A NMR method for the analysis of mixtures of alkanes with different deuterium substitutions

Abstract

13C NMR at 125.76 MHz with 1H and 2H decoupling, 2H NMR at 76.77 MHz with 1H decoupling, and 1H NMR at 500.14 MHz with 2H decoupling were employed as analytical tools to study the complex mixtures of deuterated ethanes resulting from the catalytic H–D exchange of normal ethane with gas-phase deuterium in the presence of a platinum foil. Reference samples consisting of 1:1 binary mixtures of pure normal ethane and ethane-d n ( n=1–6) were used to identify the peak positions in the 13C, 2H, and 1H NMR spectra due to each individual isotopomer, and the effect of isotopic substitution on the chemical shifts was determined in each case. While the NMR of all three nuclei worked well for the identification of the individual components of the 1:1 standard mixtures, both 1H and 2H NMR suffered from inadequate resolution when studying complex reaction mixtures because of the broadening of the lines due to 1H– 1H ( 1H NMR) and 2H– 2H ( 2H NMR) couplings. 13C NMR was therefore determined to be the method of choice for the quantitative analysis of the reaction mixtures. Using the 13C NMR results, a correlation that takes into account the primary and secondary isotope substitution effects on chemical shifts was deduced. This equation was used for the identification of the individual components of the mixtures, and integration of the individual observed resonances was then employed for quantification of their composition. This study shows that 13C NMR with 1H and 2H decoupling is a viable procedure for studying mixtures of deuterated ethanes. Furthermore, the additivity of the isotopic effects on chemical shifts and the transferability of the values obtained with ethane to other molecules makes this approach general for the analysis of other isotopomer mixtures.