A quantum molecular dynamics study of the properties of NO+(H2O)n clusters

The structures and dynamics of NO+(H2O)(n), with n = 1,2,3, have been studied using first principles Born-Oppenheimer molecular dynamics (BOMD) performed in the framework of density functional theory (DFT) with a generalized gradient approximation (GGA). The ground-state structure of NO+(H2O), in which a relatively weak bond connects NO+ and H2O, is shown to be floppy along certain degrees of freedom. When a second water molecule is added, a new solvation shell is formed via a hydrogen bond. Our investigations indicate that a third water molecule attaches to the first water molecule and completes the second solvation shell. The hydration energies are found to be 1.31, 0.87, and 0.77 eV for n = 1,2,3, respectively. The vibrational spectra at room temperature are calculated for NO+, and all three hydrated clusters. Compared to an isolated NO+ ion, a redshift of 120-200 cm(-1) is observed for the N-O vibrational mode in NO+(H2O)(n). For n = 2, new peaks, identified as O-H stretches of the first H2O molecule, appear below the O-H stretch in the second H2O molecule. The spectrum of NO+(H2O)(3), which maintains the most important features in NO+(H2O)(2), indicates the presence of a complete solvation shell. Our studies suggest that the BOMD method is an efficient method for finding the optimal geometry of NO+(H2O)(n) clusters. More importantly, BOMD simulations allow for studies of dynamical and thermodynamical properties of these clusters at finite temperature, which mimics the physical conditions in which these clusters are found in nature and in the laboratory. (C) 1998 American Institute of Physics.