81Br/79Br-Induced 13C Isotope Shifts and Line Broadening in the 13C NMR Spectrum of Methyl Bromide

Abstract

NMR studies of the 13C isotope shift between CH379Br and CH381Br and of the linewidths of the two resolved resonances were carried out as part of a wider study of the NMR properties of methyl halide molecules. The isotope shift of 1.12 ppb can be readily accounted for in terms of the very slight shortening of the C—Br bond (0.000008 Å) which occurs upon replacing 79Br by 81Br. From the isotope shift it is shown that the C—Br bond length derivative of the carbon-13 shielding is -140 ppm Å-1. There is no need to involve the change in bromine nuclear quadrupole moment to explain the isotope shift. The linewidths are influenced by scalar relaxation of the second kind and, from the observed values of 0.084 and 0.100 Hz for CH379Br and CH381Br, respectively, it is possible to deduce that J(13C79Br)=48 Hz and J(13C81Br) is 45 Hz. These results are in accordance with extrapolation from previously measured results for CBr4, CHBr3 and CH2Br2. © 1997 by John Wiley & Sons, Ltd.