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The reactivity of two metallated nitrenium ions toward various substrates was examined in the gas phase. The nitrenium ions were generated by a reaction of benzoyl azide with laser-ablated Mg + or Cu + in a Fourier transform ion cyclotron resonance mass spectrometer. The two nitrenium ions show drastically different reactivity. While the Mg-nitrenium ion reacts by radical mechanisms ( e.g ., H atom abstraction), the Cu-nitrenium ion follows non-radical pathways ( e.g. , metal ion transfer).

In order to improve the understanding of the interactions of aromatic sigma,sigma-biradicals with DNA, the reactivity of three isomeric sigma,sigma-biradicals toward four dinucleoside phosphates was studied in a mass spectrometer. The dinucleoside phosphates were evaporated into the mass spectrometer by using laser-induced acoustic desorption (LIAD). The results demonstrate that the structure of the sigma,sigma-biradical and the base sequence of the dinucleoside phosphate can have a major influence on these reactions.

A combined computational and experimental study on the gas-phase structures and reactivities of charged 1,3-didehydroarenes (meta-benzynes) demonstrates that the reactivity of such biradicals can be “tuned” by using appropriate substituents. Substituents that destabilize a specific zwitterionic resonance structure can change the reactivity of the biradical from mildly carbocationic to radical-like. These substituent effects are not the result of changes in the singlet−triplet gaps of the biradicals, but rather reflect changes in the potential energy surfaces for the dehydrocarbon separation.

Gas-phase reactivity of a positively charged aromatic σ,σ-biradical (N-methyl-6,8-didehydroquinolinium) was examined toward six aliphatic amino acids and 15 dipeptides by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR) and laser-induced acoustic desorption (LIAD). While previous studies have revealed that H-atom and NH(2) abstractions dominate the reactions of related monoradicals with aliphatic amino acids and small peptides, several additional, unprecedented reaction pathways were observed for the reactions of the biradical. For amino acids, these are 2H-atom abstraction, H(2)O abstraction, addition - CO(2), addition - HCOOH, and formation of a stable adduct. The biradical reacts with aliphatic dipeptides similarly as with aliphatic amino acids, but undergoes also one additional reaction pathway, addition/C-terminal amino acid elimination (addition - CO - NHCHR(C)). These reactions are initiated by H-atom abstraction by the biradical from the amino acid or peptide, or nucleophilic addition of an NH(2) or a HO group of the amino acid or peptide at the radical site at C-6 in the biradical. Reactions of the unquenched C-8 radical site then yield the products not observed for related monoradicals. The biradical reacts with aromatic dipeptides with an aromatic ring in N-terminus (i.e., Tyr-Leu, Phe-Val, and Phe-Pro) similarly as with aliphatic dipeptides. However, for those aromatic dipeptides that contain an aromatic ring in the C-terminus (i.e., Leu-Tyr and Ala-Phe), one additional pathway, addition/N-terminal amino acid elimination (addition - CO - NHCHR(N)), was observed. This reaction is likely initiated by radical addition of the biradical at the aromatic ring in the C-terminus. Related monoradicals add to aromatic amino acids and small peptides, which is followed by Cα-Cβ bond cleavage, resulting in side-chain abstraction by the radical. For biradicals, with one unquenched radical site after the initial addition, the reaction ultimately results in the loss of the N-terminal amino acid. Similar to monoradicals, the C-S bond in amino acids and dipeptides was found to be especially susceptible to biradical attack.