Probing supercritical water with the n-π* transition of acetone: A Monte Carlo/quantum mechanics study

Tertius L. Fonseca, Kaline Coutinho, Sylvio Canuto
2007 Journal of Chemical Physics  
The n-* electronic transition of acetone is a convenient and important probe to study supercritical water. The solvatochromic shift of this transition in supercritical water ͑adopting the experimental condition of P = 340.2 atm and T = 673 K͒ has been studied theoretically using Metropolis NPT Monte Carlo ͑MC͒ simulation and quantum mechanics ͑QM͒ calculations based on INDO/CIS and TDDFT-B3LYP/ 6-31+ G͑d͒ methods. MC simulations are used to analyze hydration shells, solute-solvent interaction,
more » ... lvent interaction, and for generating statistically relevant configurations for subsequent QM calculations of the n-* transition of acetone. The results show that the average number of hydrogen bonds between acetone and water is essentially 1 / 3 of that in normal water condition of temperature and pressure. But these hydrogen bonds have an important contribution in the solute stabilization and in the solute-solvent interaction. In addition, they respond for nearly half of the solvatochromic shift. The INDO/CIS calculations explicitly considering all valence electrons of the water molecules, using different solvation shells, up to the third shell ͑170 water molecules͒, give a solvatochromic shift of 670± 36 cm −1 in very good agreement with the experimentally inferred result of 500-700 cm −1 . It is found that the solvatochromic effect on n-* transition of acetone in the supercritical condition is essentially given by the first solvation shell. The time-dependent density-functional theory ͑TDDFT͒ calculations are also performed including all solvent molecules up to the third shell, now represented by point charges. This TDDFT-B3LYP/ 6-31+ G͑d͒ also gives a good but slightly overestimated result of 825± 65 cm −1 . For comparison the same study is also made for acetone in water at normal condition. Finally, all average results reported here are statistically converged.
doi:10.1063/1.2428293 pmid:17249885 fatcat:htkeiq7l4fhklhhpuo5rombpjq