Supersonic jet UV spectrum and nonradiative processes of the thymine analogue 5-methyl-2-hydroxypyrimidine

Abstract

We investigate the infrared and electronic absorption spectra and the excited-state nonradiative processes of supersonic jet-cooled 5-methyl-2-hydroxypyrimidine (5M2HP), the enol form of deoxythymine, using two-color resonant two-photon ionization (R2PI) and infrared–UV depletion spectroscopies. Unlike uracil and thymine, which exhibit structureless electronic absorption spectra, the vibronic spectrum of 5M2HP is structured with narrow vibronic bands, allowing for the first time to probe the excited state of a thymine analogue. The S₀ state infrared depletion spectrum shows an O–H and no N–H stretch band, identifying the spectrum as that of the enol tautomer. The S₁ ← S₀ electronic transition is ¹nπ*, as evidenced by the rotational contour of the 0⁰₀ band. Vibronic excitations of the in-plane benzene-type vibrations , , and are observed, while none are observed for the out-of plane fundamental excitations, implying that the ¹nπ* excited state of 5M2HP has a planar pyrimidine frame. From 1200 to 3600 cm⁻¹ the vibronic bands become steadily broader, signaling a coupling to a lower-lying electronic state that increases with increasing energy. At ∼3600 cm⁻¹ above the origin, the R2PI spectrum broadens completely, indicating that the two states are strongly mixed. Delayed ionization measurements show that the coupled electronic state has a >5 μs lifetime. No fluorescence has been observed from the ¹nπ* state, implying relaxation to the lower-lying long-lived state is very efficient. Separate ionization potentials are measured for the ¹nπ* state (9.178 eV) and for the long-lived state (∼9.46 eV), hence the latter lies ∼2200 cm⁻¹ below the ¹nπ* state. Time-dependent B3LYP calculations of the excited states of 5M2HP indeed predict the S₁ state to be ¹nπ* with a planar hydroxypyrimidine moiety. The T₁ (³ππ*) state is calculated to lie 3000 cm⁻¹ below the S₁ state, in excellent agreement with the experiment.