CIDNP spectroscopy

Spectra of carbenes are very useful sources of information on the structure of the free carbenes, e,g. the R—C—R angle, or the multiplicity of their lowest state. However, these data were mostly obtained under conditions different from those in solution, where chemical reactions normally occur. The spectra are usually recorded either in matrices at low temperatures, say at 4 or 77 °K, or in the gas phase. Only very few investigations of that type have been carried out in solution. The most important spectroscopic technique used in the investigations of carbenes is ESR. Other spectroscopic methods, such as flash photolysis which produces electronic spectra of carbenes, and infrared and lately CIDNP spectroscopy have been successfully employed. 

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. 

Palm, W.-U., Dreeskamp, H., Bous-Laurent, H., and Castellan, A., "The Photochemistry of a-Phenoxyacetophenones Investigated by Flash-CIDNP-Spectroscopy", Ber. Bunsenges Phys. Chem., 1992, 96(1), 50. 

In summary, the existence of these three different structure types suggests a rather delicate balance between the different minima on this interesting potential energy surface. Molecular orbital calculations at the highest levels currently practical are certainly quite useful in elucidating the nature of these structures. This is also an area where one may safely predict continuing efforts and where we anticipate additional insights by both ESR and CIDNP spectroscopy. 

M. Goez, Photo-CIDNP spectroscopy, in Annual Reports on NMR Spectroscopy, (G. A. Webb ed.). Academic Press, London, 2009, 66, pp. 77-147. 

Much less persistent radical cations with remarkable electronic structures can be produced by y-irradiation of small hydrocarbons in freon matrices at low temperatures. Analogously, such short-lived species can also be detected by CIDNP spectroscopy. Moreover, radical cations can be generated in zeolites. "... 

Why should one resort to an indirect method when a direct method exists or, to put it less provocatively, what are potential advantages of CIDNP spectroscopy over EPR spectroscopy The answer is that the independence of generation and detection of a physical observable, which is the essence of an indirect method, has three very attractive consequences. 

Very simple rules connect the polarization phases (i.e., signs) with the magnetic parameters on one hand and with the pathway from the precursor to the observed product in Chart 9.1 on the other hand. Although only semiquantitative, these relationships are extremely valuable assets of CIDNP spectroscopy. In the present context, only net effects—meaning that all NMR lines of a nucleus are scaled by the same factor—are relevant. Their phase T (E = -f 1, absorption E = — 1, emission) is given by Equation 9.5,... 

These case studies illustrate the power of CIDNP spectroscopy. Short-lived paramagnetic intermediates can be identified because their EPR spectrum remains frozen in as a polarization pattern of the nuclear spins in much longer-lived secondary species, and the pathways of their subsequent reactions can be traced out because these polarizations behave as nearly ideal labels. As the examples have shown, transformations of radical pairs into other radical pairs, with or without the participation of a third molecule as a scavenger, and transformations of biradicals can all be investigated by this method, which yields information that is often inaccessible by other techniques. 

Photolysis of tristannanes Me3SnSnR2SnMe3 (R = Me or Et) similarly results in the formation of the radicals Me3SnSnR2 and Me3Sn which have been monitored by ESR and 119Sn CIDNP spectroscopy.47... 

Abstract Photo-CIDNP spectroscopy relies on a unique interplay of magnetic interac-... 

Figure 7 Illumination methods for photo-CIDNP spectroscopy. The coil area has been shaded. Inserts from left to right, (A) normal optical fibre (B) stepwise etched optical fibre (C) quartz rod ...

Despite the fact that CIDNP spectroscopy has reached 40 years of age, novel pulse sequences were devised only recently that greatly improve the performance of these two classes of photo-CIDNP experiments with respect to their specific key problems. 

Mono- and bisacylphosphine oxides 4 and 5 (Chart 7) are efficient photoinitiators for radical polymerizations. Their phofochemistry was studied by H-, C-, and P-CIDNP spectroscopy. The primary step is the breaking of the bond between the carbonyl group and phosphorous in a triplet state for the bisacylphosphine oxides, a stepwise cleavage occurs. 

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