for the different reaction mechanisms (Figure ), namely stepwise, concerted and stepwise/alcohol-catalyzed, are in line with the previous findings. The concerted pathway is very unlikely due to a large energetic barrier, and the formation of the zwitterionic intermediate is the rate-determining step in the stepwise pathway. The subsequent proton transfer is barrier-less in the gas phase. The stepwise pathway again profits from hydrogen bonding, here exemplified by the alcohol-catalyzed reaction. The overall reaction is highly exothermic (see section S2 for details of the calculation).