Assigning the CH stretch overtone spectrum of benzene and naphthalene with extension to anthracene and tetracene using 2- and 3-quanta anharmonic quantum chemical computations.

The CH stretch overtone region (5750-6300 cm-1) of benzene and naphthalene is assigned herein using anharmonic quantum chemical computations, and the trend of how this extends to larger polycyclic aromatic hydrocarbons (PAHs) is established. The assignment of all experimental bands to specific vibrational states is performed for the first time. Resonance polyads and the inclusion of 3-quanta vibrational states are both needed to compute accurate vibrational frequencies with the proper density-of-states to match the experimental band shape. Hundreds of 3-quanta states produce the observed band structure in naphthalene, anthracene, and tetracene, and this number is expected to increase drastically for larger PAHs. The width and shape of the main peak are consistent from naphthalene to anthracene, necessitating further exploration of this trend to confirm whether it is representative of all PAHs in the CH stretch overtone region. Understanding observations of PAH sources in the 1-3 µm region from the NIRSpec instrument aboard JWST requires new computational data, and this study provides a benchmark and foundation for their computation.

The infrared absorption spectrum of phenylacetylene and its deuterated isotopologue in the mid- to far-IR.

Anharmonicity strongly influences the absorption and emission spectra of polycyclic aromatic hydrocarbon (PAH) molecules. Here, IR-UV ion-dip spectroscopy experiments together with detailed anharmonic computations reveal the presence of fundamental, overtone, as well as 2- and 3-quanta combination band transitions in the far- and mid-infrared absorption spectra of phenylacetylene and its singly deuterated isotopologue. Strong absorption features in the 400-900 cm-1 range originate from CH(D) in-plane and out-of-plane wags and bends, as well as bending motions including the C≡C and CH bonds of the acetylene substituent and the aromatic ring. For phenylacetylene, every absorption feature is assigned either directly or indirectly to a single or multiple vibrational mode(s). The measured spectrum is dense, broad, and structureless in many regions but well characterized by computations. Upon deuteration, large isotopic shifts are observed. At frequencies above 1500 cm-1 for d1-phenylacetylene, a one-to-one match is seen when comparing computations and experiments with all features assigned to combination bands and overtones. The C≡C stretch observed in phenylacetylene is not observed in d1-phenylacetylene due to a computed 40-fold drop in intensity. Overall, a careful treatment of anharmonicity that includes 2- and 3-quanta modes is found to be crucial to understand the rich details of the infrared spectrum of phenylacetylene. Based on these results, it can be expected that such an all-inclusive anharmonic treatment will also be key for unraveling the infrared spectra of PAHs in general.

Spectroscopy and Photochemistry of Aluminum-Bearing Species in the Universe.

ConspectusMetal-bearing molecules impact the chemical and physical environment of many astronomical sources such as the circumstellar envelopes of large asymptotic giant branch and red supergiant stars, the interstellar medium, and planetary atmospheres (e.g., ablation of ∼20 tons per day into the Earth's upper atmosphere). In recent decades, the number of successfully detected metal-containing molecules has increased via rotational spectroscopic observations, which are driven by theoretical and experimental investigations. Following formation, the ultimate fate of each species (stabilization, dissociation, etc.) is determined by its electronic structure and electronic spectroscopic properties as it encounters the pervasive radiation fields in the vacuum of space. Studying these properties can evince the possibility of detection and predict the impact each molecule has on its surrounding environment. Aluminum, one of the most abundant elements and metals, is distributed throughout the universe as a constituent of gas-phase molecules (e.g., AlO, AlOH, AlCl, etc.) as well as condensed onto solid dust grains such as Al2O3. Free gas-phase aluminum-bearing molecules are synthesized by nonthermal equilibrium processes such as shocks and pulsations near the stellar photosphere or via the reaction of molecules on the surface of dust grains. Recent investigations in our research group utilizing quantum chemical methods, such as coupled cluster (CCSD(T) and CCSD(T)-F12) and multireference configuration interaction (MRCI) with large basis sets, have explored a wide breadth of spectroscopy and photochemistry of small (triatomic and tetratomic) aluminum-bearing molecules, including Al-H, Al-C, Al-N, Al-O, Al-Si, Al-P, and Al-S bonds, among others. The ground-state spectroscopy (rotational and vibrational) of various aluminum-bearing molecules is discussed in the context of experimental and observational detection potentials. These detection potentials depend on various factors, such as the magnitude of the permanent dipole moment (PDM) and the population of states yielding transition frequencies in detectable ranges. Many aluminum-bearing molecules possess large PDMs and may be prime candidates for astronomical and laboratory detection. Within this discussion, interesting aspects of the ground-state molecular orbital configuration of OAlNO are shown to lead to an uncommon triplet ground state. Additionally, the electronic absorption spectrum of the quasi-isoenergetic ground-state isomers of AlOSO is discussed as a sensitive method for detecting this species and differentiating between the two isomers. Finally, photochemical mechanisms key to the production of AlO and AlOH in low-density regions and the destruction of AlCO and AlOC are also discussed in order to understand the radiation-induced formation and destruction of these molecules.

