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 S(0) state infrared depletion spectrum shows an O-H and no N-H stretch band, identifying the spectrum as that of the enol tautomer. The S(1)<--S(0) electronic transition is (1)npi*, as evidenced by the rotational contour of the 0 band. Vibronic excitations of the in-plane benzene-type vibrations nu'(6a), nu'(6b), nu'(14) and nu'(15) are observed, while none are observed for the out-of plane fundamental excitations, implying that the (1)npi* 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 approximately 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 micros lifetime. No fluorescence has been observed from the (1)npi* state, implying relaxation to the lower-lying long-lived state is very efficient. Separate ionization potentials are measured for the (1)npi* state (9.178 eV) and for the long-lived state (approximately 9.46 eV), hence the latter lies approximately 2200 cm(-1) below the (1)npi* state. Time-dependent B3LYP calculations of the excited states of 5M2HP indeed predict the S(1) state to be (1)npi* with a planar hydroxypyrimidine moiety. The T(1) ((3)pipi*) state is calculated to lie 3000 cm(-1) below the S(1) state, in excellent agreement with the experiment.
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