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 (C10H16 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-(H2O)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.