A full-dimensional simulation of the photo-dissociation of 1,3-cyclohexadiene in the manifold of three electronic states was performed via non-adiabatic surface hopping dynamics using extended multi-state complete active space second-order perturbation (XMS-CASPT2) electronic structure theory with fully analytic non-adiabatic couplings. With the 47 ± 8% product quantum yield calculated from the 136 trajectories, generally 400 fs-long, and an estimated excited lifetime of 89±9 fs, our calculations provide a detailed description of the non-adiabatic deactivation mechanism, showing the existence of an extended conical intersection seam along the reaction coordinate. The nature of the preferred reaction pathways on the ground state is discussed and extensive comparison to the previously published full dimensional dynamics calculations is provided.
Characterizing the photochemical reactivity of transient volatile organic compounds (VOCs) in our atmosphere begins with a proper understanding of their abilities to absorb sunlight. Unfortunately, the photoabsorption cross-sections for a large number of transient VOCs remain unavailable experimentally due to their short lifetime or high reactivity. While structure–activity relationships (SARs) have been successfully employed to estimate the unknown photoabsorption cross-sections of VOCs, computational photochemistry offers another promising strategy to predict not only the vertical electronic transitions of a given molecule but also the width and shape of the bands forming its absorption spectrum. In this work, we focus on the use of the nuclear ensemble approach (NEA) to determine the photoabsorption cross-section of four exemplary VOCs, namely, acrolein, methylhydroperoxide, 2-hydroperoxy-propanal, and (microsolvated) pyruvic acid. More specifically, we analyze the influence that different strategies for sampling the ground-state nuclear density—Wigner sampling and ab initio molecular dynamics with a quantum thermostat—can have on the simulated absorption spectra. We highlight the potential shortcomings of using uncoupled harmonic modes within Wigner sampling of nuclear density to describe flexible or microsolvated VOCs and some limitations of SARs for multichromophoric VOCs. Our results suggest that the NEA could constitute a powerful tool for the atmospheric community to predict the photoabsorption cross-section for transient VOCs.
The photochemistry of pyruvic acid has attracted much scientific interest because it is believed to play critical roles in atmospheric chemistry. However, under most atmospherically relevant conditions, pyruvic acid deprotonates to form its conjugate base, the photochemistry of which is essentially unknown. Here, we present a detailed study of the photochemistry of the isolated pyruvate anion and uncover that it is extremely rich. Using photoelectron imaging and computational chemistry, we show that photoexcitation by UVA light leads to the formation of CO2, CO, and CH3−. The observation of the unusual methide anion formation and its subsequent decomposition into methyl radical and a free electron may hold important consequences for atmospheric chemistry. From a mechanistic perspective, the initial decarboxylation of pyruvate necessarily differs from that in pyruvic acid, due to the missing proton in the anion.
The photochemistry of pyruvic acid has received a large attention due to its relevance to atmospheric chemistry. Pyruvic acid is produced in the troposphere from both biogenic and anthropogenic sources and is a prototypical model for the family of a-dicarbonyls. What makes the photochemistry of pyruvic acid particularly interesting from a gas-phase perceptive is its expected decarboxylation upon sunlight absorption. The exact photodynamics leading to this release of CO 2 remains elusive. In this work, we used a combination of excited-and ground-state ab initio molecular dynamics to unravel the possible mechanisms leading to the decarboxylation of pyruvic acid. Our calculations highlight the importance of a protoncoupled electron transfer mechanism taking place in the first excited electronic state and triggering a nonadiabatic transfer of the molecule to the ground electronic state. The decarboxylation takes place in the ground-electronic state with the concomitant formation of methylhydroxycarbene. We also calculate the photoabsorption cross-section and wavelengthdependent quantum yields for pyruvic acid, highlighting the limits of our theoretical formalism.
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