Biacetyl, absorption spectrum

Electronic Spectrum. The A X and B - X absorption spectrum of biacetyl is similar to that of glyoxal. The first absorption band occurs in the 470- to 340-nm region. The 0-0 band is placed at 21,983 cm l (111), and there is some vibrational structure at wavelengths greater than 400 nm. The second absorption band occurs in the 280- to 220-nm region and consists of continuous absorption only. The emission occurs only when the A state is excited. 

Solutes that Produce no New Intermediates. Methanol, ethyl alcohol, cyclohexene, cyclohexane, n-hexane, 3-methylpentane, and biacetyl also remove the absorption spectrum attributed to (CCV), but no additional new spectra are observed from 3500 to 6000 A. Millimolar concentrations of these solutes remove the long-lived portion of (CC14+) while 0.1 M of all solutes apart from cyclohexane completely removes the short and long-lived (CC14+) cyclohexane O.lAf increases the decay rate of the positive ion. Adding 10 mAf methanol and 20 mAf n-hexane decreases the ti/2... 

Figure 14. Absorption spectrum (top) and fluorescence excitation spectrum (middle) of isoquinoline vapor at room temperature, and the excitation spectrum of biacetyl phosphorescence (bottom), sensitized by energy transfer from the triplet isoquinoline. The band positions (in cmT1) are relative to the starred 0+ band at 31,925cm-1. (From ref. [45] with permission.)...

Solvent shifts are useful as a criterion to identify n,7t transitions in absorption spectra because hydrogen bonding of protic solvents with the carbonyl oxygen stabilizes the np lone pair and gives rise to a hypsochromic shift of the n,7t absorption bands see the positions of the n,7t absorption band of acetone in heptane and water (Figure 6.5, top). This contrasts with 7t,7t transitions that tend to be shifted bathochromically in polar solvents. Also, the photophysical and photochemical properties often serve to identify the nature of the lowest excited state. Lone-pair interaction in biacetyl splits the two np-orbitals giving rise to two n,7t transitions at v = 2.23 and 3.54 im In the spectrum of 1,4-naphthoquinone in methanol, the n,7t band is barely detectable as a shoulder on the red edge of the 71,71 absorption. 

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