Chemical Shift Measurements

Several inconsistencies have been noted during recent systematic studies of the validity of additivity relationships (1-6) in the C chemical shifts of substituted pyridines. These showed that substantial errors could be introduced if care is not taken to use, for a given solvent, approximately the same concentration for all solutes; in our case this applied to monosubstituted and polysubstituted pyridines and benzenes. Whereas for H NMR spectra it is customary to "lock" the field using the signal of tetramethylsilane (TMS) for C NMR spectra, on instruments such as the Varian FT-80, it is often customary to lock on to the signal of deuterium situated in a capillary concentric with the sample. The chemical shift of the solvent taken from tables or from previous measurements is then fed into the spectrometer program and the computer refers all data to the TMS scale (7-9). Although this procedure eliminates the uncertainty due to the usually unknown magnetic susceptibility of the solution, it does not take into account the fact that with ternary solutions there is a significant solvent effect of the solutes on the solvent and even on the reference compound. This makes the use of this technique unsuitable. On the other hand, the use of TMS as an internal standard for C NMR has been found to bn unsuitable for other reasons. Its very low boiling point (265°C) and our high, local average room temperature make it almost impossible to measure its concentration correctly. To solve this problem we have used hexamethyldisiloxane (HM) as an internal standard (b.p. 101°C) at a concentration of 2% w/w. The problem was then to make the data obtained with this standard comparable with those obtained with TMS. If no aromatic or complexing solvents are used, the main effects of the solvent are from the van der Waals forces. The effect of each solvent or solution can then be characterized by a factor g (9,10) which is a simple function of the refractive index, n, of the solution. Using this property, a method has been devised to convert the measurements made with HM to those made with TMS.