Computational Physics and Chemistry
Computational Chemistry and Atmospheric modeling
Great advances in computer speed and algorithms have made it possible to compute from first principles accurate rates and cross sections for chemical reactions which may be difficult or impossible to measure in the laboratory. Using the latest computational chemistry tools for electronic structure and molecular dynamics, SSI has applied a variety of computational chemistry techniques to compute fundamental reaction rates and cross sections used in modeling atmospheric chemistry, and in understanding the environment around spacecraft. Please inquire at matt(at)spectral.com or duff(at)spectral.com for details.
Potential Energy Surfaces for the Reaction N(2D) + O2 ® NO + O
References:
M. Braunstein and J. W. Duff, "Electronic Structure and Dynamics of O + CO Collisions", J. Chem. Phys., 112, 2736-2745 (2000). (Abstract O + CO)
M. Braunstein and J. W. Duff, "Theoretical Study of the N(2D) + O2(3Sg-) Reaction", J. Chem. Phys., submitted. (Abstract N + O2)
J. W. Duff and D. R. Smith, "The O+(4S)+N2 ® NO+ + N(4S) Reaction as a Source of Highly Rotationally Excited NO+ in the Thermosphere," J. Atmos. and Solar-Terr. Physics., accepted for publication, 2000.
H. Dothe, R. D. Sharma, and J. W. Duff, "On the Steady-State Assumption for the Energy Distribution Function of the Nonthermal N(4S) Atoms and the Efficiency of NO Production by these Atoms in the Terrestrial Thermosphere," Geophys. Res. Lett., 24, 3233, 1997.
J. W. Duff and R. D. Sharma, "Quasiclassical Trajectory Study of NO Vibrational Relaxation by Collisions with Atomic Oxygen," J. Chem. Soc. Faraday Trans., 93, 2645, 1997.
J. W. Duff and R. D. Sharma, "Quasiclassical Trajectory Study of the N(4S) + NO(X2P) ® N2 + O(3P) Reaction Cross Section on the Excited 3A' NNO Surface," Chem. Phys. Lett., 265, 404, 1997.
J. W. Duff and R. D. Sharma, "Quasiclassical Trajectory Study of the N(4S)+NO(X2P) ® N2 + O(3P) Reaction Rate Coefficient," Geophys. Res. Lett., 23, 2777, 1996.
J. W. Duff, F. Bien, and D. E. Paulsen, "Classical Dynamics of the N(4S) + O2 ® NO(X2P) + O(3P) Reaction," Geophys. Res. Lett., 21, 2043, 1994.
M. Braunstein and J. W. Duff, "Electronic Structure and Dynamics of O + CO Collisions", J. Chem. Phys., 112, 2736-2745 (2000).
The potential energy surfaces of the three lowest electronic triplet states of CO2 which lead to O(3P)+CO(1S+), 3A', 13A', and 23A'', have been computed at the complete-active-space-self-consistent-field plus second-order perturbation theory (CASSCF-MP2) level with a modest 631+G(d) basis. Potential energy surfaces are fit with a global functional form. The 3A' state has a well 0.9 eV deep and the 13A'' state has a 0.2 eV well with respect to the O(3P)+CO(1S+) dissociation threshold. The 3A' and 13A'' states are both bent at their minima and have a barrier at 0.2 eV and 0.3 eV above threshold, respectively. The 23A'' state is mostly repulsive, and has a saddle at C2V geometries. We have run classical trajectory calculations for O(3P)+CO(1S+) collisions using these surfaces. Results agree well with available vibrational relaxation and oxygen atom exchange measurements except at low temperature. Comparisons are also made with measured vibrational excitation cross sections and infrared emission spectra of the nascent CO products at 3.4 eV collision energy. These results show a high degree of vibrational and rotational excitation with a nearly statistical population which is evident in a distinct spectral "band-head" signature. Analysis of the trajectories show that almost all collisions which lead to oxygen atom exchange and/or vibrational energy transfer occur when the O(3P) approaches the CO at OCO angles between 80o and 140o, passes over the barrier and through the wells of the 3A' and 13A'' states, and interacts with the repulsive wall of the carbon end of the CO nearly perpendicular to the CO bond.
M. Braunstein and J. W. Duff, "Theoretical Study of the N(2D) + O2(3Sg-) Reaction", J. Chem. Phys., submitted.
Potential energy surfaces are computed for all the electronic states relevant for the reaction N(2D) + O(X3Sg-) ® O + NO at the complete-active-space-self-consistent-field plus second-order perturbation theory (CASSCF-MP2) level using a 6311G(d) basis set. For those states with barriers low enough to contribute significantly at low to moderate temperatures, adiabatic global potential energy surfaces are fit with a functional form using at least 1,000 computed ab-initio points. Quasi-classical trajectory (QCT) calculations, excluding non-adiabatic effects, are performed and rates and final state vibrational distributions are compared with available experimental data. The peaked vibrational distribution observed in the experimental data is reproduced in these calculations, slightly shifted to higher vibration. These calculations show that from low to moderate temperatures the dynamics is dominated by the 2 2A' and 1 2A'' states of NO2, which have similar bent, early entrance channel transition states. Although production of O(1D) + NO(X2P) is allowed in these calculations, the barrier connecting this adiabatic channel to products is much too high to contribute, and only the O(3P) + NO(X2P) branch is formed.