Time-resolved CIDNP

In an extension of traditional CIDNP methods, Closs and co-workers developed time-resolved CIDNP (TR-CIDNP) m the late 1970s [24, 25 and 26]. The initial time-resolved experiments had a time resolution in the... 

The first application of the time-resolved CIDNP method by Closs and co-workers involved tire Norrish 1 cleavage of benzyl phenyl ketone [24, 25]. Geminate RPs may recombine to regenerate the starting material while escaped RPs may fonn the starting ketone (12), bibenzyl (3), or benzil (4), as shown below. 

Closs G L and Miller R J 1979 Laser flash photolysis with NMR detection. Microsecond time-resolved CIDNP separation of geminate and random-phase polarization J. Am. Chem. Soc. 101 1639—41... 

Closs G L, Miller R J and Redwine O D 1985 Time-resolved CIDNP applications to radical and biradical chemistry Acc. Chem. Res. 18 196-202... 

Palm, W.-U., Dreeskamp, H., "Evidence for Singlet State 0 Cleavage in the Photoreaction of a-(2,6-Dimethoxyphenoxy)-acetophenone Inferred From Time-Resolved CIDNP Spectroscopy", J. Photochem. Photobiol., A Chemistry, 1990, 52, 439. 

One might argue that the results least subject to ambiguity are those with the shortest delay between the generation of the radical cation and its observation. In this respect, the time-resolved ODMR results of Trifunac and Qin (Fig. 24) [368] and time resolved CIDNP results observed in the author s laboratory (Fig. 25) [380], may provide the least distorted view of the species in question. Of course, neither of these experiments qualifies as the coveted direct observation. Thus, the direct observation of the elusive hexamethyl-Dewar benzene radical cation must await further scrutiny. 

Fig. 25. Time resolved CIDNP spectrum observed upon photo-induced electron transfer oxidation of hexamethyl-(Dewar)-benzene (0.02 M) by chloranil (0.02 M). The top spectrum was observed 5 ps after excitation with the frequency-tripled output (355 nm) of a Nd/YAG laser. The bottom spectrum was recorded in the dark...

Using time-resolved CIDNP [56], the products of the fast geminate reaction may be detected, too. Obviously, the CIDNP effect is predetermined as an analytical tool for the examination of the structure and energetics of radical ions [57], less for ion pairs. 

A very useful review of time resolved CIDNP and its application to radical and biradical chemistry has been prepared by Closs et al... 

Figure 1 Time-resolved CIDNP experiment on the system xanthone/triisopropylamine in acetonitrile at 294 K amine concentration 10 mM. Shown are the CIDNP signals of the terminal protons of the amine as functions of the delay Atbetween the laser flash and the sampling rf pulse (duration, 1.0 ps).

Figure 13 Storage and accumulation of a time-resolved CIDNP signal in a spin system. The picture displays the timing of the laser and the spectrometer, and the time dependence of longitudinal and transverse magnetization. Further explanation, see text. Reproduced from Ref 75 with permission copyright (2006) Taylor Francis Ltd, httpr/ www.tandfco.uk/journals.

Figure 18 Time-resolved CIDNP in the system triphenyiamine/fumarodinitriie. Left example spectra taken at three different delays between laser flash and rf pulse. H°, H", are the signals of the aromatic protons of the amine, FN and MN those of the protons of the starting olefin and the isomerized olefin (which are not observed). Right time dependence of the CIDNP signals of FN. Further explanation, see text. Reproduced from Ref. 89 with permission copyright (2006) Verlag Helvetica Chimica Acta AG.

Nof only does the observation of such a label in a producf mean fhaf fhaf producf had fhe radical pair as a precursor, buf fhe disfribufion of fhe labels befween different products affords relative reaction rates (see, e.g. Figure 17), and time-resolved CIDNP experiments that probe how fast the labels appear in the products even yield absolute rates (compare Figure 18). 

Time-resolved CIDNP experiments have been used to measure the rates of the degenerate electron exchange between the radical cation of 1,6-diphenylhexa-... 

Although the first case puts no demands on the energetics because escape is always a feasible process, no direct observation by a time-resolved CIDNP experiment seems to have been successful so far instead, it has repeatedly been reported that time-resolved photo-CIDNP experiments showed olefin radical cations or anions to be configurationally stable. The second and third cases are obviously only possible if the energy of the olefin triplet or the biradical lies below the energy of the radical ion pair. 

To study the addition of 2-hydroxy-2-propyl radicals to alkenes, Batchelor and Fischer employed a ketone as the source of these radicals, which then develop multiplet polarizations from F-pairs the normal reaction product is isopropanol. The addition of an alkene scavenges some part of the radicals and thus decreases the isopropanol polarizations. Time-resolved CIDNP experiments yield the addition rate. The advantage of analysing multiplet CIDNP instead of net CIDNP is that relaxation effects are avoided. The method was reported to be applicable for scavenging rate constants between and 10 M s . ... 

