Fragmentation of 3‐hydroxyflavone; a computational and mass spectrometric study

In a recent study of the collision‐induced dissociation of protonated and deprotonated molecules of 3‐, 5‐, 6‐, 7‐, 2′‐, 3′‐ and 4′‐hydroxyflavone, it was observed that the ratio, γ , of the propensities for cross‐ring cleavage (CRC) to ring opening (RO) varied by a factor of 660, i.e., from 0.014:1 (for deprotonated 3‐hydroxyflavone) to 9.27:1 (for deprotonated 5‐hydroxyflavone). An explanation for the variation of γ was presented in terms of experimental NMR 13 C and 1 H spectra modified by computation. Deprotonated 3‐hydroxyflavone exhibited the highest ion signal intensity for fragmentation following RO relative to that for CRC in that γ  = 0.014:1. Because no chemical computations of the fragmentation of protonated and deprotonated monohydroxyflavone molecules have been carried out thus far, the basis on which fragmentation mechanisms for deprotonated 3‐hydroxyflavone have been proposed is principally chemical intuition. The energy states of product ions formed in the fragmentation of deprotonated 3‐hydroxyflavone molecules were computed by the CBS‐4M method implemented in Gaussian 03. The overly demanding calculations needed to handle diradicals reliably and directly were circumvented by a process in which each of the bond‐breaking reactions was approximated by a process that gave two monoradicals. Bond energies were calculated, with one exception, from the approximation reactions as the energy of products minus the energy of reactants. Copyright © 2007 John Wiley & Sons, Ltd.