A theoretical and experimental study of nitric oxide complexes.

Intermolecular interactions involving molecules with unpaired electrons are a crucial part of phenomena ranging from nerve cell signalling to water oxygenation, and it is necessary to know the intermolecular potential in order to predict the preferred alignment of molecules in atmospheric chemistry and in gas-phase chemical reactions such as combustion. However, intermolecular potentials are difficult to obtain, and molecules with unpaired electrons further complicate the situation, especially when an unpaired electron can occupy two or more orbitals with similar energies.In our proposed work, we shall develop and assess the theoretical methods that we believe are the most promising for calculating intermolecular potentials of molecules with unpaired electrons, and apply the methods to interactions involving the chemically important molecule NO, whose unpaired electron can occupy two different 'pi' orbitals. These orbitals are equal in energy (degenerate) in the isolated NO molecule, but not when other molecules interact with it in weakly bound molecular complexes. We shall use a range of experimental methods to obtain information about these NO-X complexes, where X includes a number of diatomic molecules, rare gas atoms, and methane, and the NO molecule will be prepared in several different electronic and spin-orbit states.The work will involve collaboration between research groups at the Universities of Nottingham and Oxford, with experience in the calculation of intermolecular potentials, quantum chemistry of excited electronic states, and spectroscopy of Van der Waals complexes. The breadth and depth of this expertise, supported by collaborations with other leading research groups and by nationally-leading supercomputer facilities, offers the likelihood of substantial progress in this topical and exciting area of research. The spectroscopy of NO-X complexes will use microwave spectroscopy to obtain detailed information on the low-energy regions of the potential energy surfaces, and stimulated emission pumping to obtain information on the higher-energy vibrational and rotational states of the complexes in the ground electronic state. This will provide new information on the Van der Waals stretching motion and the hindered rotational motion of the complexes, and on the interplay between the spin-orbit interaction in the NO monomer and the Van der Waals interaction between the two monomers.Intermolecular potentials for the NO-X complexes will be calculated using a combination of the supermolecule method and new methods including intermolecular perturbation theory and the Maximum Overlap Method. The splitting of the spatial degeneracy by the intermolecular interaction makes these calculations non-standard and very challenging, especially for excited states and for polyatomic molecules X. From the intermolecular potentials, theoretical rotational and vibrational spectra will be predicted by solving the Schrdinger equation for the nuclear motion of the complex, including the non-Born-Oppenheimer effects that arise from coupling of the different spin-orbit states of NO by the intermolecular potential.The interplay between experiment and theory will be crucial, because the new theoretical methods can be assessed by their ability to reproduce the experimental data, and the results of the theoretical calculations will give additional, detailed, information on the potential energy surfaces, which cannot be obtained from the experiments. It is also expected, from our recent work on NO-methane and on the A states of NO-rare gas complexes, that the spectra will prove to be complicated and difficult to assign. The theoretical calculations will therefore be invaluable in understanding the experimental data.