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

Abstract

Pulsed laser photolysis (PLP) at λ=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 [σ(MEK)(λ,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 kJ mol(-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.

Analysis of Potential Energy Surface for Butanone Isomerization

“…There is only one kind ether isomer for C 2v symmetry of acetone, but butanone has another ether isomer (ethyl vinyl ether, 8a or 8b) in which O atom locates in the middle of structure (see Fig.1)). An intermediate, diradical ether (Int5), in which the C1 and C2 are bridged by the oxygen atom, is found lying 70.58 kcal/mol above butanone in Reaction (6). The transition state (TS26, 109.65 kcal/mol) connecting butanone and Int5 is actually a CH 3 CH 2 · · · OCCH 3 complex.…”

Exploring Multiple Potential Energy Surfaces: Photochemistry of Small Carbonyl Compounds

“…The first is the S 0 /T 1 ISC path for H 2 CO as discussed above [49]. The second is an H-atom roundtrip path discovered in photolysis of methyl-ethylketone CH 3 C(O)C 2 H 5 [138]. In this path, on the S 1 PES, an H atom in the ethylgroup at first transfers to the O atom to undergo an S 0 /S 1 IC in a conical intersection of a diradical isomer, and then the H atom comes back to the original position on the S 0 PES, and finally the molecule dissociates into CH 3 CO + C 2 H 5 or C 2 H 5 CO + CH 3 on the S 0 PES.…”

Exploring Norrish type I and type II reactions: an ab initio mechanistic study highlighting singlet-state mediated chemistry

“…Figure 3 shows a Norrish type I reaction scheme for 5-MHONE. We recognize that photoexcitation could promote C1-C2 or C2C3 bond extension but, for simplicity, henceforth focus on the latter extension towards (CH 3 57 and such similarities can be expected to extend to 5-MHONE also. We do recognize however that photoinduced C1-C2 fission, which leads to the formation of CH 3 + CO CH 2 CH 2 CH(CH 3 ) 2 products, may compete with C2-C3 bond fission.…”

“…and has many parallels with the corresponding CIs identified in recent ab initio studies of acetone 59 and methyl ethyl ketone. 57 The bent ground state of the isolated CH 3 CO radical 38,60 is the lower component of a Renner-Teller pair with 2 E symmetry at the C 3v geometry (with a linear C-C=O spine). Thus CI CC is best viewed as the approach to an asymptote where, for linear CH 3 CO, two singlet (the S 0 and S 1 ) and two triplet (the T 1 and T 2 ) PESs of the parent 5-MHONE molecule approach degeneracy.…”