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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.

Duality of roaming mechanism in H 2 CO The phenomenon of roaming in chemical reactions (that is, bypassing the minimum energy pathway from unlikely geometries) has attracted a great deal of attention in the chemical reaction dynamics community over the past decade and still demonstrates unexpected results. Using velocity-map imaging of state-selected H 2 products of H 2 CO photodissociation, Quinn et al. discovered the bimodal structure of rotational distribution of the other product fragment, CO. Quasiclassical trajectories showed that this bimodality originates from two distinctive reaction pathways that proceed by the trans or cis configuration of O–C–H⋯H, leading to high or low rotational excitations of CO, respectively. Whether such a mechanism is present in the many other chemical reactions for which roaming reaction pathways have been reported is yet to be determined. Science , this issue p. 1592

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+.

We have measured the two-photon EF←X (v″=3−9) Resonance-Enhanced Multiphoton Ionization (REMPI) spectrum of H2 by producing the hydrogen molecule via photolysis of H2CO. H2 is produced with rovibrational energies up to 30,000 cm−1, at a maximum observed (v″,J″) of (9,5). In the well-resolved jet-cooled H2CO spectrum, ortho and para H2 are selectively produced with near 100% selectivity. Over 470 REMPI transitions were identified in the EF−X band system, many of which have not been seen previously, including transitions to EF levels not yet accurately measured. Speed distributions were obtained for each REMPI transition using Velocity-mapped imaging, aiding in the assignment by providing a precise measurement of the internal energy of the H2 molecule.

In the interstellar medium, carbon is distributed between the gas and solid phases. However, while about half of the expected carbon abundance can be accounted for in the gas phase, there is considerable uncertainty as to the amount incorporated in interstellar dust. The aliphatic component of the carbonaceous dust is of particular interest because it produces a significant 3.4 $\mu$m absorption feature when viewed against a background radiation source. The optical depth of the 3.4 $\mu$m absorption feature is related to the number of aliphatic carbon C-H bonds along the line of sight. It is possible to estimate the column density of carbon locked up in the aliphatic hydrocarbon component of interstellar dust from quantitative analysis of the 3.4 $\mu$m interstellar absorption feature providing that the absorption coefficient of aliphatic hydrocarbons incorporated in the interstellar dust is known. We generated laboratory analogues of interstellar dust by experimentally mimicking interstellar/circumstellar conditions. The resultant spectra of these dust analogues closely match those from astronomical observations. The measurements of the absorption coefficient of aliphatic hydrocarbons incorporated in the analogues were carried out by a procedure which combined FTIR and $^{13}$C NMR spectroscopies. The absorption coefficients obtained for both interstellar analogues were found to be in close agreement (4.76(8) $\times$ 10$^{-18}$ cm group$^{-1}$ and 4.69(14) $\times$ 10$^{-18}$ cm group$^{-1}$), less than half those obtained in studies using small aliphatic molecules. The results thus obtained permit direct calibration of the astronomical observations, providing rigorous estimates of the amount of aliphatic carbon in the interstellar medium.

The Δ v = + 2 sequence of the Mulliken system of C 2 is reported for the first time. Seven new levels of the D 1 Σ u + state of C 2 , v = 5 through v = 11 , were found. The analysis reveals the rotational constants and origin of each band. The numbers are combined with the data reported for previously reported levels, v = 0 through v = 4 , to generate updated Dunham parameters for the D 1 Σ u + state: ω e = 1829.905 ( 57 ) cm −1 , ω e x e = 14.089 ( 8 ) cm −1 , ω e y e = 0 cm −1 , ω e z e = 0.00124 ( 3 ) cm −1 , B e = 1.83254 ( 46 ) cm −1 , α e = 0.01909 ( 18 ) cm −1 and β e = - 7.28 ( 14 ) × 10 - 5 cm −1 . The determined equilibrium bond length R e = 1.23804 ( 28 ) A. The fluorescence lifetimes of the new levels are also reported.

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 [Formula: see text] 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.

