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LETTERS TO THE EDITOR effects. Our results are in fair agreement with those obtained by electron diffraction; the 0-0 bond distance provides the most reliable comparison between the two methods and agrees within the experimental errors.This work was suggested by Professor E. B. Wilson, Jr., and the major portion of it was done in his laboratory.

Tris (dibenzoylmethanato) iron (III) and tris (dibenzoylmethanato) chromium (III) show exceptionally high reactivity as quenchers in the photoreduction of benzophenone by benzhydrol. The dependence of the effect on the concentrations of the quenchers suggests that, in addition to benzophenone triplets, excited singlet states of the ketone are deactivated by the chelates. Study of the effects of the two chelates on the benzophenonesensitized isomerization confirms the singlet quenching. Singlet quenching occurs at lower concentrations than would have been predicted by approximate theories.