Deuterium substitution and

Another separation method in using nonselective relaxation-rates is based on deuterium substitution, and utilizes the fact that the efficiency with which a nucleus contributes to the relaxation of a neighboring nucleus is proportional to the square of the magnetogyric ratio of the donor nucleus, that is, Phd/PHH = /3(yo/yn) - Because yo/yn — l/, replacement of a proton by a deuterium nucleus would be expected to contribute to the... 

Both positive and negative shifts have been observed with deuterium substitution, and although the directions of the observed frequency shifts are not readily understood, it is significant that C02 molecules that have similar orientations and spectroscopic features show similar shifts with deuteration. 

Lithiation of aryltriazenes followed by treatment with an electrophile provides a new approach to benzylamines. The regioselectivity of the reaction can be controlled by means of the substituents on the aryl group. The reaction consists of an intramolecular carbon-carbon bond formation with the aryl ring of a lithiated alkyl group on a 3-nitrogen atom, a 1,2-proton shift, demonstrated by deuterium substitution, and the subsequent release of nitrogen gas.15... 

The solute molecule is dissolved in the liquid crystal solvent at low concentration. A variety of nematic solvents are available, some of which are nematic at room temperature. Representative high-resolution proton NMR spectra are given in Figure 1. Because the solvent order depends on composition and temperature, it is important that temperature and composition gradients at the NMR probe be minimized if the narrow line widths of a few hertz are to be obtained. The spectra of Figure 1 show the rapid increase of spectral complexity with the number of nuclei. The spectra become almost continuous and uninterpretable at about 10 spins. Simplified proton NMR spectra can be obtained by partial deuterium substitution and decoupling.6 This has been described for cyclohexane, but has not been used extensively. Proton double resonance is also a useful experimental technique for the identification of spectral lines.6... 

The magnitudes of the isotopic changes thus found for acrolein are in the range 0 to —0.003 A for the CH bond upon deuterium substitution, and in the range 0.0001 A for the CC and CO bonds upon or substitution. It is important to note that whereas in a diatomic molecule the substitution of a heavier isotope always causes rz to decrease, in a polyatomic molecule a change of either sign is possible. This is a consequence of the opposite signs of the terms in S( ) and in... 

A complete study of optical rotation would include infrared rotatory dispersion, the effect of deuterium substitution, and other vibrational effects. Moffitt and Moscowitz (1959) have considered the vibrational structure of visible and ultra-violet rotatory dispersion curves in a general manner. We will only consider two specific questions related to the vibrations of the system. These are (1) Do vibrations effect the frequency of an electronic transition in the system (2) Do vibrations effect the total rotational strength of an electronic transition The answer to the finst question is yes the answer to the second question is no, not to our approximation. 

Evidence for the formation of the first dizinc(I) salt was observed in 1967 by introducing Zn into molten ZnClj [16]. The existence of Zn2Cl2 was proven by UV-vis, Raman, and ESR (Electron Spin Resonance) spectroscopy as well as magnetic susceptibility measurement and chemical methods however, no crysttJ structure has been obtained yet. Dizinc(l) dihydride, ZnjHj, was claimed to be produced in 1995 by the matrix-isolation technique and could be identified by spectroscopic methods involving deuterium substitution and comparisons with quantum mechanical calculations [17]. 

The pro-(R) and pro-(S) hydrogen atoms in methylene groups of a-deuterated carboxylic acids can be distinguished in derivatives with methyl mandelate 37. The conformational preference and shielding of the phenyl ring of 37 is used to assign the positions of deuterium substitution and absolute configuration. " ... 

The high depth resolution, nondestructive nature of thermal neutrons, and availability of deuterium substituted materials has brought about a proliferation in the use of neutron reflectivity in material, polymer, and biological sciences. In response to this high demand, reflectivity equipment is now available at all major neutron facilities throughout the country, be they reactor or spallation sources. 

Fig. 4.9. DifiBoing zero-point energies ofprotium- and deuterium-substituted molecules as the cause of primary kinetic isotope effects.

Mixtures of EE Cl and [EMIMjCl have also been studied [9, 10]. By analysis of the first order differences by hydrogen/deuterium substitution both on the imidazoli-um ring and the EE Cl, two intramolecular peaks were observed. These indicated the presence of [EECI2] as an asymmetric species, which, coupled with analysis of the second order differences, allowed the structure in Figure 4.1-3 to be proposed. 

