Nucleobase Pair Analogues 2-Pyridone·Uracil, 2-Pyridone·Thymine, and 2-Pyridone·5-Fluorouracil: Hydrogen-Bond Strengths and Intermolecular Vibrations

Abstract

Due to the ultrafast nonradiative decay of the uracil (U), thymine (T), and 5-fluorouracil (5-FU) nucleobases, resolved vibronic spectra of nucleobase dimers with U, T, or 5-FU chromophores have not yet been observed in the gas phase. 2-Pyridone (2PY) is a fluorescent hydrogen-bonding analogue of U with a single N−H donor and CO acceptor group. We report the mass-selected S1 ← S0 vibronic spectra of 2PY·U, 2PY·T, and 2PY·5-FU, revealing properties of their H-bonds free of stacking interactions and backbone constraints. The in-plane H-bond vibrations, ω‘, χ‘, σ‘, are observed for all three dimers; the out-of-plane vibrations β‘, τ‘ and the overtone of the stagger mode 2δ‘ are measured for 2PY·T and 2PY·U. The experimental frequencies are in good agreement with the ab initio calculated frequencies of the most stable H-bonded isomer, which involves the N1−H and C2O groups of U, T, or 5-FU as H donor/acceptor. The H-bond dissociation energies follow the sequence T ≈ U < 5-FU. However, the experimental S1 state H-bond stretching force constants increase in the sequence U < T < 5-FU. The absence of fluorescence from these dimers implies that the S1 lifetimes of 2PY in these complexes are >20 times shorter than those in the other 2PY·X complexes measured so far. Energy transfer from 2PY to U (T, 5-FU) is suggested as the cause, which implies that the nπ* S1 states of U, T, and 5-FU, which have never been directly observed, lie below the S1 ππ* state of 2PY.