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

The n-pi(*) 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 K) has been studied theoretically using Metropolis NPT Monte Carlo (MC) simulation and quantum mechanics (QM) calculations based on INDO/CIS and TDDFT-B3LYP6-31+G(d) methods. MC simulations are used to analyze hydration shells, solute-solvent interaction, and for generating statistically relevant configurations for subsequent QM calculations of the n-pi(*) transition of acetone. The results show that the average number of hydrogen bonds between acetone and water is essentially 13 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 molecules), 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-pi(*) transition of acetone in the supercritical condition is essentially given by the first solvation shell. The time-dependent density-functional theory (TDDFT) calculations are also performed including all solvent molecules up to the third shell, now represented by point charges. This TDDFT-B3LYP6-31+G(d) 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.     

Electronic properties of liquid ammonia: a sequential molecular dynamics/quantum mechanics approach

The electronic properties of liquid ammonia are investigated by a sequential molecular dynamics/quantum mechanics approach. Quantum mechanics calculations for the liquid phase are based on a reparametrized hybrid exchange-correlation functional that reproduces the electronic properties of ammonia clusters [(NH3)n; n=1-5]. For these small clusters, electron binding energies based on Green's function or electron propagator theory, coupled cluster with single, double, and perturbative triple excitations, and density functional theory (DFT) are compared. Reparametrized DFT results for the dipole moment, electron binding energies, and electronic density of states of liquid ammonia are reported. The calculated average dipole moment of liquid ammonia (2.05+/-0.09 D) corresponds to an increase of 27% compared to the gas phase value and it is 0.23 D above a prediction based on a polarizable model of liquid ammonia [Deng et al., J. Chem. Phys. 100, 7590 (1994)]. Our estimate for the ionization potential of liquid ammonia is 9.74+/-0.73 eV, which is approximately 1.0 eV below the gas phase value for the isolated molecule. The theoretical vertical electron affinity of liquid ammonia is predicted as 0.16+/-0.22 eV, in good agreement with the experimental result for the location of the bottom of the conduction band (-V 0=0.2 eV). Vertical ionization potentials and electron affinities correlate with the total dipole moment of ammonia aggregates.