Quantum chemistry for spectroscopy : a tale of three spins (S = 0, 1/2, and 1)

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Vita.Includes bibliographical references (p. 193-203).Three special topics in the field of molecular spectroscopy are investigated using a variety of computational techniques. First, large-amplitude vibrational motions on ground-state singlet (S0) potential energy surfaces are analyzed for both the acetylene/vinylidene and the HCN/HNC isomerization systems. Electronic properties such as electric dipole moments and nuclear quadrupole coupling constants are used as diagnostic markers of progress along the isomerization path. Second, the topic of electronically excited triplet states and their relevance to doorway-mediated intersystem crossing for acetylene is considered. A new diabatic characterization of the third triplet electronic state, T3, enables a vibrational analysis of data obtained from current and past experiments. The last part of this thesis reviews the techniques and ideas of electron-molecule collisions relevant to Rydberg states of diatomic molecules. Previously developed and current methods of treating the excited Rydberg electron are evaluated and extended. Each of these three topics in molecular spectroscopy is studied using ab initio approaches coupled with experimental observations or chemically intuitive models. The unique combination of quantum chemistry and spectroscopy stimulates further developments in both theory and experiment.by Bryan Matthew Wong.Ph.D

An NMR study of molecular dynamics in organic crystalline compounds

Using a combination of solid-state NMR techniques including Tip measurements, dynamic line shape analyses and 2D EXSY data, a variety of intramolecular motions including rotations of methyl, t-butyl, t-amyl, phenyl, trimethylammonium and trimethylphosphonium groups in a series of quaternary ammonium and phosphonium salts have been investigated. Where possible, activation parameters Ea, △G+/-, △H+/- and △S+/- have been derived. A range of values was recorded, especially for △S+/- which ranged from +75 JK⁻¹ mol⁻¹ for a phenyl group to -57 JK⁻¹ mol⁻¹for a t-butyl group. It was shown that ¹³p and ³¹p T1p measurements from CP/MAS spectra can give quantitative information on the kinetics of intramolecular motions that agree with line shape analysis. Recent work using X-ray crystallography has suggested that some derivatives of bicycle [3.3.1] nonane show evidence of conformational equilibria in the solid state. Using ¹³CP/MAS NMR, 22 derivatives of bicycle [3.3.1] nonane were studied, some at variable temperature. No evidence of conformational equilibria was observed in the chosen compounds. Solid-state NMR has been used to follow the kinetics of ring- chain tautomerism in a bicyclic tetrahydro-1,3-oxazine derivative. This was found to form initially the metastable chain on crystallisation, which then cyclises. The kinetics of the cyclisation were followed and the activation energy for the solid-state reaction was derived. In contrast to this, a related pyrimidine derivative has been found to form initially the metastable ring on cyclisation which quickly ring opens to the chain