The figure shows the aromatic region of the ¹³C NMR spectrum obtained on an AM-400WB spectrometer. The ratio of the components by weight is calculated using the following equation:

G_aG_b = F_aF_b · N_bN_a · M_aM_b

where G_a and G_b are the parts by weight of the components a (naphthalene) and b (phenanthrene) and N_a and N_b the numbers of nuclei that cause the signals. M_a and M_b are the molecular masses of the two components, 128.16 and 178.23 respectively. Using the measured averaged integrals F_a for naphthalene (signals at δ_C = 132.8, 127.3, 125.2, relative intensity ratio 2:4:4) and F_b for phenanthrene (signals at δ_C = 131.4, 129.6, 127.9, 126.3. 125.9, 122.1, relative intensity ratio 2:2:2:2:4:2) the naphthalene/phenanthrene ratio by weight is calculated to be 2.27/1, which corresponds to 69.5/30.5 percentage weight. This is in very good agreement with the percentage ratio of 69.1/30.9 corresponding to the weighed amounts used.

7. Comments

The integrals of NMR signals are relative measures of the numbers of nuclei producing that signal. If one component is present at much lower concentration than another, the percentage error in measurement of the quantity of this minor component could be quite high (see Exp. 3.12). It is only when the line shapes and line-widths are identical that the relative peak heights are a reliable measure of the relative concentration.

The inverse gated ¹H-decoupling experiment is described in Experiment 4.13. Small NOE enhancements build up during the acquisition time, but are dissipated during the relaxation delay, which should be at least 10 times longer than the acquisition time. In addition, the paramagnetic relaxation reagent Cr(acac)₃ shortens the ¹³C spin–lattice relaxation times.

8. Own Observations