Calibration of NMR spectra

In order to measure chemical shifts, the absorption signal of a reference compound R appearing at frequency v0R is assigned the shift zero. The chemical shift (or Larmor frequency) of chemically equivalent nuclei of a sample having their signal at frequency vos may then be measured as the frequency difference Avs in Hertz (Fig. 1.10(b), legend). 

According to the Larmor equation (1.8), chemical shifts can be related to field differences zlBs, measurable in millitesla. 

Also because of the Larmor equation (1.8), the frequency or field differences /1vs or ABS are proportional to the swept radio frequency Vj (in MHz) or the field strength of B0 (in T). Therefore, chemical shifts dvs (or ABS) obtained at different radio frequencies v, (or field strengths B0) have to be adjusted to the same radio frequency (or field) before comparison. In order to get chemical shift values which are independent of the frequency or field strength used, the d scale of chemical shifts is introduced. 5 values are obtained by dividing the frequency differences Avs (in Hz) by the frequency iq used (in MHz = 106 Hz). 

Because zfvs between nuclei of the same type is very small (several Hz) compared to r, (several MHz), the shifts on the d scale are given in ppm (parts per million = units of 10 6). Examples for the calculation of S values in ppm are given for the 13C chemical shifts of acetone in the legend of Fig. 1.10(b). 

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