The relative stabilities of the six lowest energy tautomers of the DNA base guanine have been investigated in the first excited singlet state, S-1, employing the restricted open-shell Kohn-Sham (ROKS) method. Comparison of the S-1 optimized geometries to the respective ground-state structures reveals large distortions for the keto tautomers, whereas the enol tautomers remain essentially planar. Harmonic vibrational spectra in the S-1 state have been calculated using the ROKS potential energy surfaces. Adiabatic excitation energies together with characteristic vibrational features of the individual guanine tautomers enable us to unambiguously assign recent experimental IR-UV spectra. Velocity autocorrelation functions obtained from adiabatic excited state Car-Parrinello molecular dynamics simulations demonstrate that anharmonic effects only play a minor role. (C) 2003 American Institute of Physics.