A NMR METHOD FOR THE ANALYSIS OF MIXTURES OF ALKANES WITH DIFFERENT DEUTERIUM SUBSTITUTIONS

C-13 NMR at 125.76 MHz with H-1 and H-2 decoupling, H-2 NMR at 76.77 M Hz with H-1 decoupling, and H-1 NMR at 500.14 MHz with H-2 decoupling were employed as analytical tools to study the complex mixtures of deu terated ethanes resulting from the catalytic H-D exchange of normal et hane with gas-phase deuterium in the presence of a platinum foil. Refe rence 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 i n the C-13, H-2, and H-1 NMR spectra due to each individual isotopomer , and the effect of isotopic substitution on the chemical shifts was d etermined in each case. While the NMR of all three nuclei worked well for the identification of the individual components of the 1:1 standar d mixtures, both H-1 and 2H NMR suffered from inadequate resolution wh en studying complex reaction mixtures because of the broadening of the lines due to H-1-H-1 (H-1 NMR) and H-2-H-2 (H-2 NMR) couplings. C-13 NMR was therefore determined to be the method of choice for the quanti tative analysis of the reaction mixtures. Using the C-13 NMR results, a correlation that takes into account the primary and secondary isotop e substitution effects on chemical shifts was deduced. This equation w as used for the identification of the individual components of the mix tures, and integration of the individual observed resonances was then employed for quantification of their composition. This study shows tha t C-13 NMR with H-1 and H-2 decoupling is a viable procedure for study ing mixtures of deuterated ethanes. Furthermore, the additivity of the isotopic effects on chemical shifts and the transferability of the va lues obtained with ethane to other molecules makes this approach gener al for the analysis of other isotopomer mixtures. (C) 1997 Elsevier Sc ience B.V.