BR-81 BR-79-INDUCED C-13 ISOTOPE SHIFTS AND LINE BROADENING IN THE C-13 NMR-SPECTRUM OF METHYL-BROMIDE/

NMR studies of the C-13 isotope shift between (CH3Br)-Br-79 and (CH3Br )-Br-81 and of the linewidths of the two resolved resonances were carr ied out as part of a wider study of the NMR properties of methyl halid e molecules. The isotope shift of 1.12 ppb can be readily accounted fo r in terms of the very slight shortening of the C-Br bond (0.000 008 A ngstrom) which occurs upon replacing Br-79 by Br-81. From the isotope shift it is shown that the C-Br bond length derivative of the carbon-1 3 shielding is -140 ppm Angstrom(-1). There is no need to involve the change in bromine nuclear quadrupole moment to explain the isotope shi ft, The linewidths are influenced by scaler relaxation of the second k ind and, from the observed values of 0.084 and 0.100 Hz for (CH3Br)-Br -79 and (CH3Br)-Br-81, respectively, it is possible to deduce that J(C -13 Br-79) = 48 Hz and J(C-13 Br-81) is, 45 Hz. These results are in a ccordance with extrapolation from previously measured results for CBr4 , CHBr3 and CH2Br2. (C) 1997 by John Wiley & Sons, Ltd.