11:00 AM - 1:00 PM
[PCG20-P04] Direct ab-initio MD Study on the photo-ionization dynamics of molecules adsorbed on water ice
Keywords:water ice in interstellar cloud, the adsorbed molecule, the photo-ionization dynamics, Direct ab-initio MD Study
The mechanism of molecular evolution in interstellar space is currently an important topic in astrochemistry and astrophysics. Reaction probability of bi-molecular collision reactions increases significantly in the adsorption of the ice surface due to the increase in collision probability. Hence, the reaction probability on a 2-dimensional surface is strikingly larger than that in 3-dimensional space. The reaction of an ammonia molecule plays an important role in the initial synthesis of amino acids in molecular clouds and in space. In particular, the ionization of ammonia and related compounds caused by cosmic rays is a basic reaction in amino acid synthesis. Hence, studying the photo-reaction of ammonia on an ice surface would give important information about amino acid synthesis.
In the present study, to elucidate the effects the ice surface on the reaction mechanism, the reaction of an ammonia dimer cation (NH3)2+ adsorbed on a water ice surface following its ionization were investigated using direct ab-initio molecular dynamics (AIMD) combined with our own n-layered integrated molecular orbital and molecular mechanics (ONIOM) methods. The ice surface was modeled by two water layers composed of 48 water molecules (H2O)m (m = 48). The ammonia dimer was put on the central region of the model surface (H2O)48. Finally, the whole geometry of (NH3)2(H2O)48 was optimized using ONIOM (our own n-layered integrated molecular orbital and molecular mechanics) method. The level of theory was expressed by the (MP2/6-311++G(d,p): PM3).
The trajectories of (NH3)2+(H2O)48 following the ionization of (NH3)2(H2O)48 was calculated at the (MP2/6-311++G(d,p):PM3) level under the assumption of vertical ionization from the neutral state. The trajectory calculations of (NH3)2+(H2O)48 were performed using the condition of constant total energy.
In addition to the optimized geometry of (NH3)2(H2O)48, twenty structures of (NH3)2(H2O)48 generated around the equilibrium point were examined in the direct AIMD calculations. The intermolecular distance between (NH3)d and (NH3)a varied in the range 3.150–3.250 Å, twenty structures of (NH3)2(H2O)48 were selected, and the trajectories of ionic system were started from the chosen geometrical configurations of the ammonia dimer. The range of intermolecular distance was chosen from astrophysical condition at 10 K. The kinetic energy and angular momentum of each atom were assumed to be zero at time zero. The sampling method used is described in previous papers.
The geometry optimization of structure of ammonia dimer on the model ice surface (NH3)2(H2O)48 was performed at the (MP2/6-311++G(d,p):PM3) level of theory. The ammonia dimer binds to the central region of a hexagonal site on the ice surface. The ammonia dimer was connected by three protons from the surface water molecules. The donor ammonia molecule (NH3)d was connected by one hydrogen bond (bond length 1.790 Å) from the surface water molecule, while the acceptor ammonia molecule (NH3)a was bounded by two bonds with the distance of 2.588 and 2.730 Å. The intermolecular distance between (NH3)d and (NH3)a were calculated to be RNN= 3.174 Å.
In the present study, to elucidate the effects the ice surface on the reaction mechanism, the reaction of an ammonia dimer cation (NH3)2+ adsorbed on a water ice surface following its ionization were investigated using direct ab-initio molecular dynamics (AIMD) combined with our own n-layered integrated molecular orbital and molecular mechanics (ONIOM) methods. The ice surface was modeled by two water layers composed of 48 water molecules (H2O)m (m = 48). The ammonia dimer was put on the central region of the model surface (H2O)48. Finally, the whole geometry of (NH3)2(H2O)48 was optimized using ONIOM (our own n-layered integrated molecular orbital and molecular mechanics) method. The level of theory was expressed by the (MP2/6-311++G(d,p): PM3).
The trajectories of (NH3)2+(H2O)48 following the ionization of (NH3)2(H2O)48 was calculated at the (MP2/6-311++G(d,p):PM3) level under the assumption of vertical ionization from the neutral state. The trajectory calculations of (NH3)2+(H2O)48 were performed using the condition of constant total energy.
In addition to the optimized geometry of (NH3)2(H2O)48, twenty structures of (NH3)2(H2O)48 generated around the equilibrium point were examined in the direct AIMD calculations. The intermolecular distance between (NH3)d and (NH3)a varied in the range 3.150–3.250 Å, twenty structures of (NH3)2(H2O)48 were selected, and the trajectories of ionic system were started from the chosen geometrical configurations of the ammonia dimer. The range of intermolecular distance was chosen from astrophysical condition at 10 K. The kinetic energy and angular momentum of each atom were assumed to be zero at time zero. The sampling method used is described in previous papers.
The geometry optimization of structure of ammonia dimer on the model ice surface (NH3)2(H2O)48 was performed at the (MP2/6-311++G(d,p):PM3) level of theory. The ammonia dimer binds to the central region of a hexagonal site on the ice surface. The ammonia dimer was connected by three protons from the surface water molecules. The donor ammonia molecule (NH3)d was connected by one hydrogen bond (bond length 1.790 Å) from the surface water molecule, while the acceptor ammonia molecule (NH3)a was bounded by two bonds with the distance of 2.588 and 2.730 Å. The intermolecular distance between (NH3)d and (NH3)a were calculated to be RNN= 3.174 Å.