Ultraviolet Absorption of Tetramethyl-1,3-Cyclobutanedione

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Abstract
The near‐ultraviolet spectrum of the cyclic 1,3‐dione, tetramethyl‐1,3‐cyclobutanedione, has maxima near both 3400 and 3000 Å. On the basis of absorption intensity (low), solvent shift (transition energy increases with Z), and position (with respect to cyclobutanone), the two transitions can be explained as n→π1*+π2* (lower transition energy) and n→π1*—π2* (higher transition energy). The assignment is similar to that used by Sidman and McClure for the 1,2‐dione, diacetyl. It is pointed out that s‐cis‐1,2‐diones should have low‐energy transitions at longer wavelengths than s‐trans‐1,2‐diones, and further, that a cyclic peroxy structure isomeric with the s‐cis‐1,2‐dione might possess sufficient stability for isolation. Evidence from early German literature is cited to show that 1,2‐benzoquinone does indeed exist in two isomeric forms; one, a red quinonoid form and a second, a colorless unstable form.
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