M. Carvalho, F. Gozzo, M. Mendes
Jul 10, 1998
Citations
0
Influential Citations
23
Citations
Journal
Chemistry: A European Journal
Abstract
Low-energy collision-induc- ed dissociation (CID) and ion - mole- cule reactions with 2-methyl-1,3-dioxo- lane (MD) performed by pentaquadru- pole (QqQqQ) mass spectrometry were applied to locate the charge site in isomeric heteroaromatic cations. The 2-, 3-, and 4-pyridyl cations are indistin- guishable by CID. However, as suggest- ed by MS 3 experiments and ab initio calculations, the 2-pyridyl cation reacts extensively with MD by a transacetali- zation-like mechanism to afford a bicy- clic dihydrooxazolopyridyl cation. The 3- and 4-pyridyl cations, on the contrary, react predominantly with MD by proton transfer, as does the analogous phenyl cation. The 2-, 4-, and 5-pyrimidyl cati- ons display characteristic CID behavior. In addition, the 2-pyrimidyl cation re- acts extensively with MD by the trans- acetalization-like mechanism, whereas proton transfer occurs predominantly for the 4- and 5-pyrimidyl cations. The ions thought to be the 2- and 3-furanyl and 2- and 3-thiophenyl cations show in- distinguishable CID and ion - molecule behavior. This is most likely the result of their inherent instability in the gas phase and their spontaneous isomerization to the corresponding butynoyl and butyne- thioyl cations HCCHCH2CO a and HCCHCH2CS a . These isomeriza- tions, which are considerably exother- mic according to G2(MP2) ab initio calculations, are indicated by a series of experimental results. The ions dissociate upon CID by loss of CO or CS and undergo transacetalization with MD. Most informative is the participation of HCCHCH2CS a in a transacetaliza- tion/dissociation sequence with replace- ment of sulfur by oxygen, which is structurally diagnostic for thioacylium ions. It is therefore possible to locate the charge site of the 2-pyridyl and the three 2-, 4-, and 5-pyrimidyl cations and to identify the isomeric precursors from which they are derived. However, rapid isomerization to the common HCCH- CH2-CO(S) a ion eliminates character- istic chemical behavior that could result from different charge locations in the heteroaromatic 2- and 3-furanyl and 2- and 3-thiophenyl cations.