Other Nuclei

We have seen earlier in this chapter that certain nuclei, other than carbon and hydrogen, also have nuclear spin and are NMR-active. The fact that there are other NMR-active nuclei leads to the question can we run spectra specifically to see these other nuclei The answer is yes. In Table 

2 we can see which nuclei have spin and are NMR-active so that, in theory, the NMR spectrum of any of these nuclei can be recorded. The nuclei of greatest interest to organic chemists, besides H and are N (and N), and Nuclei with 100% abundance, and 

Although you would expect deuterium (D) NMR to be every bit as useful as NMR, it is not carried out routinely as deuterium has a natural abundance of 0.015% and very low sensitivity. Furthermore, as the spin is greater than %, it is a quadrupolar nucleus and the signals are usually broad. Deuterium shows signals in the same chemical shift range as H (5 0-12), and NMR is most often used in experiments using D-labelled compounds to investigate reaction mechanisms. 

Hayashi, K. Akasaka, and H. Hatano, Biochim. Biophys. Acta, 1979, 588, 181. 

studies have shown that the regiospecificity of the tritium label in [n- H]D-glucopyranose (n = 1, 2, 3, 4, 5, and 6), available commercially in aqueous solution, was 100% in each case. After storage some label appears in the water or as a non-glucose impurity. 

A conformational study of nucleic acid phosphate esters has been carried out using n.m.r. and the temperature-dependent chemical shifts used to give information about the geometry of the phosphodiester linkage. Compounds studied were 3 - and 5-O-methyl phosphates of uridine and adenosine, and the 3-0-methyl phosphate of 6-A-dimethyladenosine. It has been reported that the n.m.r. of myo-inositol hexakisphosphate has four resonances that shift and broaden as a function of pH. ° Reference to the use of P n.m.r. in the configurational determination of cyanomethyl-branched phosphonates is made in Chapter 16. 

Deuterium-induced downfield shifts in n.m.r. spectra of labelled D-glucose 6-phosphate have been determined.  

That the aldehydo-group of streptomycin does not cyclize with either the methylamino- or the guanidino-groups to form a four-ring structure has been shown by n.m.r. spectroscopy. Instead it is suggested that the group is probably hydrated and stabilized by the C-3 hydroxy-group of L-streptose,  

Alfoldi, V. Bflik, and L. Petrus, Collect. Czech. Chem. Commun., 1980, 45,123. 

Thioanalogues of adenosine phosphates have been studied by n.m.r. and the chemical shifts were compared to those of the oxygenated compounds. The effects of changing pH and the concentration of added Mg + led to the conclusion that chemical shift data cannot yield unequivocal information concerning the absolute structure of the metal complexes of nucleosides but can be used to monitor changes in chelation, for example, in binding to enzymes.  

Configurational assignments for 2,3-di- and 2,3,4-tri-deoxy phosphonates of the type (18), formed by hydrogenation of compounds of type (2), have been 

NMR can, in principle, provide complementary information on motional processes in liquid crystals. The dipole-dipole interaction between a C-H pair and the quadrupolar interaction when the proton is replaced with a deuteron share the same principal interaction axis. In the case where the carbon is not directly bonded to a proton, there is still dipole-dipole relaxation by nearby protons, but it is also necessary to include an additional relaxation mechanism, the modulation of the chemical shift anisotropy. Proton spin decoupling is necessary to give well-resolved chemically shifted lines in the mesophase of liquid crystals. Furthermore, it is not practical to determine individual spectral density parameters from measured relaxation rates. Proton-proton dipolar interactions may not be ignored even when observation is exclusively confined to the resonant spin [5.31]. This is because proton relaxation causes population flow among the proton spin levels through dipolar (or scalar) coupling. As a consequence, cross- 

FIGURE 5.4. Plots of spectral densities vs. Tc — T in the nematic phase of 5CB-di5. Open symbols denote 15.3 MHz data, while closed symbols denote 46 MHz data. Q and A denote and J2 (2a ), while O and V denote 

