Reactions femtosecond dynamics

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). 

At present, the direct study of transition state structures has been possible thanks to the enormous advances made in femtochemistry. For instance, the existence of biradical species has been established by direct femtosecond studies of the transition state structures. For the retro-Diels-Alder reaction, femtosecond dynamic studies seem to indicate that both the biradical and the concerted processes take place. ... 

Beil A, Luckhaus D, Quack M and Stohner J 1997 Intramolecular vibrational redistribution and unimolecular reactions concepts and new results on the femtosecond dynamics and statistics in CHBrCIF Ber. Bunsenges. Phys. Chem. 101 311-28... 

In a recent experimental study of the femtosecond dynamics of a Diels-Alder reaction in the gas phase it has been suggested that both concerted and stepwise trajectories are present simultaneously It is interesting to read the heated debates between Houk and Dewar on the... 

Both molecular dynamics studies and femtosecond laser spectroscopy results show that molecules with a sufficient amount of energy to react often vibrate until the nuclei follow a path that leads to the reaction coordinate. Dynamical calculations, called trajectory calculations, are an application of the molecular dynamics method that can be performed at semiempirical or ah initio levels of theory. See Chapter 19 for further details. 

Cheng YM, Pu SC, Yu YC et al (2005) Spectroscopy and femtosecond dynamics of 7-N, N-diethylamino-3-hydroxyflavone. The correlation of dipole moments among various states to rationalize the excited-state proton transfer reaction. J Phys Chem A 109 11696-11706... 

Recently the two-step decomposition of azomethane was proved in the study of the femtosecond dynamics of this reaction [68]. The intermediate CH3N2 radical was detected and isolated in time. The reaction was found to occur via the occurrence of the first and the second C—N bond breakages. The lifetime of CH3N2 radical is very short, i.e., 70fsec. The quantum-chemical calculations of cis- and /nmv-azomcthanc dissociation was performed [69]. 

Figure 9. Femtosecond dynamics of an elementary reaction (I2 — 21) in solvent (Ar) cages. The study was made in clusters for two types of excitation to the dissociative A state and to the predissociative B state. The potentials in the gas phase govern a much different time scale for bond breakage (femtosecond for A state and picosecond for B state). Based on the experimental transients, three snapshots of the dynamics are shown with the help of molecular dynamics simulations at the top. The bond breakage time, relative to solvent rearrangement, plays a crucial role in the subsequent recombination (caging) dynamics. Experimental transients for the A and B states and molecular dynamics simulations are shown.

Figure 13. Femtosecond dynamics of dissociation (Nal) reaction. Bottom Experimental observations of wavepacket motion, made by detection of the activated complexes [Nal] or the free Na atoms. Top Potential energy curves (left) and the exact quantum calculations (right) showing the wavepacket as it changes in time and space. The corresponding changes in bond character are also noted covalent (at 160 fs), covalent/ionic (at 500 fs), ionic (at 700 fs), and back to covalent (at 1.3 ps).

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). 

For exchange reactions, the femtosecond dynamics of bond breaking and bond making were examined in the following system ... 

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).

Figure 16. Femtosecond dynamics of addition/cleavage reaction of the cyclobutane-ethylene system. Bottom Experimental observation of the intermediate diradical by mass spectrometry. Top The PES showing the nonconcerted nature of the reaction, together with three snapshots of the structures at to (initial), tj (diradical) and (/ (final). The parent precursor is also shown.

It appears, therefore, that real-time studies of these reactions should allow one to examine the nature of the transformation and the validity of the diradical hypothesis. We recently reported direct studies of the femtosecond dynamics of the transient diradical structures. The aim was at freezing the diradicals in time in the course of the reaction. Various precursors were used to generate the diradicals and to monitor the formation and the decay dynamics of the reaction intermediate(s). The parent (cyclopentanone) or the intermediate species was identified distinctly using time-of-flight mass spectrometry. The concept behind the experiment and some of the results are given in Fig. 16. 

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. 

