12C carbon

Carbon, a common element in the outer crust of the earth, and the main component of all biological and organic substances, occurs in three isotopic forms carbon-12 or C-12 for short (whose chemical symbol is 12C), carbon-13 or C-13 (13C), and carbon-14 or C-14 (14C) (see Fig. 8 and Table 66). 

Relative mass (m) mass of an atom, a molecule or an ion divided by one-twelfth of the mass of the 12C carbon atom. It is thus dimensionless. Thus m = (mass of a molecular or ionic species)/(mass of 12C) x (1/12). 

As we saw in Chapter 2, the first quantitative information about atomic masses came from the work of Dalton, Gay-Lussac, Lavoisier, Avogadro, Cannizzaro, and Berzelius. By observing the proportions in which elements combine to form various compounds, nineteenth-century chemists calculated relative atomic masses. The modern system of atomic masses, instituted in 1961, is based on 12C (carbon-12) as the standard. In this system I2C is assigned a mass of exactly 12 atomic mass units (amu), and the masses of all other atoms are given relative to this standard. 

Smoke lsmok [ME, fr. OE smoca akin to OE smeocan to emit smoke, MHGr smouch smoke, and prob. to Gk smychein to smolder] (before 12c) Carbon or soot particles less than 0.1 pm in size which result from the incomplete combustion of carbonaceous materials such as coal or oil. Air suspension (aerosol) particles, often originating from combustion or sublimation. 

A further point about mass spectrometry, noticeable in the spectrum of propane (Figure 12.2), is that the peak for the molecular ion is not at the highest m/z value. There is also a small peak at M + l because of the presence of different isotopes in the molecules. Although 12C is the most abundant carbon isotope, a small amount (1.10% natural abundance) of 13C is also present. Thus, a certain... 

Table 2.1 may be useful for calculating the number of carbon, bromine, chlorine, and sulfur atoms in the molecular formula. This table shows that for every 100 12C atoms there are 1.1 13C atoms. Also, for every 100 32S atoms, there are 0.8 33S atoms and 4.4 34S atoms. The following examples... 

Molecular weight the average weight of a molecule measured in units equivalent to 1/12 of Carbon 12 (12C). 

The defining characteristic of an atom of a chemical element is the number of protons in its nucleus. A given element may have different isotopes, which are nuclei with the same numbers of protons but different numbers of neutrons. For example, 12C and 14C are two isotopes of carbon. The nuclei of both isotopes contain six protons. However, 12C has six neutrons, whereas 14C has eight neutrons. In general, it is the number of protons and electrons that determines chemical properties of an element. Thus, the different isotopes of an element are usually chemically indistinguishable. These isotopes, however, have different masses. 

The atomic weight of an element is the relative mass of an average atom of the element compared with 12C, which has an atomic weight of exactly 12. Thus, since a sulfur atom has a mass jj times that of a carbon atom, the atomic weight of sulfur is... 

Not long ago this could have been determined only by extremely laborious, and often equivocal, selective degradation experiments but the coming of carbon n.m.r. spectroscopy has now made all the difference. Neither the 12C nor the 14C carbon isotopes produce an n.m.r. signal but the 13C isotope, which occurs in ordinary carbon to... 

Other even more cunning methods have been devised to suppress the water signal in samples that have a large water content (e.g., bio-fluid samples) such as the WET and the WATERGATE pulse sequences. Other sequences have been devised to cope with signals from carbon-bound hydrogens. Some of these actually collapse the 13C satellites into the main 12C peak prior to suppression. Such a sequence would be useful if you were forced to acquire a spectrum in a nondeuterated solvent. 

The result is a type of onion-like model of the star with an iron-nickel core in the centre. The situation is somewhat different for smaller stars the path branches at the point where carbon burning (12C +12 C) begins. While the heavier stars are not affected by this process, the smaller ones (4-8 solar masses) are completely torn apart by carbon burning. 

Another important method for the detection of early life forms involves the ratio of the two carbon isotopes 12C and 13C. The lighter isotope 12C ( normal carbon ) is incorporated preferentially into biomolecules, so that the value of the ratio of 13C to 12C can indicate the presence of material which was formed in living things. It must of course be certain that other, non-biological processes can be excluded, though it is questionable whether this is always possible, in particular when we are unsure of the geological processes which occurred 3 1 billion years ago. 

Corey et al. investigated the kinetic isotopic effect (KIE) in asymmetric dihydroxylation. 12C/13C KIE was measured for the dihydroxylation of styrene, p-nitrostyrene, and 4-methoxy-benzoate (Figure 7).197 The observed similar 12C/13C isotopic effect of two olefinic carbons... 

The chemical analysis has revealed that rather low C/O ratios are found in metal-poor extragalactic carbon stars, as found for galactic carbon stars of the solar vicinity. Furthermore, the three analyzed stars show similar s-elements enhancements [ls/Fe]=0.8-1.3 and [hs/Fe]=l.l-1.7. This leads to new constraints for evolutionary models. For instance, the derived C/O and 13C/12C ratios are lower than model predictions at low metallicity. On the contrary, theoretical predictions of neutrons exposures for the production of the s-elements are compatible with observations (see Fig. 1). Finally, from their known distances, we have estimated the luminosities and masses of the three stars. It results that SMC-B30 and Sgr-C3 are most probably intrinsic carbon stars while Sgr-Cl could be extrinsic. 

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