Fragmentation of Allylmethylsulfide by Chemical Ionization: Dependence on Humidity and Inhibiting Role of Water

We report on a previously unknown
reaction mechanism involving
water in the fragmentation reaction following chemical ionization.
This result stems from a study presented here on the humidity-dependent
and energy-dependent endoergic fragmentation of allyl methyl sulfide
(AMS) upon protonation in a proton transfer reaction-mass spectrometer
(PTR-MS). The fragmentation pathways were studied with experimental
(PTR-MS) and quantum chemical methods (polarizable continuum model
(PCM), microhydration, studied at the MP2/6-311+G­(3df,2p)//MP2/6-31G­(d,p)
level of theory). We report in detail on the energy profiles, reaction
mechanisms, and proton affinities (G4MP2 calculations). In the discovered
reaction mechanism, water reduces the fragmentation of protonated
species in chemical ionization. It does so by direct interaction with
the protonated species via covalent binding (C3H5+) or via association (AMS·H+). This stabilizes
intermediate complexes and thus overall increases the activation energy
for fragmentation. Water thereby acts as a reusable inhibitor (anticatalyst)
in chemical ionization. Moreover, according to the quantum chemical
(QC) results, when water is present in abundance it has the opposite
effect and enhances fragmentation. The underlying reason is a concentration-dependent
change in the reaction principle from active inhibition of fragmentation
to solvation, which then enhances fragmentation. This amphoteric behavior
of water is found for the fragmentation of C3H5+ to C3H3+, and similarly
for the fragmentation of AMS·H+ to C3H5+. The results support humidity-dependent quantification
efforts for PTR-MS and chemical ionization mass spectrometry (CIMS).
Moreover, the results should allow for a better understanding of ion-chemistry
in the presence of water.