Imaging the molecular channel in acetaldehyde photodissociation: roaming and transition state mechanisms

Dissociative photoionization of acetaldehyde in the 10.2-19.5 eV VUV range

The valence-shell dissociative photoionization of acetaldehyde has been investigated by means of the photoion photoelectron coincidence technique in conjunction with tuneable synchrotron radiation. The experimental results consist of threshold photoelectron spectra for the parent ion and for each fragment ion in the 10.2-19.5 eV photon energy range, along with (ion, e) kinetic energy coincidence diagrams obtained from measurements at fixed photon energies. The results are complemented by high-level ab initio calculations of potential energy curves as a function of the C-H bond distance. The nudged elastic band (NEB) method has been employed to connect the parent ion Franck-Condon region to the formation of the HCO+, CH3+ and CH4+ ion fragments. Appearance energies have been determined for six fragment ions with an improved accuracy, including two fragmentation channels, which to the best of our knowledge have not been reported previously, i.e. the formation of CH2CO+, lying at 13.10 ± 0.05 eV, and the formation of CH2+ at 15.1 ± 0.1 eV. Based on both experimental and theoretical results, the dissociation dynamics following ionization of acetaldehyde into the different fragmentation channels are discussed.

Roaming in acetaldehyde

We investigate roaming in the photodissociation of acetaldehyde (CH3CHO), providing insights into the contrasting roaming dynamics observed for this molecule compared to formaldehyde. We carry out trajectory studies for full-dimensional acetaldehyde, supplemented with an analysis of a two-degree-of-freedom restricted model and obtain evidence for two distinct roaming pathways. Trajectories exhibit roaming at both shorter (9-11.5 au) and larger (14.5-22.9 au) maximum CH3-HCO separations, characterized by differing amounts of HCO rotation. No roaming trajectories were found in the intervening gap region. The roaming dynamics near 14.5-22.9 au are well-reproduced by the restricted model and involve passage through a centrifugal barrier, analogous to formaldehyde roaming. However, the shorter-range 9-11.5 au roaming appears unique to acetaldehyde and is likely facilitated by repulsive interactions absent in the simplified models. Phase space analysis reveals that this additional roaming pathway is inaccessible in the reduced dimensionality system. The findings suggest that acetaldehyde's increased propensity for roaming compared to formaldehyde may arise from the presence of multiple distinct roaming mechanisms rather than solely the higher roaming fragment mass.

A high-level ab initio study of the photodissociation of acetaldehyde

Acetaldehyde is a very relevant atmospheric species whose photodissociation has been extensively studied in the first absorption band both experimentally and theoretically. Very few works have been reported on acetaldehyde photodissociation at higher excitation energies. In this work, the photodissociation dynamics of acetaldehyde is investigated by means of high-level multireference configuration interaction ab initio calculations. Five different fragmentation pathways of acetaldehyde are explored by calculating the potential-energy curves of the ground and several excited electronic states along the corresponding dissociating bond distances. The excitation energy range covered in the study is up to 10 eV, nearly the ionization energy of acetaldehyde. We intend to rationalize the available experimental results and, in particular, to elucidate why some of the studied fragmentation pathways are experimentally observed in the different excitation energy regions and some others are not. Based on the shape of the calculated potential curves, we are able to explain the main findings of the available experiments, also suggesting possible dynamical dissociation mechanisms in the different energy regions. Thus, the reported potential curves are envisioned as a useful tool to interpret the currently available experiments as well as future ones on acetaldehyde photodissociation at excitation wavelengths in the range studied here.

Direct Observation of a Roaming Intermediate and Its Dynamics

Chemical reactions are often characterized by their transition state, which defines the critical geometry the molecule must pass through to move from reactants to products. Roaming provides an alternative picture, where in a dissociation reaction, the bond breaking is frustrated and a loosely bound intermediate is formed. Following bond breaking, the two partners are seen to roam around each other at distances of several Ångstroms, forming a loosely bound, and structurally ill-defined, intermediate that can subsequently lead to reactive or unreactive collisions. Here, we present a direct and time-resolved experimental measurement of roaming. By measuring the photoelectron spectrum of UV-excited acetaldehyde with a femtosecond extreme ultraviolet pulse, we captured spectral signatures of all of the key reactive structures, including that of the roaming intermediate. This provided a direct experimental measurement of the roaming process and allowed us to identify the time scales by which the roaming intermediate is formed and removed and the electronic potential surfaces upon which roaming proceeds.