In the photoinduced cleavage of N-alkoxyamines, where the transient and the persistent radical are an alkyl and an aminoxyl radical, Fischer et al. used time-resolved CIDNP to clarify the mechanism of cross-termination and to determine the rate constant of the recombination. They concluded that disproportionation plays no role in this system, and that no unknovm side reactions occur. Later, Ananchenko and Fischer compared the recombinations of the transient ferf-butyl and 2-hydroxy-... 

The chemistry underlying the polarization processes of tryptophan, tyrosine, and histidine (electron transfer for the first two amino acids, hydrogen transfer for the third) has long been known. For the frequently used sensitizer 2, 2 -dipyridyl 16, the kinetics of the quenching by tryptophan, tyrosine, and histidine have recently been studied in detail by time-resolved CIDNP experiments the effects of added surfactants on the polarizations of tyrosine with flavins 17 or other sensitizers have been investigated. Several low-field CIDNP investigations of the field-dependence have been carried out, with a view to optimizing... 

The determination of surface accessibilities in a protein is by now a well-established application of photo-ClDNP. Its operating principle is that a sensitizer in the bulk solution is photoexcited, forms a radical pair with an amino acid that is exposed to the solution, and so causes CIDNP to arise amino acids not accessible to the dye remain impolarized. To avoid disruptions of the structure the photore-action(s) must be cyclic. As this leads to exchange cancellation, one either observes the polarizations that remain because of relaxation in the free radicals or samples the geminate polarizations in a time-resolved CIDNP experiment. The latter appears preferable for quantitative conclusions as it also removes other artefacts, but cannot be applied to the pulse-labelling and related experiments described below. Commonly employed dyes are 2, 2 -dipyridyl 16 or flavins 17. As already mentioned in the preceding section, only tryptophan 10, tyrosine 11, and histidine 12 are polarizable. However, the reduction of the information content in a crowded protein spectrum by this selectivity is a much desired blessing rather than a drawback. 

Figure 11. Time-resolved CIDNP spectra in a system described by the mechanism of Chart VI photoreaction of the acceptor anthraquinone 12 (8 x 10-4 M) with the donor /V,/V-dimethylaniline 13 (3.2 x 10 4M) in acetonitrile-d,. Experimental parameters T — 257 K, excitation wavelength 343 nm. Shown is the dependence of the signal I of the dimethylamino protons (marked with in the formula) on the delay time f0 between laser flash and acquisition pulse. [Adapted from ref. [86a] with permission. Copyright 1990 Elsevier Science Publishers B.V.]...

Finally, CIDNP results were taken as evidence for benzoquinone-mediated photoinduced electron transfer in a donor-acceptor system [92], and the rates of electron self-exchange and nuclear spin relaxation of the radical anion of C60 have been measured by time-resolved CIDNP [93],... 

The contributions of radical ions DH + and neutral radicals D to the product polarizations were also separated by variable-field stationary CIDNP [94a], time-resolved CIDNP, and SNP [94b]. From an evaluation of the CIDNP memory effect, the rate of in-cage proton transfer from the radical cation of triethylamine to the radical anion of trans-stilbene was estimated to be 3 x 107s-1 [94F],... 

Two time-resolved CIDNP investigations of hydrogen transfer were reported, in which the rates of hydrogen exchange between carbonyl compounds (benzaldehyde and benzophenone [97a] benzoquinone [97b]) and their ketyl radicals were measured. The experiments also yielded the homogeneous recombination rate of the radicals. 

Radical Additions. Batchelor and Fischer [109] measured absolute rate constants kaM for addition of 2-hydroxy-2-propyl radicals 21 to alkenes by a new method, using time-resolved CIDNP. Photoexcited 2,4-dihydroxy-2,4-dimethylpentan-3-one 20 served as a source of these radicals. Chart XI summarizes the relevant steps of the underlying reaction scheme in a simplified manner. Their key idea was to evaluate not the net polarizations, which may be strongly distorted by nuclear spin relaxation in the radicals, but the multiplet polarizations, which had previously [110] been shown to be uninfluenced by relaxation at the paramagnetic stage provided that the... 

To keep within the kinetic window of time-resolved CIDNP, the reaction rate is controlled by the alkene concentration. The method was reported to... 

Time-Resolved CIDNP. - Petrova et al , have used time-resolved photo-CIDNP in the dibenzyl-ketone-b-cyclodextrin inclusion complex. The photodecay of the inclusion complex generates a benzyl radical that is held in the cyclodextrin cavity for a time exceeding the nuclear spin relaxation time, from... 

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