Spectroscopic observation of the activated complex

Until very recently there were no direct spectroscopic observations on activated complexes, for they have a very fleeting existence and often survive for only a few picoseconds. In a typical experiment designed to detect an activated complex, a femtosecond laser pulse is used to excite a molecule to a dissociative state, and then a second femtosecond pulse is fired at an interval after the dissociating pulse. The frequency of the second pulse is set at an absorption of one of the free fragmentation products, so its absorption is a measure of the abundance of the dissociation product. For example, when ICN is dissociated by the first pulse, the emergence of CN from the photoactiv ated state can be monitored by watching the growth of the free CN absorption (or, more commonly, its laser-induced fluorescence). In this way it has been found that the CN signal remains zero until the fragments have separated by about 600 pm, which takes about 205 fs. Some sense of the progress that has been made in the study of the intimate mechanism of chemical reactions can be obtained by considering the decay of the ion pair Na⁺I⁻. As shown in Fig. 24.25, excitation of the ionic species with a femtosecond laser pulse forms an excited state that corresponds to a covalently bonded NaI molecule. The system can be described with two potential energy surfaces, one largely ‘ionic’ and another ‘covalent’, which cross at an internuclear separation of 693 pm. A short laser pulse is composed of a wide range of frequencies, which excite many vibrational states of NaI simultaneously. Consequently, the electronically excited complex exists as a superposition of states, or a localized wave packet (Section 8.6), which oscillates between the ‘covalent’ and ‘ionic’ potential energy surfaces, as shown in Fig. 24.25. The complex can also dissociate, shown as movement of the wave packet toward very long internuclear separation along the dissociative surface. However, not every outward-going swing leads to dissociation because there is a chance that the I atom can be harpooned again, in which case it fails to make good its escape. The dynamics of the system is probed by a second laser pulse with a frequency that corresponds to the absorption frequency of the free Na product or to the frequency at which Na absorbs when it is a part of the complex. The latter frequency depends on the Na···I distance, so an absorption (in practice, a laser-induced fluorescence) is obtained each time the wave packet returns to that separation.

Fig. 24.25 Excitation of the ion pair Na+I− forms an excited state with covalent character. Also shown is movement between a ‘covalent’ surface (in green) and an ‘ionic’ surface (in purple) of the wave packet formed by laser excitation.

A typical set of results is shown in Fig. 24.26. The bound Na absorption intensity shows up as a series of pulses that recur in about 1 ps, showing that the wave packet oscillates with about that period. The decline in intensity shows the rate at which the complex can dissociate as the two atoms swing away from each other. The free Na absorption also grows in an oscillating manner, showing the periodicity of wave packet oscillation, each swing of which gives it a chance to dissociate. The precise period of the oscillation in NaI is 1.25 ps, corresponding to a vibrational wavenumber of 27 cm−1 (recall that the activated complex theory assumes that such a vibration has a very low frequency). The complex survives for about ten oscillations. In contrast, although the oscillation frequency of NaBr is similar, it barely survives one oscillation. Femtosecond spectroscopy has also been used to examine analogues of the activated complex involved in bimolecular reactions. Thus, a molecular beam can be used to produce a van der Waals molecule (Section 18.6), such as IH···OCO. The HI bond can be dissociated by a femtosecond pulse, and the H atom is ejected towards the O atom of the neighbouring CO₂ molecule to form HOCO. Hence, the van der Waals molecule is a source of a species that resembles the activated complex of the reaction

H+CO2→[HOCO]‡→HO+CO

The probe pulse is tuned to the OH radical, which enables the evolution of [HOCO]‡ to be studied in real time. Femtosecond transition state spectroscopy has also been used to study more complex reactions, such as the Diels–Alder reaction, nucleophilic substitution reactions, and pericyclic addition and cleavage reactions. Biological process that are open to study by femtosecond spectroscopy include the energy-converting processes of photosynthesis and the photo stimulated processes of vision. In other experiments, the Photoejection of carbon monoxide from myoglobin and the attach ment of O₂ to the exposed site have been studied to obtain rate constants for the two processes.

Fig. 24.26 Femtosecond spectroscopic results for the reaction in which sodium iodide separates inot Na and I. The lower curve is the absorption of the electronically excited complex and the upper curve is the absorption of free Na atoms (Adapted from A.H. Zewail, Science 242, 1645 (1988)).

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مواضيع ذات صلة

Surface composition

Surface growth

Experimental results

Electron tunnelling

Controlling chemical reactions with lasers

Quantum mechanical scattering theory

Classical trajectories

Attractive and repulsive surfaces

The direction of attack and separation

Potential energy surfaces

State-to-state dynamics

Experimental probes of reactive collisions