Interfacial Water Mediates Oligomerization Pathways of Monoterpene Carbocations

The air-water interface plays central roles in "on-droplet" synthesis, living systems, and the atmosphere; however, what makes reactions at the interface specific is largely unknown. Here, we examined carbocationic reactions of monoterpene (C 10 H 16 isomer) on an acidic water microjet by using spray ionization mass spectrometry. Gaseous monoterpenes are trapped in the uppermost layers of a water surface via proton transfer and then undergo a chain-propagation reaction. The oligomerization pathway of β-pinene ( β-P ), which showed prompt chain-propagation, is examined by simultaneous exposure to camphene ( CMP ). ( CMP )H + is the most stable isomer formed via rearrangement of ( β-P )H + in the gas phase; however, no co-oligomerization was observed. This indicates that the oligomerization of ( β-P )H + proceeded via ring-opening isomerization. Quantum chemical calculations for [carbocation-(H 2 O) n =1,2 ] complexes revealed that the ring-opened isomer is stabilized by hydrogen-π bonds. We propose that partial hydration is a key factor that makes the interfacial reaction unique.