The Ab Initio Potential Energy Surface And Quantum Dynamics Of NH₂+H₂→NH₃+H Reaction

During the research of chemical process and understanding towards the atomic and molecular reactivity, molecular dynamics plays an important subject. The NH4system is an typical small molecular system, research on this system would acquire more understanding to the elementary reactions. To perform high-precision quantum dynamic calculations perform on this reaction system would require an accurate potential energy surface.In this article we use the modified Shepard interpolation potential energy surface constructed under UCCSD(T)/aug-cc-pVDZ level of theory by Collins.et.al. as the foundation, recalculated the energies, the first order derivative and the Hessian matrix of the data point at UCCSD (T)/aug-cc-pVTZ level of ab initio theory. The origin data sets were obtained by picking data through quasi-classical trajectory theory and already dynamically converged.Initial-state-selected time-dependent wave packet dynamics studies have been performed for the H2+NH2→H+NH3reaction with a seven-dimensional model on the new ab initio potential energy surface. In the seven-dimensional model, NH2group keeps C2v symmetry and the two NH bonds are fixed at their equilibrium values. The total reaction probabilities are calculated when (1) the two reactants are initially at their ground states;(2) NH2bending mode is excited, and (3) H2on its first vibrational excited state. The integral cross sections are also reported for these initial states with centrifugal-sudden approximation. Strong quantum tunneling effect were observed at the ground state, the NH2and the H2vibrational excite state showed little and negative effect to the reaction, respectively. The kinetic energy level increase promotion effect to the reaction is more significant, this reaction is a typical "early barrier" reaction, according to the Polany’s Law.Thermal rate constants are calculated for the temperature range of200-2000K and compared with the previous calculated values and available experimental data. The calculated thermal rate constants between293K to730K showed perfect agreements with experiment results, however, the calculate thermal rate constants at high temperature region (900-2000K) still needs further improvement.To have more accurate description of the high temperature reaction, the expand of the Collins’ PES were carried out later, after the expand and the amendment of the aVTZ PES, there were6600data point contained in our new PES, after the error statistical, we believe that our new potential energy surface of NH4system can take out a better description of the reaction.MP2/cc-pVTZ level of conformational optimization on the interstellar molecules HC4N by ab initio method, a total of23singlet and5triplet state stable configuration along with21transition state were reached, we took calculation of their zero-point energy, vibrational frequencies, infrared frequencies, dipole moment, and rotational constants along with other parameters, the frequency calculation ensured the transition states had one and only imaginary frequency, and also perform IRC scan to confirm the connection of both stable configurations and the transition state. UCCSD (T)/cc-pVTZ//G3(MP2) level of calculation were carried out to obtain high-precision single-point energy to confirm the relative energy order and the conformation transition barrier. The results indicated that the lowest singlet isomer has a conformation with three carbon atoms forming three-membered ring between hydrogen and-CN group, and the lowest triplet isomer adopts a linear conformation and lies above the lowest singlet isomer. After analysis, the ab initio calculation had a good agreement with the experiment data, There were five dynamic and kinetic stable isomers of HC4N molecule obtained after analysis and discussion, named R31,B1,C1,R32and R35, respectively.The throughout examination to the conformational transit potential energy surface of the interstellar molecule HC4N may provide some useful information for the HC4N and other cyanopolyynes in the future laboratory or interstellar identification.