The 1H and 13C chemical shifts of 5–5 lignin model dimers: An evaluation of DFT functionals

Abstract

The calculations of 1H and 13C NMR chemical shifts were performed on three 5–5 lignin dimers, prominent substructures in softwood lignins, to compare with experimental data. Initially, 10 DFT functionals (B3LYP, B3PW91, BPV86, CAM-B3LYP, HCTH, HSEH1PBE, mPW1PW91, PBEPBE, TPSSTPSS, and ωB97XD) combined with the gage-including atomic orbital (GIAO) method and basic set 6–31G(d,p) were tested on 3,3′-(6,6′-dihydroxy-5,5′-dimethoxy-[1,1′-biphenyl]-3,3′-diyl)dipropionic acid (1), efficiently synthesized from ferulic acid. HSEH1PBE, mPW1PW91, and ωB97XD were found to be the three best performing functionals with strong correlations (r2 ≥ 0.9988) and low errors (CMAEs ≤ 0.0611 ppm for 1H and CMAEs ≤ 1.19 ppm for 13C). These functionals were also well-performed for the 1H and 13C shift calculations of dimers 3,3′-dimethoxy-5,5′-dimethyl-[1,1′-biphenyl]-2,2′-diol (2) and 5,5′-diallyl-3,3′-dimethoxy-[1,1′-biphenyl]-2,2′-diol (3). Overall, the ωB97XD functional consistently provided the calculated 1H and 13C chemical shifts with a high level of accuracy.