Fragmentation pathways arising from protonation at different sites in aminoalkyl-substituted 3-hydroxy-1,2,5-oxadiazoles (3-hydroxyfurazans)

RationaleThe gas-phase fragmentation chemistry of multifunctional cations is highly influenced by the site of protonation. Possible relationships between protonation site and fragmentation processes were studied using 4-aminoalkyl-3-hydroxyfurazans. For these heterocyclic amines, the starting points for competing fragmentation pathways varied with protonation at multiple sites in two tautomers. MethodsMass spectra were acquired using electrospray ionization (positive mode) coupled to triple quadrupole and ion trap mass spectrometers; precursor-product ion relationships were studied by collision-induced dissociation. Quantum mechanical computations were performed at the MP2/6-311++G(2d,p)//B97X-D/6-311+G(d) level of theory. ResultsProminent successive losses of NO and CO and competing losses of CH2=NH or NH3 were observed as fragmentation processes. The lowest barrier computed for the initial step in a fragmentation pathway was associated with the [M+H](+) ion protonated at N5 in the heterocyclic ring, whereas an alternative ring cleavage leading to complementary product ions was initiated by protonation of the ring at N2. Side-chain protonation led to loss of NH3 without cleavage of the 3-hydroxyfurazan ring. ConclusionsThe product ions obtained by the competing fragmentation processes varied with the site of protonation. Interestingly, the most abundant product ions observed at low collision energies were formed by cleavage of protonated molecules possessing more internal energy than other isomers.