Search

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+g 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.

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 correlated speed and rotational energy distributions of the CO fragment from photodissociation of CH3CHO have been measured at a range of wavelengths from 308 to 328 nm. The distributions are bimodal, showing low J, slow speed, and high J, fast speed components. The cold component disappears for λ > 325 nm. This threshold corresponds to C–H bond cleavage and we assign these CO products as arising from roaming of a H-atom about a CH3CO core. We attribute the hot component to CO formed through CH3-roaming. No evidence was observed for the presence of a transition state mechanism. This is the first time two distinct roaming channels have been observed from the same electronic state. The results support the growing understanding that roaming can be significant in chemical reactions and outweigh traditional pathways.

The A(1B1)–(1A1) transition of jet-cooled CCl2 has been investigated using both laser-induced fluorescence (LIF) excitation and dispersed fluorescence (DF) spectroscopy. DF spectra were taken from over 90 emitting vibronic states over the breadth of the LIF spectrum. 68 ground state vibrational levels were assigned in the C35Cl2 isotopomer and 47 in C35Cl37Cl. The ground state frequencies were fit to an anharmonic potential yielding: ω″1 = 736 cm−1, ω″2 = 338 cm−1, ν″3 = 760 cm−1, x″11 = −3.19 cm−1, x″22 = −0.29 cm−1, x″12 = −1.80 cm−1, x″13 = −5.70 cm−1 and x″23 = −3.80 cm−1. This is the first gas phase measurement of ν″3. Despite the close frequency match between ν″1 and 2ν″2 there was no evidence for Fermi resonance. The intensities of transitions in the DF spectra were analysed to identify the nature of the emitting state. Levels below T00 + 2500 cm−1 in the A state could be assigned simply as combinations and overtones of ν′1 and ν′2. Between T00 + 2500 and T00 + 5300 cm−1 the emission revealed an increasing mixing between levels within the same polyad, i.e., (m,n,0), (m + 1,n − 2,0), (m + 2,n − 4,0) etc, presumably due to Fermi resonance. This mixing becomes so extensive that simple assignment of the emitting states is not possible. Above T00 + 5300 cm−1 only K′ = 0 states could be identified, indicating that the Renner–Teller intersection between the and A states had been exceeded. This intersection is therefore 22 600 cm−1 above the ground state, in excellent agreement with a previous theoretical calculation of 23 000 cm−1.

Reported here are three aspects of molecular stereochemistry associated with polar molecules in an exotic solvent, namely, superfluid liquid helium. Infrared spectroscopy is used to study the nature of the solvent–solute and solute–solute interactions in this unique environment. First, we use an external dc electric field to orient the helium solvated polar molecules in the laboratory frame of reference and we characterize this orientation using pendular state spectroscopy. Second, we present results that demonstrate non-equilibrium self-assembly of polar molecules into linear chain structures, due to the mutual orientation imposed by the associated dipole–dipole interactions. Finally, the spectroscopic results provide information on the anisotropic interactions experienced by the solvated molecules, which result in significant molecular orientation in the droplet frame of reference. Studies of this type provide important new insights into the nature of the interactions between the molecules and the superfluid solvent.

The resonant two-color two-photon ionization and laser induced fluorescence excitation spectra of the 1-phenylallyl (cinnamyl) and inden-2-ylmethyl radicals are reported. The 1-phenylallyl radical is found to fluoresce with low yield, permitting only a coarse dispersed fluorescence spectrum, while the inden-2-ylmethyl radical yields sufficient fluorescence to obtain ground-state vibrational frequencies and two-dimensional fluorescence spectra. Computed ionization energies and thermochemical properties including radical stabilization energies are reported for a range of resonance-stabilized radicals, including the phenylpropargyl, vinylpropargyl and phenylallyl radicals.