Interfacial Water Mediates Oligomerization Pathways of Monoterpene Carbocations

Abstract

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.

Hydronium Ion Acidity Above and Below the Interface of Aqueous Microdroplets

“…Significantly, the gas-phase basicities of ISO and TMA are widely different, but their titration curves are identical within experimental error. Similar titration curves were obtained with other gas-phase olefins and terpenes. − We infer that the common equivalence point at pH 1/2 ∼ 3.6 is a property of OTL water rather than of the gases being protonated.…”

“…This was confirmed by the isotope effects, sometimes very large, observed in the protonation/deuteration of several gases on H 2 O/D 2 O microdroplets, ,, and by the sensitive dependence of the extent of protonation (and polymerization) of olefins on molecular structure. The relative rates of chain propagation, chain transfer, and hydride abstraction reactions of the carbocations resulting from the protonation of olefins proved to be quite sensitive to inductive and conjugative effects. − , …”

Aqueous Microdroplets Capture Elusive Carbocations

“…One may wonder why the intermediate carbocations survived in the aqueous microdroplet although they are highly susceptible to be attacked by the nucleophilic water forming alcohol. It appears that this microdroplet phenomenon is strikingly different from that of the corresponding bulk phase. ,− Indeed, this observation can be attributed to the emerging field of microdroplet chemistry demonstrating the unusual process that could occur in a tiny droplet. − ,,,− …”

Fates of Organic Hydroperoxides in Atmospheric Condensed Phases

“…3 For example, hydroxymethyl hydroperoxide, the smallest α-HH, has a large Henry's law constant of 5.0 × 10 5 M atm −1 at 295 K. 30 Monoterpene-or sesquiterpene-derived α-HHs are even less volatile. α-HHs are also produced at air− water interfaces by ozonolysis of terpenes or of oxidizedterpenes taken up into acidic water surfaces, 46,62,63 and α-HHs generated by this route diffuse into the bulk liquid before undergoing decomposition or other reactions. Thus, α-HHs, as well as other ROOHs, are considered to reside mostly in condensed phases in the atmosphere.…”