Intramolecular hole-transfer in protonated anthracene.

Excitation spectra of protonated and deuteronated anthracene are obtained by triple-resonance dissociation spectroscopy. Very cold cations, protonated/deuteronated exclusively at the 9-position, are generated from two-colour two-photon threshold ionisation of 9-dihydroanthracenyl radicals (C14H11). The excitation spectra reveal rich structure, not resolved in previous studies, that is assigned based on anharmonic and Herzberg-Teller coupling calculations. This work reveals that the excitation of protonated anthracene induces a symmetry-breaking intramolecular charge-transfer process along a Marcus-Hush coordinate, where the positively charged hole hops from the central bridging sp2 carbon, onto one of the aromatic rings. Signatures of this charge-transfer event are observed in the excitation spectrum, through active Herzberg-Teller progressions.

Photophysical oxidation of HCHO produces HO2 radicals.

Formaldehyde, HCHO, is the highest-volume carbonyl in the atmosphere. It absorbs sunlight at wavelengths shorter than 330 nm and photolyses to form H and HCO radicals, which then react with O2 to form HO2. Here we show HCHO has an additional HO2 formation pathway. At photolysis energies below the energetic threshold for radical formation we directly detect HO2 at low pressures by cavity ring-down spectroscopy and indirectly detect HO2 at 1 bar by Fourier-transform infrared spectroscopy end-product analysis. Supported by electronic structure theory and master equation simulations, we attribute this HO2 to photophysical oxidation (PPO): photoexcited HCHO relaxes non-radiatively to the ground electronic state where the far-from-equilibrium, vibrationally activated HCHO molecules react with thermal O2. PPO is likely to be a general mechanism in tropospheric chemistry and, unlike photolysis, PPO will increase with increasing O2 pressure.

POPTARTS: A New Method to Determine Quantum Yields in a Molecular Beam.

A new technique is reported to determine absolute photodissociation quantum yields, ϕdiss, in a molecular beam. The technique relies on a molecule having two available product channels, where a species in channel A can be converted photolytically to a species in channel B. The relative decrease in the species from channel A and the relative increase in species from B provide a direct measure of the relative product yield of each channel, with no external calibration required. In the event that only channels A and B exist, or at least dominate, then the sum rule ϕA + ϕB = 1 can be used to convert relative quantum yields into absolute yields. The technique is demonstrated using the well-understood and characterized photochemistry of HCHO. Formaldehyde photolysis at wavelengths near 310 nm produces either H + HCO (channel A) or H2 + CO (channel B). HCO can then be photolyzed with high efficiency into H + CO. The product state distributions for HCO from channel A, CO from channel B, and CO from the secondary HCO photolysis event are all well-known; this is not a requirement but is utilized here to demonstrate the veracity of the technique. The zero-pressure quantum yields of HCO from HCHO photolysis via the 2341 and 2151 states of HCHO are determined to be 0.66 and 0.74, respectively, which are in excellent agreement with the established quantum yields at atmospheric pressure and support the conclusion that HCHO quantum yields at these photolysis energies are not pressure dependent.

PAH Growth in Flames and Space: Formation of the Phenalenyl Radical.

Polycyclic aromatic hydrocarbons (PAHs) are intermediates in the formation of soot particles and interstellar grains. However, their formation mechanisms in combustion and interstellar environments are not fully understood. The production of tricyclic PAHs and, in particular, the conversion of a PAH containing a five-membered ring to one with a six-membered ring are of interest to explain PAH abundances in combustion processes. In the present work, resonant ionization mass spectrometry in conjunction with isotopic labeling is used to investigate the formation of the phenalenyl radical from acenaphthylene and methane in an electrical discharge. We show that in this environment the CH cycloaddition mechanism converts a five-membered ring to a six-membered ring. This mechanism can occur in tandem with other PAH formation mechanisms such as hydrogen abstraction/acetylene addition (HACA) to produce larger PAHs in flames and the interstellar medium.

Photodissociation dynamics of CF3CHO: C-C bond cleavage.

