Stabilities of nanohydrated thymine radical cations: insights from multiphoton ionization experiments and ab initio calculations

Abstract

Abstract. Multi-photon ionization experiments have been carried out on thymine-water clusters in the gas phase. Metastable H2O loss from T + (H2O)n was observed at n ≥ 3 only. Ab initio quantum-chemical calculations of a large range of optimized T + (H2O)n conformers have been performed up to n = 4, enabling binding energies of water to be derived. These decrease smoothly with n, consistent with the general trend of increasing metastable H2O loss in the experimental data. The lowest-energy conformers of T + (H2O)3 and T + (H2O)4 feature intermolecular bonding via charge-dipole interactions, in contrast with the purely hydrogen-bonded neutrals. We found no evidence for a closed hydration shell at n = 4, also contrasting with studies of neutral clusters.

Ab Initio Study of the Stepwise versus Concerted Fragmentation Pathways in Microhydrated Thymine Radical Cations

“…In the case of the monohydrated thymine, our calculations show that the most stable structure corresponds to the M 0 aggregate as presented in Figure . In this configuration the water molecule O A is attached to thymine by means of two hydrogen bonds to N 1 H and O 8 , in agreement with previous theoretical studies . Interestingly, in the case of the monohydrated cation the water molecule experiences a rotation that results in the loss of the hydrogen bond to the O 8 atom.…”

“…Close and co‐workers, Khistyaev et al . and Pandey et al . investigated this effect theoretically showing that the decrease in the ionisation energy is accompanied by a rather large structural rearrangement of water around the thymine.…”

“…To compute the structure of microhydrated thymine cation, two strategies are possible: 1) one could consider different structures of mono‐ and dihydrated structures in the configurational space and then compute the relative stability of each structure, and 2) take the stable neutral configurations as starting point for the geometry optimisation of the cation structure calculations . Since our study covers many aspects of microhydrated thymine ionisation and fragmentation, the most stable neutral microhydrated thymine configurations were taken as starting structures for the calculations involving the cation.…”

“…In the case of the dihydrated thymine, we compared four stable neutral structures, including three already proposed. Structure is the most stable structure according to our calculations and to previous theoretical studies . For , two water molecules, denoted O A and O B , form a dimer and are locked to thymine by means of two hydrogen bonds in the sites N 1 H and O 8 (Figure ).…”

Dynamics of systems on the nanoscale

“…Results of experimental study of multi-photon ionization of thymine-water clusters in the gas phase are presented and discussed in reference [2] 4 feature intermolecular bonding via chargedipole interactions, in contrast with the purely hydrogenbonded neutrals. We found no evidence for a closed hydration shell at n = 4, also contrasting with studies of neutral clusters.…”