Three-state surface hopping calculations of acetaldehyde photodissociation to CH3 + HCO on ab initio potential surfaces

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.

A highly accurate full-dimensional ab initio potential surface for the rearrangement of methylhydroxycarbene (H3C-C-OH)

We report here a full-dimensional machine learning global potential surface (PES) for the rearrangement of methylhydroxycarbene (H₃C-C-OH, 1t). The PES is trained with the fundamental invariant neural network (FI-NN) method on 91 564 ab initio energies calculated at the UCCSD(T)-F12a/cc-pVTZ level of theory, covering three possible product channels. FI-NN PES has the correct symmetry properties with respect to permutation of four identical hydrogen atoms and is suitable for dynamics studies of the 1t rearrangement. The averaged root mean square error (RMSE) is 11.4 meV. Six important reaction pathways, as well as the energies and vibrational frequencies at the stationary geometries on these pathways are accurately preproduced by our FI-NN PES. To demonstrate the capacity of the PES, we calculated the rate coefficient of hydrogen migration in -CH₃ (path A) and hydrogen migration of -OH (path B) with instanton theory on this PES. Our calculations predicted the half-life of 1t to be 95 min, which is excellent in agreement with experimental observations.

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.

Photodissociation Dynamics of CO-Forming Channels on the Ground-State Surface of Methyl Formate at 248 nm: Direct Dynamics Study and Assessment of Generalized Multicenter Impulsive Models

The photodissociation dynamics of methyl formate in the electronic ground state S₀, initiated by a 248 nm-wavelength laser, is studied by direct dynamics simulations. We analyze five channels, where four of them have as products CH₃OH + CO, one leading to the formation of three fragments, H₂CO + H₂ + CO, and a channel characterized by a roaming transition state. The analysis of energy distribution among the degrees of freedom of the product and the comparison with experimental results previously published by other groups provide the ingredients to distinguish the examined dissociation pathways. The interpretation of the results proves that the characterization of dissociation mechanisms must rely on a dynamics approach involving multiple electronic states, including considerations on the features of the S₁/S₀ conical intersection. Here, we also assess the generalized multicenter impulsive model, GMCIM, that has been designed for dissociation processes with exit barriers, and the energy distribution in the products is predicted on the basis of information from the saddle points and the intrinsic reaction coordinates. Main features, advantages, limits, and future perspectives of the method are reported and discussed.

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.

Quasiclassical trajectory calculations of CD3CHO dissociation to CD2H + DCO on a global potential energy surface

We present a quasiclassical trajectory study of the photodissociation of CD3CHO based on a global ab initio-based potential energy surface. Calculations are performed at the total energy corresponding to the photolysis wavelength of 280[Formula: see text]nm. In addition to the major radical and molecular products, CD[Formula: see text] and CD3H [Formula: see text] CO, respectively, this paper focuses on the unusual radical channel CD2H [Formula: see text] DCO, which requires a D/H exchange process before the conventional C–C bond cleavage. Five D/H exchange mechanisms are reported, which are related to the isomerizations from acetaldehyde to vinyl alcohol and back, to oxirane and back, and to the intermediate (CD–CHD–OD) and back. These D/H exchange mechanisms are in good agreement with the experimental findings [Heazlewood BR, Maccarone AT, Andrews DU, Osborn DL, Harding LB, Klippenstein SJ, Jordan MJT, Kable SH, Nat Chem 3:443, 2011].

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.

Theoretical Studies on the Kinetics of Multi-Channel Gas-Phase Unimolecular Decomposition of Acetaldehyde

Theoretical kinetic studies are performed on the multichannel thermal decomposition of acetaldehyde. The geometries of the stationary points on the potential energy surface of the reaction are optimized at the MP2(full)/6-311++G(2d,2p) level of theory. More accurate energies are obtained by single point energy calculations at the CCSD(T,full)/augh-cc-pVTZ+2df, CBS-Q and G4 levels of theory. Here, by application of steady-state approximation to the thermally activated species CH₃CHO* and CH₂CHOH* and performance of statistical mechanical manipulations, expressions for the rate constants for different product channels are derived. Special attempts are made to compute accurate energy-specific rate coefficients for different channels by using semiclassical transition state theory. It is found that the isomerization of CH₃CHO to the enol-form CH₂CHOH plays a significant role in the unimolecular reaction of CH₃CHO. The possible products of the reaction are formed via unimolecular decomposition of CH₃CHO and CH₂CHOH. The computed rate coefficients reveal that the dominant channel at low temperatures and high pressures is the formation of CH₂CHOH due to the low barrier height for CH₃CHO → CH₂CHOH isomerization process. However, at high temperatures, the product channel CH₃ + CHO becomes dominant.

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.

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.