The photodissociation dynamics of jet-cooled trifluoroacetaldehyde (CF3CHO) into radical products, CF3 + HCO, was explored using velocity mapped ion imaging over the wavelength range 297.5 nm ≤λ≤ 342.8 nm (33 613-29 172 cm-1) covering the entire section of the absorption spectrum accessible with solar actinic wavelengths at the ground level. After initial excitation to the first excited singlet state, S1, the radical dissociation proceeds largely via the first excited triplet state, T1, at excitation energies above the T1 barrier. By combining velocity-mapped ion imaging with high-level theory, we place this barrier at 368.3 ± 2.4 kJ mol-1 (30 780 ± 200 cm-1). After exciting to S1 at energies below this barrier, the dissociation proceeds exclusively via the ground electronic state, S0. The dissociation threshold is determined to be 335.7 ± 1.8 kJ mol-1 (28 060 ± 150 cm-1). Using laser-induced fluorescence spectroscopy, the origin of the S1 ← S0 transition is assigned at 28 903 cm-1. The S0 dissociation channel is active at the S1 origin, but the yield significantly increases above 29 100 cm-1 due to enhanced intersystem crossing or internal conversion.

Photodissociation of dicarbon: How nature breaks an unusual multiple bond.

The dicarbon molecule (C2) is found in flames, comets, stars, and the diffuse interstellar medium. In comets, it is responsible for the green color of the coma, but it is not found in the tail. It has long been held to photodissociate in sunlight with a lifetime precluding observation in the tail, but the mechanism was not known. Here we directly observe photodissociation of C2 From the speed of the recoiling carbon atoms, a bond dissociation energy of 602.804(29) kJ·mol[Formula: see text] is determined, with an uncertainty comparable to its more experimentally accessible N2 and O2 counterparts. The value is within 0.03 kJ·mol-1 of high-level quantum theory. This work shows that, to break the quadruple bond of C2 using sunlight, the molecule must absorb two photons and undergo two "forbidden" transitions.

Rotational resonances in the H2CO roaming reaction are revealed by detailed correlations.

Since its discovery 16 years ago, roaming has become a ubiquitous mechanism in molecular photochemistry. Its general features are now understood, but little detail is known about how the potential energy surface (PES) determines reaction outcomes. We performed detailed experiments on formaldehyde (H2CO) photodissociation and determined fully correlated quantum state distributions of the molecular hydrogen and carbon monoxide products. These experiments reveal previously undetected bimodal carbon monoxide rotational distributions. Insights from classical trajectory calculations demonstrate that these features arise from resonances as the PES directs the reaction into cis and trans O-C-H···H critical geometries, which produce rebound and stripping mechanisms, respectively. These subtle and pervasive effects demonstrate additional complexity in this prototypical roaming reaction, which we expect to be general. They also provide detailed benchmarks for predictive theories of roaming.

Structural Effects on the Norrish Type I α-Bond Cleavage of Tropospherically Important Carbonyls.

Norrish Type I (NTI) α-bond cleavage is the dominant photolysis mechanism in small carbonyls and is an important source of radicals in the troposphere. In nonsymmetric species two cleavages are possible, NTIa and NTIb, forming larger and smaller alkyl radicals, respectively. For a data set of 20 small, atmospherically relevant carbonyls we predict NTIa and NTIb thresholds on the S0, S1, and T1 electronic states. The calculated NTIa T1 thresholds give a mean absolute deviation (MAD) of 5.8 kJ/mol with respect to the available experimental thresholds of five carbonyls. In addition, the intrinsic barrier heights to dissociation on the S0, S1, and T1 electronic states are predicted. We find RI-B2GP-PLYP/def2-TZVP calculations on S0 and unrestricted RI-B2GP-PLYP/def2-TZVP calculations on T1 give MADs of 6.1 kJ/mol for S0 asymptotic energies and 6.3 kJ/mol for S0 → T1 0-0 excitation energies, with respect to available experimental data. A composite method is used to determine S1 thresholds, with bt-STEOM-CCSD/cc-pVQZ calculation of vertical excitation energies and TD-RI-B3LYP/def2-TZVP calculations on S1, which achieves a MAD of 7.2 kJ/mol, with respect to experimental 0-0 excitation energies. Our calculations suggest, with the exception of bifunctional carbonyls and enones, NTI reactions on S1 are unlikely to be important at tropospherically relevant photolysis energies (<400 kJ/mol). In contrast, at these energies almost all possible NTI channels on T1 are open, and all barrierless S0 NTI dissociations are accessible. Our calculations allow a number of structural effects on both 0-0 excitation energies and intrinsic reaction barriers, on a given electronic state, to be elucidated and rationalized.

Quantum-Induced Symmetry Breaking in the Deuterated Dihydroanthracenyl Radical.

The hydrogen-atom adduct with anthracene, 9-dihydroanthracenyl radical (C14H11), and its deuterated analogue have been identified by laser spectroscopy coupled to time-of-flight mass spectrometry, supported by time-dependent density functional theory calculations. The electronic spectrum of 9-dihydroanthracenyl radical exhibits an origin band at 19115 cm-1 and its ionization energy was determined to be 6.346(1) eV. The spectra reveal a low-frequency vibrational progression corresponding to a mode described by a butterfly inversion. In the deuterated analogue, a zero-point-energy imbalance along this coordinate is found to lead to a doubling of the observed spectral lines in the progression. This is attributed to quantum-induced symmetry breaking as previously observed in isotopologues of CH5+.