The observed femtosecond dynamics of this dissociative CT reaction is related to the nature of bonding. Upon excitation to the CT state, an electron in the highest occupied molecular orbital (HOMO) of benzene (ir) is promoted to the lowest occupied molecular orbital (LUMO) of I2 (a ). Vertical electron attachment of ground state I2 is expected to produce molecular iodine anions in some high vibrational levels below the dissociation limit. In other words, after the electron transfer, the I—I bond is weakened but not yet broken. While vibrating, the entire I2 and benzene complex begins an excursion motion within die coulombic field and the system proceeds... 

Femtosecond Dynamics of Reactions Elementary Processes of Controlled Solvation, A. H. 

The rebound mechanism, though in a modified version, has been recently supported by theoretical calculations of KIF using the density functional theory (Yoshizawa et al., 2000). The calculations demonstrate that the transition state for the H-atom abstraction from ethane involves a linear [FeO.H...C] array a resultant radical species with a spin density of nearly one is bound to an iron-hydroxy complex, followed by recombination and release of product ethanol. According to the calculation of the reaction energy profile, the carbon radical species is not a stable reaction intermediate with a finite lifetime. The calculated KIF at 300 K is in the range of 7-13 in accord with experimental data and is predicted to be significantly dependent on temperature and substituents. It was also shown from femtosecond dynamic calculations in the FeOVCH4 system that the direct abstraction mechanism can occur in 100-200 fs. 

Pig. The femtosecond dynamics of the iodine atom abstraction reaction. The corresponding structures are shown with emphasis on three molecular photographs att0, tc and tt as the reaction proceeds to completion. The calculated well depth and barrier height are also noted. 

E. W.-G. Diau, S. De Fey ter, A. H. Zewail, Femtosecond Dynamics of Retro Diels-Alder Reactions the Concept of Concertedness. Chem. Phys. Lett. 1999, 304, 134-144. 

Hence there is no gas-phase experiment yet which fully encompasses all aspects of an electron-transfer reaction in solution. In solution, the solvent acts first as a polarization medium, which affects the energetics of direct transfers from the donor to the acceptor. It can also act as a transport medium for indirect electron transfers. The first aspect has been addressed in various cluster experiments [276]. The second aspect was addressed more recently by considering the femtosecond dynamics of iodide-(water) anion clusters, as reviewed below [277]. Finally, clusters present the advantage of isolating one reaction pair free from secondary collisions, except those, which are desired, with the solvent molecules (or atoms). The latter consideration motivated the cluster isolated chemical reaction (CICR) technique reviewed in Section 2.8.3. 

In their recent landmark femtosecond-resolved mass spectrometry studies, Zewail and coworkers have used mass spectrometry for monitoring the time-resolved unimolecular fragmentation of neutral norbomene and norbomadiene. In both cases, the RDA reactions occurred, but only in the norbomadiene case was the well-known H loss giving rise to C7H7+ ions found to compete. Still, non-concertedness and biradicaloid character of the intermediates is being addressed by femtosecond dynamic studies. In this context, Kompa and coworkers" have compared the expulsion of H+ from femtosecond-laser-irradiated... 

De Feyter, S., Diau, E. W. G., Zewail, A. H., Femtosecond Dynamics of Norrish Type II Reactions Nonconcerted Hydrogen transfer and Diradical Intermediacy, Angew. Chem. Int. Ed. 2000, 39, 260 263. 

Cavanagh R R, King D S, Stephenson J C and Heinz T F 1993 Dynamics of nonthermal reactions— femtosecond surface chemistry J. Phys. Chem. 97 786... 

Zewail A H 1995 Femtosecond dynamics of reactions elementary processes of controlled solvation... 

For chemical reactions in solution, the solvent plays an important role in the elementary processes of bond making and breaking. For example, it may enhance bond formation by trapping reactive species in a solvent cage on the time-scale of the reaction it also may act as a chaperone that stabilizes energetic species. One of the most studied reactions in the condensed phase is that of dissociation of neutral iodine molecules most recently, it has been studied using ultrafast lasers to investigate its femtosecond dynamics. 

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