title: A Monte Carlo-quantum mechanics study of the lowest n-pi* and pi-pi* states of uracil in water.
abstract: The solvatochromic shifts of the n-pi(*) and pi-pi(*) states of uracil in water are analyzed using a combined and sequential Monte Carlo/quantum mechanics (MC/QM) approach. The role of the solute polarization and electronic delocalization into the solvent region are investigated. Electronic polarization of the solute is obtained using the HF/6-31G(d), the polarizable continuum model (PCM) and an iterative procedure using MP2/aug-cc-pVDZ in the MC/QM. The in-water dipole moment of uracil is obtained, respectively, as 5.12 D, 6.12 D and 7.01 +/- 0.05 D. This latter result, corresponding to an increase of 60% with respect to the gas phase value, is used in the classical potential of the MC simulation to obtain statistically uncorrelated configurations for subsequent QM calculations of the ultraviolet-visible absorption spectrum of uracil in water. QM calculations are performed at the time-dependent density-functional theory (TD-DFT) combined with the B3LYP and B3PW91 functionals, multiconfigurational (CASSCF) and the semi-empirical all-valence electron INDO/CIS methods. Using 60 solute-solvent configurations with the explicit inclusion of 200 water molecules the solvatochromic shift is obtained as a blue shift of 0.50 eV for the n-pi(*) state and a red shift of 0.19 eV for the pi-pi(*) state, in good agreement with experimentally-inferred values. These results are compared with TD-DFT results in conjunction with PCM approaches and the importance of solute polarization and wave function delocalization over the solvent region is discussed. Our results suggest that the elusive n-pi(*) state of uracil in water lies around 255 nm hidden by the intense and broad pi-pi(*) transition with a maximum at 260 nm, inverting the relative locations of these states compared to the gas phase. This is further supported by considering the in-water dipole moment changes upon excitation, as obtained from CASSCF calculations.
authors.affiliation: Instituto de Física, Universidade de São Paulo, CP 66318, São Paulo, 05315-970, SP Brazil.
authors.forename: Valdemir
authors.initials: V
authors.lastname: Ludwig
authors.forename: Kaline
authors.initials: K
authors.lastname: Coutinho
authors.forename: Sylvio
authors.initials: S
authors.lastname: Canuto