Spectroscopy and Photochemistry of OAlNO and Implications for New Metal Chemistry in the Atmosphere.

A new aluminum-bearing species, OAlNO, which has the potential to impact the chemistry of the Earth's upper atmosphere, is characterized via high-level, ab initio, spectroscopic methods. Meteor-ablated aluminum atoms are quickly oxidized to aluminum oxide (AlO) in the mesosphere and lower thermosphere (MLT), where a steady-state layer of AlO then builds up. Concurrent formation of nitric oxide (NO) in the same region of the atmosphere will lead to the bimolecular formation of the OAlNO molecule. Molecular orbital analysis provides fundamental insights into the chemical bonding and energetic arrangement of the triplet (1 3A″) ground state and singlet (1 1A') excited-state species of OAlNO. Additionally, unpaired electrons on the terminal oxygen atom of triplet (1 3A″) OAlNO cause it to be reactive to atmospheric species, potentially impacting climate science and high-altitude chemistry. The triplet (1 3A″) ground-state species exhibits a large permanent dipole moment useful for rotational spectroscopic detection; however, similar rotational constants to the singlet (1 1A') excited-state species will hamper differentiation in a spectrum. Strong infrared intensities will assist in detection and discrimination of the different spin states and isomers. Repulsive electronic excited states of OAlNO will lead to photolysis of the Al-N bond and formation of various electronic states of AlO + NO through nonadiabatic pathways. Reaction through the OAlNO intermediate represents a means for the production of electronically excited AlO, leading to new chemistry in the atmosphere. Excitation to higher-lying electronic states will lead to fluorescence with a minor Stokes shift, useful for laboratory investigation. Such physical properties of this molecule will allow for new, unexplored chemical pathways in the MLT to be considered.

Modeling the Ground- and Excited-State Unimolecular Decay of the Simplest Fluorinated Criegee Intermediate, HFCOO, Formed from the Ozonolysis of Hydrofluoroolefin Refrigerants.

Hydrofluoroolefins (HFO) are fourth-generation refrigerants designed to function as efficient refrigerants with no ozone depletion potential and zero global warming potential. Despite extensive studies on their chemical and physical properties, the ground- and excited-state chemistry of their atmospheric oxidation products is less well understood. This study focuses on the ground- and excited-state chemistry of the simplest fluorinated Criegee intermediate (CI), fluoroformaldehyde oxide (HFCOO), which is the simplest fluorinated CI formed from the ozonolysis of HFOs. HFCOO contains syn- and anti-conformers, which have Boltzmann populations of, respectively, 87 and 13% at 298 K. For both conformers, the calculated ground-state reaction energy profiles associated with cyclization to form fluorodioxirane is lower than the equivalent unimolecular decay path in the simplest CI, H2COO, with anti-HFCOO returning a barrier height more than half of that of H2COO. The excited-state dynamics reveal that photoexcitation to the bright S2 state of syn-HFCOO and anti-HFCOO is expected to undergo a prompt O-O fission─with the former conformer expected to dissociate with an almost unity quantum yield and to form both O (1D) + HFCO (S0) and O (3P) + HFCO (T1) products. In contrast, photoexcitation of anti-HFCOO is expected to undergo an O-O bond fission with a non-unity quantum yield. The fraction of photoexcited anti-HFCOO that dissociates is predicted to exclusively form O (1D) + HFCO (S0) products, which is in sharp contrast to H2COO. The wider implications of our results are discussed from both physical and atmospheric chemistry perspectives.

Rapid Allylic 1,6 H-Atom Transfer in an Unsaturated Criegee Intermediate.

A novel allylic 1,6 hydrogen-atom-transfer mechanism is established through infrared activation of the 2-butenal oxide Criegee intermediate, resulting in very rapid unimolecular decay to hydroxyl (OH) radical products. A new precursor, Z/E-1,3-diiodobut-1-ene, is synthesized and photolyzed in the presence of oxygen to generate a new four-carbon Criegee intermediate with extended conjugation across the vinyl and carbonyl oxide groups that facilitates rapid allylic 1,6 H-atom transfer. A low-energy reaction pathway involving isomerization of 2-butenal oxide from a lower-energy (tZZ) conformer to a higher-energy (cZZ) conformer followed by 1,6 hydrogen transfer via a seven-membered ring transition state is predicted theoretically and shown experimentally to yield OH products. The low-lying (tZZ) conformer of 2-butenal oxide is identified based on computed anharmonic frequencies and intensities of its conformers. Experimental IR action spectra recorded in the fundamental CH stretch region with OH product detection by UV laser-induced fluorescence reveal a distinctive IR transition of the low-lying (tZZ) conformer at 2996 cm-1 that results in rapid unimolecular decay to OH products. Statistical RRKM calculations involving a combination of conformational isomerization and unimolecular decay via 1,6 H-transfer yield an effective decay rate keff(E) on the order of 108 s-1 at ca. 3000 cm-1 in good accord with the experiment. Unimolecular decay proceeds with significant enhancement due to quantum mechanical tunneling. A rapid thermal decay rate of ca. 106 s-1 is predicted by master-equation modeling of 2-butenal oxide at 298 K, 1 bar. This novel unimolecular decay pathway is expected to increase the nonphotolytic production of OH radicals upon alkene ozonolysis in the troposphere.