Time-Resolved Pair-Correlated Imaging of the Photodissociation of Acetaldehyde at 267 nm: Pathway Partitioning

Photodissociation of acetaldehyde (CH₃CHO) by UV excitation involves interwoven multiple reaction pathways, including nonradiative decay, isomerization, transition-state pathway, roaming, and other dissociation mechanisms. Recently, we employed picosecond time-resolved, pair-correlated product imaging in a study of acetaldehyde photodissociation at 267 nm to disentangle those competing mechanisms and to elucidate the possible roaming pathways (Yang, C. H.; Chem. Sci. 2020, 11, 6423-6430). Here, we complement the pair-correlated product speed distribution of CO(v = 0) at the high-j side of the CO rotational state distribution in the CO + CH₄ channel and detail the two-dimensional data analysis of the time-resolved images. As a result, extensive comparisons with other studies can be made and the branching fractions of the previously assigned TS_cc(S₀), non-TS_cc(S₀), and CI(S₁/S₀) pathways for the CO(v = 0) + CH₄ molecular channel are evaluated to be 0.74 ± 0.08, 0.15 ± 0.02, and 0.11 ± 0.02, respectively. Together with the macroscopic branching ratio between the molecular (CO + CH₄) and radical (CH₃ + HCO) channels at 267 nm from the literature, a global view of the microscopic pathways can then be delineated, which provides invaluable insights and should pave the way for further studies of this interesting system.

Roaming Reactions and Dynamics in the van der Waals Region

Roaming reactions were first clearly identified in photodissociation of formaldehyde 15 years ago, and roaming dynamics are now recognized as a universal aspect of chemical reactivity. These reactions typically involve frustrated near-dissociation of a quasibound system to radical fragments, followed by reorientation at long range and intramolecular abstraction. The consequences can be unexpected formation of molecular products, depletion of the radical pool in chemical systems, and formation of products with unusual internal state distributions. In this review, I examine some current aspects of roaming reactions with an emphasis on experimental results, focusing on possible quantum effects in roaming and roaming dynamics in bimolecular systems. These considerations lead to a more inclusive definition of roaming reactions as those for which key dynamics take place at long range.

Roaming Dynamics and Conformational Memory in Photolysis of Formic Acid at 193 nm Using Time-resolved Fourier-transform Infrared Emission Spectroscopy

In photodissociation of trans-formic acid (HCOOH) at 193 nm, we have observed two molecular channels of CO + H₂O and CO₂ + H₂ by using 1 μs-resolved Fourier-transform infrared emission spectroscopy. With the aid of spectral simulation, the CO spectra are rotationally resolved for each vibrational state (v = 1-8). Each of the resulting vibrational and rotational population distributions is characteristic of two Boltzmann profiles with different temperatures, originating from either transition state pathway or OH-roaming to form the same CO + H₂O products. The H₂O roaming co-product is also spectrally simulated to understand the interplay with the CO product in the internal energy partitioning. Accordingly, this work has evaluated the internal energy disposal for the CO and H₂O roaming products; especially the vibrational-state dependence of the roaming signature is reported for the first time. Further, given a 1 μs resolution, the temporal dependence of the CO/CO₂ product ratio at v ≥ 1 rises from 3 to 10 of study, thereby characterizing the effect of conformational memory and well reconciling with the disputed results reported previously between absorption and emission methods.

Real-time tracking of the entangled pathways in the multichannel photodissociation of acetaldehyde

The roaming mechanism, an unconventional reaction path, was discovered more than a decade ago in the studies of formaldehyde photodissociation, H₂CO → H₂ + CO. Since then, observations of roaming have been claimed in numerous photochemical processes. A closer examination of the presented data, however, revealed that evidence for roaming is not always unequivocal, and some of the conclusions could be misleading. We report here an in-depth, joint experimental and theoretical study of the title reaction. By tracking the time-evolution of the pair-correlated product state distributions, we decipher the competing, interwoven reaction pathways that lead to the radical (CH₃ + HCO) and molecular (CH₄ + CO) products. Possible roaming pathways are then elucidated and a more precise descriptor of the phenomenon is delineated.

Photodissociation of iso-propoxy (i-C3H7O) radical at 248 nm

Photodissociation of the i-C₃H₇O radical is investigated using fast beam photofragment translational spectroscopy.

