Near Ultraviolet Absorption Spectra of Acrolein and Crotonaldehyde

The positions vibrational structures and blueshifts of absorption spectra of acrolein and crotonaldehyde in 33300∼25000 cm−1 region suggest that the transition is of the n-π* type. They are similar in vibrational structure except that the crotonaldehyde absorption is shifted towards higher frequencies with respect to that of acrolein. The vapor frequency of the 0–0 band is 25873 cm−1 for acrolein, smaller by 654 cm−1 than that of crotonaldehyde. The acrolein absorption has 1427, 1271 and 498 cm−1 progressions, which are assigned to the C=C stretching frequency, the C=O stretching frequency and skeletal bending frequency in the excited state, respectively. , For the crotonaldehyde there are 1360, 1237 and 450 cm−1 progressions, which are assigned to the C=C stretching frequency, the C=O stretching and the skeletal bending vibration in the excited state, respectively According to our calculation, the elongation q in the carbonyl bond length during the n-π* transition, is estimated to be approximately 0.1 Å.