Ultrafast and radiationless electronic excited state decay of uracil and thymine cations: computing the effects of dynamic electron correlation

Abstract

In this article we characterise the radiationless decay of the first few electronic excited states of the cations of DNA/RNA nucleobases uracil and thymine, including the effects of dynamic electron correlation on energies and geometries (optimised with XMS-CASPT2).

Computing the Ultrafast and Radiationless Electronic Excited State Decay of Cytosine and 5‐methyl‐cytosine Cations: Uncovering the Role of Dynamic Electron Correlation

“…Whilst D 0 and D 1 vertical ionisation and D 0 adiabatic CASPT2 averaged values fall within the registered experimental data, larger differences of ∼0.3 and ∼0.4 eV are found for D 2 and D 3 , respectively. Similar to what we have found in recent works on cationic uracil and thymine, the use of the IPEA shift blue‐shifts the estimates by ∼0.2 eV and overestimates the experimental data when used in conjunction with multistate CASPT2 variants (either MS or XMS), as these already feature ionisation energies closer to the experimental range within IPEA=0.0 a.u.…”

“…A small energy gap can be observed at the CASSCF level at the effective 3‐state degeneracy, displaying a well‐separated D 1 state at the . Dynamic electron correlation therefore has the opposite effect in cytosine from that previously reported in uracil, helping to bring the electronic states together and effectively promoting a 3‐state degeneracy instead of splitting the energy levels and disrupting this crossing. Significant differences are also observed when comparing the energy of the state at the FC and geometries, showing a 0.40 eV stabilisation at CASSCF and a more pronounced 1.06 eV at the XMS‐CASPT2 level, which appears to be correlated to the overall bond length shortenings observed with the latter level of theory.…”

“…Overall, CASSCF and XMS‐CASPT2 geometries are shown to be qualitatively similar for all different key structures characterised, which potentially validates the use of CASSCF as a potential cost‐effective alternative to simulate photoionisations in cytosine. It is also worth noting that the differences induced due to the inclusion of dynamic electron correlation are much smaller than those observed in closely related systems such as uracil and thymine, where XMS‐CASPT2 geometries were shown to deviate more prominently from CASSCF estimates, particularly at conical intersections.…”

“…In this article we aim to explore the ultrafast radiationless decay of excited state cationic cytosine and 5‐methyl‐cytosine with dynamically correlated electronic structure methods, and to provide spectral fingerprints that may allow monitoring these photo‐processes experimentally in the near future. We have already studied the other pyrimidine bases; our purpose here is to understand what their similarities and differences are.…”

“…The ionisation of cytosine derivatives has been a topic of scarce study in the literature, with the only references found by us being relative to the specific UV/Vis absorption spectral fingerprints of cytosine +[22] as well as their IR features . Very little is also known about the cationic excited state decay relaxation, particularly comparing to other similar systems like the RNA nucleobase uracil . The only works available in the literature are characterisation of the Dyson norms of cytosine and the evolution of the cationic manifold along the relaxation of the bright and initially accessed singlet decay channel by Matsika and co‐workers, which cover different facets of potential ionisation mechanisms while not depicting the actual cation generation and its excited state decay relaxation channels.…”