title: Ultraviolet spectroscopy of fundamental lignin subunits: guaiacol, 4-methylguaiacol, syringol, and 4-methylsyringol.
abstract: Ultraviolet spectroscopy of the G- and S-type lignin subunits, guaiacol (G) and syringol (S), along with their para-methylated derivatives 4-methylguaiacol (4-MG) and 4-methylsyringol (4-MS), has been carried out in the cold, isolated environment of a supersonic jet. The excitation spectra and dispersed fluorescence (DFL) spectra of G and 4-MG show strong S0-S1 origins and Franck-Condon activity involving both the ring modes typical of aromatic derivatives, and the four lowest frequency out-of-plane modes (a") and lowest in-plane mode (a') involving the OH and OCH3 groups. The four low-frequency out-of-plane modes undergo extensive Duschinsky mixing between the ground and excited state. In 4-MG, combination bands involving methyl rotor levels with out-of-plane modes appeared with surprisingly high intensity, indicating a high degree of hindered rotor-vibration coupling in both S0 and S1. These mixing effects accompany the change in geometry upon π-π∗ electronic excitation going from a planar ground state to a non-planar excited state. Time-dependent density functional theory (TDDFT M05-2X∕6-311++G(d,p)) calculations predict a geometric distortion along the out-of-plane oxygen flapping coordinate, yielding a double minimum potential in S1 with a barrier to planarity of 195 cm(-1) in G. The excitation spectrum of S and 4-MS showed a much higher degree of spectral congestion and a larger geometry change evident by a shifted intensity distribution peaking ∼300 cm(-1) above the electronic origin. TDDFT calculations predict a larger geometry change in S compared with G, with the OH and H-bonded methoxy groups displaced in opposite directions above∕below the ring plane. Dispersed fluorescence from all S1 excited state levels in S∕4-MS yield only broad emission peaking far to the red of the excitation wavelength (-4500 cm(-1)). Several hypotheses regarding the source of this broad, redshifted emission were tested, but the cause remains unclear. p-Methylation was found to significantly redshift the UV absorption in both 4-MG and 4-MS, and methyl rotor transitions were assigned in both allowing for the determination of the shape and barrier heights of their respective potentials. These results provide a foundation for the discrimination of G- and S-chromophores in lignin oligomers, and demonstrate the potential for site-selective absorption.
authors.affiliation: Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, USA.
authors.forename: Jacob C
authors.initials: JC
authors.lastname: Dean
authors.forename: Polina
authors.initials: P
authors.lastname: Navotnaya
authors.forename: Alexander P
authors.initials: AP
authors.lastname: Parobek
authors.forename: Rachel M
authors.initials: RM
authors.lastname: Clayton
authors.forename: Timothy S
authors.initials: TS
authors.lastname: Zwier
mesh_terms: D003408:Cresols / Q000737:chemistry* / Q000302:isolation & purification; D006139:Guaiacol / Q000737:chemistry* / Q000302:isolation & purification; D008031:Lignin / Q000031:analogs & derivatives / Q000737:chemistry* / Q000302:isolation & purification; D015394:Molecular Structure; D011748:Pyrogallol / Q000031:analogs & derivatives* / Q000737:chemistry / Q000302:isolation & purification; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet
chemical_list: D003408:Cresols; D011748:Pyrogallol; C010120:pyrogallol 1,3-dimethyl ether; D006139:Guaiacol; D008031:Lignin; C025390:creosol