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Apart from pyridine N-oxide ( 1a ), the C 5 H 5 NO family of stable molecules comprises, 2-, 3- and 4-hydroxypyridine ( 2a , 3a and 4a ) as well as their keto counterparts 2-, 3- and 4(1 H )-pyridone ( 2b , 3b and 4b ). This study focuses on the characterisation of their radical cations and a number of stable H-shift isomers, which are α- or β-distonic ions. This was done by using a combination of mass spectrometric experiments and computational chemistry, at the B3LYP/CBSB7 level of theory. The ionic species were identified on the basis of both their collision-induced dissociation (CID) characteristics and specific associative ion–molecule reactions with dimethyl disulfide and tert -butyl isocyanide as substrates. The distonic ions ( 1b + , 2c + , 2d + and 3c + ) were obtained by dissociative electron impact ionisation and subjected to neutralisation–reionisation mass spectrometry (NRMS). From CID spectra of the intense survivor ions, it follows that the neutral counterparts of the α-distonic ions 2c + and 3c + are viable chemical species in the rarefied gas phase. The energy-rich ylide type neutrals 1b , on the other hand, readily isomerise into pyridine N-oxide, 1a , or else dissociate. The neutral counterpart of the β-distonic ion 2d + only has a marginal stability and part of these neutrals are proposed to isomerise into energy-rich 2-pyridone molecules 2b . This is in agreement with the computational results. However, ionised 2-pyridone cannot readily be differentiated from its enol isomer 2-hydroxypyridine. In contrast, the keto isomers of ionised 3- and 4-hydroxypyridine display characteristically different CID spectra.

The potential energy surface comprising ionized pyrimidine, 1 ·+ , and eight of its hydrogen-shift isomers, as well as that of the corresponding neutrals was explored at a level of theory (B3LYP/TZVP) that has proven adequate for related species. The computations predicted that among the isomers there are four C 4 H 4 N 2 ·+ distonic radical cations, 2 ·+ – 5 ·+ , of comparable stability to 1 ·+ , and transition state calculations indicated that high barriers separate these stable ions. Thus, the ions 2 ·+ – 5 ·+ should also be viable chemical species, and indeed mass spectrometry based experiments lead to the generation and characterization of three of the four, that is 2 ·+ , 3 ·+ , and 4 ·+ , as stable ions in the gas phase. Ions 2 ·+ – 4 ·+ were identified on the basis of their collision-induced dissociation characteristics in the mass spectrometer. The ions 2 ·+ and 3 ·+ obtained by dissociative electron impact ionization were subjected to neutralization–reionization mass spectrometry (NRMS). From collision-induced dissociation spectra of the intense NRMS survivor ions, it follows that the neutral ylide/carbene counterparts, i.e. pyrimidine-4-ylidene, 2 , and pyrimidine-2-ylidene, 3 , have lifetimes of at least microseconds in the rarefied gas phase. The interpretation of the experimental observations that 2 and 3 are viable chemical species in gaseous environs was supported computationally. According to the calculations the neutral isomers 2 – 5 each represent a minimum separated by high hydrogen-shift barriers, although situated some 50 kcal/mol higher in energy than 1 , pyrimidine itself. However, molecules 4 and 5 remained elusive since ions 4 · + , only obtainable by a collision-induced dissociation process, were not amenable to NR experiments and a viable strategy to produce a beam of pure ions 5 ·+ could not be realized.

The decarbonylation reactions of ionized 2-acetylpyridine, 2-acetylpyrazine, and 2-acetylthiazole have been investigated using the multiple collision technique of neutralization–reionization/collision-induced dissociation mass spectrometry and related techniques. The resultant heterocyclic C6H7N·+, C5H6N2·+, and C4H5NS·+ ions were identified as 2-methylene-1,2-dihydropyridine, 6·+, 2-methylene-1,2-dihydropyrazine, 9·+, and 2-methylene-2,3-dihydrothiazole, 12·+, respectively. This result refutes proposals in the older literature that the decarbonylation would involve a methyl transfer yielding the 2-methyl species 2-methylpyridine (2·+), 2-methylpyrazine (8·+), and 2-methylthiazole (11·+). Literature proposals for a methyl transfer in the dissociation of ionized dimethyl-2,3-pyridinedicarboxylate and methyl-4-pyridinecarboxylate were also examined, but could not be substantiated either. However, the proposed gas phase synthesis of the N-methylpyridinium ylide, 1·+, from ionized methyl-2-pyridinethiocarboxylate did provide evidence for a genuine methyl migration. To reinforce these conclusions, the dissociation characteristics of isomers structurally related to 6·+ (3·+– 5·+ and 7·+), to 9·+ (10·+) and to 12·+ (13·+– 15·+) were also considered. Exploratory quantum chemical calculations (at the B3LYP/6-31G∗ level of theory) indicate that 6·+ and 12·+ are among the most stable isomers in the C6H7N·+ and C4H5NS·+ systems, lying 24 and 19 kcal/mol lower in energy than 2·+ and 11·+, respectively. Their neutral counterparts, however, are considerably less stable: 6 is calculated to be 27 kcal/mol higher in energy than 2. Nevertheless, from neutralization–reionization experiments it follows that the neutral counterparts of the ionized decarbonylation products 6, 9, and 12 are stable molecules on the microsecond time scale. Significant 1,3-hydrogen shift barriers hinder the interconversion of both the ions and the neutrals into their 2-methyl substituted counterparts, thus accounting for their stability in the dilute gas phase.

The [C 4 ,H 4 ,N 2 ] potential energy hypersurface relating to pyrazine and its hydrogen shift isomers has been investigated computationally using hybrid Hartree–Fock/density functional theory and through a variety of tandem mass spectrometry experiments (metastable ion, collision-induced dissociation, and neutralization reionization mass spectrometry). In addition to the conventional pyrazine structure 1 , its α-ylide 2 , β-ylide 3 , and the 1,4-diradical 4 were generated and characterized through neutralization reionization mass spectrometry experiments. Also, the corresponding radical cations 1 ·+ – 4 ·+ were accessible by dissociative electron ionization of the appropriate pyrazine esters. Quantum chemical calculations at the B3LYP/TZVP level of theory reveal that all these species correspond to minima that are separated by significant barriers thus preventing facile isomerization. As an additional, albeit high lying isomer, the 1,3-diradical 5 was computationally identified. In the case of the radical cations the energy differences between the various isomers are much smaller than for the corresponding neutrals; however, pyrazine represents in both cases the most stable species.