Femtochemistry of Norrish Type-I Reactions: I. Experimental and Theoretical Studies of Acetone and Related Ketones on the S_1 Surface
The dissociation dynamics of two acetone isotopomers ([D_0]- and [D_6]acetone) after 93 kcal mol^(−1) (307 nm) excitation to the S_1(n,π^*) state have been investigated using femtosecond pump–probe mass spectrometry. We found that the nuclear motions of the molecule on the S_1 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 S_1 surface. For longer times, the direct observation of the build-up of the acetyl radical confirms that the S_1α-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 S_0, S_1, and T_1 states of acetone and six other related aliphatic ketones. For acetone, the S_1 energy barrier along the single α-positioned carbon–carbon (α-CC) bond-dissociation coordinate (to reach the S_0/S_1 conical intersection) was calculated to be 18 kcal mol^(−1) (∼110 kcal mol^(−1) above the S_0 minimum) for the first step of the nonconcerted α-CC bond cleavage; the concerted path is energetically unfavorable, consistent with experiments. The S_1 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 T_1 surface was calculated to be only 5 kcal mol^(−1) (∼90 kcal mol^(−1) above the S_0 minimum), which is substantially lower than the barrier on the S_1 surface. Based on the calculations, the α-cleavage reaction mechanism of acetone occurring on the S_0, S_1, and T_1 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 S_1 dynamics of acetone below the barrier is governed by a rate-limiting S_1 → T_1 intersystem crossing process followed by α-cleavage on the T_1 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.