Vibrational spectra of guaiacylglycerol-β-guaiacyl ether: Experiment and theory

As an important inter-unit of lignin, guaiacylglycerol-β-guaiacyl (GG) ether has been synthesized, and characterized using terahertz time-domain spectroscopy in the frequency range of 5-85 cm^-1. Seven absorption peaks have been observed. Among these peaks, the 49.8 cm^-1 and 57.6 cm^-1 vibrations are propose to be characteristic absorption peaks of GG ether. Raman spectra were also measured in the range of 50-3500 cm^-1. The vibrations of the two lowest energy forms, i.e., erythro 1r4s and threo 1s4s, were calculated using density functional theory at the B3LYP/6-311G^∗ level and assigned according to potential energy distribution. In addition, the contents of erythro and threo forms in GG sample could be estimated by comparing the waveform similarities between theoretical and observed curves in the 33.0-80.0 cm^-1 range. Results showed that the observed curve of GG sample is a combination of erythro 1s4r and threo 1s4s. The four absorption vibrations below 33.0 cm^-1 could be assigned to phonon, inter-molecular modes and/or hydrogen bond vibrations. Terahertz spectra and Raman spectra, together with theoretical calculations, could be powerful methods for predicting contents of different isomers in sample.