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Carbon isotopes split

Table 1 summarizes some of the important properties of the carbon isotopes. Note that only the rare ( 1%), naturally occurring, stable carbon isotope, namely, C, has a nuclear spin and is observable by NMR. The organic chemist is fortunate that 99% of natural carbon is the isotope C with no nuclear spin, so that proton and carbon-13 NMR spectra of organic compounds are not complicated by spin - spin splitting arising fi om adjacent carbon atoms. The radioisotope C is made by thermal neutron irradiation of lithium or aluminum nitride (equation 1). It decays back to stable yN by jS emission, with a half-life of 5570 years (equation 2). Cosmic rays generate thermal neutrons, which leads to the formation of C02 in the atmosphere (equation 1). Metabolism of... [Pg.627]

Another important difference is that since the main carbon isotope, does not possess nuclear spin, most hydrogen atom spectra do not have any splitting from the carbon atom to which the hydrogen is attached. The few hydrogen atoms which are attached to atoms and hence are split produce tiny peaks which are usually lost in the background electronic noise on the spectrum. Consequently, we need consider only to coupling, unless we have present other atoms with nuclear spin such as fluorine or phosphorus. [Pg.149]

Figure 9 gives a brief overview of the sample preparation procedure used in onr laboratory. One point of note is that drying is carried out at 40 °C, instead of the 80-KX) C that is normal in many laboratories. Experience from organic geochemistry indicates that the lower temperature is preferable to ensure that no volatile components are lost before analysis. Splits from the dry, powdered sample are also used for carbon isotopic analysis and RockEval pyrolysis (Talbot Livingstone, 1989 Talbot Laerdal, 2000 Fig. 9), in fact it is our practice to perform the analysis first, as %N data from the elemental analyser allows us to estimate how much sample needs to be weighed for the N-isotope determination. As little as 100 /xg N are required for analysis. [Pg.417]

The observed shift difference between the deuteriated carbon and the P-carbon was corrected for intrinsic isotope shifts which were taken from model compounds. This gave an equilibrium isotope splitting of 8 = 0.589 ppm at 25°C. The approximate formula for calculating the equilibrium constant is = (A -I- 5)/(A — 48) assuming equivalent olehnic positions. Using A = 90 ppm as an approximate shift difference between the metal-substituted and olehnic carbons, one obtains K = 1.034 at 25°C. [Pg.87]

The temperature-independence of the shifts and the small ratio of the observed isotope splitting to the estimated shift difference (100 ppm) between the averaged C-1/C-2 carbons in a titanium-alkylidene-olehn structure [36] were interpreted as being consistent with a symmetrical but easily distorted titanacyclobutane structure [35] resting at the minimum of a broad shallow potential energy surface which allows easy distortion towards a transition state for the metathesis reaction. [Pg.91]

Alternative structural models which could involve a rapid equilibrium between two different carbon sites were ruled out since only small, temperature-independent, intrinsic upheld isotope shifts (0.2 ppm per D) were observed in the spectrum. Structure [71] with two C—H—Os interactions was also ruled out because for this case different isotope splitting patterns are expected than are observed. Similar equilibrium isotope effects as in [70] have been observed in the homologous ethyl complex (Shapley et al., 1986). [Pg.111]

The l-methyl-2-trideuteriomethyl-2-bicyclo[2.1.1]hexyl cation [116] has a definite equilibrium isotope effect. The averaged C-1 and C-2 carbons show an isotope splitting of 46.5 ppm at -128°C. The carbon bonded to the CDs-group is shifted to higher field. The enthalpy difference (AH° = — 50 cal mol" per D) for the isotopic equilibrium was calculated from the temperature dependence of the equilibrium constant between... [Pg.132]

Contrary to the parent ion [139] the geminal protons at the averaged methylene carbons in [130] are not distinct. Averaging of the geminal protons occurs via a bent methylcyclobutyl cation with sufficient lifetime to allow inversion by way of a planar cyclobutyl cation transition state [131]. The two isotope effects for exo- and endo-Dare different in size and sign, but are averaged and thus smaller in Di-[130]. The size of the isotope splitting observed in the nmr spectrum of Di-[130] corresponds to the arithmetic mean of the two isotope effects observed in exo- and e /o-Di-[139]. [Pg.141]

The C nmr spectrum of the mixture of exo- and endo-C D cations convincingly confirms the observation of two different isotope effects. As in the proton spectrum, the methylene resonance in the e rfo-D-cation showed the larger isotopic perturbation. With respect to the protio-ion, the CHD-carbon moved downfield and two CHj-carbons moved upiield. The shift between them varies with temperature from 7.05 ppm ( —133°C) to 3.82 ppm (— 87°C). The. vo-D-cation showed smaller isotope splittings between 3.16ppm (—I18 C) and 2.78ppm ( —87°C) in the opposite direction, with the CHD-carbon moved upheld and the CH2-carbons moved downfield. [Pg.144]

The nmr spectrum of cation [141] shows a very large isotope splitting of 5 = 167.7 ppm at — 138°C for the C /methine-carbon pair. The equilibrium constant K= 1.264—1.158 between — 138°C and — 71°C and AH° = — 68 2 cal mol" per D were calculated using A = 277 ppm as the shift for the and C—H carbons (Kates, 1978). These data are probably more accurate than those determined from proton spectra because a larger temperature range was investigated and the assumed A agrees within 1 ppm with the shift difference obtained recently from the solid-state nmr spectrum of the unlabelled cation (Myhre et al., 1984). [Pg.148]

Topper and Stetten (T2) fed rats on galactose isotopically labeled with Cu on carbon atom 1. They found that the glycogen formed in the liver was isotopically labeled and 70% of the C14 was on C-l of the glucose units. They concluded that galactose could not be split into 2 triose units which would then recombine, but that a Walden-type inversion must occur at C-4. [Pg.26]

Because the nucleus is isotopically rare, it is extremely unlikely that any two adjacent carbon atoms in a molecule will both be As a consequence, coupling is not observed in NMR spectra i.e. there is no signal multiplicity or splitting in a NMR spectrum due coupling. couples strongly to any... [Pg.65]


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See also in sourсe #XX -- [ Pg.278 ]




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