Conformation-specific spectroscopy and dynamics of lignin monomers and bichromophores
Abstract
The theme of this dissertation is the spectroscopic characterization of lignin monomers and flexible bichromophores using double-resonance ultraviolet and infrared spectroscopies. Herein a combined experimental and computational approach is employed to investigate in some detail the potential energy surfaces of complex molecules, which possess multiple conformational minima and pathways to isomerization. Experimentally, the powerful combination of laser-based spectroscopy and supersonic jet-cooling is applied to isolated molecules in the gas-phase. The cooling environment of the supersonic expansion allows for investigation of conformational isomers free from interference from others present, as well as, solvent and other interferers. Computationally, cutting-edge conformational searching algorithms, density functional methods, and ab initio calculations are utilized to aide in the interpretation of the experimentally recorded spectral data. More specifically, the initial focus of this dissertation is the spectroscopic characterization of the three monomer units of lignin, a natural aromatic biopolymer present in all vascular plants. The three monomer units, sinapyl alcohol, coniferyl alcohol, and p-coumaryl alcohol, make up, in varying concentrations, all known lignin oligomers. The body of data presented regarding the lignin monomer units provides a basis for further studies of larger oligomeric forms of lignin. The second focus of this dissertation is the spectroscopic investigation of flexible bichromophores, which play a role in 'soot' formation during fuel combustion. Particular interest was rooted in the flexible nature of the bichromophores, bis-(4-hydroxyphenyl)methane and 2-benzylphenol. The built in flexibility of the methylene linkage between aromatic chromophores leads to multiple isomeric forms, and reaction pathways. Furthermore, these two bichromophores are natural points of extension to the previously studied bichromophoric molecules diphenylmethane and bis-(2-hydroxyphenyl)methane. Here, all the might of single-conformation spectroscopy was utilized to elegantly determine not only the conformational minima present, but the thresholds (barriers) to isomerization between the minima, as well.
Degree
Ph.D.
Advisors
Zwier, Purdue University.
Subject Area
Physical chemistry
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