Signals in the NMR spectrum are measured on a scale independent of the applied magnetic field, called the chemical shift and represented by the letter $\delta$. Regardless of the magnetic field at which the spectrophotometer works, the signals of a chemical compound are always obtained at the same values of $\delta$. \begin{equation} \delta=\frac{\nu_{sample}-\nu_{reference}}{\nu_{reference}}\cdot 10^6 \end{equation} By definition is taken as zero of the scale of chemical shift the signal from tetramethylsilane ($Si(CH_3)_4$). Let's calculate the chemical shift for $CH_3Br$ knowing that in a 90 MHz device the absorption frequency occurs at 90 000 237 Hz. \begin{equation} \delta =\frac{90 000 237 - 90 000 000}{ 90 000 000}\cdot 10^{6}=2.63 \end{equation} In a spectrophotometer working at 300 MHz the absorption occurs at 300 000 790 Hz, repeating the previous calculation the same chemical shift is obtained.\ \ The more unshielded hydrogens come out at larger displacements: $CH_3Br\rightarrow \delta =2.63$; $CH_2Br_2\rightarrow \delta =$4.90; $CHBr_3\rightarrow \delta = $6.82

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