"Fragmentation of allylmethylsulfide by chemical ionization: dependence on humidity and inhibiting role of water.",
J Phys Chem A
, vol. 117, pp. 5149–5160, Jun, 2013.
<p>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.</p>
[Schuhfried2013] "Sulfides: chemical ionization induced fragmentation studied with proton transfer reaction-mass spectrometry and density functional calculations.",
J Mass Spectrom
, vol. 48, no. 3: Institut für Ionenphysik und Angewandte Physik, Leopold Franzens Universität Innsbruck, Technikerstr. 25, A-6020, Innsbruck, Austria., pp. 367–378, Mar, 2013.
We report the energy-dependent fragmentation patterns upon protonation of eight sulfides (organosulfur compounds) in Proton Transfer Reaction-Mass Spectrometry (PTR-MS). Studies were carried out, both, experimentally with PTR-MS, and with theoretical quantum-chemical methods. Charge retention usually occurred at the sulfur-containing fragment for short chain sulfides. An exception to this is found in the unsaturated monosulfide allylmethyl sulfide (AMS), which preferentially fragmented to a carbo-cation at m/z 41, C3H5(+). Quantum chemical calculations (DFT with the M062X functional 6-31G(d,p) basis sets) for the fragmentation reaction pathways of AMS indicated that the most stable protonated AMS cation at m/z 89 is a protonated (cyclic) thiirane, and that the fragmentation reaction pathways of AMS in the drift tube are kinetically controlled. The protonated parent ion MH(+) is the predominant product in PTR-MS, except for diethyl disulfide at high collisional energies. The saturated monosulfides R-S-R' (with R<R') have little or no fragmentation, at the same time the most abundant fragment ion is the smaller R-S(+) fragment. The saturated disulfides R-S-S-R display more fragmentation than the saturated monosulfides, the most common fragments are disulfide containing fragments or long-chain carbo-cations. The results rationalize fragmentation data for saturated monosulfides and disulfides and represent a detailed analysis of the fragmentation of an unsaturated sulfide. Apart from the theoretical interest, the results are in support of the quantitative analysis of sulfides with PTR-MS, all the more so as PTR-MS is one of a few techniques that allow for ultra-low quantitative analysis of sulfides.