Low-energy collision-induced dissociation (CID) and ion-molecule react
ions with 2-methyl-1,3-dioxolane (MD) performed by pentaquadrupole (Qq
QqQ) mass spectrometry were applied to locate the charge site in isome
ric heteroaromatic cations. The 2-, 3-, and 4-pyridyl cations are indi
stinguishable by CID. However, as suggest ed by MS3 experiments and ab
initio calculations, the 2-pyridyl cation reacts extensively with MD
by a transacetalization-like mechanism to afford a bicyclic dihydrooxa
zolopyridyl cation. The 3- and 4-pyridyl cations, on the contrary reac
t predominantly with MD by proton transfer, as does the analogous phen
yl cation. The 2-. 4-, and 5-pyrimidyl cations display characteristic
CID behavior. In addition, the 2-pyrimidyl cation reacts extensively w
ith MD by the transacetalization-like mechanism, whereas proton transf
er occurs predominantly for the 4- and 5-pyrimidyl cations. The ions t
hought to be the 2- and 3-furanyl and 2- and 3-thiophenyl cations show
indistinguishable CID and ion-molecule behavior. This is most likely
the result of their inherent instability in the gas phase and their sp
ontaneous isomerization to the corresponding butynoyl and butynethioyl
cations HC=CHCH2C=O+ and HC=CHCH2C=S+. These isomerizations, which ar
e considerably exothermic according to G2(MP2) ab initio calculations,
are indicated by a series of experimental results. The ions dissociat
e upon CID by loss of CO or CS and undergo transacetalization with MD.
Most informative is the participation of HC=CHCH2C=S+ in a transaceta
lization/dissociation sequence with replacement of sulfur by oxygen, w
hich 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 fr
om which they are derived. However, rapid isomerization to the common
HC=CHCH2-C=O(S)(+) ion eliminates characteristic chemical behavior tha
t could result from different charge locations in the heteroaromatic 2
- and 3-furanyl and 2- and 3-thiophenyl cations.