Chemistry

Event Title

Ultraviolet Spectroscopy of Lignin Chromophores

Presenter Information

Alexander Parobek, Purdue University

Document Type

Oral Presentation

Location

Indianapolis, IN

Subject Area

Chemistry

Start Date

11-4-2014 1:15 PM

End Date

11-4-2014 3:00 PM

Description

Electronic excitation and dispersed fluorescence spectroscopy has been performed on four aromatic chromophores of the complex biopolymer lignin: guaiacol (G), syringol (S), 4-methylguaiacol (4-MG), and 4-methylsyringol (4-MS). Dispersed fluorescence data derived from G and S indicate Frank-Condon activity accompanied by extensive degrees of Duschinsky mixing for the lowest frequency out-of-plane modes. Time-dependent density functional theory (TDDFT) M05-2X/6-311++G(d,p) calculations were utilized to determine mode-specific vibrational frequencies, and the geometric distortions associated with the excitation process. Assignments have been made for the observed vibronic structure in the excitation and dispersed fluorescence spectra, providing evidence for the geometry change that accompanies electronic excitation in these molecules. TDDFT calculations of S and 4-MS predicted larger degrees of geometry change when compared to their respective G derivatives. Resulting excitation spectra of S and 4-MS reflected calculations with more pronounced geometry changes and spectral congestion. However, dispersed fluorescence yielded only a broad, red-shifted emission. Possible reasons for this red-shifted emission will be discussed. Additionally, para-methylation has proven to red-shift UV adsorption and to provide additional spectral congestion, while maintaining similar degrees of geometry change. Transitions ascribed to energy levels of the methyl rotors are observed and used to determine the barriers to hindered formation of the methyl group in ground and excited states. The implications of these ultraviolet experiments provide critical insight into the site-specific excitations associated with different monomers of lignin.

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Apr 11th, 1:15 PM Apr 11th, 3:00 PM

Ultraviolet Spectroscopy of Lignin Chromophores

Indianapolis, IN

Electronic excitation and dispersed fluorescence spectroscopy has been performed on four aromatic chromophores of the complex biopolymer lignin: guaiacol (G), syringol (S), 4-methylguaiacol (4-MG), and 4-methylsyringol (4-MS). Dispersed fluorescence data derived from G and S indicate Frank-Condon activity accompanied by extensive degrees of Duschinsky mixing for the lowest frequency out-of-plane modes. Time-dependent density functional theory (TDDFT) M05-2X/6-311++G(d,p) calculations were utilized to determine mode-specific vibrational frequencies, and the geometric distortions associated with the excitation process. Assignments have been made for the observed vibronic structure in the excitation and dispersed fluorescence spectra, providing evidence for the geometry change that accompanies electronic excitation in these molecules. TDDFT calculations of S and 4-MS predicted larger degrees of geometry change when compared to their respective G derivatives. Resulting excitation spectra of S and 4-MS reflected calculations with more pronounced geometry changes and spectral congestion. However, dispersed fluorescence yielded only a broad, red-shifted emission. Possible reasons for this red-shifted emission will be discussed. Additionally, para-methylation has proven to red-shift UV adsorption and to provide additional spectral congestion, while maintaining similar degrees of geometry change. Transitions ascribed to energy levels of the methyl rotors are observed and used to determine the barriers to hindered formation of the methyl group in ground and excited states. The implications of these ultraviolet experiments provide critical insight into the site-specific excitations associated with different monomers of lignin.