Insights into the Ultrafast Dynamics of CH2 OO and CH3 CHOO Following Excitation to the Bright 1 ππ* State: The Role of Singlet and Triplet States.

Criegee intermediates make up a class of molecules that are of significant atmospheric importance. Understanding their electronically excited states guides experimental detection and provides insight into whether solar photolysis plays a role in their removal from the troposphere. The latter is particularly important for large and functionalized Criegee intermediates. In this study, the excited state chemistry of two small Criegee intermediates, formaldehyde oxide (CH2 OO) and acetaldehyde oxide (CH3 CHOO), was modeled to compare their specific dynamics and mechanisms following excitation to the bright ππ* state and to assess the involvement of triplet states to the excited state decay process. Following excitation to the bright ππ* state, the photoexcited population exclusively evolves to form oxygen plus aldehyde products without the involvement of triplet states. This occurs despite the presence of a more thermodynamically stable triplet path and several singlet/triplet energy crossings at the Franck-Condon geometry and contrasts with the photodynamics of related systems such as acetaldehyde and acetone. This work sets the foundations to study Criegee intermediates with greater molecular complexity, wherein a bathochromic shift in the electron absorption profiles may ensure greater removal via solar photolysis.

Spectroscopic Characterization of HSO2• and HOSO• Intermediates Involved in SO2 Geoengineering.

Sulfur-containing radicals HSO2• and HOSO• are key intermediates involved in stratospheric sulfur geoengineering by SO2 injection. The spectroscopic characterization and photochemistry of both radicals are crucial to understanding the chemical impact of SO2 chemistry in the stratosphere. On the basis of the efficient generation of HOSO• by flash pyrolysis of gaseous sulfinic acid, CHF2S(O)OH, a strong absorption is observed at 270 nm along with a shoulder up to 350 nm for HOSO• isolated in low-temperature noble gas matrixes (Ar and Ne). These mainly arise from the excitations from the ground state (X2A) to the C2A/D2A and A2A/B2A states, respectively. Upon a 266 nm laser irradiation, the broad absorption band in the range 320-500 nm for HSO2• appears, and it corresponds to the combination of three excitations from the X2A state to the first (A2A), second (B2A), and third (C2A) excited states. Assignment of the UV-vis spectra is consistent with the photochemistry of HOSO• and HSO2• as observed by matrix-isolation IR spectroscopy and also by the agreement with high-level ab initio calculations.

The states that hide in the shadows: the potential role of conical intersections in the ground state unimolecular decay of a Criegee intermediate.

Criegee intermediates are of great significance to Earth's troposphere - implicated in altering the tropospheric oxidation cycle and in forming low volatility products that typically condense to form secondary organic aerosols (SOAs). As such, their chemistry has attracted vast attention in recent years. In particular, the unimolecular decay of thermal and vibrationally-excited Criegee intermediates has been the focus of several experimental and computational studies, and it is now recognized that Criegee intermediates undergo unimolecular decay to form OH radicals. In this contribution we reveal insight into the chemistry of Criegee intermediates by highlighting the hitherto neglected multi-state contribution to the ground state unimolecular decay dynamics of the Criegee intermediate products. The two key intermediates of present focus are dioxirane and vinylhydroperoxide - known to be active intermediates that mediate the unimolecular decay of CH2OO and CH3CHOO, respectively. In both cases the unimolecular decay path encounters conical intersections, which may play a pivotal role in the ensuing dynamics. This hitherto unrecognized phenomenon may be vital in the way in which the reactivity of Criegee intermediates are modelled and is likely to affect the ensuing dynamics associated with the unimolecular decay of a given Criegee intermediate.

Photochemistry of NH2NO2 and implications for chemistry in the atmosphere.

