Photodissociation of protonated β-diketones in the gas phase

The gas phase photodissociation spectra of four protonated β-diketones were obtained and compared with the absorption spectra of the corresponding ions in solution. Protonated 2,4-pentanedione was observed to undergo the photodissociation process C₅H₉O₂]⁺ +hν → CH₃CO]⁺ +C₃H₆O with a λmax at 276±10 nm compared with a solution absorption maximum at 286 nm. Protonated 2,4-hexanedione was observed to undergo the photodissociation processes C₆H₁₁O₂]⁺ +hν → CH₃CO]⁺ +C₄H₈O and C₆H₁₁O₂]⁺ +hν → C₂H₅CO]⁺ +C₃H₆O with a λmax at 279±10 nm compared with a solution absorption maximum at 288 nm. Protonated 3-methyl-2,4-pentanedione was observed to undergo the photodissociation process C₆H₁₁O₂]⁺ +hν → CH₃CO]⁺ +C₄H₈O with a λmax at 295±10 nm compared with a solution absorption maximum at 305 nm. Protonated 1,1,1-trifluoro-2,4-pentanedione was observed to undergo the photodissociation process C₅H₆F₃O₂]⁺ +hν → CF₃H+C₄H₅O₂]⁺ with a λmax at 273±10 nm compared with a solution absorption maximum at 288 nm. The CH₃CO]⁺ and C₂H₅CO]⁺ produced photochemically with the first three ions react to regenerate the protonated β-diketone leading to a photostationary state. Photodissociation of the protonated alkyl β-diketones is believed to occur from the protonated keto form, whereas photodissociation of protonated 1,1,1-trifluoro-2,4-pentanedione is believed to occur from the protonated enol form. Mechanisms for the observed photodissociation processes are proposed and comparisons with results from related techniques are presented.