Exceptional resolution and new signals detected in the 13C NMR spectra of alkanes

Abstract

AbstractThe relative ease with which very high resolution 13C NMR spectra can now be obtained because of advances in NMR technology is shown. Spectra of four alkanes were obtained at 62.9 MHz with careful shimming, low‐power decoupling and a digital resolution of less than 0.1 Hz. Signals as little as 3 ppb apart can be clearly distinguished at this moderate field strength without resolution enhancement. Consequently, additional information can be obtained. For n‐hexadecane, six, not the usual five, signals are evident; the chemical shifts of C‐6, C‐7 and C‐8 are within 0.16 Hz (2.5 ppb) of each other and are not all the same. For 2‐methyltridecane, all 13 signals are visible; all the signals can be assigned with confidence except those for C‐7 and C‐8, which differ by only 4 ppb. For 7‐ methylhexadecane, all 17 signals are visible; many can be assigned to a specific carbon, including the signals for C‐3 and C‐14 (both of which are in CH3CH2CH2CH2CH2CH2— environments), which differ by only 37 ppb. For the two pairs of enantiomers of 2,6,10‐trimethyldodecane, all but one carbon give a pair of resonances; the two signals for C‐3 are only 2.9 ppb apart. The potential limitations on molecular size and field strength affecting line width are considered. The same solvent, concentration and temperature must be used if chemical shift data for different compounds are to be meaningfully compared in detail.