Ion-molecule reactions of the mass-selected distonic radical cation
+CH
2-O-CH
2 · (
1) with several heterocyclic compounds have been investigated by multiple stage mass spectro- metric experiments performed in a pentaquadrupole mass spectrometer. Reactions with pyridine, 2-, 3-, and 4-ethyl, 2-methoxy, and 2-
n-propyl pyridine occur mainly by transfer of CH
2 +· to the nitrogen, which yields distonic
N-methylene-pyridinium radical cations. The MS
3 spectra of these products display very characteristic collision-induced dissociation chemistry, which is greatly affected by the position of the substituent in the pyridine ring. Ortho isomers undergo a δ-cleavage cyclization process induced by the free-radical character of the
N-methylene group that yields bicyclic pyridinium cations. On the other hand, extensive CH
2 +· transfer followed by rapid hydrogen atom loss, that is, a net CH
+ transfer, occurs not to the heteroatoms, but to the aromatic ring of furan, thiophene, pyrrole, and
N-methyl pyrrole. The reaction proceeds through five- to six-membered ring expansion, which yields the pyrilium, thiapyrilium,
N-protonated, and
N-methylated pyridine cations, respectively, as indicated by MS
3 scans. Ion 1 fails to transfer CH
2 +· to tetrahydrofuran, whereas a new α-distonic sulfur ion is formed in reactions with tetrahydrothiophene. Unstable
N-methylene distonic ions, likely formed by transfer of CH
2 +· to the nitrogen of piperidine and pyrrolidine, undergo rapid fragmentation by loss of the α-NH hydrogen to yield closed-shell immonium cations. The most thermodynamically favorable products are formed in these reactions, as estimated by ab initio calculations at the MP2/6-31G(
d,p)//6-31G(
d,p) + ZPE level of theory.
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