Chemically induced magnetic spin

Another burst of activity in free radical research occurred in the 1960s and 1970s, after several reports of anomalous intensities in the EPR spectra of photochemically or radiolytically produced radicals, and in the NMR spectra of the products from free radical reactions in solution." " These so-called chemically induced magnetic spin polarization (CIDNP and CIDEP) phenomena provided a wealth of mechanistic, kinetic, dynamic, and structural information and were a cornerstone of carbon-centered free radical research for the better part of three decades. The umbrella term for this area of research is spin chemistry, which is defined as the chemistry of spin-selective processes. 

A.R. Lepley, G.L. C loss. Eds.. Chemically Induced Magnetic Polarization, Wiley, New York, 1973 K.M. Salikhov, Yu.N. Molin. R.Z. Sagdccv, A.L. Buchachcnko, Spin Polarization and Magnetic Effects in Radical Reactions. Elsevier, Amsierdam, 1984. 

NMR spin relaxation is not a spontaneous process, it requires stimulation by a suitable fluctuating field to induce an appropriate spin transition to reestablish equilibrium magnetization. There are four main mechanisms for obtaining relaxation dipole-dipole (most significant relaxation mechanism for I = 1/2 nuclei), chemical shift anisotropy, spin rotation, and quadrupolar (most significant relaxation mechanism for I > 1/2 nuclei) (Claridge, 1999). 

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. ... 

The ion pair spin multiplicity may be a valuable tool to affect the BET rates and to probe the ion pair dynamics via magnetic field effects [36], Even weak magnetic fields are known to influence relative probabilities of singlet and triplet reactions [34], Chemically induced dynamic nuclear polarization (CIDNP) is a particularly informative technique [12]. Many bond scission reactions and rearrangements in cyclic radical ions have been successfully explored using this approach. Both structural data (spin densities) and approximate kinetic informations are indirectly available from such experiments [12]. 

Chemically induced dynamic nuclear polarization (CIDNP) is a nuclear magnetic resonance method based on the observation of transient signals, typically substantially enhanced, in either absorption of emission. These effects are induced as a result of magnetic interactions in radical or radical ion pairs on the nanosecond time scale. This method requires acquisition of an NMR spectrum during (or within a few seconds of) the generation of the radical ion pairs. The CIDNP technique is applied in solution, typically at room temperature, and lends itself to modest time resolution. The first CIDNP effects were reported in 1967, and their potential as a mechanistic tool for radical pair reactions was soon recognized [117, 118]. Nuclear spin polarization effects were discovered in reactions of neutral radicals and experiments in the author s laboratory established that similar eflects could also be induced in radical ions [119-121]. 

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. 

When a free radical is produced in a chemical reaction, the orientation of the electron spins with respect to the external magnetic field is random, so that in effect the electron spin states of the newly formed radicals are saturated. Thus if the newly formed radical is sufficiently short-lived, so that any nuclear polarization induced in the radical has not had time to decay before it becomes a diamagnetic molecule, when the n.m.r. signal is observable, an enhanced nuclear signal may be produced. Furthermore the relaxation of the electrons in the newly formed radicals may also induce nuclear spin polarization in other molecules in the solution. 

Chemically-induced spin polarization was one of the last truly new physical phenomena in chemistry to be discovered and explained during this century. So unusual were the observations and so ground-breaking the theoretical descriptions that, over a very short time period, the chemist s way of thinking about free radical reactions and how to study them was fundamentally changed. After the earliest experimental reports of unusual phases of electron paramagnetic resonance (EPR) (1963) [1] and nuclear magnetic resonance (NMR)... 

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