A Monte Carlo-quantum mechanics study of the lowest n–π* and π–π* 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.

Assessing the accuracy of many-body expansions for the computation of solvatochromic shifts

“…From the test set used, the solvatochromic shift of the π → π * acrolein excitation is the case where this approach proved of greater usefulness, successfully capturing the effect of the environment and improving the computational estimate. A strong dependency on the quantum system size has also been observed for similar valence excitations in other molecules, such as in the uracyl molecule 85 . The many-body expansion here reviewed could help to elucidate this effect.…”

UV–Visible Absorption and Emission Energies in Condensed Phase by PCM/TD‐DFT Methods

“…As a consequence, the excited states of the nucleic acid bases (nucleobases) have been thoroughly investigated by many experimental and computational studies, both in the gas phase and in solution, providing several indications of the performances of the different computational approaches. Uracil (see Figure 1.1) is probably the nucleobase examined more in detail [107][108][109][110][111][112][113][114][115][116][117][118][119][120][121][122][123]. Experiments reveal the presence of a strong absorption band at $4.8-5.0 eV (depending on the solvent), and computations ascribe this feature to a p/p à transitions (hereafter Sp), which mainly corresponds to a HOMO !…”

“…The VEEs of uracil in water solution have also been studied by other approaches [117][118][119][120]. MRCI/cc-pVDZ/MM calculations (considering 257 water molecules as fixed charges, whose position is averaged by means of MD simulation) predicts that in water Sp VEE is red shifted by $0.05-0.1 eVand Sn VEE blue shifted by $0.4 eV with respect to the gas phase [117].…”

2-Aminopurine non-radiative decay and emission in aqueous solution: A theoretical study

“…Some of these studies use a continuum approach for the inclusion of the solvent effects [19,20]. However, it has been clearly established that the specific solute-solvent interaction largely affects the relative energy and properties of the electronic excited states [22]. In order to understand the distinct relaxation and emission properties of 2-aminopurine, we have used the sequential Monte Carlo quantum mechanics (S-MC/QM) approach in which the hydrated environment is modeled by the inclusion of different shells of water molecules mimicked by a distribution of point charges.…”