title: UV photofragmentation and IR spectroscopy of cold, G-type β-O-4 and β-β dilignol-alkali metal complexes: structure and linkage-dependent photofragmentation.
abstract: Ultraviolet photofragmentation spectroscopy and infrared spectroscopy were performed on two prototypical guaiacyl (G)-type dilignols containing β-O-4 and β-β linkages, complexed with either lithium or sodium cations. The complexes were generated by nanoelectrospray ionization, introduced into a multistage mass spectrometer, and subsequently cooled in a 22-pole cold ion trap to T ≈ 10 K. A combination of UV photofragment spectroscopy and IR-UV double resonance spectroscopy was used to characterize the preferred mode of binding of the alkali metal cations and the structural changes so induced. Based on a combination of spectral evidence provided by the UV and IR spectra, the Li(+) and Na(+) cations are deduced to preferably bind to both dilignols via their linkages, which constitute unique, oxygen-rich binding pockets for the cations. The UV spectra reflect this binding motif in their extensive Franck-Condon activity involving low-frequency puckering motions of the linkages in response to electronic excitation. In the pinoresinol•Li(+)/Na(+) complexes involving the β-β linkage, the spectra also showed an inherent spectral broadening. The photofragment mass spectra are unique for each dilignol•Li(+)/Na(+) complex, many of which are also complementary to those produced by collision-induced dissociation (CID), indicating the presence of unique excited state processes that direct the fragmentation. These results suggest the potential for site-selective fragmentation and for uncovering fragmentation pathways only accessed by resonant UV excitation of cold lignin ions.
authors.affiliation: Department of Chemistry, Purdue University , West Lafayette, Indiana 47907-2084, United States.
authors.forename: Jacob C
authors.initials: JC
authors.lastname: Dean
authors.forename: Nicole L
authors.initials: NL
authors.lastname: Burke
authors.forename: John R
authors.initials: JR
authors.lastname: Hopkins
authors.forename: James G
authors.initials: JG
authors.lastname: Redwine
authors.forename: P V
authors.initials: PV
authors.lastname: Ramachandran
authors.forename: Scott A
authors.initials: SA
authors.lastname: McLuckey
authors.forename: Timothy S
authors.initials: TS
authors.lastname: Zwier
mesh_terms: D003080:Cold Temperature; D008094:Lithium / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D055668:Photochemical Processes; D011789:Quantum Theory; D012964:Sodium / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013056:Spectrophotometry, Ultraviolet
chemical_list: D009942:Organometallic Compounds; D008094:Lithium; D012964:Sodium