Electronic structure and spectroscopy of excited and open-shell species

UnrestrictedRadical systems, or systems with unpaired electrons, play fundamental roles in chemistry, biology, and physics. Relevant examples include photochemical breakdown of air pollution in the lower atmosphere, carbon-carbon bond formation in synthetic and pharmaceutical chemistry, and formation of fundamental organic molecules in the interstellar medium. The work presented in this thesis focuses on the electronic structure and spectroscopy of open-shell species; the organization is as follows. Chapter 1 provides a short background and overview. Chapter 2 presents a study of the excited states of several vinylic hydrocarbon radicals, with a focus on 3p Rydberg states and the effects of increased alkyl length on excitation energies and quantum defects. Chapters 3 and 4 extend some of these conclusions to the diazomethane molecule, work which was done in close collaboration with the Reisler group at USC. Chapter 5 focuses ondiazirine, a three-membered cyclic isomer of diazomethane. The agreement between theory and experiment afforded a detailed insight into the photochemical dissociation of diazirine via a two-photon process.; Chapter 6 explores the electronic structure of the 1,2,3-tridehydrobenzene triradical. Despite a nominally antibonding character in one of the electronic states, signifcant distortion of nuclear framework was observed that maximized overlap between meta carbons. Experimental isolation of this system by our coworkers Winkler et al. allowed comparison to harmonic frequencies calculated for two nearly-degenerate electronic states, leading to conclusive assignment ofthe ground state. Chapters 7 and 8 present our results on the hydroxycarbene diradicals. Full-dimensional potential energy surfaces were obtained. Anharmonic vibrational energies and wavefunctions were calculated on the surfaces, as well as intensities for two types of spectroscopies. With the full-dimensional dipole moment surface, infrared spectra were obtained. Franck-Condon factors (photoelectron spectra) were calculated between neutral and cation surfaces; the photoelectron spectrum shows qualitative differences between the two isomers, which will facilitate a future experimental identification. Lastly, Chapter 9 presents future work ideas, which concern the variational approach and block diagonalization techniques for the molecular Hamiltonian. The two Appendices provide user information and code structure for ezVibe, a program developed in our group for solving the vibrational problem on potential energy surfaces