Spinning of samples

Section 13.14 C has a nuclear- spin of 5 but only about 1% of all the carbons in a sample are C. Nevertheless, high-quality C NMR spectra can be obtained by pulse FT techniques and are a useful complement to F1 NMR spectra. 

Despite the fact that natural elemental sulfur contains 0.75% of the isotope [4] with a nuclear spin of 7 = /2 no NMR spectra of elemental sulfur have ever been reported. Such spectra are however well-known for compounds containing just one or two sulfur atoms [5]. Electron spin resonance spectra of irradiated elemental sulfur samples and of quenched sulfur vapor have been reviewed elsewhere [6-8]. 

The crystal field interaction gives rise to an energy splitting into a number of Kramers doublets. In the case of high-spin Fe " with spin S = 5/2, there are three Kramers doublets, each of which give rise to separate contributions in the Mossbauer spectra of samples with slow paramagnetic relaxation. For 1 = 0 and a = 0, they can be labeled l/2), 3/2) and 5/2). 

Each spectroscopic technique (electronic, vibra-tional/rotational, resonance, etc.) has strengths and weaknesses, which determine its utility for studying polymer additives, either as pure materials or in polymers. The applicability depends on a variety of factors the identity of the particular additive(s) (known/unknown) the amount of sample available the analysis time desired the identity of the polymer matrix and the need for quantitation. The most relevant spectroscopic methods commonly used for studying polymers (excluding surfaces) are IR, Raman (vibrational), NMR, ESR (spin resonance), UV/VIS, fluorescence (electronic) and x-ray or electron scattering. 

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