An Investigation of Weak CH···O Hydrogen Bonds in Maltose Anomers by a Combination of Calculation and Experimental Solid-State NMR Spectroscopy

Two-dimensional ¹H−¹³C MAS-J-HMQC solid-state NMR spectra of the two anomeric forms of maltose at natural abundance are presented. The experimental ¹H chemical shifts of the CH and CH₂ protons are assigned using first-principles chemical shift calculations that employ a plane-wave pseudopotential approach. Further calculations show that the calculated change in the ¹H chemical shift when comparing the full crystal and an isolated molecule is a quantitative measure of intermolecular C−H···O weak hydrogen bonding. Notably, a clear correlation between a large chemical shift change (up to 2 ppm) and both a short H···O distance (<2.7 Å) and a CHO bond angle greater than 130° is observed, thus showing that directionality is important in C−H···O hydrogen bonding.