Preferred protonation site of a series of sulfa drugs in the gas phase revealed by IR spectroscopy Thomas Uhlemann, Giel Berden and Jos Oomens Eur. Phys. J. D, 75 1 (2021) 23 Published online: 21 January 2021 DOI: 10.1140/epjd/s10053-020-00027-x