Between Protons and Other Nuclei

Couplings between protons and the other commonly studied magnetic nuclei ( F, are continually being found and correlated. 

Silicon-29-proton coupling constants have been studied by Ebsworth and his co-workers and directly bonded Si-H coupling constants were shown to follow the relationship— 

In an extensive study of methyltin halides [(CH3),SnX4 jt, X = C1 or Br, x = l to 4] by double-resonance techniques, the Sn- C coupling constants have been determined as well as the 

Tin-119-proton coupling ranges between -I-541 [Sn(CH3)4] and +99-3 c./sec. [(CH3)2SnCl2], and coupling ranges from 

The dependance of N-H coupling constants on the configuration of aldoximes has been observed. In unhindered oximes both syn (33) and anti (34) forms are present. In the syn form the value of is between 2 and 4 c./sec., a higher (10-16 c./sec.) value being observed 

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The quantum description of N coupled protons in Hilbert space is given by a spin Hamiltonian of dimension 2 equalling the number of direct product spin-i states. Two experimental tools have been used for the decoupling of spin interactions, RF irradiation and MAS. In the following, any discussion of sample spinning assumes MAS conditions. MAS effectively eliminates the CSA and DDfcetero interaction between protons and other spin-i nuclei. 

This second chapter on establishing correlations through the chemical bond concentrates on techniques which correlate different nuclides, so-called heteronuclear shift correlations. For an organic chemist this means, in the vast majority of cases, establishing connectivities between proton and carbon nuclei and as such the techniques encountered in the sections which follow are concerned primarily with these. That is not to say that the techniques are not suitable for correlating other nuclides, for example with or P, or even P with and so on. Indeed many of the modem techniques used routinely in the chemical laboratory were originally implemented as methods for correlations in proteins and peptides. The principal techniques described in the sections that follow are summarised in Table 6.1. 

Some features of the EPR spectrum increase with magnetic field, such as the separation between g factors. Others, such as the separation between hyperfine splittings, do not change, to first order. By altering the microwave frequency it is possible to resolve the effects of multiple electron-nuclear and electron-electron interactions. The EPR spectra of organic radicals at X-band (9 GHz) are dominated by hyperfine splittings by protons and other nuclei. At W-band (96 GHz) the spectra are dominated by the anisotropy of the g factor. 

In still other NMR studies of mixtures, Douglass and McBrierty have considered the homogeneity of polyvinylidene fluoride - poly methylmethacrylate blends.(38) Here the energy exchange between proton and fluorine magnetic spins is efficient only if the nuclei are adjacent. The results indicate, when taken around 40°C, that either the fluorine nuclei in a 40 polyvinylidene fluoride/60 poly methylmethacrylate blend are... 

As the weak interaction is the slowest of all, it was the first to find itself unable to keep up with the rapid expansion of the Universe. The neutrinos it produces, which serve as an indicator of the weak interaction, were the first to experience decoupling, the particle equivalent of social exclusion. By the first second, expansion-cooled neutrinos ceased to interact with other matter in the form of protons and neutrons. This left the latter free to organise themselves into nuclei. Indeed, fertile reactions soon got under way between protons and neutrons. However, the instability of species with atomic masses between 5 and 8 quickly put paid to this first attempt at nuclear architecture. The two species of nucleon, protons and neutrons, were distributed over a narrow range of nuclei from hydrogen to lithium-7, but in a quite unequal way. 

Fig. 42. Second moment of the broadline H NMR spectrum at 40 MHz of zeolite H-Y (194) versus the internuclear distance between the proton and other magnetic nuclei.

In addition to providing motional information there is another benefit to be derived from CP studies which will be exemplified further in Section 8 and which depends on the fact that the effectiveness of the CP phenomenon is proportional to r 3, where r is the distance between the / and S nuclei. In other words, the efficiency of transfer of magnetization falls off extremely rapidly as the distance between / and S increases. Bulk Si nuclei in Si02, for example, will be relatively distinct from any proton species, whereas Si nuclei at the surface will be relatively close to surface OH groups (or any chemically bound organic species). The use of cross-polarization Si n.m.r. will therefore enable one to examine Si nuclei at the surface relative to those in the bulk material. 

Recall from Section 1.4 that almost all the mass of an atom is concentrated in a very small volume in the nucleus. The small size of the nucleus (which occupies less than one trillionth of the space in the atom) and the strong forces between the protons and neutrons that make it up largely isolate its behavior from the outside world of electrons and other nuclei. This greatly simplifies our analysis of nuclear chemistry, allowing us to examine single nuclei without concern for the atoms, ions, or molecules in which they may be found. 

There are several other problems associated with quantitation using cross polarization techniques. In determining the aromaticity of humic substances, cross polarization is achieved by applying r.f. magnetic fields at the resonant frequency of both 13C and IH nuclei. By adjusting their relative intensities, contact between the two populations is established and cross polarization occurs between protons and carbons. However, the decay of polarization of the population of nuclei occurs at a rate determined by a time constant T. (H) which is Independent of the rate of cross polarization. 

It must be realized that the effect of spin-spin coupling of protons to other magnetic nuclei may be observed in the proton spectra, and hence some idea of the magnitudes of coupling between protons and some commonly occurring magnetic nuclei may be useful in interpreting proton spectra. 

Clearly the homonuclear and the heteronuclear experiments could be combined in the reverse order i.e. HSQC-NOESY and HSQC-TOCSY. The main advantage in these schemes relates to N-edited experiments in which the narrower amide proton spectral width is sampled during f and the full proton spectral width is collected during t. On the other hand water suppression is more effective when HSQC follows NOESY. Also the sensitivity enhancement can be incorporated into the NOESY-HSQC experiment. Eor the TOCSY-HSQC or HSQC-TOCSY it does not matter because both the TOCSY and HSQC sequences can be implemented with the sensitivity enhancement. It should be mentioned that the TOCSY type of transfer is more effective between C nuclei than between protons and therefore the HCCH-TOCSY experiment is preferred when a doubly labeled sample is available. 

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