On the dissociation of the 2-pentanone ion studied by threshold photoelectron photoion coincidence spectroscopy

Abstract

The photodissociation of 2-pentanone has been studied by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy, in which ion time-of-flight (TOF) mass spectra are recorded as a function of the photon energy in the range of 9.6–12.2 eV. 2-Pentanone ions dissociate via four competitive channels: ethylene (C 2H 4) loss to produce the propen-2-ol ion, n-propyl ( C 3H 7) loss to produce the acetyl ion, and two parallel methyl ( CH 3) loss channels, producing the butanoyl ion at low energies and the but-3-en-2-ol ion at higher energies. The latter dissociates further via ethylene (C 2H 4) loss providing a second pathway to the acetyl ion. This final dissociation channel is observed experimentally by the appearance of an asymmetric ion peak in the time-of-flight (TOF) distribution at photon energies greater than 11.5 eV. The ion TOF distributions and breakdown diagram have been modeled in terms of the statistical RRKM theory for unimolecular reactions, yielding the 0 K dissociation onsets of 10.239 ± 0.015 eV for the butanoyl ion and 10.259 ± 0.019 eV for the propen-2-ol (acetone enolate) ion. By relating the measured onsets with other well established heats of formation, the 298 K heat of formation of the butanoyl and propen-2-ol ions were determined to be 586.9 ± 2.1 and 680.7 ± 1.8 kJ/mol, respectively. The acetone enolate ion is thus 37 kJ/mol more stable than the acetone ion, a value supported by G3B3, and CBS-QB3 calculations. The but-3-en-2-ol ion was found to lose ethylene to produce the acetyl ion without an energy barrier.