Recent studies have indicated that nitramide (NH2NO2) may be formed more plentifully in the atmosphere than previously thought, while also being a missing source of the greenhouse gas nitrous oxide (N2O) via catalyzed isomerization. To validate the importance of NH2NO2 in the Earth's atmosphere, the ground and first electronic excited states of NH2NO2 were characterized and its photochemistry was investigated using multireference and coupled cluster methods. NH2NO2 is non-planar and of singlet multiplicity in the ground state while exhibiting large out-of-plane rotation in the triplet first excited state. One-dimensional cuts of the adiabatic potential energy surface calculated using the MRCI+Q method show low-lying singlet electronic states with minima in their potential along the N-N and N-O bond coordinates. Due to vertical excitation energies in the 225-180 nm region, photochemical processes will not compete in the troposphere, causing N2O production to be the predicted major removal process of NH2NO2. In the upper atmosphere, photodissociation to form NH2NO + O (3P) is suggested to be a major photochemical removal pathway.

Spectroscopic Characterization of the First and Second Excited States of the HOSO Radical.

The spectroscopic properties of the ground and first two excited states of the HOSO radical are investigated using the internally contracted multireference configuration interaction method, including the Davidson correction (MRCI+Q) and explicit treatment of the electron correlation (MRCI-F12). The vertical and adiabatic excitation energies are also determined. The results reveal that both the 1 2A and 2 2A electronic states contain minima in their potential energy surfaces. The first excited state 1 2A possesses a nonplanar structure and has an adiabatic excitation energy of 1.45 eV (855 nm), lying in the near-infrared region. The second excited state 2 2A has a planar geometry and an adiabatic excitation energy of 2.91 eV (426 nm) existing in the visible region. The calculated oscillator strengths for the vertical electronic excitations to the 1 2A (327 nm) and 2 2A (270 nm) states are 0.003 and 0.022, respectively, indicating experimental intensity should be observed. The small but non-negligible Franck-Condon factors for excitations ∼300 nm, and the broad and intense absorption feature in the 225-275 nm region suggest that detection of the HOSO radical with electronic spectroscopy may be feasible.

Photodissociation Dynamics of CH2OO on Multiple Potential Energy Surfaces: Experiment and Theory.

UV excitation of the CH2OO Criegee intermediate across most of the broad span of the (B 1A')-(X 1A') spectrum results in prompt dissociation to two energetically accessible asymptotes: O (1D) + H2CO (X 1A1) and O (3P) + H2CO (a 3A''). Dissociation proceeds on multiple singlet potential energy surfaces that are coupled by two regions of conical intersection (CoIn). Velocity map imaging (VMI) studies reveal a bimodal total kinetic energy release (TKER) distribution for the O (1D) + H2CO (X 1A1) products with the major and minor components accounting for ca. 40% and ca. 20% on average of the available energy (Eavl), respectively. The unexpected low TKER component corresponds to highly internally excited H2CO (X 1A1) products accommodating ca. 80% of Eavl. Full dimensional trajectory calculations suggest that the bimodal TKER distribution of the O (1D) + H2CO (X 1A1) products originates from two different dynamical pathways: a primary pathway (69%) evolving through one CoIn region to products and a smaller component (20%) sampling both CoIn regions enroute to products. Those that access both CoIn regions likely give rise to the more highly internally excited H2CO (X 1A1) products. The remaining trajectories (11%) dissociate to O (3P) + H2CO (a 3A'') products after traversing through both CoIn regions. The complementary experimental and theoretical investigation provides insight on the photodissociation of CH2OO via multiple dissociation pathways through two regions of CoIn that control the branching and energy distributions of products.

CH Stretch Activation of CH3CHOO: Deep Tunneling to Hydroxyl Radical Products.

Alkene ozonolysis, an important source of hydroxyl (OH) radicals in the Earth's troposphere, proceeds through unimolecular decay of Criegee intermediates. In this work, infrared activation of the methyl-substituted Criegee intermediate, syn-CH3CHOO, in the CH stretch fundamental region (2850-3150 cm-1) is shown to result in unimolecular decay to OH radical products. These excitation energies correspond to only half of the transition state barrier height, and thus the resultant 1,4 H atom transfer that leads to OH products occurs exclusively by quantum mechanical tunneling. Infrared action spectra recorded with UV laser-induced fluorescence detection of the OH products reveal the four CH stretch fundamentals and CO stretch overtone predicted to have strong transition strength. The vibrational band origins, relative intensities, and transition types derived from rotational band contour analyses are in good accord with theory. Distinctly different Lorentzian line broadening of the observed features is attributed to mode-specific anharmonic couplings predicted theoretically between spectroscopically bright and nearby dark states. The measured OH product state distribution shows a strong λ-doublet preference arising from pπ orbital alignment, which is indicative of the vinyl hydroperoxide intermediate along the reaction pathway. The unimolecular decay of syn-CH3CHOO at ca. 3000 cm-1 is predicted to be quite slow (ca. 105 s-1) using statistical Rice-Ramsperger-Kassel-Marcus theory with tunneling and much slower than observed at higher energies.