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Abstract
The vibrational spectra of small, organic radicals are measured and interpreted. A major focus is the effective use of ab initio quantum chemistry to describe anharmonic resonance polyads in the CH stretching fundamental region (about 2800 – 3150 cm-1). An accurate, cost-effective protocol is employed that uses vibrational perturbation theory, based on hybrid quartic force fields, with large effective Hamiltonian treatments of resonances. The Second-Order Vibrational Perturbation Theory with Resonances (VPT2+K) approach is shown to be very successful at modeling the CH stretch regions of isoprene and is notably much more successful at predicting relative intensities than approaches with less extensive resonance treatments. Clear vibrational signatures of the higher energy gauche isoprene isomer are reported for the first time, and improved thermochemical predictions are made with the aid of coupled-cluster theory. Vibrationally resolved CH stretching spectra of the propyl radicals and ethylperoxy radical are measured in helium droplets. These represent the highest quality vibrational spectra of these species to date, revealing numerous overtone and combination band transitions. VPT2+K describes these radicals with varying levels of success, depending on the severity of their large amplitude motion. New theoretical predictions are presented for the tert-butylperoxy and HO4+ systems. The structure, energetics, and vibrational spectroscopy of both the ground and lowest electronically excited state of tert-butylperoxy radical are modeled with high-level coupled cluster theory. Theoretical studies of the HO4+ system are focused on understanding its argon-tagged infrared photodissociation spectrum, also reported here. Various new isomers are located with a combination of multiconfigurational methods and single-reference coupled-cluster theory. The spectra of HO4+ and DO4+ are understood as arising from these new isomers, which are not traditional proton-bound dimers.