A study in quantum chemistry through applications and pedagogy
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Quantum chemical theory enables a computational investigation of the electronic structure and properties of small molecules with an accuracy that frequently matches, or even exceeds, experiment. A theoretical investigation of the World War I chemical weapon phosgene (carbonyl chloride) utilizing such methods is presented. The results include a description of phosgene's molecular geometry and fundamental frequencies, which were acquired with the most rigorous coupled-cluster methods currently available. Another small, high-symmetry system of inquiry is thioformaldehyde sulfide, the sulfur analog of carbonyl oxide. This latter species is better known as the Criegee intermediate, an ephemeral molecule that plays an essential role in many relevant processes, including those of the atmosphere. Due to its transience, carbonyl oxide long eluded experimental interrogation. Efforts to investigate its mechanism of reactivity have included studies of the chalcogen analog thioformaldehyde sulfide, which has yet to be isolated in the gas phase. A map of thioformaldehyde sulfide's potential energy surface, as it undergoes cyclization to the the valence isomer dithiirane, is presented, aided by natural bond orbital methods. Finally, a detailed, pedagogical treatment of perhaps the single most important set of entities in quantum chemistry, the molecular integrals, is included. The materials included in this chapter are designed to help students and researchers better grasp this elusive subject via detailed and readable descriptions and exercises.