Comparison of Multistandard and TMS-Standard Calculated NMR Shifts for Coniferyl Alcohol and Application of the Multistandard Method to Lignin Dimers

Abstract

Coniferyl alcohol is a monomeric building block of lignin, the second most abundant biopolymer. During lignification, the monomer forms a variety of linkages through free radical additions. A large NMR database has been constructed that reports the (1)H and (13)C chemical shifts for thousands of lignin oligomers. Herein, Boltzmann averaged (1)H and (13)C GIAO NMR calculations were performed on coniferyl alcohol and four of its dimers, β-O-4, β-β, β-5, and 5-5, to compare the calculated chemical shifts with experiment. Six B3LYP/6-311++G(d,p) energy-minimized conformational isomers of coniferyl alcohol were subjected to single-point GIAO NMR calculations. Initially, four NMR shift calculation methods were compared: three were performed using the TMS-standard method at the HF/6-311+G(2d,p), B3LYP/6-311+G(2d,p), and mPW1PW91/6-31G(d) theory levels, and the fourth was performed with a multistandard approach using a mPW1PW91/6-31G(d) theory level. For the multistandard method, benzene was used as the standard for aromatic C and H atoms and methanol was used for aliphatic C and H atoms. The hydroxyl-H of methanol was used as the standard for hydroxyl-H atoms. The Boltzmann averaged results for six conformers showed that the multistandard method is more accurate for coniferyl alcohol and its dimers than the often used TMS-standard method, based on the mean unsigned, root-mean-squared, and maximum errors, as well as linear correlations between observed and calculated values. The (13)C results were more accurate than the (1)H results, due to poorer agreement between calculated hydroxyl-H results and observed data. Further Boltzmann-averaged, multistandard NMR calculations compared the (13)C and (1)H chemical shifts with experiment for the four stereoisomers of the β-O-4 dimer, as well as the 5-5, β-5, and β-β dimers of coniferyl alcohol. The (13)C results correlated well with experiment (r(2)>0.99) for all dimers and showed small statistical errors, compared with experiment. The correlation with experiment for (1)H NMR was generally inferior to the (13)C NMR results for the dimers.

Cellulose Structural Polymorphism in Plant Primary Cell Walls Investigated by High-Field 2D Solid-State NMR Spectroscopy and Density Functional Theory Calculations

“…Chemical shifts were calculated using the multi-reference method. Methanol was the secondary chemical shift standard, because it produces δ 13 C in better agreement with experiments 38, 39, 49, 50 . An empirical correction of 49.5 ppm 51 was used for the difference between the δ 13 C of methanol and TMS 49 .…”

Structural factors affecting 13C NMR chemical shifts of cellulose: a computational study

“…Chemical shifts were calculated using the multi-reference method. Methanol was the secondary standard to calculate the 13 C chemical shift, because it produces d 13 C in better agreement with experiment (Kubicki et al 2013Sarotti and Pellegrinet 2009;Watts et al 2011Watts et al , 2014). An empirical correction of 49.5 ppm (Gottlieb et al 1997) was used for the difference between the d 13 C in methanol and TMS commonly used as an experimental 13 C NMR standard (Sarotti and Pellegrinet 2009).…”

Recent innovations in analytical methods for the qualitative and quantitative assessment of lignin

“…As noted in Section 2.8, theoretical methods were utilized for lignin analysis and aided in validation of vibrational spectra and conformations. Boltzmann-averaged, multi-standard NMR calculations were used for evaluating the 13 C gauge including atomic orbital (GIAO) and 1 H chemical shifts with experimental data for the four stereoisomers of the β-O-4 dimer, as well as the 5-5, β-5, and β-β dimers of coniferyl alcohol [198]. The 13 C results show an excellent correlation with experiment (R 2 40.99).…”