Thermal reaction CIDNP

For thermal reactions a variable temperature probe is necessary since optimum polarized spectra are usually obtained in reactions having a half-life for radical formation in the range 1-5 minutes. Reactant concentrations are usually in the range normally used in n.m.r. spectroscopy, although the enhancement of intensity in the polarized spectrum means that CIDNP can be detected at much lower concentrations. Accumulation of spectra from rapid repetitive scans can sometimes be valuable in detecting weak signals. 

Finally, it should be pointed out that methods used to study short-lived chemical intermediates in fast thermal reactions may be applicable also to photochemical studies. Radical intermediates, however generated, can be studied by CIDNP ichemically induced dynamic nuclear spin polarization), in which the n.m.r. spectrum of the reaction mixture is recorded during the reaction period. II a substrate is continuously irradiated with ultraviolet/visible light in the cavity of an n.m.r. spectrometer, the resulting n.m.r. spectrum of the substrate/product mixture exhibits intensity variations as compared with the normal spectrum—intensity enhancement, reduction or even reversal (i.e. emissionl. Note that the spectrum involved is not... 

More detailed consideration of light absorption and consequent chemical changes is left to Chapter 13, but it is appropriate here to summarize briefly the types of compounds that are convenient photochemical radical sources. Many of the substances we have been discussing as thermal radical sources absorb light in the visible or ultraviolet and can be decomposed photochemically. The azoalkanes are particularly versatile they absorb around 350 nm and decompose cleanly to nitrogen and two radicals just as in the thermal reaction. As we have already noted, a preliminary photochemical isomerization to the cis isomer precedes the homolysis, which is actually a thermal decomposition of this unstable form.78 CIDNP observations confirm a stepwise decomposition pathway, and clarify the various reactions of the radicals produced.79... 

Under continuous uv irradiation, the observed steady-state polarization (whether by cw or by FT spectrometers) may be substantially modified by various nuclear relaxation processes. For example, Closs and Czeropski (35,36) have demonstrated that CIDNP can be transferred from a group of polarized nuclei to another group not originally polarized. Both the dipolar and the scalar relaxation mechanisms (of the nuclear Overhauser effects) can be operative. The extremely interesting case of intramolecular dipolar nuclear cross relaxation reported by Closs and Czeropski (35) involves the thermal reaction of... 

Although CIDNP effects were first observed in thermal reactions, photochemical generation of radical pairs gives much better control, so it is not surprising that photo-CIDNP studies account for almost all CIDNP work published today. 

In 1973, Sagdeev et al. [16] reported that the NMR intensities of several products after thermal reactions of substituted benzyl chlorides with n-butyllithium in solution were appreciably changed by magnetic fields less than 2.5 T. They explained the MFEs of these thermal reactions by the HFC mechanism of the radical pair mechanism. This interpretation was more plausible than the above-mentioned one because CIDNP had been observed in this type of reactirais [17]. In 1974, Brocklehurst et al. [18] observed MFEs on the intesities of fluorescence and transient absorption in pulse radiolaysis of fluorene in squalane at room temperature. They found that the fluorescence intensity and the singlet yield observed 100 ns... 

Diacyl peroxides undergo thermal and photochemical decomposition to give radical intermediates (for a recent review, see Hiatt, 1971). Mechanistically the reactions are well understood as a result of the many investigations of products and kinetics of thermal decomposition (reviewed by DeTar, 1967 Cubbon, 1970). Not surprisingly, therefore, one of the earliest reports of CIDNP concerned the thermal decomposition of benzoyl peroxide (Bargon et al., 1967 Bargon and Fischer, 1967) and peroxide decompositions have been used more widely than any other class of reaction in testing theories of the phenomenon. 

While the in-cage product enol in reaction 64 is not thermally stable, its transient spectrum can be detected by CIDNP. 

By measuring the intensities in CIDNP patterns and the values KL corresponding to thermal equilibrium of the same system, can be determined experimentally. The usual procedure for the determination of la I " volves rapid quenching of the reaction and allowance for thermal equilibration. 

It should be noted that [g(C H3)-g(CH3COO )] is used for the Ag value in Eq. (4-25) because this radical pair is the most important in this reaction. Such phenomena is called the memory effect. Similarly, we can explain the enhanced absorptive CIDNP observed for CH3CI and CH4 during this reaction and the emissive CIDNP observed for benzene during the thermal decomposition of dibenzoylperoxide shown in Fig. 4-4. 

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