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

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 H-1 and C-13 chemical shifts for thousands of lignin oligomers. Herein, Boltzmann averaged H-1 and C-13 GIAO NMR calculations were performed on coniferyl alcohol and four of its dimers, beta-O-4, beta-beta, beta-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 C-13 results were more accurate than the H-1 results, due to poorer agreement between calculated hydroxyl-H results and observed data. Further Boltzmann-averaged, multistandard NMR calculations compared the C-13 and H-1 chemical shifts with experiment for the four stereoisomers of the beta-O-4 dimer, as well as the 5-5, beta-5, and beta-beta dimers of coniferyl alcohol. The C-13 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 H-1 NMR was generally inferior to the C-13 NMR results for the dimers.