Bimolecular clock reactions

Some unimolecular clock reactions are given in Table 7.1 and bimolecular clock reactions in Table 7.2, together with their kinetics. Rate constants at any... 

The radicals formed by imimolecular rearrangement or fragmentation of the primary radicals arc often termed secondary radicals. Often the absolute rate constants for secondary radical formation are known or can be accurately determined. These reactions may then be used as radical clocks",R2° lo calibrate the absolute rate constants for the bimolecular reactions of the primary radicals (e.g. addition to monomers - see 3.4). However, care must be taken since the rate constants of some clock reactions (e.g. f-butoxy [3-scission21) are medium dependent (see 3.4.2.1.1). 

The technique has been used to determine rate constants for a number of radical reactions in solution, notably ring closure and ring fission processes which serve as clock reactions in conventional radical kinetics [45]. As an example, the bimolecular reaction of the cyclohexadienyl radical with molecular iodine is shown in Figure 11. The straight line behavior demonstrates a pseudo-first order... 

Scherer N F, Khundkar L R, Bernstein R B and Zewail A H 1987 Real-time picosecond clocking of the collision complex in a bimolecular reaction the birth of OH from H + CO2 J. Chem. Phys. 87 1451-3... 

In these circumstances, where routine kinetic measurements are uninformative and direct measurements of the product-forming steps difficult, comparative methods, involving competition between a calibrated and a non-calibrated reaction, come into their own. Experimentally, ratios of products from reaction cascades involving a key competition between a first-order and a second-order processes are measured as a function of trapping agent concentration. Relative rates are converted to absolute rates from the rate of the known reaction. The principle is much the same as the Jencks clock for carbenium ion lifetimes (see Section 3.2.1). However, in radical chemistry Newcomb prefers to restrict the term clock to a calibrated unimolecular reaction of a radical, but such restriction obscures the parallel with the Jencks clock, where the calibrated reaction is a bimolecular diffusional combination with and the unknown reaction a pseudounimolecular reaction of carbenium ion with solvent. Whatever the terminology, the practical usefulness of the method stems from the possibility of applying the same absolute rate data to all reactions of the same chemical type, as discussed in Section 7.1. 

The assumption that one radical is an appropriate model for another is most sound when one is using a clock to calibrate a bimolecular reaction and the local environment of the clock is similar to that of the radical of interest. For example, the rate constant found for reaction of the 5-hexenyl radical with a specific trapping agent should be a good approximation of the rate constant for reaction of another primary alkyl radical, especially one without substituents at C2. For most synthetic applications, the small errors in rate constants from this assumption will be unimportant. 

The evolution of kinetic scales has been highly dependent on radical clock and, more generally, indirect competition kinetic studies [6], These types of studies provide ratios of rate constants as discussed above. One can build an extensive series of relative rate constants for unimolecular clocks and bimolecular reactions, and the relative rate constants often are determined with very good to excellent precision. At some point, however, absolute rate constants are necessary to provide real values for the entire kinetic scale. These absolute kinetic values are the major source of error in the kinetics, but the absolute values are becoming more precise and, one certainly hopes, more accurate as increasingly refined techniques are introduced and multiple methods are applied in studies of specific reactions. 

When the probe reaction being calibrated is a unimolecular process, one measures the rate constant of a radical clock directly for the initial absolute kinetic values, and, thus, the method is inverted in approach from that used for alkyl radical kinetics. LFP studies of unimolecular process give more precise data than those of bimolecular processes, and the approach typically starts with inherently good kinetic data. The synthetic efforts necessary for production of appropriate radical precursors are a drawback to this method, but it is, nonetheless, useful for establishing absolute kinetics for some classes of radicals where little kinetic information was available, such as nitrogen-centered radicals discussed later. 

The pioneering work on the calibration of intramolecular cy-clization of the 5-hexenyl radical by Ingold and co-workers provided the basis for the development of a large number of radical clocks." These are now used both for the calibration of rate constants for intermolecular radical reactions and as mechanistic probes to test for the intermediacy of radical intermediates in a variety of processes. Furthermore, the ready availability of bimolecular rate constants from competitive product studies using free radical clocks without the use of time-resolved experiments has greatly enhanced the synthetic utility of free radical chemistry. The same concept has recently been extended to radical ion chemistry. For example, rate constants for carbon—carbon bond cleavage reactions of a variety of radical cations and anions derived from substituted diarylethanes have been measured by direct time-resolved techniques. " ... 

Real-Time Clocking of Bimolecular Reactions Applications to H + CO2. 

Scherer NF, Sipes C, Bernstein RB, Zewail AH (1990) Real-time clocking of bimolecular reactions application to H - - CO2. J Chem Phys 92 5239... 

An important subject of this paper is that of reagent preparation and stereoselectivity. Experimental techniques for production of oriented (and of aligned) molecules and their theoretical description are outlined. Reactive asymmetry experiments using oriented molecule beams are briefly reviewed, and new approaches described. Results are presented of recent experiments which take advantage of the inherent mutual orientation of the molecules in a van der Waals complex, i.e., "precursor geometry-limited" bimolecular reactions. Finally, a new technique is described for real time, picosecond, clocking of the collision complex in such bimolecular reactions. Results are reported for the time-dependent birth of OH from H + CO2. 

---