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Quant 13C
NMR
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IntroductionQuantitative NMR (nuclear magnetic resonance) is useful for determining impurities in materials down to 0.1-0.01%. While quantitative 1H NMR spectroscopy is easily implemented, quantitative 13C NMR is a little more difficult. The difficulties arise primarily from the long and variable relaxation times of the carbon nucleus. Another complication of course is that the 13C isotope of carbon has a natural abundance of ~1.1%. To overcome these difficulties, 13C NMR with inverse-gated 1H decoupling is typically used, employing a 90˚ observe pulse, with long interpulse delay, generating non-NOE enhanced 13C spectra. This results in very long acquisition times. However when examining molecules that lack quaternary carbon atoms, a significant gain in sensitivity can be achieved by observing the quantitative 13C spectra with full NOE enhancement (nuclear Overhauser effect), decoupling 1H throughout the pulse sequence. Such is the case with poly(ethylene) and poly(propylene) described here. Our 500 MHz Bruker Avance III NMR system is equipped with a variable temperature probe (-80 to +150°C) which is helpful in analyzing polymers in solution. The poly(ethylene) and poly(propylene) samples discussed here were dissolved in 1,2,4-trichlorobenzene and analyzed at 130°C. |
Poly(ethylene) and
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Quantitative 13C NMR of Poly(propylene)
