Monolignol lithium cation basicity estimates and lithium adduct ion optimized geometries

Abstract

Mass spectrometric analysis of lignin for developing biomaterials requires advances of characterization techniques. Positive ion mass spectrometry of lignin model compounds using lithium has recently been explored as a viable alternative to current negative mode techniques. To date, little is known about the impact of lithium adduct ion formation on relative response factors of lignin and lignin decomposition products. In this contribution, we report estimates of lithium cation basicity for synthetic monolignols H, G and S using Cooks’ kinetic method on a linear quadrupole ion trap mass spectrometer. Optimized geometries and interaction energies have also been calculated by DFT methods to quantify the electrostatic cation coordination. Based on a combination of experimental and computational evidence, lithium appears to preferentially bind to the phenol and methoxy substituents on the aromatic ring of monolignols. The strength of this interaction increases with the number of methoxy substituents (S > G > H). This work serves as a basis of understanding for future work in developing lithium adducted lignin mass spectrometric analytical methods.