Organic compounds isotopes

Therefore, this chapter provides a summarized data on the preparation of organic ion-radicals as independent particles that can be free or bound with counterions in ion pairs. The chapter considers liquid-phase equilibria in electron transfer reactions and compares electrode and liquid-phase processes for the same organic compounds. Isotope-containing molecules have specific features as ion-radical precursors, therefore, the generation of the corresponding ion-radicals is considered in Section 2.6 of this chapter. This chapter also pays some attention to the peculiarities of ion-radical formation in living organisms. 

Not only the molecular ion peak but all the peaks m the mass spectrum of benzene are accompanied by a smaller peak one mass unit higher Indeed because all organic com pounds contain carbon and most contain hydrogen similar isotopic clusters will appear m the mass spectra of all organic compounds... 

Isotopic clusters are especially apparent when atoms such as bromine and chlorine are present m an organic compound The natural ratios of isotopes m these elements are... 

Isopropyl group (Section 2 13) The group (CH3)2CH— Isotactic polymer (Section 7 15) A stereoregular polymer in which the substituent at each successive chirality center is on the same side of the zigzag carbon chain Isotopic cluster (Section 13 22) In mass spectrometry a group of peaks that differ in m/z because they incorporate differ ent isotopes of their component elements lUPAC nomenclature (Section 2 11) The most widely used method of naming organic compounds It uses a set of rules proposed and periodically revised by the International Union of Pure and Applied Chemistry... 

Infrared Spectrophotometry. The isotope effect on the vibrational spectmm of D2O makes infrared spectrophotometry the method of choice for deuterium analysis. It is as rapid as mass spectrometry, does not suffer from memory effects, and requites less expensive laboratory equipment. Measurement at either the O—H fundamental vibration at 2.94 p.m (O—H) or 3.82 p.m (O—D) can be used. This method is equally appticable to low concentrations of D2O in H2O, or the reverse (86,87). Absorption in the near infrared can also be used (88,89) and this procedure is particularly useful (see Infrared and raman spectroscopy Spectroscopy). The D/H ratio in the nonexchangeable positions in organic compounds can be determined by a combination of exchange and spectrophotometric methods (90). 

Irreversible processes are mainly appHed for the separation of heavy stable isotopes, where the separation factors of the more reversible methods, eg, distillation, absorption, or chemical exchange, are so low that the diffusion separation methods become economically more attractive. Although appHcation of these processes is presented in terms of isotope separation, the results are equally vaUd for the description of separation processes for any ideal mixture of very similar constituents such as close-cut petroleum fractions, members of a homologous series of organic compounds, isomeric chemical compounds, or biological materials. 

The measurements of concentration gradients at surfaces or in multilayer specimens by neutron reflectivity requires contrast in the reflectivity fiDr the neutrons. Under most circumstances this means that one of the components must be labeled. Normally this is done is by isotopic substitution of protons with deuterons. This means that reflectivity studies are usually performed on model systems that are designed to behave identically to systems of more practical interest. In a few cases, however (for organic compounds containing fluorine, for example) sufficient contrast is present without labeling. 

Table 2.1. Isotopic distributions of some elements found in organic compounds...

A. I. Shatenshtein, Isotopic Exchange and the Replacement of Hydrogen in Organic Compounds, Consultants Bureau, New York, 1962, p. 105. 

Studies of the molar volumes of perdeuteriated organic compounds might be expected to be informative about non-bonded intermolecular forces and their manifestations, and such studies might be considered to obviate the necessity of investigating steric isotope effects in reacting systems. The results from non-reacting systems could then be simply applied to the initial and transition states in order to account for a kinetic steric isotope effect. 

Elements such as C, N, O, S, and Cl that are components of many organic compounds exist naturally as mixtures of stable isotopes. The ratios of these in a compound reflect the different rates of reaction at isotopically labeled positions, and therefore reflect the fractionation—biotic or abiotic—by which it was synthesized or to which the compound has been subjected. Techniques have been developed whereby the ratios C/ C (5 C), (5 N), (5 0),... 

Holt BD, NC Sturchio, TA Abrajano, LJ Heraty (1997) Conversion of chlorinated volatile organic compounds to carbon dioxide and methyl chloride for isotopic analysis of carbon and chlorine. Anal Chem 69 2727-2733. 

GC-C-IRMS instrumentation enables the compound-specific isotope analysis of individual organic compounds, for example, n-alkanes, fatty acids, sterols and amino acids, extracted and purified from bulk organic materials. The principle caveat of compound-specific work is the requirement for chemical modification, or derivatisation, of compounds containing polar functional groups primarily to enhance their volatility prior to introduction to the GC-C-IRMS instrument. Figure 14.7 summarises the most commonly employed procedures for derivatisation of polar, nonvolatile compounds for compound-specific stable isotope analysis using GC-C-IRMS. 

Begley, I. S. and Scrimgeour, C.M. (1997) High precision 82H and 8lsO measurement for water and volatile organic compounds by continuous flow pyrolysis isotope ratio mass spectrometry. 

Rieley, G. (1994) Derivatization of organic compounds prior to gas chromatographic combustion isotope ratio mass spectrometric analysis identification of isotope fractionation processes. Analyst 119, 915 919. 

EPA. 1980b. Volatile organic compounds by purge-and-trap isotope dilution GC-MS. Method 1624. Washington, DC U.S. Environmental Protection Agency. 

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