Spectrum isotope effect

A SSIMS spectrum, like any other mass spectrum, consists of a series of peaks of dif ferent intensity (i. e. ion current) occurring at certain mass numbers. The masses can be allocated on the basis of atomic or molecular mass-to-charge ratio. Many of the more prominent secondary ions from metal and semiconductor surfaces are singly charged atomic ions, which makes allocation of mass numbers slightly easier. Masses can be identified as arising either from the substrate material itself from deliberately introduced molecular or other species on the surface, or from contaminations and impurities on the surface. Complications in allocation often arise from isotopic effects. Although some elements have only one principal isotope, for many others the natural isotopic abundance can make identification difficult. 

For E2 eliminations in 2-phenylethyl systems with several different leaving groups, both the primary isotope effect and Hammett p values for the reactions are known. Deduce from these data the relationship between the location on the E2 transition state spectrum and the nature of the leaving group i.e., deduce which system has the most El-like transition state and which has the most Elcb-like. Explain your reasoning. 

DEPT-NMR spectrum. 6-methyl-5-hepten-2-ol, 451 Detergent, structure of, 1065 Deuterium isotope effect, 386-387 El reaction and, 392 E2 reaction and, 386-387 Dewar benzene. 1201 Dextromethorphan, structure of, 294 Dextrorotatory, 295 Dextrose, structure of. 973 Dialkylamine, pKa of, 852 Diastereomers, 302-303 kinds of, 310-311 Diastereotopic (NMR), 456... 

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

One way to determine just where a given reaction stands on the El-E2-ElcB spectrum is to study isotope effects, which ought to tell something about the behavior of bonds in the transition state. For example, CH3CH2NMe3 showed a nitrogen isotope effect of 1.017, while PhCH2CH2NMe3" gave a corres-... 

Interest has been shown by several groups on the effect of solvent and of added anions upon the oxidation of alcohols. The oxidation of isopropanol proceeds 2500 times faster in 86.5 % acetic acid than in water at the same hydrogen ion concentration . The kinetics and primary kinetic isotope effect are essentially the same as in water. Addition of chloride ion strongly inhibits the oxidation and the spectrum of chromic acid is modified. The effect of chloride ion was rationalised in terms of the equilibrium,... 

Because fluorine is relatively sensitive to its environment and has such a large range of chemical shifts, considerable changes in chemical shift can be observed when a nearby atom is replaced by an isotope. For example, replacement of 12C by 13C for the atom to which the fluorine is attached, gives rise to a quite measurable shift, usually to lower frequency. A consequence of this isotope effect is the observation that the 13C satellites in a fluorine spectrum are not symmetrical about the 12C—F resonance. 

FIGURE 2.11. 19F NMR spectrum of l,6-difhiorohexane-l,l-d2, demonstrating the deuterium isotope effect on the fluorine chemical shift... 

FIGURE 2.12. CH2F/CD2F region of the 13C NMR spectrum of 1,6-difluorohexane-1,1 -d2 demonstrating deuterium isotope effect on 13C chemical shifts... 

So if this all sounds a bit bleak, what s the good news Well, strangely, there is quite a lot. For a start, let s not forget that had the 13C nucleus been the predominant carbon isotope, the development of the whole NMR technique itself would have been held back massively and possibly even totally overlooked as proton spectra would have been too complex to interpret. Whimsical speculation aside, chemical shift prediction is far more reliable for 13C than it is for proton NMR and there are chemical shift databases available to help you that are actually very useful (see Chapter 14). This is because 13 C shifts are less prone to the effects of molecular anisotropy than proton shifts as carbon atoms are more internal to a molecule than the protons and also because as the carbon chemical shifts are spread across approximately 200 ppm of the field (as opposed to the approx. 13 ppm of the proton spectrum), the effects are proportionately less dramatic. This large range of chemical shifts also means that it is relatively unlikely that two 13C nuclei are exactly coincident, though it does happen. 

Figure 18 shows the spectrum of C3D6 in the CH deformation region. We would expect the normal isotope effect to shift C—D deformations about 400 cm-1, that is, completely out of this region. Thus, the observed bands are due to C—C vibrations. The band at 1473 cm-1 (with a shoulder at 1460 cm-1) can only correspond to the 1545 cm-1 band in C3H6. The... 

A considerable amount of the strain in l,8-bis(dimethylamino)naphtha-lene is relieved by protonation and the N—H N bond length (260 pm) in the protonated amine shows that the molecule is able to adopt a conformation [55] with an intramolecular hydrogen bond (Truter and Vickery, 1972). The infra-red spectrum of protonated l,8-bis(dimethylamino)naphthalene and the chemical shift (5 19.5) of the acidic proton in the nmr spectrum confirm the presence of an intramolecular hydrogen bond (Alder et al., 1968). The magnitude of the isotope effect on the chemical shift (Altman et al., 1978) and the appearance of two Nls peaks in the photoelectron spectrum... 

The spectrum of GeF2 trapped in a neon matrix is shown in Fig. 3. The ratio of GeF2/rare gas in the matrix was 1 1000. When new matrices were prepared similar spectra were obtained, even when the ratio of diluent was changed or the temperature of deposition was altered. This indicated that the splitting seen in the spectrum was due to isotope effects and was not due to matrix effects. As can be seen the intensities of the various peaks are in the same ratio as the abundant isotopes of germanium, providing additional evidence that the splitting is due to isotope effects. 

Prakash et al. (1985) used the deuterium isotope effect on the l3C NMR spectrum of [47] to provide further evidence for the symmetrical, homoaromatic nature of this ion. They prepared the specifically deuterated trishomocyclopropenyl cation [57] by superacid treatment of the corresponding alcohol [58]. The 13C NMR spectrum of [57] displayed a triplet for the deuterated methine only 0.2 ppm to higher field than the undeuterated methine, indicating only an isotopic perturbation of resonance and not a rapidly equilibrating classical ion system (see Siehl, 1987). 

In the spectrum of pure H2O (or D2O), the intermolecular coupling which broadens the free molecule transitions vi and 3 leads to severe overlap of these and (possibly) of the overtone 21 2, thereby complicating interpretation of the observed spectrum. Relatively effective decoupling may be brought about by partial isotopic substitution. In a solution HOD/H2O or HOD/D2O, where either... 

The last discovery of an isotope of an element in the second row of the periodic table was that of 15N. This is credited to R. Naude (1929). In the band spectrum of NO, he observed band heads for not only lsNieO but also 14N180 and 14N1tO on the basis of the expected isotope effect on the reduced mass of the molecule. 

Mulliken, R. S. The isotope effect in band spectra, Part I. Phys. Rev. 25, 119-138 (1925a). Mulliken, R. S. The isotope effect in band spectra, II the spectrum of boron monoxide. Phys. Rev. 25, 259-296 (1925b). 

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