UV photofragmentation and IR spectroscopy of cold, G-type β-O-4 and β-β dilignol-alkali metal complexes: structure and linkage-dependent photofragmentation

J Phys Chem A. 2015 Mar 12;119(10):1917-32. doi: 10.1021/jp512603n. Epub 2015 Feb 19.

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.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Cold Temperature*
  • Lithium / chemistry*
  • Models, Molecular
  • Molecular Conformation
  • Organometallic Compounds / chemistry*
  • Photochemical Processes*
  • Quantum Theory
  • Sodium / chemistry*
  • Spectrophotometry, Infrared
  • Spectrophotometry, Ultraviolet

Substances

  • Organometallic Compounds
  • Lithium
  • Sodium