The reaction of H + phenol and H/D + toluene has been studied in a supersonic expansion after electric discharge. The (1 + 1') resonance-enhanced multiphoton ionization (REMPI) spectra of the reaction products, at m/z = parent + 1, or parent + 2 amu, were measured by scanning the first (resonance) laser. The resulting spectra are highly structured. Ionization energies were measured by scanning the second (ionization) laser, while the first laser was tuned to a specific transition. Theoretical calculations, benchmarked to the well-studied H + benzene → cyclohexadienyl radical reaction, were performed. The spectrum arising from the reaction of H + phenol is attributed solely to the ortho-hydroxy-cyclohexadienyl radical, which was found in two conformers (syn and anti). Similarly, the reaction of H/D + toluene formed solely the ortho isomer. The preference for the ortho isomer at 100-200 K in the molecular beam is attributed to kinetic, not thermodynamic effects, caused by an entrance channel barrier that is ∼5 kJ mol(-1) lower for ortho than for other isomers. Based on these results, we predict that the reaction of H + phenol and H + toluene should still favour the ortho isomer under elevated temperature conditions in the early stages of combustion (200-400 °C).

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 Cs 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 amb...

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 tro...

We report the observation of a new band system of C2, namely, the 4(3)Πg-a(3)Πu system. The bands, observed by resonant 2-photon ionization spectroscopy and time-of-flight mass spectrometry, were identified through a synergy of high-level ab initio computation and double-resonance spectroscopy. Two bands are firmly identified, 1-3 and 0-2, allowing the 4(3)Πg origin to be placed at 51496.44 cm(-1). The 4(3)Πg state is characterized as having a single bond, with a vibrational frequency of about 1268 cm(-1), and an equilibrium bond length of 1.57 Å. The state is predicted to exhibit a barrier to dissociation, with a rotational constant that unusually increases with vibrational excitation up to a maximum before decreasing at higher vibrational excitation. The new band system allows us to probe the a(3)Πu state of C2 through a straightforward 1 + 1 REMPI scheme.

The lowest 13 vibrational levels, v = 0–12, of the eΠg3 state of the C2 molecule have been measured by laser-induced fluorescence of new bands of the Fox-Herzberg system. The newly observed levels, v = 5–12, which span the eΠg3 electronic state up to and beyond the first dissociation threshold of C2, were analyzed to afford highly accurate molecular constants, including band origins, and rotational and spin-orbit constants. The spin-orbit coupling constants of the previously published lowest five levels are revised in sign and magnitude, requiring an overhaul of previously published molecular constants. The analysis is supported by high level ab initio calculations. Lifetimes of all observed levels were recorded and found to be in excellent agreement with ab initio predicted values up to v = 11. v = 12 was found to exhibit a much reduced lifetime and fluorescence quantum yield, which is attributed to the onset of predissociation. This brackets the dissociation energy of ground state XΣg+1 C2 between 6.180...

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 abs...

The 33Πg state of the dicarbon molecule, C2, has been identified for the first time by a combination of resonant ionization spectroscopy, mass spectrometry, and high-level ab initio quantum chemical calculations. This marks the discovery of the final valence triplet state of C2 spectroscopically accessible from the lowest triplet state. It is found to be vibronically coupled to the recently discovered 43Πg state, necessitating vibronic calculations beyond the Born-Oppenheimer approximation to reconcile calculated rotational constants with observations. The 33Πg state of C2 is observed to have a much shorter fluorescence lifetime than expected, possibly pointing to predissociation by coupling to the unbound d3Πg state.

The photodissociation dynamics of acetone (CH3)2CO, cooled in a molecular beam, have been explored over the wavelength range 266–312 nm. Nascent CH3 fragments were detected by resonance-enhanced multiphoton ionization, followed by mass-selected ion imaging. For photolysis at λ = 306 nm, the image shows a sharp ring, which, when converted to a translational energy distribution, reveals a narrow Gaussian peak with a maximum at 90% of the available energy. As the photolysis energy is increased, the distribution slowly broadens and shifts to higher recoil translational energy. The fraction of available energy in translation energy decreases in favour of internal energy of the CH3CO fragment. These observations are consistent with a dynamical model in which the energy of the exit channel barrier on the T1 surface evolves mostly into relative translational energy. Energy in excess of the barrier is partitioned statistically into all degrees of freedom. No evidence was found for any other dynamical pathway produ...

Resonant two-photon threshold ionization spectroscopy is employed to determine the ionization energy of C2 to 5 meV precision, about two orders of magnitude more precise than the previously accepted value. Through exploration of the ionization threshold after pumping the 0-3 band of the newly discovered 4(3)Πg ← a(3)Πu band system of C2, the ionization energy of the lowest rovibronic level of the a(3)Πu state was determined to be 11.791(5) eV. Accounting for spin-orbit and rotational effects, we calculate that the ionization energy of the forbidden origin of the a(3)Πu state is 11.790(5) eV, in excellent agreement with quantum thermochemical calculations which give 11.788(10) eV. The experimentally derived ionization energy of X(1)Σg(+) state C2 is 11.866(5) eV.