Photochemistry of Methyl Ethyl Ketone: Quantum Yields and S1/S0-Diradical Mechanism of Photodissociation

Pulsed laser photolysis (PLP) at lambda = 248 and 308 nm coupled with gas-chromatographic analysis is applied to determine the photodissociation quantum yield (QY) of methyl ethyl ketone (MEK). Temperature dependent UV absorption cross-sections [sigma MEK(lambda,T)] are also determined. At 308 nm, the QY decreases with decreasing temperature (T = 323-233 K) and with increasing pressure (P = 67-998 mbar synthetic air). Stern-Volmer (SV) analysis of the T and P dependent QYs provides the experimental estimate of E-S1 = 398 + 9 kJmol(-1) (= 300 + 6 nm) for the barrier of the first excited singlet state (S-1). The QY at 248 nm is close to unity and independent of pressure (T = 298 K). Theoretical reaction pathways are examined systematically on the basis of CASPT2/6-31 + G* calculations. Among three possible pathways, a S-1/S-0-diradical mechanism, which involves H atom transfer on the S-1 surface, followed by a nonadiabatic transition at a diradical isomer of MEK, explains the experimental data very well. Therefore, this unusual mechanism, which is not seen in any smaller carbonyl compounds, is proposed as an important pathway for the MEK dissociation. Our study supports the view that both the absorption cross-sections and the QYs of carbonyls have significant temperature dependences that should be taken into account for accurate modelling of atmospheric chemistry.