Probing Competing Relaxation Pathways in Malonaldehyde with Transient X-Ray Absorption Spectroscopy

Abstract

Excited-state intramolecular hydrogen transfer (ESIHT) is a fundamental reaction relevant to chemistry and biology. Malonaldehyde is the simplest example of ESIHT, yet only little is known experimentally about its excited-state dynamics. Several competing relaxation pathways have been proposed, including internal conversion mediated by ESIHT and C=C torsional motion as well as intersystem crossing. We perform an in silico transient X-ray absorption spectroscopy (TRXAS) experiment at the oxygen K-edge to investigate its potential to monitor the proposed ultrafast decay pathways in malonaldehyde upon photoexcitation to its bright S2(pp * ) state. We employ both restricted active space perturbation theory and algebraicdiagrammatic construction for the polarization propagator along interpolated reaction coordinates as well as representative trajectories from ab initio multiple spawning simulations to compute the TRXAS signals from the lowest valence states. Our study suggests that oxygen Kedge TRXAS can distinctly fingerprint the passage through the H-transfer intersection and the concomitant population transfer to the S1(np * ) state. Potential intersystem crossing to T1(pp * ) is detectable from reappearance of the double pre-edge signature and reversed intensities. Moreover, the torsional deactivation pathway induces transient charge redistribution from the enol side towards the central C-atom and manifests itself as substantial shifts of the pre-edge features. Given the continuous advances in X-ray light sources, our study proposes an experimental route to disentangle ultrafast excited-state decay channels in this prototypical ESIHT system and provides a pathway-specific mapping of the TRXAS signal to facilitate the interpretation of future experiments.

XABOOM: An X-ray Absorption Benchmark of Organic Molecules Based on Carbon, Nitrogen, and Oxygen 1s → π* Transitions

“…As such, the results presented here are compared to reference values obtained using EOM-CCSD and ADC(2)-x, two methods which have been shown to yield good agreement with experiments on top of having a solid theoretical foundation. 24 , 36 , 43 , 51 − 59 Note that these methods yield accurate relative features, albeit with element-dependent absolute errors—as an example, the discrepancies with respect to experiments for CO/HCN/CO amount to approximately −0.1/–0.6/–1.1 eV for ADC(2)-x and 0.3/0.7/1.2 eV for EOM-CCSD, with relativistic effects accounted for.…”

“…Due to difficulties in obtaining appropriate experimental reference values (as discussed in detail below) and inability of obtaining full configuration interaction estimates for the systems investigated, we instead chose to use fc-CVS-EOM-CCSD and CVS-ADC(2)-x as our references, which have been demonstrated to yield results in very good agreement with experiments for small- and medium-sized systems. 36 , 43 , 51 − 53 , 55 − 59 Using both methods and paying attention to cases where they show noticeable discrepancies, we achieve theoretical estimates of sufficient quality for this benchmark study. Should more accurate methods capable of considering both transition energies and intensities be made computationally affordable for the systems considered here, we encourage a future critical assessment of our selection.…”

“…We refer to recent studies for illustration of this broader applicability and limitations of propagator approaches under such circumstances. 59 , 110 …”

An assessment of different electronic structure approaches for modeling time-resolved x-ray absorption spectroscopy

“…List et al have recently used ADC(2)-x, along with restricted active-space second-order perturbation theory (RASPT2), to study competing relaxation pathways in malonaldehyde by TR-XAS simulations. 94 …”

Perturbing the O–H…O Hydrogen Bond in 1-oxo-3-hydroxy-2-propene

“…Various aspects of 1-oxo-3-hydroxy-2-propene have been investigated, including substituent effects [ 20 , 21 ], solvent effects in which water molecules interact with this molecule [ 22 ], excited state properties [ 23 ], and the important problem of proton tunneling [ 23 , 24 , 25 , 26 , 27 ]. Both vibrational [ 28 ] and rotational [ 29 , 30 , 31 ] spectroscopic studies have been carried out on this molecule.…”