New Karplus equations for 2JHH, 3JHH, 2JCH, 3JCH, 3JCOCH, 3JCSCH, and 3JCCCH in some aldohexopyranoside derivatives as determined using NMR spectroscopy and density functional theory calculations Original Research Article

The 1H–13C coupling constants of methyl α- and β-pyranosides of d-glucose and d-galactose have been measured by one-dimensional and two-dimensional 1H–13C heteronuclear zero and double quantum, phase sensitive J-HMBC spectra to determine a complete set of coupling constants (1JCH, 2JCH, 3JCH, 2JHH, and 3JHH) within the exocyclic hydroxymethyl group (CH2OH) for each compound. In parallel with these experimental studies, structure, energy, and potential energy surfaces of the hydroxymethyl group for these compounds were determined employing quantum mechanical calculations at the B3LYP level using the 6-311++G∗∗ basis set. Values of the vicinal coupling constants involving 1H and 13C in the C5–C6 (ω) and C6–O6 (θ) torsion angles in the aldohexopyranoside model compounds were calculated with water as the solvent using the PCM method. To test the relationship between 3JCXCH (X = C, O, S) and torsion angle C1–X (φ) around the anomeric center, the conformations of 24 derivatives of glucose and galactose, which represent sequences of atoms at the anomeric center of C-glycosides (C–C bond), O-glycosides (C–O bond), thioglycosides (C–S bond), glycosylamines (C–N bond), and glycosyl halides (C–halogen (F/Cl) bond) have been calculated. Nonlinear regression analysis of the coupling constants 1JC1,H1, 2JC5,H6R, 2JC5,H6S, 2JC6,H5, 3JC4,H6R, 3JC4,H6S, 2JH6R,H5, and 3JH5,H6R as well as 3JCXCH (X = C, O, S) on the dihedral angles ω, θ, and φ have yielded new Karplus equations. Good agreement between calculated and experimentally measured coupling constants revealed that the DFT method was able to accurately predict J-couplings in aqueous solutions.