Roaming Dynamics in the Photodissociation of Formic Acid at 230 nm

Roaming dynamics is observed in the photodissociation of formic acid (HCOOH) at 230 nm by using the slice imaging method. In combination with rotational state selective (2 + 1) resonance-enhanced multiphoton ionization of the CO fragments, the speed distributions of the CO fragments exhibit a low recoil velocity at low rotational levels of J = 9 and 20, while the velocity distributions of CO at high rotational levels of J = 30 and 48 show a relatively large recoil velocity. The experimental results indicate that the roaming of OH radical should be related with the formation of CO + H₂O channel at the present photolysis energy. Unlike the roaming pathways occurring in H₂CO that can be described by loose flat potential, our CO speed distribution analysis suggests the presence of a "tight" flat potential in the roaming dynamics of HCOOH molecules.

A generalized unimolecular impulsive model for curved reaction path

This work aims to introduce a generalized impulsive model for unimolecular dissociation processes. This model allows us to take into account the curvature of the reaction path explicitly. It is a generalization of the previously developed multi-center impulsive model [P.-Y. Tsai and K.-C. Lin, J. Phys. Chem. A 119, 29 (2015)]. Several limitations of conventional impulsive models are eliminated by this study: (1) Unlike conventional impulsive models, in which a single molecular geometry is responsible for the impulse determination, the gradients on the whole dissociation path are taken into account. The model can treat dissociation pathways with large curvatures and loose saddle points. (2) The method can describe the vibrational excitation of polyatomic fragments due to the bond formation by multi-center impulse. (3) The available energy in conventional impulsive models is separated into uncoupled statistical and impulsive energy reservoirs, while the interplay between these reservoirs is allowed in the new model. (4) The quantum state correlation between fragments can be preserved in analysis. Dissociations of several molecular systems including the roaming pathways of formaldehyde, nitrate radical, acetaldehyde, and glyoxal are chosen as benchmarks. The predicted photofragment energy and vector distributions are consistent with the experimental results reported previously. In these examples, the capability of the new model to treat the curved dissociation path, loose saddle points, polyatomic fragments, and multiple-body dissociation is verified. As a cheaper computational tool with respect to ab initio on-the-fly direct dynamic simulations, this model can provide detailed information on the energy disposal, quantum state correlation, and stereodynamics in unimolecular dissociation processes.

Photodissociation of CH3CHO at 248 nm: identification of the channels of roaming, triple fragmentation and the transition state

Quasi-classical trajectory (QCT) calculations are performed on the molecular products CO + CH₄via the tight transition state (TS) and global minimum configurations. With the aid of this theoretical evidence, we have re-examined the experimental results published previously to clarify the controversial issue of photodissociation dynamics of CH₃CHO at 248 nm. For the CO (v = 0 and 1) bimodal rotational distributions obtained previously [K.-C. Hung, P.-Y. Tsai, H.-K. Li, and K.-C. Lin, J. Chem. Phys., 2014, 140, 064313], the low-rotational (J) component is re-assigned to the contribution of triple fragmentation (H + CO + CH₃), whereas the high-J component is ascribed to the CH₃-roaming pathway. The H-roaming pathway is not found in the calculations. Further, the QCT results have confirmed that the CO vibrational population especially at higher states and the low-energy component of CH₄ vibrational bimodality obtained experimentally are mainly produced following the TS pathway, which has never been identified before. While taking into account both the theoretical and experimental results, the ratio of the molecular products (CO(v = 1) + CH₄) obtained by the triple fragmentation/roaming/TS processes is evaluated to be 0.23 : 1 : 0.29.

Perspective: chemical dynamics simulations of non-statistical reaction dynamics

Non-statistical chemical dynamics are exemplified by disagreements with the transition state (TS), RRKM and phase space theories of chemical kinetics and dynamics. The intrinsic reaction coordinate (IRC) is often used for the former two theories, and non-statistical dynamics arising from non-IRC dynamics are often important. In this perspective, non-statistical dynamics are discussed for chemical reactions, with results primarily obtained from chemical dynamics simulations and to a lesser extent from experiment. The non-statistical dynamical properties discussed are: post-TS dynamics, including potential energy surface bifurcations, product energy partitioning in unimolecular dissociation and avoiding exit-channel potential energy minima; non-RRKM unimolecular decomposition; non-IRC dynamics; direct mechanisms for bimolecular reactions with pre- and/or post-reaction potential energy minima; non-TS theory barrier recrossings; and roaming dynamics.This article is part of the themed issue 'Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces'.

Competing pathways in the near-UV photochemistry of acetaldehyde

Time-resolved ion imaging measurements have been performed to explore the photochemistry of acetaldehyde at photolysis wavelengths spanning the range 265–328 nm.

