Natural Isotopic Abundances

Figure Bl.11.1. Resonance frequencies for different nuclei in a field of 14.1 T. Widths indicate the quoted range of shifts for each nucleus, and heights mdicate relative sensitivities at the natural isotopic abundance, on a log scale covering approximately six orders of magnitude. Nuclei resonatmg below 140 MHz are not shown.

Natural Isotopic Abundances. The relative abundances of natural isotopes produce peaks one or more mass units larger than the parent ion (Table 7.75a). For a compound C H O N, a formula allows one to calculate the percent of the heavy isotope contributions from a monoisotopic peak, Pto the Pm + 1 peak ... 

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. 

Table 2.5 Natural isotopic abundances and monoisotopic masses of common elements...

Recently the 29Si hyperfine interaction from the nearest and next-nearest neighbors has been studied (Kiefl et al., 1988b). The experiments are difficult because of the low natural isotopic abundance (4.7%) of 29Si... 

Nucleus Nuclear spin quantum number / Magnetic moment, (ampere square metre x 1027) Resonance frequency in MHz at 1.4092 TESLA Relative sensitivity at the natural isotopic abundance... 

Carbon-13 spectra are inherently less complex than proton spectra for two reasons, viz. (a) chemical shifts between 13C nuclei in different chemical environments can differ by as much as 200 ppm whereas proton shift differences are seldom more than 10 ppm (b) as the natural isotopic abundance of 13C is only 1.1%, coupling between 13C nuclei themselves is not observed (but see 2-D INADEQUATE spectrum, p. 420). 

Average Mass The mass of an ion, atom, or molecule calculated using the masses of all isotopes of each element weighted for their natural isotopic abundance. See also related entries on accurate mass dalton molar mass monoisotopic mass nominal mass unified atomic mass unit. 

Table 5.4 represents natural isotopic abundances of the most important (for organic compounds) elements. This table is a fragment of the complete table of isotopes of... 

TABLE 5.4. Natural Isotopic Abundances of Widespread Chemical Elements... 

Element Isotope ratio1 Natural isotopic abundance (%) Substances studied Application... 

Example Isotopic enrichment is a standard means to enhance the response of an analyte in nuclear magnetic resonance (NMR). Such measures gain importance if extremely low solubility is combined with a large number of carbons, as is often the case with [60]fullerene compounds. [19] The molecular ion signals, IVT, of Qo with natural isotopic abundance and of C-enriched Cgo are shown below (Fig. 3.11 for EI-MS of [60]fullerenes cf. Refs. [20-22]). From these mass spectra, the enrichment can be determined by use of Eq. 3.1. For Qo of natural isotopic abundance we obtain Mrceo = 60 x 12.0108 u = 720.65 u. Applying Eq. 

Fig. 3.11. Comparison of the molecular ion signals, M , of [60]fullerene with natural isotopic abundance and of C-enriched Cgo- By courtesy of W. Kratschmer, Max Planck Institute for Nuclear Physics, Heidelberg.

Because of the favorable cross-peak multiplet fine-structure, the HSQC experiment offers superior spectral resolution over the HMQC (heteronuclear multiple quantum coherence) experiment [13, 14], On the other hand, the HMQC experiment works with fewer pulses and is thus less prone to pulse imperfections. The real advantage of the HSQC experiment is for measurements of samples at natural isotopic abundance and without the use of pulsed field gradients, since the HSQC experiment lends itself to purging with a spin-lock pulse. Spin-lock purging in the HMQC experiment... 

Fig. 2. N HSQC spectrum of a 75 mM solution of Pro -cyclosporin in CDCI3 at natural isotope abundance using the pulse sequence of fig. 1 without N decoupling during acquisition, t = 5.7 ms, SL = 2.5 ms. An additional, short spin-lock pulse was used right before signal detection [8]. The projections are shown at the top and on the left. (Reproduced by permission of Academic Press from...

Selecting the C-bound protons before performing a homonuclear two-dimensional experiment enables to measure small heteronuclear coupling constants [16]. Such an experiment with a sample of natural isotopic abundance was first published by Otting and Wuthrich in 1990, where the half-filter element with spin-lock purge pulse was used to select the C-bound protons in a small protein in aqueous solution [6]. Later applications illustrated the usefulness of the same half-filter element with smaller molecules [17, 18]. 

Excellent results are routinely obtained when applying PFGs to the transverse C coherence in a C HMQC experiment which is conducted at natural isotopic abundance [27]. Under those circumstances, however, half of the coherence transfer pathways are rejected by the PFGs. More complicated acquisition schemes and additional pulses in the sequence are required to restore the full sensitivity [26, 28, 29]. Spin-lock purge pulses may not purge as well as PFGs, but they are easier to use and don t interfere with a phase-sensitive recording. 

An important paper by Salomon, Clennan and coworkers on dialkyl peroxides , where also one ozonide was reported, appeared in 1985. In this paper, a correlation among and 0 chemical shifts was established, and the influence of several factors like concentration, temperature, solvent and, naturally, chemical structure was thoroughly studied but confined to dialkyl peroxides. There was a gap of several years before the appearance of a further note reporting data of seven 1,2,4-trioxolanes (ozonides), 1-4, and of the 1,2,4,5-tetroxane 5. Their 0 NMR data are given in Table 2. Spectra were obtained at natural isotopic abundance, in toluene at 27 °C. 

Following this work on NMR spectra of ozonides, there is an extensive paper by the Griesbaum group" where 35 ozonides (6-14 with different stereochemistries) have been studied. The widely different structures examined allowed the influences of structural features on "O NMR spectra of ozonides to be shown. Five structurally different types of ozonides can be recognized symmetrically tetrasubstituted (type 6), unsymmetrically tetrasubstituted (type 7), unsymmetrically tri- and tetrasubstituted (type 8), unsymmetrically disubstituted (type 9-13) and bicyclic ozonides (type 14). "O NMR chemical shifts of peroxidic and ethereal oxygens are collected in Table 3. All spectra were obtained at natural isotopic abundance, in benzene-dg solution mainly at 25 °C, although in some cases higher temperatures had to be used. These experimental conditions make for an easy comparison with the previously discussed data. 

Substituted pyridines Insensitive nuclei enhanced by polarization transfer procedure (INEPT) for 1SN NMR with natural isotope abundance 2J, 3J and 4/(15N, H) coupling constants obtained in (CD3)2CO and DMSO-d6 81OMR(16)170... 

Applications to Biological Samples. - Methods of distance measurements were compared for four doubly spin-labelled derivatives of human carbonic anhydrase.53 The distances between the spin labels were obtained from continuous wave spectra by analysis of the relative intensity of the half-field transition, Fourier deconvolution of the line-shape broadening, and computer simulation of line-shape changes. For variants with interspin distances greater than 18 A, the DEER method also was used. For each variant, at least two methods were applicable and reasonable agreement between distances obtained by different methods was obtained. The useful distance ranges for the techniques employed at X-band with natural isotope abundance spin labels were estimated to be half-field transition (5-10 A), line-shape simulation (up to 15 A), Fourier deconvolution (8 - 20 A), and four-pulse DEER (> 18 A).53... 

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