Proton polarized

It may be concluded that during the contact time in the competing process for the energy in the various spin systems, the carbon atoms are trying to reach thermal equilibrium with the proton polarization, which is in itself decreasing with a time constant, (Tig, H). In fact the protons undergo spin diffusion and can be treated together, whereas the carbon atoms behave individually. Therefore one implication is that we can also expect to obtain a C-13 spin polarization proportional to the proton polarization. 

Studies of alkyl halide-lithium alkyl reactions have been almost wholly concerned with proton polarization. The one exception to date is an investigation of polarization in the reaction of n-butyl lithium with p-fluorobenzyl chloride giving p,p -difluorobibenzyl (A/E multi-plet) and l-fluoro-4-pentylbenzene (E/A) (Rakshys, 1970). Surprisingly H-polarization is not observed. 

Likewise, the original proton polarization can be transferred to other magnetically active heteronuclei, notably to 13C, 15N, 19F, 29Si, 31P or appropriate isotopes of various transition metals. This is especially attractive because of the frequently low sensitivity of many heteronuclei, in particular of those with low magnetic moments [8]. 

It has been postulated [39] and demonstrated previously that Hetero-PHIP for nuclei such as 13C, 29Si, and 31P can result in a signal enhancement (SE) > 10 [8, 40, 41], particularly if the reactions are carried out at low magnetic fields (i.e., under ALTADENA conditions). 31P INEPT(+n/4) experiments have also been carried out to transfer the initial proton polarization to 31P [8]. 

Transfer of the initial proton polarization is not confined to other protons or 13C, but the signals of other heteronuclei (2H, 15N, 29Si, 31P) in the hydrogenation products can also become substantially enhanced, thereby also increasing their receptivity. Accordingly, the transfer of the PHIP-derived high spin order to 19F has been accomplished using a set of chemically similar fluorinated styrene and ethynylbenzene derivatives. 

Likewise, the initial proton polarization may be transferred to other magnetically active heteronuclei, most attractively to those associated with a low y-value of their nucleus (i.e., to 15N, 29Si), and similarly difficult ones, using heteroatom PHIP at low magnetic fields [8, 45]. 

For all the reasons mentioned above, it is more desirable to obtain PASADENA spectra when applying the PHIP method, at least initially. Nevertheless, in certain cases ALTADENA spectra can be very helpful, too, especially for a quick screening, because these experiments can be performed without any specialized NMR probes. Rather, in these cases, standard NMR equipment will suffice totally. Furthermore, in other cases, the (normally unwanted) isotropic mixing can be used advantageously to obtain information about other additional aspects of the system. A typical such case results when it is desirable to transfer the original proton polar-... 

As shown in Fig. 7.1, at large distances, the system can be considered as a neutral hydrogen atom plus an isolated proton. The field of the proton polarizes the hydrogen atom to induce a dipole. The interaction between the proton and the induced dipole generates a van der Waals force. The van der Waals force can be treated as a classical phenomenon by introducing a phenomenological polarizability a ... 

B This pulse sequence, the ID DEFT (Distorsionless Enhancement by Polarization Transfer) experiment, was developed to measure carbon chemical shifts with enhanced sensitivity and to determine at the same time their multiplicities, to differentiate between CH, CH, CH and C. It is a heteronuclear multiple pulse experiment with pulses applied to perturb both carbon and proton spins. Il consists of a preparation, a mixing (used to transfer proton polarization to the directly bound carbons) and a detection period. 

The antiphase relationship of the C13 —C13 doublet signals in the INADEQUATE spectrum can be eliminated by an additional spin-echo sequence (— 1/4Jcc — 180° — 1 /4 Jcc —) before the 90 monitor pulse [58]. The sensitivity of the experiment may be improved by the application of stronger magnetic fields or by using proton polarization transfer techniques [59]. 

The first combined 13C- H MAS/CP experiment was performed by Schaefer and Stejskal (23). The double-resonance procedure decouples the strong heteronuclear dipolar interactions and indirect J couplings, while the weak 13C signal is enhanced by proton polarization transfer. The residual spectral width of several kHz arises from 13C chemical shift anistropy and weak 13C- 3C dipolar interactions between 1.1 % abundant 13C nuclei. Both of these interactions are removed by MAS. 

This evident similarity of proton polarizations observed for 9 and 10 shows that, in the case of 9, XH CIDNP effects could only be employed to trace the fate of COCHMe2 and CHMe2 radicals and the pathways of formation of the reaction products that do not contain organotin function (Table 3). To facilitate analysis of the structure and multiplicity of the initial radical pair and the reaction pathways of tin-centered radicals, it is much more convenient to employ 13C and 119Sn CIDNP techniques. 

E - emission, A - absorption denote proton polarizations observed in equations 11 and 12... 

In order to study the molecular aggregation during the volume relaxation of network epoxies, CP/MAS carbon-13 (natural abundance) NMR was utilized. The Hartman-Hahn cross-polarization technique 129) was used with a cross contact time of 1 mi llisecond for transfer of proton polarization to carbon nuclei. The protondecoupling was achieved at the radio frequency of 56.4 MHz. Carbon-13 14.2 MHz spectra were measured in a 1.4 Tesla magnetic field. Room temperature (23 °C) experiments were performed at 54.7° MAS at 1 KHz. The spinner was constructed using an Andrew-type rotor driven by compressed air. 

A generalized CP pulse sequence is shown in Fig. 4.5.2, with vertical displacements indicating transmitter power and the horizontal axis indicating elapsed time. A CP pulse sequence begins with a pulse delay, td, to allow recovery of proton polarization along the static field of the NMR magnet. This can be achieved if td greatly exceeds T,(H), the time constant for recovery of equilibrium polarization. 

We have seen that the 180° 13C pulse reordered the 13C populations, but transfer of proton polarization to the 13C system occurred only when the and 13C 90° pulses were applied at time 4. We chose to use a matrix that represented the concurrent application of both pulses. However, these are independent, their Hamiltonians commute, and we could have considered instead the application of the two pulses separately—in either order. Here we examine the effect of applying the H pulse. 

Observation of the polarization effects of the reaction product, germacyclopropene 22, confirms its formation from paramagnetic precursors. However, in the case under study, one could hardly imagine the generation of a radical pair, and therefore it is reasonable to suggest that the intermediate step of the insertion of 16 into the triple bond of thiacycloheptyne 21 involves the formation of a 1,3-biradical. Analysis of the observed proton polarization effects (Table 8 and Figure 16) in accordance with the existing rules allows us to propose the reaction mechanism in Scheme 11. 

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