This chapter concludes by pointing out that relaxation of multispin proton systems played a major role in the early days of NMR relaxation measurements on liquid crystals [5.34]. In particular, the detection of director fluctuations [5.35] by means of its characteristic frequency dependence in proton Ti [5.36-5.39] started intensive NMR research on liquid crystals. Since there are many inequivalent proton species in a liquid crystalline molecule, it is impossible to distinguish various atomic sites from a broad proton lineshape, which is a result of strong dipolar couplings. Moreover, translation self-diffusion also modulates the intermolecular dipole-dipole interactions and contributes to proton relaxation in liquid crystals [5.40, 5.41]. Partially deuterated liquid crystals may be used to reduce the number of inequivalent proton species. Proton spin relaxation studies remain as a possible method of probing intermolecular interactions or translational motions in liquid crystals. 

Abragam, Principles of Nuclear Magnetism (Clarendon Press, Oxford, 1961). 

Nuclear Magnetic Resonance of Liquid Crystals, edited by J.W. Emsley (D. Reidel Publishing Co., Dordrecht, 1985), Chap. 11. 

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The shifts for other nuclei are not usually so simple to interpret, because more orbitals are involved, and... 

The diagonal elements of the core Ham iltonian simply represent theenergy ofan electron in an atom ic orbital of the correspondin g atom, plus the attraction ofan electron in th at atom ic orbital (on atom A) for the other nuclei (atoms D). 

Every electron in a molecule has a Coulombic attraction to its own nucleus Hii = J iH idx. In addition, it has an athaction to all other nuclei in the molecule Hij = J Coulombic attraction between nuclei and the electrons normally... 

Not only is pulsed FT NMR the best method for obtaining proton spectra it is the only practical method for many other nuclei including It also makes possible a large number of sophisticated techniques that have revolutionized NMR spectroscopy... 

Other Nuclei. Although most nmr experiments continue to involve H, or both, many other nuclei may also be utilized Several factors, including the value of I for the nucleus, the magnitude of the quadmpolar moment, the natural abundance and magnetogyric ratio of the isotope, or the possibihty of preparing enriched samples, need to be considered. The product of the isotopic parameters can be compared to the corresponding value for providing a measure of relative sensitivity or receptivity. Table 1 summarizes these factors for a number of isotopes. More complete information may... 

In the following 55 problems, the chemical shift value (ppm) is given in the scale below the spectra and the coupling constant (Hz) is written immediately above or below the appropriate multi-plet. Proton NMR data are italicised throughout in order to distinguish them from the parameters of other nuclei ( C, N). 

Uranium in the fuel of a nuclear power plant is designated U. The 92 protons and 143 neutrons in a U nucleus sum to 235, the number in the U notation. Through interaction with a neutron the 92 protons and 144 neutrons involved are rearranged into other nuclei. Typically, this rearrangement is depicted as... 

Some heavy nuclei will fission spontaneously. Others can be induced to fission through interaction with a neutron. In both spontaneous nuclear fission and induced nuclear fission the pool of neutrons and protons is conseiwed. For example, the nucleus "" Cf (Californium) fissions spontaneously. The 98 protons and 154 neutrons in the nucleus of Cf are reconfigured into other nuclei. Usually a few neu-... 

Cyclitol Spectra at 220 MHz with the Superconducting Solenoid. In 1964, Nelson and Weaver (34) at Varian Associates constructed a superconducting solenoid with which proton spectra can be observed at 51.7 kilogauss (220 MHz.) or even higher fields. Other nuclei have been observed at suitable field/frequency combinations. 

Of these, two ( gC, gC) are stable in the sense that they do not decompose over time. In contrast, the three lighter nuclei ( C, LgC, gC) and the three heavier nuclei ( 0, jC, gC) are unstable as time passes they decompose to other nuclei. 