That Little Extra Kick: Nonadiabatic Effects in Acetaldehyde Photodissociation

The effect of nonadiabatic transitions on branching ratios, kinetic and internal energy distribution of fragments, and reaction mechanisms observed in acetaldehyde photodissociation is investigated by nonadiabatic molecular dynamics (NAMD) simulations using time-dependent hybrid density functional theory and Tully surface hopping. Homolytic bond breaking is approximately captured by allowing spin symmetry to break. The NAMD simulations reveal that nonadiabatic transitions selectively enhance the kinetic energy of certain internal degrees of freedom within approximately 50 fs. Branching ratios from NAMD and conventional "hot" Born-Oppenheimer molecular dynamics (BOMD) are similar and qualitatively agree with experiment. However, as opposed to the BOMD simulations, NAMD captures the high-energy tail of the experimental kinetic energy distribution. The extra "kick" of the nuclei in the direction of the nonadiabatic coupling vector results from nonadiabatic transitions close to conical intersections. From a mechanistic perspective, the nonadiabatic effects favor asynchronous over synchronous fragmentation and tend to suppress roaming.

Regulation of nonadiabatic processes in the photolysis of some carbonyl compounds

An energy scheme involving S_o → S₁ excitation, followed by dissociation channels via diabatic coupling, internal conversion, transition state and roaming mechanisms.

Dynamical, spectroscopic and computational imaging of bond breaking in photodissociation: roaming and role of conical intersections

Recent experimental and theoretical advances in the study of the dissociation of excited molecules are revealing unexpected mechanisms, when their outcomes are tackled by combining (i) space-time ion imaging of translational features, with (ii) spectroscopic probing of rotational and vibrational distributions; crucial is the assistance of (iii) the quantum chemistry of structural investigations of rearrangements of chemical bonds, and of (iv) the simulations of molecular dynamics to follow the evolution of selective bond stretching and breaking. Here we present results of such an integrated approach to methyl formate, HCOOCH3, the simplest of esters; the main focus is on the rotovibrationally excited CO (v=1) product and in general on the energy distribution in the fragments. Previous laser studies of dissociation into CO and CH3OH at a sequence of various wavelengths discovered signatures of a roaming mechanism by the late arrival of CO (v=0) products in time-of-flight ion imaging. Subsequent detailed investigations as a function of excitation energy provided the assessment of the threshold, which opens for triple breakdown into CO and further fragments H and CH3O, as spectroscopically characterized by ion imaging and FTIR respectively. Accompanying quantum mechanical electronic structure calculations and classical molecular dynamics simulations clarify the origin of these fragments through "roaming" pathways involving incipient radical intermediates at energies below the triple fragmentation threshold: a specific role is played by nonadiabatic transitions at a conical intersection between ground and excited states; alternative pathways focalize our attention to regions of the potential energy surfaces other than those in the neighbourhoods of saddle points along minimum energy paths: eventually this leads us to look for avenues in reaction kinetics beyond those of venerable transition state theories.

Insight into photofragment vector correlation by a multi-center impulsive model

A multi-center impulsive model has been recently developed to characterize the dynamic feature of fragment vector correlation in photodissociation of formaldehyde, H₂CO → CO + H₂, via both transition state and roaming pathways.

Roaming as the dominant mechanism for molecular products in the photodissociation of large aliphatic aldehydes

Photodissociation of isobutyraldehyde (C₃H₇CHO) at 248 nm is investigated using time-resolved Fourier-transform infrared emission spectroscopy to demonstrate the growing importance of the roaming pathway with increasing molecular size of aliphatic aldehydes.

Photodissociation of Propionaldehyde at 248 nm: Roaming Pathway as an Increasingly Important Role in Large Aliphatic Aldehydes

Time-resolved Fourier transform infrared emission spectroscopy is employed in the photolysis of propionaldehyde (CH3CH2CHO) at 248 nm to characterize the role of the roaming pathway. High-resolution spectra of CO are analyzed to yield a single Boltzmann rotational distribution for each vibrational level (ν = 1-4) with small rotational and large vibrational energy disposals. A roaming saddle point is found containing two far separated moieties of HCO and CH3CH2 with a weak interaction between them. Quasiclassical trajectory calculations on this configuration yield the CO energy flow behavior, consistent with the findings. The rate constant along the roaming pathway is evaluated to be larger by >1-2 orders of magnitude than those along tight transition state or three-body dissociation pathways. This work implies that the roaming mechanism plays an increasingly important role in aliphatic aldehydes as the molecular size becomes larger.

Two roaming pathways in the photolysis of CH3CHO between 328 and 308 nm

We attribute the two product-state distributions previously seen in CH₃CHO photodissociation to CH₃-roaming and H-roaming, unifying all previous experimental results.