To Boldly Look Where No One Has Looked Before: Identifying the Primary Photoproducts of Acetylacetone.

We investigate the gas-phase photochemistry of the enolone tautomer of acetylacetone (pentane-2,4-dione) following S(ππ*) ← S excitation at λ = 266 and 248 nm, using three complementary time-resolved spectroscopic methods. Contrary to earlier reports, which claimed to study one-photon excitation of acetylacetone and found OH and CH as the only important gas-phase products, we detect 15 unique primary photoproducts and demonstrate that five of them, including OH and CH, arise solely by multiphoton excitation. We assign the one-photon products to six photochemical channels and show that the most significant pathway is phototautomerization to the diketone form, which is likely an intermediate in several of the other product channels. Furthermore, we measure the equilibrium constant of the tautomerization of the enolone to diketone on S from 320 to 600 K and extract Δ H = 4.1 ± 0.3 kcal·mol and Δ S = 6.8 ± 0.5 cal·mol·K using a van't Hoff analysis. We correct the C-OH bond dissociation energy in acetylacetone, previously determined as 90 kcal·mol by theory and experiment, to a new value of 121.7 kcal·mol. Our experiments and electronic structure calculations provide evidence that some of the product channels, including phototautomerization, occur on S, while others likely occur on excited triplet surfaces. Although the large oscillator strength of the S ← S transition results from the (ππ*) excitation of the C═C-C═O backbone, similar to conjugated polyenes, the participation of triplets in the dissociation pathways of acetylacetone appears to have more in common with ketone photochemistry.