NASA NTRS1997
Lester and coworkers have experimentally characterized complexes of OH in the X and A states with H2 and N2. Recently, we have carried out ab initio calculations of relevant portions of the ground state and excited state potential energy surfaces for these systems, including the conical intersection regions, which are responsible for electronic quenching of the A state of OH by H2 and N2. Both of these systems have weakly bound complexes in the X state and strongly bound complexes in the A state. The OH-H2 complex is T-shaped, while the OH-N2 complex is collinear. In both cases the H end of OH is oriented toward the H2 or N2 molecule, respectively. Rotation of the OH about its center of mass involves only a small barrier and rotation by 1800 so that the 0 end of OH is oriented toward the H2 or N2 molecule leads to conical intersections with the ground state surface. Since there is about 95 kcal/mol of available energy after crossing to the ground state surface, chemical reactions on the ground state surface are possible, in addition to electronic quenching. In the case of OH-H2, the conical intersection is much lower in energy than the OH (A state) + H2 asymptotic energy and the A state complex can be characterized as a hanging well on the upper cone of the conical intersection. Passage through the conical intersection places the system on the ground state potential energy surface with the possibility of going to OH (X state) + H2 (i.e. electronic quenching) or to H2O + H (reaction). For OH-N2, the A state complex is also a hanging well on the upper cone of the conical intersection, but the conical intersection is only slightly below the OH (A state) + H2 asymptotic energy. Passage through the conical intersection can lead to OH (X state) + N2 (i.e. electronic quenching), but so far reactive pathways have not been found.