Enhanced absorptive signal

The first reports of the observation of transient emission and enhanced absorption signals in the H-n.m.r. spectra of solutions in which radical reactions were taking place appeared in 1967. The importance of the phenomenon, named Chemically Induced Dynamic Nuclear Spin Polarization (CIDNP), in radical chemistry was quickly recognized. Since that time, an explosive growth in the number of publications on the subject has occurred and CIDNP has been detected in H, C, N, and P as well as H-n.m.r. spectra. Nevertheless, the number of groups engaged in research in this area is comparatively small. This may be a consequence of the apparent complexity of the subject. It is the purpose of this review to describe in a quahtative way the origin of CIDNP and to survey the published applications of the phenomenon in... 

The effects observed for 23 (Figure 8) are particularly clear-cut, since the spectrum is fully resolved." The key to the structure of 23 lies in the prominent enhanced absorption signals of H3 (5.2 ppm) and (-0.2 ppm) and the strong emission of (2,4 ppm), H4 (2.2 ppm), and H5 (1.5 ppm). This polarization pattern supports a spepies with spin density on C3 and C6, indicating the delocalization of spin and charge into the lateral cyclopropane bond. Weakly enhanced absorption observed for H2 (5.8 ppm), (0.8 ppm), and Hi (1.75 ppm), and weak emission for H5 further support this structure type. ... 

Consider the 13C— H bond as a two-spin system. CH coupling occurs between one nucleus with small population difference (13C) and another one with large polarization (1H). Fig. 2.43(a) illustrates this situation by the number of dots on the energy levels. Population inversion of the proton levels 1 and 3 connected by the transition 1H1 is achieved by an appropriate 180° pulse, which turns the double cone of precession shown in Fig. 2.1 upside down. Thereafter, the inverted proton population difference controls both carbon-13 transitions (Fig. 2.43(b)). This is the polarization or population transfer making up an enhanced absorption signal for one transition (e.g. 13Ci in Fig. 2.43 (b)) and an enhanced emission on the other (e.g. 13C2 in Fig. 2.43(b)). 

M. Katyiar, J.R. Abelson Methods to enhance absorption signals in infrared reflectance spectroscopy A comparison using optical simulations. J. Vac. Sci. Technol. A 13, 2005 (1995)... 

In this section, several typical CINPD spectra will be shown. These spectra can be explained by Kaptain s rules. Typical net absorptive and emissive CINDP signals were observed during the thermal decomposition of acetyl peroxide (AP) in hexachloroacetone at 110 °C as shown in Fig. 4-8. Here, enhanced absorptive signals were observed for CH3CI and CH4 and emissive ones for CH3COOCH3 and CH3-CH3. 

Conversely, radicals with an excess of nuclear spins are left over in solution, leading to escape products for which the NMR spectrum shows enhanced absorption signals. 

Clearly, if a situation were achieved such that exceeded Np, the excess energy could be absorbed by the rf field and this would appear as an emission signal in the n.m.r. spectrum. On the other hand, if Np could be made to exceed by more than the Boltzmann factor, then enhanced absorption would be observed. N.m.r. spectra showing such effects are referred to as polarized spectra because they arise from polarization of nuclear spins. The effects are transient because, once the perturbing influence which gives rise to the non-Boltzmann distribution (and which can be either physical or chemical) ceases, the thermal equilibrium distribution of nuclear spin states is re-established within a few seconds. 

Some Cl product signals from hot band transitions due to the enhanced absorption of the vibrationally excited CH2CI radical are observed, particularly for excitation to the lowest l2Ai state in the wavelength region longer than 247nm.114,116 A recent theoretical study has... 

The reaction of durene with H NOj under the conditions just mentioned is another specific but principally important case. The reaction is accompanied with a strong enhanced absorption in N NMR spectrum with respect to a signal belonging to the product, that is, nitrodurene. Durene and naphthalene have very similar standard potentials (2.07 and 2.08 V, respectively, in AN see Ridd 1991). A significant difference between them is that, with durene, much of the nitration supposedly arises from ipso attack followed by rearrangement according to Scheme 4.41. 

The spectra shown in Figure 4.4 were obtained from a reaction of fert-butyl-lithium with l-bromobutane. ° One deduces that both the fert-butyl and the butyl radical were produced and that they reacted in disproportionation reactions to give in part isobutylene and 1-butene, respectively. In the spectrum recorded at 30 s, vinyl proton signals from 1-butene are in emission (5 5.0 and 5 6.1) and enhanced absorption (5 5.6). The isobutylene vinyl proton signal at 5 4.6 is in emission on the left side and enhanced absorption is on the right side. This phenomenon is known as a multiplet effect, and it is due to differences in ISC rates for radical pairs containing rcrt-butyl radicals with different proton nuclear spins. Note that the tert-butyl-lithium sample contained an impurity of isobutylene before the reaction, and the amount of isobutylene was increased after the reaction. 

Among NMR methods providing insight into radical ions, chemically induced dynamic nuclear polarization (CIDNP) has proved especially useful it results in enhanced transient signals, in absorption or emission CIDNP effects were first reported in 1967 their application was soon extended to radical ions. The method lends itself to modest time resolution. 

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