Journal of Chemical Physics, Vol.100, No.5, 3791-3802, 1994
Monte-Carlo Simulation of Acetonitrile Clusters (CH3CN)(N), N=2-256 - Melting Transitions and Even/Odd Character of Small Clusters (N=2-9), Heat-Capacities, Density Profiles, Fractal Dimension, Intracluster Dimerization, and Dipole Orientation
The thermodynamic and structural properties of acetonitrile clusters [CH3CN](N), N=2-15, 20, 30, 60, 128, and 256 have been investigated using Monte Carlo simulation. Interactions in the small clusters (N less than or equal to 9) are dominated by antiparallel pairing of the molecular dipoles. The simulations reveal rigid <----> fluid (melting) transitions with a remarkable even-cad alternation in the transition temperatures for the N=2-9 clusters. The higher melting temperatures of the even-N clusters arise as consequences of the antiparallel paired dipoles which provide favorable electrostatic interactions. Even-odd alternation has also been observed in the configurational energies and heat capacities and the percentage of molecules,possessing an antiparallel nearest neighbor. These observations are consistent with the fact that Coulomb potential terms dominate the interaction energies in clusters with N < 12. The average density in clusters with N > 60 is fairly well represented by the bulk liquid density. Order parameters characterizing dipole orientation indicate that the molecular dipoles tend to lie flat on the cluster surface for N > 30. Significant dimerization within the clusters suggests evaporation of molecules via dimers and an enhancement of evaporative loss over condensation and this may explain the slower nucleation rates observed for acetonitrile compared to the predictions of the classical nucleation theory.
Keywords:MOLECULAR-DYNAMICS SIMULATION;VAPOR-PHASE NUCLEATION;SMALL WATER CLUSTERS;RARE-GAS CLUSTERS;LIQUID ACETONITRILE;THERMODYNAMIC PROPERTIES;NEUTRON-DIFFRACTION;COMPUTER-SIMULATION;PHYSICAL CLUSTERS;POLAR-MOLECULES