THE EFFECT OF RING-DEUTERIUM SUBSTITUTION ON THE RATE AND PRODUCTS OF THE REARRANGEMENT OF HYDRAZOBENZENE IN 90% ETHANOL AT 0 °C... 

The methyl groups were then deuterium-substituted. Ethanolysis of (CD3)2CHBr gave a mixture of CD3CH = CD2 and (CD3)2CHOC2H5. The elimination pathway had hAd = 6.7, whereas the substitution pathway showed no measurable kie. Clearly, these observations imply nearly complete proton abstraction in the one transition state and the lack of C-H involvement in the other. 

The work of Melander and Carter (1964) on 2,2 -dibromo-4,4 -di-carboxybiphenyl-6,6 -d2 (1) has been referred to above in the introductory and theoretical sections, where it was pointed out that the availability of two detailed theoretical computations of the inversion barrier (Westheimer and Mayer, 1946, Westheimer, 1947 Hewlett, 1960) made this system especially attractive for the study of steric isotope efifects. Furthermore, in the preferred initial-state conformation the two bromines are probably in van der Waals contact (cf. Hampsoii and Weissberger, 1936 Bastiansen, 1950), and thus initial-state steric effects are unaffected by deuterium substitution in the 6 and 6 positions. The barrier calculations provided two different theoretical values for the non-bonded H Br distance in the transition state which, together with the corresponding H Br potential function, could be inserted in equation (10) to yield values for A AH. For... 

Deuterium Substitution. The a and P secondary isotope effects affect the rate in various ways (p. 298). The measurement of a secondary isotope effects provides a means of distinguishing between SnI and Sn2 mechanisms, since for Sn2 reactions the values range from 0.95 to 1.06 per a D, while for S l reactions the values are higher. This method is especially good because it provides the minimum of perturbation of the system under study changing from a H to a D hardly affects the reaction, while other probes, such as changing a substituent or the polarity of the solvent, may have a much more complex effect. 

Thus, identification of all pairwise, interproton relaxation-contribution terms, py (in s ), for a molecule by factorization from the experimentally measured / , values can provide a unique method for calculating interproton distances, which are readily related to molecular structure and conformation. When the concept of pairwise additivity of the relaxation contributions seems to break down, as with a complex molecule having many interconnecting, relaxation pathways, there are reliable separation techniques, such as deuterium substitution in key positions, and a combination of nonselective and selective relaxation-rates, that may be used to distinguish between pairwise, dipolar interactions. Moreover, with the development of the Fourier-transform technique, and the availability of highly sophisticated, n.m.r. spectrometers, it has become possible to measure, routinely, nonselective and selective relaxation-rates of any resonance that can be clearly resolved in a n.m.r. spectrum. 

The second separation method involves n.O.e. experiments in combination with non-selective relaxation-rate measurements. One example concerns the orientation of the anomeric hydroxyl group of molecule 2 in Me2SO solution. By measuring nonselective spin-lattice relaxation-rat s and n.0.e. values for OH-1, H-1, H-2, H-3, and H-4, and solving the system of Eq. 13, the various py values were calculated. Using these and the correlation time, t, obtained by C relaxation measurements, the various interproton distances were calculated. The distances between the ring protons of 2, as well as the computer-simulated values for the H-l,OH and H-2,OH distances was commensurate with a dihedral angle of 60 30° for the H-l-C-l-OH array, as had also been deduced by the deuterium-substitution method mentioned earlier. 

However, the relaxation contributions obtained from Eq. 22 were not satisfactorily compared with those obtained from specific, deuterium-substitution experiments and single- and double-selective relaxation-rates. Moreover, the errors estimated for the triple-pulse experiments were very much larger than those observed for the other techniques. This point will be discussed next. 

Combinations of non-selective and/or single-selective relaxation-rates, or both, with n.0.e. values may conveniently be performed with reliable results, especially when other methods seem impractical. However, these experiments are time-consuming, as they entail the determination of a rather large number of experimental values. Moreover, the n.O.e. parameters carry their own systematic and random errors, which are magnified in the calculation of interproton distances. The deuterium-substitution method requires specific deuteration at a strategic position, which, in many cases, may be inconvenient or impractical. Also, this technique is valid only when the relaxation rates obtained after deuterium substitution are at least 5% enhanced, relative to the relaxation rates of the unsubstituted compound, and it requires that, for a meaningful experiment, the following condition " be satisfied. 

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