Fig. 2

Infrared spectra of glycosyl cations reveal their conformations. Comparison of the experimental (blue) and theoretical (grey) IR fingerprinting region of a glucose, b mannose, and c galactose variants of glycosyl cations. The experimental spectra were recorded using cold-ion IR spectroscopy in helium nanodroplets and the theoretical spectra were derived using dispersion-corrected hybrid density-functional PBE0+D3 in 6-311+G(d,p) basis set. The highly diagnostic spectral region above 1450 cm⁻¹ is characterized by carbonyl vibrations as labeled; the fingerprint region below 1450 cm⁻¹ mostly contains coupled C–O, C–C, C–OMe, and C–H vibrations. On the right, the assigned ring puckers are schematically shown. The corresponding high-resolution structures are shown in Supplementary Figs. 4–6