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.

Zero-point energy conservation in classical trajectory simulations: Application to H2CO.

A new approach for preventing zero-point energy (ZPE) violation in quasi-classical trajectory (QCT) simulations is presented and applied to H2CO "roaming" reactions. Zero-point energy may be problematic in roaming reactions because they occur at or near bond dissociation thresholds and these channels may be incorrectly open or closed depending on if, or how, ZPE has been treated. Here we run QCT simulations on a "ZPE-corrected" potential energy surface defined as the sum of the molecular potential energy surface (PES) and the global harmonic ZPE surface. Five different harmonic ZPE estimates are examined with four, on average, giving values within 4 kJ/mol-chemical accuracy-for H2CO. The local harmonic ZPE, at arbitrary molecular configurations, is subsequently defined in terms of "projected" Cartesian coordinates and a global ZPE "surface" is constructed using Shepard interpolation. This, combined with a second-order modified Shepard interpolated PES, V, allows us to construct a proof-of-concept ZPE-corrected PES for H2CO, V eff, at no additional computational cost to the PES itself. Both V and V eff are used to model product state distributions from the H + HCO → H2 + CO abstraction reaction, which are shown to reproduce the literature roaming product state distributions. Our ZPE-corrected PES allows all trajectories to be analysed, whereas, in previous simulations, a significant proportion was discarded because of ZPE violation. We find ZPE has little effect on product rotational distributions, validating previous QCT simulations. Running trajectories on V, however, shifts the product kinetic energy release to higher energy than on V eff and classical simulations of kinetic energy release should therefore be viewed with caution.

Jet-Cooled Spectroscopy of ortho-Hydroxycyclohexadienyl Radicals.

The electronic spectra of the ortho-hydroxycyclohexadienyl radical have been observed following the supersonic expansion of the electric discharge products of phenol and water. Hydrogen atoms, split from water, add to the phenol ring at the ortho position, generating syn and anti rotamers with respect to the hydroxyl group. The D1 ← D0 transitions were recorded by resonance-enhanced multiphoton ionization spectroscopy. The spectrum of each isomer was isolated through hole-burning spectroscopy. The assignment and symmetry of the excited state are evaluated through ab initio calculations and are employed to assign each spectrum. Both rotamers are calculated to have a puckered ring in the excited state, leading to C1 symmetry. The spectrum of the anti isomer is assigned well using this symmetry; however, the syn isomer is assigned better in the C s symmetry of the ground state. We use Duschinsky matrices as a tool for the spectroscopist to determine which point group to use when ab initio calculations are ambiguous.

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.

Photodissociation dynamics of propanal and isobutanal: The Norrish Type I pathway.

The Norrish Type I photodissociation of two aliphatic aldehydes, propanal and isobutanal, has been investigated using velocity-map imaging. The HCO photoproduct of this reaction was probed using a 1+1 resonance-enhanced multiphoton ionization scheme via the 3p2Π Rydberg state. The velocity map images of HCO+ were collected across a range of photolysis energies for both species from 30 500 to 33 000 cm-1 (λ = 312-327 nm). The corresponding translational energy distributions show that the majority of the available energy goes into the translational motion of the products (55%-68%) with this fraction increasing as the T1 barrier is approached. Analysis of the translational energy distributions was also used to determine the aldehyde α C-C bond dissociation energies which were found to be 339.8 ± 2.5 and 331.2 ± 2.5 kJ/mol for propanal and isobutanal, respectively. These values were also found to be in good agreement with the computed dissociation energies using G4 and CCSD(T)/aug-cc-pVTZ//M062X/aug-cc-pVTZ levels of theory. Furthermore, these dissociation energies, combined with the known ΔfH (0 K) of the reaction products, provided the ΔfH (0 K) of propanal and isobutanal which were calculated to be -167.3 ± 2.5 and -184.0 ± 2.5 kJ/mol, respectively.

Multihydroxy-Anthraquinone Derivatives as Free Radical and Cationic Photoinitiators of Various Photopolymerizations under Green LED.

