UV photolysis of α-cyclohexanedione in the gas phase.

Ultraviolet absorption spectrum of alpha-cyclohexanedione (alpha-CHD) vapor in the wavelength range of 220-320 nm has been recorded in a 1 m long path gas cell at room temperature. With the aid of theoretical calculation, the band has been assigned to the S-2 <- S-0 transition of largely pi pi* type. The absorption cross section at the band maximum (similar to 258 nm) is nearly 3 orders of magnitude larger compared to that for the S-2 <- S-0 transition of a linear alpha-diketo prototype, 2,3-pentanedione. The photolysis was performed by exciting the sample vapor near this band maximum, using the 253.7 nm line of a mercury vapor lamp, and the products were analyzed by mass spectrometry as well as by infrared spectroscopy. The identified products are cyclopentanone, carbon monoxide, ketene, ethylene, and 4-pentenal. Geometry optimization at the CIS/6-311++G** level predicts that the carbonyl group is pyramidally distorted in the excited SI and S2 states, but the alpha-CHD ring does not show dissociative character. Potential energy curves with respect to a ring rupture coordinate (C-C bond between two carbonyl groups) for S-0, S-1, S-2, T-1, T-2, and T-3 states have been generated by partially optimizing the ground state geometry at DFT/B3LYP/6-311++G** level and calculating the vertical transition energies to the excited states by TDDFT method. Our analysis reveals that the reactions can take place at higher vibrational levels of So as well as T1 states.