolet photoionization cross section of the hydroxyl radical

The absolute photoionization spectrum of the hydroxyl (OH) radical from 12.513 to 14.213 eV was measured by multiplexed photoionization mass spectrometry with time-resolved radical kinetics. Tunable vacuum ultraviolet (VUV) synchrotron radiation was generated at the Advanced Light Source. OH radicals were generated from the reaction of O(^1D) + H_2O in a flow reactor in He at 8 Torr. The initial O(^1D) concentration, where the atom was formed by pulsed laser photolysis of ozone, was determined from the measured depletion of a known concentration of ozone. Concentrations of OH and O(^3P) were obtained by fitting observed time traces with a kinetics model constructed with literature rate coefficients. The absolute cross section of OH was determined to be σ(13.436 eV) = 3.2 ± 1.0 Mb and σ(14.193 eV) = 4.7 ± 1.6 Mb relative to the known cross section for O(^3P) at 14.193 eV. The absolute photoionization spectrum was obtained by recording a spectrum at a resolution of 8 meV (50 meV steps) and scaling to the single-energy cross sections. We computed the absolute VUV photoionization spectrum of OH and O(^3P) using equation-of-motion coupled-cluster Dyson orbitals and a Coulomb photoelectron wave function and found good agreement with the observed absolute photoionization spectra

Conserved and Distinct Modes of CREB/ATF Transcription Factor Regulation by PP2A/B56γ and Genotoxic Stress

Activating transcription factor 1 (ATF1) and the closely related proteins CREB (cyclic AMP resonse element binding protein) and CREM (cyclic AMP response element modulator) constitute a subfamily of bZIP transcription factors that play critical roles in the regulation of cellular growth, metabolism, and survival. Previous studies demonstrated that CREB is phosphorylated on a cluster of conserved Ser residues, including Ser-111 and Ser-121, in response to DNA damage through the coordinated actions of the ataxia-telangiectasia-mutated (ATM) protein kinase and casein kinases 1 and 2 (CK1/2). Here, we show that DNA damage-induced phosphorylation by ATM is a general feature of CREB and ATF1. ATF1 harbors a conserved ATM/CK cluster that is constitutively and stoichiometrically phosphorylated by CK1 and CK2 in asynchronously growing cells. Exposure to DNA damage further induced ATF1 phosphorylation on Ser-51 by ATM in a manner that required prior phosphorylation of the upstream CK residues. Hyperphosphorylated ATF1 showed a 4-fold reduced affinity for CREB-binding protein. We further show that PP2A, in conjunction with its targeting subunit B56γ, antagonized ATM and CK1/2-dependent phosphorylation of CREB and ATF1 in cellulo. Finally, we show that CK sites in CREB are phosphorylated during cellular growth and that phosphorylation of these residues reduces the threshold of DNA damage required for ATM-dependent phosphorylation of the inhibitory Ser-121 residue. These studies define overlapping and distinct modes of CREB and ATF1 regulation by phosphorylation that may ensure concerted changes in gene expression mediated by these factors

BOND INSERTION IN METAL–CARBON DIOXIDE ANIONIC CLUSTERS STUDIED BY INFRARED PHOTODISSOCIATION SPECTROSCOPY

C–O bond breaking is an important process in the activation of \chem{CO_2} that can be catalyzed by the presence of a metal. In this talk, we investigate the factors that lead to bond insertion in [M(\chem{CO_2})$_{y}$]$^{-}$ gas phase clusters, specifically addressing differences amongst the metals M = Ni, Fe, and Ti. Gas phase anionic clusters were generated using laser ablation of a metal target in the presence of a \chem{CO_2} expansion, and the infrared photodissociation spectra were measured from 950–2400 \wn. Metal carbonyl vibrational signatures were used to infer bond insertion, and computational chemistry simulations were used to assess the feasibility of bond breaking in these systems