Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125, USA.
Chemphyschem. 2001 May 18;2(5):273-93. doi: 10.1002/1439-7641(20010518)2:5<273::AID-CPHC273>3.0.CO;2-H.
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.
在 93 kcal/mol（307nm）激发到 S1（n，π*）态后，通过飞秒泵浦探针质谱法研究了两种丙酮同位素（[D0]-和[D6]丙酮）的离解动力学。我们发现，分子在 S1 表面上的核运动涉及两个时间尺度。初始飞秒运动对应于波包在 S1 表面上的 Franck-Condon 区域之外的退相位。对于更长的时间，乙酰基自由基的直接观察证实，丙酮的 S1α-断裂动力学在纳秒时间尺度上。进行了密度泛函理论和从头计算，以表征丙酮和其他六种相关脂肪酮的 S0、S1 和 T1 态的势能面。对于丙酮，沿着单α位碳-碳（α-CC）键解离坐标（到达 S0/S1 锥形交叉）的 S1 能垒被计算为 18 kcal/mol（（S0 最低能量以上约 110 kcal/mol）对于非协同α-CC 键断裂的第一步；协同路径在能量上是不利的，与实验结果一致。通过在α位甲基取代，发现其他脂肪酮的 S1 势垒高度明显低于丙酮。在 T1 表面上，丙酮的α-CC 键离解能垒仅为 5 kcal/mol（（S0 最低能量以上约 90 kcal/mol），明显低于 S1 表面上的势垒。基于这些计算，可以通过价键理论框架内的简单物理图像更好地理解发生在 S0、S1 和 T1 表面上的丙酮的α-断裂反应机制。理论计算支持这样的结论，即在 S1 势垒以下观察到的丙酮的纳秒级 S1 动力学是由限速 S1→T1 系间交叉过程控制的，随后在 T1 表面上进行α-断裂。然而，在高能下，α-断裂可以通过 S1 表面上的势垒交叉进行，这在随附论文中的环丁酮中得到了证明。
