A combined nuclear magnetic resonance and computational study of monohydroxyflavones applied to product ion mass spectra

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Abstract
A method is presented for the estimation of 13 C‐chemical shifts for carbon atoms in protonated and deprotonated molecules; in principle, this method can be applied to ions in general. Experimental 13 C‐chemical shifts were found to vary linearly with computed atomic charges using the PM3 method. Pseudo‐ 13 C‐chemical shifts for atoms in protonated and deprotonated molecules can be estimated from computed atomic charges for such atoms using the above linear relationship. The pseudo‐ 13 C‐chemical shifts obtained were applied to the rationalization of product ion mass spectra of protonated and deprotonated molecules of flavone and 3‐, 5‐, 6‐, 7‐, 2′‐, 3′‐, and 4′‐hydroxyflavones, where product ion formation is due to either cross‐ring cleavage of the C‐ring (retro‐Diels‐Alder reaction) or to cleavage of a C‐ring bond followed by loss of either a small neutral molecule or a radical. The total product ion abundance ratio of C‐ring cross cleavage to C‐ring bond cleavage, γ , varied by a factor of 660 for deprotonated monohydroxyflavones, i.e., from 0.014:1 to 9.27:1. The magnitude of γ , which is dependent on the relative bond orders within the C‐ring of the protonated and deprotonated molecules of monohydroxyflavones, can be rationalized on the basis of the magnitudes of the 13 C‐ and 1 H‐chemical shifts as determined by nuclear magnetic resonance spectroscopy. Copyright © 2007 John Wiley & Sons, Ltd.

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