Combining ab initio multiconfigurational and Free Energy Gradient methods to study the p–p * excited state structure and properties of uracil in water Carlos Bistafa a , Herbert C. Georg b , Sylvio Canuto a,⇑ a Instituto de Física, Universidade de São Paulo, CP 66318, 05314-970 São Paulo, SP, Brazil b Instituto de Física, Universidade Federal de Goiás, CP 131, 74001-970 Goiânia, GO, Brazil article info Article history: Received 3 February 2014 Received in revised form 22 April 2014 Accepted 23 April 2014 Available online xxxx Keywords: Solvent effects Uracil in water Free Energy Gradient Absorption and emission spectra Stokes shift abstract The Free Energy Gradient method is used to obtain the p–p * excited state structure of uracil in aqueous environment. The geometry calculations are made at the CASSCF level. The CASPT2 method is employed to calculate absorption and emission energies of uracil in gas and in aqueous solution. An average discrete electrostatic model is used to include the solvent effect. The results for the calculated absorption and emission transitions are found in good agreement with experiment both in gas phase and in water. The solvent effect on the Stokes shift is also calculated in good agreement with experiment. These agreements lend additional credence to the structures obtained for the lowest p–p * excited state. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction There has been an increasing interest in the understanding of the properties and dynamics of molecular excited states. As both the absorption and emission of the electromagnetic radiation are sensitive to the environment, this justifies the increasing interest on the role of solvent effects. Important applications with biologi- cal interest are provided by fluorescent probes, such as Laurdan and Prodan, that show great sensitivity to the solvent environment [1]. In biological processes, the DNA and RNA nucleobases, as thy- mine [2], adenine [3,4] and uracil [5–9] have dominated recent theoretical studies, because of the low quantum yield of emission associated to an intense absorption. This implies the existence of possible non-radiative decays and has stimulated the development of methodologies to study the structure and dynamics of excited states [10]. Such studies have shown some recent and interesting developments to include the effects of the solvent environment. In this work we address to the solvent effects on the excited p–p * state of uracil. Among the nucleobases, uracil (Fig. 1) has been the subject of some previous studies, since the absorption and emission spectra have been observed in several solvents [2,11,12]. Previous theoretical studies focused in describing the absorption spectrum [6,13,14] agreeing that the dominant, most intense, transition has a p–p * character, with a close and weak n–p * transition. The order of these transitions depends on the environment, but in gas phase the n–p * is located lowest [8,14,15]. As in solvent n–p * transitions are expected to blue shift whereas p–p * are expected to red shift there is the possibility of reversal and indeed this seems to be the case in aqueous environment, but not in acetonitrile [16]. This possible reversal raises some interesting photophysical prop- erties that have been addressed before [6]. Upon excitation to the bright p–p * excited state uracil undergoes important photophysi- cal dynamics responsible for its known photostability. The clarification of the precise mechanisms and the dynamics of the low-lying excited states have been the subject of some important theoretical investigations [6–9,16]. In these, one important aspect is the role of the solvent in the deactivation mechanism and in the structure and position of the vertical and relaxed p–p * excited state. Although several studies have been performed before, they were mostly done for isolated uracil. In this work we are devoted to study the structure of the p–p * excited state in gas and in aque- ous environment and on the nature of the emission transition. Regarding the structural aspects Serrano-Andrés and coworkers [7] have used the complete active space self-consistent field (CASS- CF) method [17] and the second-order corrected (CASPT2) [18], with an active space including all p orbitals and found the excited p–p * state to have a planar structure after full geometry relaxation. http://dx.doi.org/10.1016/j.comptc.2014.04.024 2210-271X/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: canuto@if.usp.br (S. Canuto). Computational and Theoretical Chemistry xxx (2014) xxx–xxx Contents lists available at ScienceDirect Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc Please cite this article in press as: C. Bistafa et al., Comput. Theoret. Chem. (2014), http://dx.doi.org/10.1016/j.comptc.2014.04.024