Femtosecond reaction mechanism

It has been possible to determine transition structures computationally for many years, although not always easy. Experimentally, it has only recently become possible to examine reaction mechanisms directly using femtosecond pulsed laser spectroscopy. It will be some time before these techniques can be applied to all the compounds that are accessible computationally. Furthermore, these experimental techniques yield vibrational information rather than an actual geometry for the transition structure. 

Employing all of the TR results in conjunction with results from femtosecond time-resolved transient absorption (fs-TA) and femtosecond time-resolved Kerr gated fluorescence (fs-KTRF) experiments enables a reaction mechanism to be developed... 

A fundamental goal of chemical research has always been to understand the reaction mechanisms leading to specific reaction products. Reaction mechanisms, in turn, are a consequence of the structural dynamics of molecules participating in the chemical process, with atomic motions occurring on the ultrafast timescale of femtoseconds (10 s) and picoseconds (10" s). Although kinetic studies often allow reaction mechanisms as well as the kind and properties of reaction intermediates to be determined, the obtained information is not sufficient to deduce the ultrafast molecular dynamics. Because these ultrafast motions are the essence of every chemical process, detailed knowledge about their nature is of fundamental importance. 

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. 

Yang H, Asplund MC, Kotz KT, Wilkens MJ, Frei H, Harris CB. Reaction mechanism of silicon-hydrogen bond activation studied using femtosecond to nanosecond IR spectroscopy and ab initio methods. J Am Chem Soc 1998 120(39) 10154-10165. 

We first discuss the overall chemical process predicted, followed by a discussion of reaction mechanisms. Under the simulation conditions, the HMX was in a highly reactive dense fluid phase. There are important differences between the dense fluid (supercritical) phase and the solid phase, which is stable at standard conditions. Namely, the dense fluid phase cannot accommodate long-lived voids, bubbles, or other static defects, since it has no surface tension. Instead numerous fluctuations in the local environment occur within a timescale of 10s of femtoseconds. The fast reactivity of the dense fluid phase and the short spatial coherence length make it well suited for molecular dynamics study with a finite system for a limited period of time. Under the simulation conditions chemical reactions occurred within 50 fs. Stable molecular species were formed in less than a picosecond. We report the results of the simulation for up to 55 picoseconds. Figs. 11 (a-d) display the product formation of H2O, N2, CO2 and CO, respectively. The concentration, C(t), is represented by the actual number of product molecules formed at the corresponding time (. Each point on the graphs (open circles) represents a 250 fs averaged interval. The number of the molecules in the simulation was sufficient to capture clear trends in the chemical composition of the species studied. These concentrations were in turn fit to an expression of the form C(/) = C(l- e ), where C is the equilibrium concentration and b is the effective rate constant. From this fit to the data, we estimate effective reaction rates for the formation of H2O, N2, CO2, and CO to be 0.48, 0.08,0.05, and 0.11 ps, respectively. 

The impact which femtosecond spectroscopy is also having on the study of the reaction mechanisms of biological systems is finally illustrated by an investigation of the cis/trans isomerization of rhodopsin 66 —> 67,... 

Until very recently chemists could only guess at the details of reaction mechanisms. New developments such as femtosecond spectroscopy (see Femto-chemistry on page 718) and scanning tunneling microscopy (STM see Seeing Atoms on page 22) have enabled scientists to begin to see the details of chemical reactions. 

Thus, by using femtosecond laser pulses to excite a surface, one can separate electron- from phonon-induced surface processes. This method can be used to unravel reaction mechanisms whose characteristic time lies within the sub-picosecond time-scale. A model example of such an ultrafast surface reaction is that of CO/O [Ru(0001)]. 

IR measurements. These labile adsorbed species may be detected using unusual measurements such as femtosecond laser excitation followed by nanosecond time resolved FTIR measurements. On the contrary, heavy adsorbed species that are not involved in the main reaction concentrate at the catalyst surface and can be easily detected. It is possible to forecast that, if a species is determined to be highly concentrated and stable in reaction conditions, it is either the true catalyst (e.g. sometimes carbonaceous materials deposited at the surface during the induction period), or a poison (e.g. coke that is a main cause of deactivation of acidic catalysts), or finally, a spectator species. In any case, the information obtained in operando conditions is always interesting and sometimes useful for revealing the reaction mechanisms. 

Thus, the predicted reaction mechanism is very dependant on the method onployed in the calculations. The long wavelength transition of singlet CH3N in the A state was calculated at 287 nm with oscillator strength f = 5 x 10" . Therefore, spectroscopic detection with pico- or femtosecond time-resolution should be performed to solve this problan. 

Chou PT, Pu SC, Cheng YM et al (2005) Femtosecond dynamics on excited-state proton/ charge-transfer reaction in 4 -N, N-diethylamino-3-hydroxyflavone. The role of dipolar vectors in constructing a rational mechanism. J Phys Chem A 109 3777-3787... 

The energy transfer occurs by means of the Coulombic long-range mechanism (Section 6.6), which ultimately redistributes the excitation energy via the adjacent pigment molecules to the reaction centre. Excitation of the reaction centre is over within a few femtoseconds. 

The reaction pathways by which the net transfer of a hydrogen atom from an amine to a photoexcited ketone has been extensively examined in the nanosecond [23, 25-30], picosecond [20, 22, 31-33], and femtosecond [24] time domains. The following mechanism, as it pertains to the photochemical reduction of benzophenone (Bp) by N, A-dimethylaniline (DMA), is derived from these numerous studies. Only an overview of the mechanism will be presented. The details of the studies leading to the mechanism will not be given for specifics, the reader is referred to the original literature. 

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