Spectroscopy chemically induced dynamic

Time-resolved laser flash ESR spectroscopy generates radicals with nonequilibrium spin populations and causes spectra with unusual signal directions and intensities. The signals may show absorption, emission, or both and be enhanced as much as 100-fold. Deviations from Boltzmann intensities, first noted in 1963, are known as chemically induced dynamic electron polarization (CIDEP). Because the splitting pattern of the intermediate remains unaffected, the CIDEP enhancement facilitates the detection of short-lived radicals. A related technique, fluorescence detected magnetic resonance (FDMR) offers improved time resolution and its sensitivity exceeds that of ESR. The FDMR experiment probes short-lived radical ion pairs, which form reaction products in electronically excited states that decay radiatively. ... 

One of the most startling developments in nmr spectroscopy since its inception has been the discovery of chemically induced dynamic nuclear polarization or CIDNP. An especially dramatic example is provided by irradiation of 3,3-dimethyl-2-butanone with ultraviolet light. 

The text assumes elementary knowledge of the common organic spectroscopic techniques. Nevertheless, we have included a description of the recently developed method of chemically induced dynamic nuclear polarization (CIDNP), which has already proved to be of great importance in the study of radical reactions and which has not yet found its way into books covering spectroscopy of organic compounds. 

Since the heroic early mechanistic investigations, there have been two developments of major significance in radical chemistry. The first was the advent of electron spin resonance (ESR) spectroscopy (and the associated technique of chemically induced dynamic nuclear polarisation, CIDNP) [24], which provided structural as well as kinetic information the second is the more recent development of a wide range of synthetically useful radical reactions [20]. Another recent development, the combination of the pulse radiolysis and laser-flash photolysis techniques, is enormously powerful for the study of radicals but beyond the scope of this book. 

Appropriate modifications of the ESR spectrometer and generation of free radicals by flash photolysis allow time-resolved (TR) ESR spectroscopy [71]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They may be enhanced as much as one hundredfold and may appear in absorption, emission, or in a combination of both modes. These spectra indicate the intermediacy of radicals with substantial deviations from equilibrium populations. Significantly, the splitting pattern characteristic for the spin density distribution of the intermediate remains unaffected thus, the CIDEP (chemically induced dynamic electron polarization) enhancement facilitates the detection of short-lived radicals at low concentrations. 

Chemically induced dynamic nuclear polarization (CIDNP) has been used to discriminate radical and nonradical processes in cycloadditions of electron-rich alkenes and electron-poor carbonyl components <1998MI9>. Spectroscopic observation of the fragmentation of an oxetane radical anion revealed the generation of the most stable alkene radical <2003JOC10103, 2006PPS51>. Transient absorption spectroscopy has been employed to monitor the Paterno-Biichi cycloaddition of benzophenone and furan <2004JA2838>. 

Whereas NMR spectroscopy is generally applied to the diamagnetic allyl, pentadienyl, or cyclopentadienyl cations or anions, this technique is generally not applicable to the analogous radical species, although CIDNP (chemically induced dynamic nuclear polarization) has been observed for allyl140 and pentadienyl radical species139. More useful for the radicals, naturally, is EPR spectroscopy. 

ELECTRON PARAMAGNETIC RESONANCE SPECTROSCOPY AND CHEMICALLY INDUCED DYNAMIC NUCLEAR POLARIZATION... 

The presence of speeies able to initiate polymerization as observed by other spectroscopic tools such as electron spin resonance spectroscopy (ESR), emission spectroscopy, transient absorption spectroscopy or chemically induced dynamic nuclear polarization (CIDNP). 

CIDNP (chemically induced dynamic nuclear polarization) Non-Boltzmann nuclear spin state distribution produced in thermal or photochemical reactions, usually from colligation and diffusion, or disproportionation of radical pairs, and detected by nuclear magnetic resonance spectroscopy by enhanced absorption or emission signals. 

The photochemical cleavage of naphthylmethyl alkanoates in methanol is reported to proceed by homolytic cleavage to naphthylmethyl radical and acyloxy radical,the latter decarboxylates in competition with electron transfer to give naphthylmethyl cation and carboxylate anion. Using known rates of electron transfer as a clock the rate constants for decarboxylation of the acyloxy radicals has been estimated.The light induced homolysis of 1-chloromethyl-naphthalene has also been studied using chemically induced dynamic electron polarisation (CIDEP) spectroscopy to detect the naphthyl-... 

The overall multiplicity of geminate radical pairs formed by bond fission is the same as that of the precursor excited state. Remarkably, the multiplicity of the precursor can often be established by NMR spectroscopy of the final products thanks to a phenomenon called chemically induced dynamic nuclear polarization (CIDNP, Special Topic 5.3). 

Further chemically induced dynamic nuclear polarization (CIDNP) studies have appeared concerning the photoreduction of acetone by propan-2-ol, including the use of a variety of deuteriated components.19 The enol of acetone has been detected by such methods,20 and Bargon and Seifert21 have now observed by CIDNP spectroscopy the ends from acetaldehyde (giving vinyl alcohol) and several aliphatic aldehydes. Vinyl alcohol, for example, is generated in a spin-polarized state by photoreduction of [2He]acetone with ethanol in DaO (Scheme 3). 

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