Calculations of chemical shifts

Electric field theory is used to predict the 1H, 13C and 19F chemical shifts of a series of halogenated ethanes and the 19F shifts of some cyclobutanes. The quantity 3PI/r 6 is used to describe the time-averaged square of the electric field at the nucleus from fluctuating point-dipoles elsewhere in the molecule. The results are compared with predictions made by an alternative method, using P/Ir 3, to define the electric fields. The calculations show that, whatever the theoretical justification or lack of it, the former quantity is often more widely applicable than the latter.