Photodissociation dynamics of propanal and isobutanal: The Norrish Type I pathway

The Norrish Type I photodissociation of two aliphatic aldehydes, propanal and isobutanal, has been investigated using velocity-map imaging. The HCO photoproduct of this reaction was probed using a 1+1 resonance-enhanced multiphoton ionization scheme via the 3p²Π Rydberg state. The velocity map images of HCO⁺ were collected across a range of photolysis energies for both species from 30 500 to 33 000 cm^-1 (λ = 312-327 nm). The corresponding translational energy distributions show that the majority of the available energy goes into the translational motion of the products (55%-68%) with this fraction increasing as the T₁ barrier is approached. Analysis of the translational energy distributions was also used to determine the aldehyde α C-C bond dissociation energies which were found to be 339.8 ± 2.5 and 331.2 ± 2.5 kJ/mol for propanal and isobutanal, respectively. These values were also found to be in good agreement with the computed dissociation energies using G4 and CCSD(T)/aug-cc-pVTZ//M062X/aug-cc-pVTZ levels of theory. Furthermore, these dissociation energies, combined with the known Δ_fH (0 K) of the reaction products, provided the Δ_fH (0 K) of propanal and isobutanal which were calculated to be -167.3 ± 2.5 and -184.0 ± 2.5 kJ/mol, respectively.