Solvatochromic effects using Monte Carlo simulation and semi-empirical quantum mechanical calculations

The sequential QM/MM methodology is used to describe the solvent effects on the electronic absorption spectra of organic molecules in solution. The structure of the liquid is generated by Monte Carlo computer simulation. Configurations composed by the solute and several solvent molecules are selected for a posteriori quantum mechanical calculations of the spectra. Situations are considered where a large number of solvent molecules are necessary to describe the solvation problem. The examples considered here involve supermolecular systems composed of ca. 1500-2000 valence electrons, justifying the need for a semi-empirical approach. The electronic spectrum is then calculated using the INDO/CIS method. The solvatochromic shifts of pyrimidine in water and of beta-carotene in acetone and isopentane are considered. These exemplify the situations of a polar molecule in a polar Correspondence/Reprint request: Dr. Sylvio Canuto, Instituto de Física, Universidade de São Paulo, CP 66318 05315-970, São Paulo, SP, Brazil. E-mail: [email protected] Kaline Coutinho et al. 64 environment and of a non-polar molecule in both polar and non-polar environments. An additional example is considered where the absorption spectrum of acetone is analysed in a low-dense condition of a supercritical water environment. Good agreements with experimental shifts are obtained in all cases. The relative importance of the inner and outer solvation shells is analyzed. The case of beta-carotene is a persistent and difficult problem because the spectrum involves a π-πexcitation between two states of zero dipole moment. For the case of acetone in supercritical water analysis is made of the decrease in the solute-solvent hydrogen bonds and their role on the calculated solvatochromic shift. This solvatochromism is considered both in relation to the gas phase (blue shift) and the normal liquid water spectra (red shift). The success of the present approach emphasizes the importance of the combined use of quantum mechanics and statistical mechanics and the usefulness of the semi-empirical method employed.