Femtosecond Transition State Dynamics

The pump-and-probe technique has proved to be very well suited for studying shortlived transient states of molecular systems that had been excited by a short laser pulse before they dissociate  

An illustrative example is the photodissociation of excited Nal molecules, which has been studied in detail by Zewail et al. [819]. 

The adiabatic potential diagram of Nal (Fig. 6.99) is characterized by an avoided crossing between the repulsive potential of the two interacting neutral atoms Na -I-1 and the Coulomb potential of the ions Na + which is mainly responsible for the strong binding of Nal at small intemuclear distances R. If Nal is excited into the repulsive state by a short laser pulse at the wavelength A. i, the excited molecules start to move toward larger values of R with a velocity v R) = [(2/ix)(E - E R)V.  

When the excited system [Nal] reaches the avoided crossing zt R = Rc it may either stay on the potential Vi(/ ) and oscillate back and forth between Ri and R2, or it may tunnel to the potential curve Vo(R), where it separates into Na +1. 

The time behavior of the system can be probed by a probe pulse with the wavelength A-2 tuned to the transition from Vi R) into the excited state V2(R) that dissociates into Na +1. At the fixed wavelength X2 = 2Ttcla 2 the dissociating system 

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Zewail A H 1991 Femtosecond transition-state dynamics Faraday Discuss. Chem. Soc. 91 207-37... 

Zewail, A. H. Femtosecond transition-state dynamics, Faraday Disc.Chem.Soc., 91 (1991) 207-237. 

Lee S-Y 1995 Wave-packet model of dynamic dispersed and integrated pump-probe signals in femtosecond transition state spectroscopy Femtosecond Chemistry ed J Manz and L Wdste (Heidelberg VCH)... 

A. H. Zewail If we solve for the molecular Hamiltonian, we will be theorists I do, of course, understand the point by Prof. Quack and the answer comes from the nature of the system and the experimental approach. For example, in elementary systems studied by femtosecond transition-state spectroscopy one can actually clock the motion and deduce the potentials. In complex systems we utilize a variety of template-state detection to examine the dynamics, and, like every other approach, you/we use a variety of input to reach the final answer. Solving the structure of a protein by X-ray diffraction may appear impossible, but by using a number of variant diffractions, such as the heavy atom, one obtains the final answer. 

Bowman, R.M., Dantus, M., and Zewail, A.H. (1990). Femtosecond transition-state spectroscopy of iodine From strongly bound to repulsive surface dynamics, Chem. Phys. Lett. 161, 297-302. 

R. M. Bowman, M. Dantus, and A. H. Zewail, Chem. Phys. Lett., 161, 297 (1989). Femtosecond Transition-State Spectroscopy of Iodine From Strongly Bound to Repulsive Surface Dynamics. 

Photoinduced reactions in the liquid phase are influenced much more by collisions than those in the gas phase. In order to study such reactions on a time scale below the mean collision time, femtosecond spectroscopy is needed. One example is the exploration of transition-state dynamics and rotational dynamics of the fragments in the photolysis of mercuric iodide Hgl2 in ethanol solution [1415, 1416]. 

M. Danfus, G. Roberts Femtosecond transition state spectroscopy and chemical reaction dynamics. Commen. At. Mol. Rhys. 26, 131 (1991)... 

S. Pedersen, L. Banares, A.H. Zewail Femtosecond vibrational transition state dynamics in a chemical reaction. J. Chem. Phys. 97, 8801 (1992)... 

There has already been a great deal of preliminary work on the dynamics and spectroscopy of the transition state [74-76], but Ref 73b now signals the beginning of a new era, that of real time femtosecond transition-state spectroscopy/dynamics. 

Recently, Zewail and co-workers have combined the approaches of photodetachment and ultrafast spectroscopy to investigate the reaction dynamics of planar COT.iii They used a femtosecond photon pulse to carry out ionization of the COT ring-inversion transition state, generated by photodetachment as shown in Figure 5.4. From the photoionization efficiency, they were able to investigate the time-resolved dynamics of the transition state reaction, and observe the ring-inversion reaction of the planar COT to the tub-like D2d geometry on the femtosecond time scale. Thus, with the advent of new mass spectrometric techniques, it is now possible to examine detailed reaction dynamics in addition to traditional state properties." ... 

Reaction dynamics on the femtosecond time scale are now studied in all phases of matter, including physical, chemical, and biological systems (see Fig. 1). Perhaps the most important concepts to have emerged from studies over the past 20 years are the five we summarize in Fig. 2. These concepts are fundamental to the elementary processes of chemistry—bond breaking and bond making—and are central to the nature of the dynamics of the chemical bond, specifically intramolecular vibrational-energy redistribution, reaction rates, and transition states. 

Figure 14. (a) Potential-energy surfaces, with a trajectory showing the coherent vibrational motion as the diatom separates from the I atom. Two snapshots of the wavepacket motion (quantum molecular dynamics calculations) are shown for the same reaction at / = 0 and t = 600 fs. (b) Femtosecond dynamics of barrier reactions, IHgl system. Experimental observations of the vibrational (femtosecond) and rotational (picosecond) motions for the barrier (saddle-point transition state) descent, [IHgl] - Hgl(vib, rot) + I, are shown. The vibrational coherence in the reaction trajectories (oscillations) is observed in both polarizations of FTS. The rotational orientation can be seen in the decay of FTS spectra (parallel) and buildup of FTS (perpendicular) as the Hgl rotates during bond breakage (bottom). 

Figure 15. Femtosecond dynamics of the Br + I2 - Brf + I exchange reaction. Here, the collision complex is long lived, tc = 53 ps. As shown by the molecular dynamics, the [Brill complex is trapped in the transition-state region the reaction may also involve avoided crossings (see text).

The mass spectra obtained at different femtosecond time delays show the changes of the precursor and intermediate species. At negative times there is no signal present. At time zero, the parent mass (84 amu) of the precursor cyclopentanone is observed whereas the intermediate mass of 56 amu is not apparent. As the time delay increases, a decrease of the 84 mass signal was observed for the 56 mass, first an increase and then a decrease of the signal was observed. The 56 mass corresponds to the parent minus the mass of CO, and its dynamics directly reflect the nature of the transition-state region. 

The product we monitor is again the I atom using femtosecond-resolved mass spectrometry (the other product is the Bzl species). We also monitor the initial complex evolution. The initial femtosecond pulse prepares the system in the transition state of the harpoon region, that is, Bz+h. The iodine atom is liberated either by continuing on the harpoon PES and/or by electron transfer from iodine (I2-) to Bz+ and dissociation of neutral I2 to iodine atoms. We have studied the femtosecond dynamics of both channels (Fig. 17) by resolving their different kinetic energies and temporal behavior. The mechanism for the elementary steps of this century-old reaction is now clear. 

Transition States of Charge-Transfer Reactions Femtosecond Dynamics and the Concept of Harpooning in the Bimolecular Reaction of Benzene with Iodine, P. Y. Cheng, D. Zhong, and A. H. Zewail, J. Chem. Phys. 103, 5153 (1995). 

A review of direct observation of the transition state has traced the development of the femtosecond reaction dynamic technique, which has been used to demonstrate that the retro-Diels-Alder reaction can proceed by a stepwise mechanism as well as the usual concerted process.28 The oxide anion accelerated retro-Diels-Alder reaction has also been reviewed29 and the promise of this mild reaction for synthetic application has been emphasized. 

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