Femtochemistry of norrish type-I reactions: I. Experimental and theoretical studies of acetone and related ketones on the S1 surface.

The dissociation dynamics of two acetone isotopomers ([D0 ]- and [D6 ]acetone) after 93 kcal mol(-1) (307 nm) excitation to the S1 (n,π*) state have been investigated using femtosecond pump-probe mass spectrometry. We found that the nuclear motions of the molecule on the S1 surface involve two time scales. The initial femtosecond motion corresponds to the dephasing of the wave packet out of the Franck-Condon region on the S1 surface. For longer times, the direct observation of the build-up of the acetyl radical confirms that the S1 α-cleavage dynamics of acetone is on the nanosecond time scale. Density functional theory and ab initio calculations have been carried out to characterize the potential energy surfaces for the S0 , S1 , and T1 states of acetone and six other related aliphatic ketones. For acetone, the S1 energy barrier along the single α-positioned carbon-carbon (α-CC) bond-dissociation coordinate (to reach the S0 /S1 conical intersection) was calculated to be 18 kcal mol(-1) (∼110 kcal mol(-1) above the S0 minimum) for the first step of the nonconcerted α-CC bond cleavage; the concerted path is energetically unfavorable, consistent with experiments. The S1 barrier heights for other aliphatic ketones were found to be substantially lower than that of acetone by methyl substitutions at the α-position. The α-CC bond dissociation energy barrier of acetone on the T1 surface was calculated to be only 5 kcal mol(-1) (∼90 kcal mol(-1) above the S0 minimum), which is substantially lower than the barrier on the S1 surface. Based on the calculations, the α-cleavage reaction mechanism of acetone occurring on the S0 , S1 , and T1 surfaces can be better understood via a simple physical picture within the framework of valence-bond theory. The theoretical calculations support the conclusion that the observed nanosecond-scale S1 dynamics of acetone below the barrier is governed by a rate-limiting S1 →T1 intersystem crossing process followed by α-cleavage on the T1 surface. However, at high energies, the α-cleavage can proceed by barrier crossing on the S1 surface, a situation which is demonstrated for cyclobutanone in the accompanying paper.