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.

Observation of Rainbows in the Rotationally Inelastic Scattering of NO with CH4.

Using a combination of velocity-map imaging and resonance-enhanced multiphoton ionization detection with crossed molecular beam scattering, the dynamics of rotational energy transfer have been examined for NO in collisions with CH4 at a mean collision energy of 700 cm-1. The images of NO scattered into individual rotational (jNO') and spin-orbit (Ω) levels typically exhibit a single broad maximum that gradually shifts from the forward to the backward scattering direction with increasing rotational excitation (i.e., larger ΔjNO). The rotational rainbow angles calculated with a two-dimensional hard ellipse model show reasonable agreement with the observed angles corresponding to the maxima in the differential cross sections extracted from the images for higher ΔjNO transitions, but there are clear discrepancies for lower ΔjNO (in particular, final rotational levels with jNO' = 7.5 and 8.5). The sharply forward scattered angular distributions for these lower ΔjNO transitions better agree with the predictions of an L-type rainbow model. The more highly rotationally excited NO appears to coincide with low rotational excitation of the co-product CH4, indicating a degree of rotational product-pair anticorrelation in this bimolecular scattering.

Collision Energy Dependence of the Competing Mechanisms of Reaction of Chlorine Atoms with Propene.

Quasi-classical trajectory simulations examine the reaction of Cl with propene across a range of collision energies, from 7 to 28 kJ mol-1. The majority (70% at 7 kJ mol-1, 86% at 14 kJ mol-1, and 93% at 28 kJ mol-1) of reactive trajectories produce HCl by direct abstraction of a hydrogen atom from the methyl group of propene, but the remainder involve a variety of delayed mechanisms. Among these longer-lived trajectories, transient formation of an energized 1-chloropropyl radical intermediate is predominant, with only a minor contribution from the 2-chloropropyl radical and roaming pathways. The branching ratios between these intermediate states are largely invariant to collision energy, although the overall proportion of indirect trajectories increases at lower collision energies. The greater role for longer-lived trajectories is reflected in the computed product scattering angle distributions, which become more isotropic at lower energies. However, the distributions of population over vibrational and rotational states of the product HCl do not change with collision energy because they are controlled by the dynamics late along the reaction path.

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.

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.