This paper reports on an experimental study of the C=O stretching vibration frequency in a large number of quinonoid compounds, both substituted and unsubstituted. For nonsubstituted ortho and para quinones, relationships between the carbonyl frequency and (a) the number of fused rings, (b) the oxidation‐reduction potential, and (c) the index of free valence on the parent hydrocarbon are pointed out and discussed. It is shown that for a six‐atom quinonoid ring the C=O frequency is, to a first approximation, a function of the C=O bond order. For substituted quinones, the variation in the carbonyl frequency is related to the induction effect of the substituent in the same way as is the variation in oxidation‐reduction potential. Certain hydroxy‐substituted quinones show spectral anomalies which do not seem to fit into the usual category of hydrogen bonded compounds.

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The oxidation‐reduction potential under “normal” conditions is proportional to the drop in free energy in going from the quinone plus hydrogen form to the hydroquinone form and can be thought of as a measure of the oxidizing power of the quinone (see reference 13).
7.
This index, computed for each carbon atom, is the concentration of unpaired electrons in the neighborhood of each atom and is a measure of the reactivity of the hydrocarbon at each atom (see reference 8).
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15.
Bond order as used here is defined to be the number of pairs of pi electrons which are localized between two atoms to form a particular bond (see reference 8).
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(a) (Note added in proof).—We have recently run a sample of thioxanthone in CCl4 solution and have found the carbonyl frequency at 1649 cm−1. The explanation for this lower value when sulfur replaces oxygen in xanthone is still under discussion.
(b) In the previous report, reference 1, p. 390, Fig. 2, a typographical error was made in numbering the carbons in fluorenone, the intended numbering being the standard one.
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The curves for the other two groups of compounds were not based on enough points for their slopes to be significant.
23.
The six‐micron region absorption of dihydroxynaphthacenequinone (1792 cm−1) previously reported and assigned to the carbonyl vibration (see footnote 1) is anomalous. This frequency would be expected to lie far below that of naphthacenequinone (XV) (1682 cm−1) (see Table IV). This compound and its band interpretation are thus in question and are being restudied.
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