Instead of producing new kinds of substances by combination of atoms, the element uranium has combined with a neutron and as a result has split into two other elements—barium and krypton—plus three more neutrons. Atoms of a given element are characterized by their atomic number, the number of units of positive charge on the nucleus. For one element to change into another element the nucleus must be altered. In our example the uranium nucleus, as a result of reacting with a neutron, splits or fissions into two other nuclei and releases, in addition, neutrons. ... 

Actually, then, by our symbol jjU we are representing not an atom, but a nucleus. Our equation is written in terms of nuclei and particles associated with them. This nuclear equation tells us nothing about what compound ol uranium was bombarded with neutrons or what compound of barium is formed. We are summarizing only the nuclear changes. During the nuclear change there is much disruption of other atoms because of the tremendous amounts of energy liberated. We do not know in detail what happens but eventually we return to electrically neutral substances (chemical compounds) and the neutrons are consumed by other nuclei. 

Similar calculations can be made for other nuclei. A significant comparison between nuclear binding energies can be made if we divide the total binding energy of each nucleus by the... 

The probability per site of forming a nucleus on an infinite substrate in the absence of other nuclei is taken to be equivalent to the nucleation rate, i. This obviously assumes that neighbouring patches do not collide during their formation, which is fully consistent with the nucleation model (see Sect. 3.4.4). 

The usable time ranges can be estimated from the following considerations. The slow and fast exchange limits correspond to r-1 relaxation times are roughly 10-3 < r < 1 s for protons, or less for other nuclei with larger (8 v) values. 

Carbon-12 is the principal isotope of carbon, but a small proportion of carbon-14 is present in all living organisms. Its nuclei are produced when nitrogen nuclei in the atmosphere are bombarded by neutrons formed in the collisions of cosmic rays with other nuclei ... 

FIGURE 17.24 A self-sustaining chain reaction, in which neutrons are the chain carriers, takes place when induced fission produces more than one neutron per fission event. These newly produced neutrons can stimulate fission in increasingly greater numbers of other nuclei. 

Other nuclei besides the proton have a nuclear spin of 2 and, in principle at least, may be suitable for nuclear magnetic... 

Both net and multiplet effects must normally be considered except in two special cases (i) when = 0 and only multiplet effects are observed and (ii) when ai = 0 in which case there is no CIDNP to observe. In addition, if there is no coupling between a given nucleus or nuclei and any other nuclei in the product, the n.m.r. spectrum will be a single peak, which of necessity can show only net polarization. 

CIDNP studies of the decomposition have centred mainly on thermal decompositions photochemical decomposition has generally been less intensively investigated. While most reports of polarization refer to n.m.r. spectra, a number of papers have described polarization of other nuclei, (Kaptein, 1971b Kaptein et al., 1972), (Lippmaa el al., 1970a, b, 1971 Kaptem, 1971b Kaptein et al., 1972 Kessenikh et al., 1971), and F (Kobrina et al., 1972) contained in the peroxide reactant. Additionally, polarization of P has been reported in the products of decomposition of benzoyl peroxide in phosphorus-containing solvents (Levin et al., 1970). 

Eq. (1) has potential application to other types of measurements of substituent effects besides those specifically considered in this paper e.g., nmr coupling constants and shifts for other nuclei, ir and uv spectral shifts and intensities. We caution (with emphasis) in these applications the needed use of data sets of high quality, both with respect to the precision of the measurement and substituents considered (i.e., a full complement of substituent o/ and Or properties must be encompassed for a meaningful correlation to be obtained). There is, of course, no requirement that all data sets will be uniquely fitted by eq. (1) using one of the four or scales of Table V. For example, the data for the ionization of the conjugate acids of pyridine-N-oxides (30), HjO, 25° is found to fit equally well the or(ba.) or Or scales (SD=. 14 /=. 072). The data (31) for the rates of alkaline ("OMe) cleavage of ArSnMea are not fitted to acceptable precision (fs >. 23) by any of the Or parameters. This data set is nevertheless indicated... 

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