Eric W.-G. Diau

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.

Femtosecond dynamics of valence-bond isomers of azines: transition states and conical intersections

In this Letter, we report the ultrafast dynamics of isomerization and ring opening of azines, using femtosecond-resolved mass spectrometry. The experimental results and theoretical DFT/ab initio calculations elucidate the reaction mechanism and indicate the crucial role of conical intersections in driving these forbidden, ground-state processes. The global motion, initiated by fs wavepackets, is an important concept for nonradiative and reactive processes in polyatomics.

Direct observation of the femtosecond nonradiative dynamics of azulene in a molecular beam: The anomalous behavior in the isolated molecule

Using femtosecond-resolved mass spectrometry in a molecular beam, we report real-time observation of the nonradiative, anomalous dynamics of azulene. We studied both S_2 and S_1 state dynamics. The motion of the wave packet in S_1 involves two time scales, a dephasing time of less than 100 fs and a 900±100 fs internal conversion. We discuss the dynamical picture in relation to the molecular structures and the conical intersection, and we compare with theory.

Femtosecond dynamics of retro Diels–Alder reactions: the concept of concertedness

Using femtosecond-resolved mass spectrometry in a molecular beam, we report real-time studies of the retro Diels–Alder reactions of cyclohexene, norbornene and bicyclo[2,2,2]oct-2-ene. The experimental results and theoretical calculations elucidate the influence of conical intersections on ground-state reaction trajectories. We address the issue of concertedness on the actual time scale of the nuclear motion.

Femtochemistry of Norrish Type‐I Reactions: II. The Anomalous Predissociation Dynamics of Cyclobutanone on the S1 Surface

The anomalous nonradiative dynamics for three cyclobutanone isotopomers ([D0 ]-, 3,3-[D2 ]-, and 2,2,4,4-[D4 ]cyclobutanone) have been investigated using femtosecond (fs) time-resolved mass spectrometry. We have found that the internal motions of the molecules in the S1 state above the dissociation threshold involve two time scales. The fast motion has a time constant of <50 fs, while the slow motion has a time constant of 5.0±1.0, 9.0±1.5, and 6.8±1.0 ps for the [D0 ], [D2 ], and [D4 ] species, respectively. Density functional theory and ab initio calculations have been performed to characterize the potential energy surfaces for the S0 , S1 (n,π*), and T1 (n,π*) states. The dynamic picture for bond breakage is the following: The fast motion represents the rapid dephasing of the initial wave packet out of the Franck-Condon region, whereas the slow motion reflects the α-cleavage dynamics of the Norrish type-I reaction. The redistribution of the internal energy from the initially activated out-of-plane bending modes into the in-plane ring-opening reaction coordinate defines the time scale for intramolecular vibrational energy redistribution (IVR), and the observed picosecond-scale (ps) decay gives the rate of IVR/bond cleavage across the barrier. The observed prominent isotope effect for both [D2 ] and [D4 ] isotopomers imply the significance of the ring-puckering and the CO out-of-plane wagging motions to the S1 α-cleavage dynamics. The ethylene and ketene (C2 products)-as well as CO and cyclopropane (C3 products)-product ratios can be understood by the involvement of an S0 /S1 conical intersection revealed in our calculations. This proposed dynamic picture for the photochemistry of cyclobutanone on the S1 surface can account not only for the abnormally sharp decrease in fluorescence quantum yield and lifetime but also for the dramatic change in the C3 :C2 product ratio as a function of increasing excitation energy, as reported by Lee and co-workers (J. C. Hemminger, E. K. C. Lee, J. Chem. Phys. 1972, 56, 5284-5295; K. Y. Tang, E. K. C. Lee, J. Phys. Chem. 1976, 80, 1833-1836).

Femtosecond dynamics of diradicals: transition states, entropic configurations and stereochemistry

With femtosecond-resolved mass spectrometry, we report real-time studies of the dynamics of reactive diradicals: trimethylene, tetramethylene and structurally-constrained (by a bridge) tetramethylene. These comparative studies elucidate the role of transition states, entropic configurations and IVR on the global potential energy surface. The critical time scale for rotational clocks in stereochemistry is illustrated in the reaction mechanism for cyclization and fragmentation products.

Femtosecond β-cleavage dynamics: Observation of the diradical intermediate in the nonconcerted reactions of cyclic ethers

Femtosecond (fs) dynamics of reactions of cyclic ethers, symmetric and asymmetric structures, are reported. The diradical intermediates and their beta-cleavages, which involve simultaneous C-C, C-H sigma-bond breakage and C-O, C-C pi-bond formation, are observed and studied by fs-resolved mass spectrometry. To compare with experiments, we present density functional theory calculations of the potential energy surface and microcanonical rates and product distributions.

Femtosecond dynamics of hydrogen elimination : benzene formation from cyclohexadiene

Using femtosecond-resolved mass spectrometry in a molecular beam, we report real-time study of the hydrogen elimination reaction of 1,4-cyclohexadiene. The experimental observation of the ultrafast stepwise H-elimination elucidates the reaction dynamics and mechanism. With density-functional theory (ground-state) calculations, the nature of the reaction (multiple) pathways is examined. With the help of recent conical-intersection calculations, the excited-state and ground-state pathways are correlated. From these experimental and theoretical results we provide a unifying picture of the thermochemistry, photochemistry and the stereochemistry observed in the condensed phase.