Solvatochromic Shifts in Uracil: A Combined MD-QM/MM Study

Uracil is a commonly occurring pyrimidine derivative found in RNA where it base pairs with adenine. Rationalizing the electronic properties of uracil in both gas phase and aqueous solution is of fundamental importance because of the significant biological role played by this molecule. This paper presents accurate predictions of the solvatochromic shifts of the lowest pi -> pi(star) and n -> pi(star) vertical electronic excitation energies in uracil due to an aqueous solution. The calculations are conducted using a recently developed combined quantum mechanics/molecular mechanics (QM/MM) method, and nuclear dynamical effects are accounted for through molecular dynamics simulations. The electronic structure is described using either density functional theory employing the CAM-B3LYP exchange-correlation functional or the coupled cluster singles and approximate doubles (CC2) method. The predicted solvatochromic shifts using CAM-B3LYP/MM and CC2/MM are -0.12 +/- 0.01 eV and -0.20 +/- 0.01 eV, respectively, for the pi -> pi(star) transition and 0.42 +/- 0.03 eV and 0.43 +/- 0.03 eV, respectively, for the n -> pi(star) transition. Our best estimate of the solvatochromic shifts are derived using a self-consistent polarizable model in both the MID and QM/MM simulations and are -0.29 +/- 0.01 eV and 0.45 +/- 0.03 eV for the pi -> pi(star) and n -> pi(star) transitions, respectively. The estimate is based on CC2 with electrostatic corrections defined from CAM-B3LYP and dispersion corrections derived from CC2 model system calculations. These solvatochromic shifts are in excellent agreement with experimental data, indicating the importance of explicit inclusion of polarization effects in MD-based QM/MM methods.