Do Water Molecules Always Stabilize Resonances? Microhydration Effects on Thymine Shape Resonances

TL;DR

This study examines how microhydration influences thymine's low-lying shape resonances, revealing that water molecules can significantly stabilize these resonances through complex interactions.

Contribution

It provides a detailed computational analysis of the effects of microhydration on thymine's shape resonances, highlighting the roles of hydrogen bonding and basis-set effects.

Findings

Resonance lifetimes increase with hydration, e.g., from 39 fs to 110 fs.

Resonance shifts are influenced by hydrogen bonding, electrostatics, and basis-set effects.

Explicit water molecules cause genuine stabilization of thymine's resonance states.

Abstract

We investigate microhydration effects on the three low-lying {\pi}* shape resonances of thymine using the Resonance via Pad\'e approach in combination with the DLPNO-EA-EOM-CCSD method. For isolated thymine, the calculated resonance positions are benchmarked against projected CAP-EA-EOM-CCSD calculations and compared with available theoretical and experimental data. Upon hydration, the 1{\pi}* and 2{\pi}* resonances undergo systematic stabilization accompanied by significant increases in their lifetimes, whereas the 3{\pi}* resonance exhibits a more complex behavior. In particular, the lifetime of the lowest resonance increases from 39 fs in isolated thymine to 110 fs in the thymine(H2O)3 cluster. Detailed analysis reveals that the observed resonance shifts arise from competing contributions involving hydrogen bonding, electrostatic interactions, microsolvation-induced geometric distortion, and finite-basis-set effects. Ghost-atom calculations demonstrate that diffuse basis functions associated with nearby water molecules contribute appreciably to the apparent stabilization, while explicit inclusion of water molecules leads to genuine physical stabilization of the resonance states. Furthermore, calculations on multiple conformers of the monohydrated cluster show that resonance positions and lifetimes depend strongly on the local hydrogen-bonding arrangement and microsolvation geometry. These findings demonstrate that resonance stabilization in microhydrated nucleobases is governed by a subtle interplay between geometry, basis-set effects, and intermolecular interactions.