Scheme 8.
Main candidates for product ion and neutral losses obtained after fragmentation of 9-NO2-LA to form a hexacycle. Ions 11 and 12 were selected from accurate free energy determinations because they displayed the lowest energies during semiempirical product ion calculations. Free energies are shown in Table 1. Ion 11 is the proposed ion products obtained during fragmentation in addition to the neutral fragment 13. Neutral fragment 13 is a methylene group shorter than neutral ion 8 formed during fragmentation rendering the five-member heterocycle