Unexpected hydrogen dissociation in thymine: predictions from a novel coupled cluster theory

TL;DR

This study uses a novel coupled cluster method to simulate thymine's ultrafast excited state dynamics, revealing a new hydrogen dissociation pathway and confirming key spectral features, thus resolving longstanding debates.

Contribution

The paper introduces a new coupled cluster approach for simulating thymine's excited states, predicting a previously unconfirmed hydrogen dissociation pathway.

Findings

Confirmed ultrafast ππ* to nπ* transition with a 41 fs lifetime

Predicted a new πσ* pathway leading to hydrogen dissociation

Achieved quantitative agreement with experimental X-ray spectra

Abstract

The fate of thymine upon excitation by ultraviolet radiation has been the subject of intense debate over the past three decades. Today, it is widely believed that its ultrafast excited state decay stems from a radiationless transition from the bright ${\pi}{\pi}^*$ state to a dark $n{\pi}^*$ state. However, conflicting theoretical predictions have made the experimental data difficult to interpret. Here we simulate the ultrafast dynamics in thymine at the highest level of theory to date, performing wavepacket dynamics with a new coupled cluster method. Our simulation confirms an ultrafast ${\pi}{\pi}^*$ to $n{\pi}^*$ transition (${\tau} = 41 \pm 14$ fs). Furthermore, the predicted oxygen-edge X-ray absorption spectra agree quantitatively with the experimental results. Our simulation also predicts an as-yet uncharacterized photochemical pathway: a ${\pi}{\sigma}^*$ channel that leads to hydrogen dissociation at one of the two N-H bonds in thymine. Similar behavior has been identified in other heteroaromatic compounds, including adenine, and several authors have speculated that a similar pathway may exist in thymine. However, this was never confirmed theoretically or experimentally. This prediction calls for renewed efforts to experimentally identify or exclude the presence of this channel.