Multihydroxy-anthraquinone derivatives [i.e., 1,2,4-trihydroxyanthraquinone (124-THAQ), 1,2,7-trihydroxyanthraquinone (127-THAQ), and 1,2,5,8-tetrahydroxyanthraquinone (1258-THAQ)] can interact with various additives [e.g., iodonium salt, tertiary amine, N-vinylcarbazole, and 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine] under household green LED irradiation to generate active species (cations and radicals). The relevant photochemical mechanism is investigated using quantum chemistry, fluorescence, cyclic voltammetry, laser flash photolysis, steady state photolysis, and electron spin resonance spin-trapping techniques. Furthermore, the multihydroxy-anthraquinone derivative-based photoinitiating systems are capable of initiating cationic photopolymerization of epoxides or divinyl ethers under green LED, and the relevant photoinitiation ability is consistent with the photochemical reactivity (i.e., 124-THAQ-based photoinitiating system exhibits highest reactivity and photoinitiation ability). More interestingly, multihydroxy-anthraquinone derivative-based photoinitiating systems can initiate free radical crosslinking or controlled (i.e., reversible addition-fragmentation chain transfer) photopolymerization of methacrylates under green LED. It reveals that multihydroxy-anthraquinone derivatives can be used as versatile photoinitiators for various types of photopolymerization reactions.

Interconversion of Methyltropyl and Xylyl Radicals: A Pathway Unavailable to the Benzyl-Tropyl Rearrangement.

The products of an electrical discharge containing toluene are interrogated using resonance-enhanced multiphoton ionization and laser-induced fluorescence spectroscopies. A previously unreported electronic spectrum recorded at m/z = 105, with a putative origin band at 26053 cm-1, is assigned to methyltropyl radical, which appears to be a major product of the toluene discharge, plausibly arising from CH insertion. All three o-, m-, and p-xylyl isomers are also identified. These isomers are detected in electrical discharges containing various xylenes, where it is also found that interconversion occurs: A discharge of o-xylene produces some m-xylyl; a discharge of m-xylene produces some o-xylyl; and a discharge of p-xylene produces all three isomers. No α-methylbenzyl was detected, but styrene was. These observations are supported by state-of-the-art quantum chemical calculations, which reveal an isomerization pathway between methyltropyl and xylyl radicals for which there is no analogue in the canonical tropyl-benzyl isomerization.

Formaldehyde roaming dynamics: Comparison of quasi-classical trajectory calculations and experiments.

The photodissociation dynamics of roaming in formaldehyde are studied by comparing quasi-classical trajectory calculations performed on a new potential energy surface (PES) to new and detailed experimental results detailing the CO + H2 product state distributions and their correlations. The new PES proves to be a significant improvement over the past one, now more than a decade old. The new experiments probe both the CO and H2 products of the formaldehyde dissociation. The experimental and trajectory data offer unprecedented detail about the correlations between internal states of the CO and H2 dissociation products as well as information on how these distributions are different for the roaming and transition-state pathways. The data investigated include, for dissociation on the formaldehyde 2143 band, (a) the speed distributions for individual vibrational/rotational states of the CO products, providing information about the correlated internal energy distributions of the H2 product, and (b) the rotational and vibrational distributions for the CO and H2 products as well as the contributions to each from both the transition state and roaming channels. The agreement between the trajectory and experimental data is quite satisfactory, although minor differences are noted. The general agreement provides support for future use of the experimental techniques and the new PES in understanding the dynamics of photodissociative processes.

The energy dependence of CO(v,J) produced from H2CO via the transition state, roaming, and triple fragmentation channels.

The dynamics of CO production from photolysis of H2CO have been explored over a 8000 cm-1 energy range (345 nm-266 nm). Two-dimensional ion imaging, which simultaneously measures the speed and angular momentum distribution of a photofragment, was used to characterise the distribution of rotational and translational energy and to quantify the branching fraction of roaming, transition state (TS), and triple fragmentation (3F) pathways. The rotational distribution for the TS channel broadens significantly with increasing energy, while the distribution is relatively constant for the roaming channel. The branching fraction from roaming is also relatively constant at 20% of the observed CO. Above the 3F threshold, roaming decreases in favour of triple fragmentation. Combining the present data with our previous study on the H-atom branching fractions and published quantum yields for radical and molecular channels, absolute quantum yields were determined for all five dissociation channels for the entire S1←S0 absorption band, covering almost 8000 cm-1 of excitation energy. The S0 radical and TS molecular channels are the most important over this energy range. The absolute quantum yield of roaming is fairly constant ∼5% at all energies. The T1 radical channel is important (20%-40%) between 1500 and 4000 cm-1 above the H + HCO threshold, but becomes unimportant at higher energy. Triple fragmentation increases rapidly above its threshold reaching a maximum of 5% of the total product yield at the highest energy.

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.