Here we will interpret the 1H and 13C NMR spectrum of an example molecule: 12,14-ditbutylbenzo[g]chrysene (fig. 1) in THF-d8. The molecule chosen yields complex 1H and 13C-NMR spectra and 2D techniques are ideal for its interpretation.
Fig. 1. Molecular structure of 12,14-ditbutylbenzo[g]chrysene
A partial assignment is possible without resorting to 2D methods. The 1H-NMR spectrum (fig. 2) shows two large peaks in the aliphatic methyl region (0.7 to 2 ppm) with integrals of nine that correspond to the tbutyls. One can go further and state that steric hindrance makes the signal at 0.92 ppm is slightly broader (due to faster relaxation) and this shifts it to a lower chemical shift than the other tbutyl. Therefore the signal at 0.92 ppm corresponds to tbu14 while the signal at 1.40 ppm corresponds to tbu12
Fig. 2. 1H-NMR spectrum of 12,14-ditbutylbenzo[g]chrysene in THF-d8
A partial assignment can be obtained from the aromatic region (fig. 3) using the coupling patterns. Each ring has its own group of protons. Ignoring the small, long-range (more than three-bond) couplings, this region is expected to consist of two groups of four protons (H1 to 4 and H4 to 8). Each group consists of two AX protons (H1, 4, 5 and 8) and two AXY protons (H2, 3, 6 and 7). There is a group of two AX protons (H9 and 10) and two other protons (H11 and H13) that are expected to display only long-range coupling.
Fig. 3. Aromatic region of the 1H-NMR spectrum of 12,14-ditbutylbenzo[g]chrysene in THF-d8
The two doublets at 7.76 and 8.32 ppm (shown in red in fig. 4) show a clear AB roofing effect and the absence of long-range couplings indicates that they are alone in their group. Therefore they correspond to H9 and H10 but one cannot tell which is which. There remain four AX doublets at 8.17, 8.54, 8.56 and 8.62 ppm of which the ones at 8.54 and 8.56 ppm overlap. There are also four triplets (AXY) at 7.34 and 7.44 ppm and overlapping at 7.55 and 7.59 ppm. The roofing effect clearly shows that the triplets at 7.34 and 7.44 ppm are directly coupled as are the other two. Each of the AXY multiplets is coupled to an AX multiplet but one cannot tell which ones because their coupling constants are so similar. Nonetheless, there is a hint at asymmetric second-order splittings (in addition to symmetric long-range couplings) on the doublets at 8.56 and 8.62 ppm. This suggests that they are coupled to the overlapping triplets at 7.55 and 7.59 ppm. However, it is still unknown as to which rings the green and blue groups belong. The remaining two long-range doublets (shown in yellow in fig. 4) at 7.59 (overlapping an AXY triplet) and 7.86 ppm are H11 and H13, but again, one does not know which is which. This is as far as 1D analysis of the 1H spectrum can reasonably be taken.
Fig. 4. Aromatic region of the 1H-NMR spectrum color coded according to rings
Further analysis of the proton spectrum requires 2D experiments. COSY and TOCSY experiments can be used to confirm the provisional assignment of the proton groups and COSY would yield slightly more connectivity information. However, for this molecule, NOESY or ROESY give the full connectivity including that between groups and include all the information obtainable from COSY and TOCSY. The analysis is continued here using a NOESY experiment (fig. 5) while the application of COSY, TOCSY and ROESY to this molecule is discussed elsewhere.
Fig. 5. 2D NOESY spectrum of 12,14-ditbutylbenzo[g]chrysene
The aromatic region (fig. 6) of the spectrum shows three bond correlations that include a COSY type (through bond) component in dispersive phase and negative pure phase signals for correlations between ring systems. These can be used to determine which protons are neighbors. For example the proton at 8.17 ppm is next to the proton at 7.34 ppm on the same ring (see fig. 7 for color coded NOESY accoring to rings as in fig. 8), a fact that could not be easily determined from the 1D spectrum. The proton at 8.56 is on the ring next to the signal at 8.32 ppm, a fact that could not be determined from the 1D or the COSY spectrum. This shows the proton at 8.56 ppm is in a group of four protons next to a group of two protons. The only case in this molecule is the correlation between H8 and 9 which are now known to be at 8.56 and 8.32 ppm respectively.
Fig. 6. Aromatic region of the 2D NOESY spectrum of 12,14-ditbutylbenzo[g]chrysene
Continuing the connectivity, we can assign H10 as 7.76 ppm, H11 as 7.60 ppm and H13 as 7.86 ppm. In the opposite direction, H7 is at 7.59 ppm, H6 at 7.55 ppm, H5 at 8.62 ppm, H4 at 8.54 ppm, H3 at 7.44 ppm, H2 at 7.34 ppm and H1 at 8.17 ppm.
Fig. 7. Aromatic region of the 2D NOESY spectrum of 12,14-ditbutylbenzo[g]chrysene showing connectivity and separation into four color-coded proton groups
Fig. 8. Structure of 12,14-ditbutylbenzo[g]chrysene showing color coding of rings
All that remains is to assign the tbutyls. tbu14 at 0.92 ppm correlates with H1 and 13 while tbu12 at 1.40 ppm correlates with H11 and 13 in the NOESY spectrum (fig. 9) confirming the previous assignment.
Fig. 9. tButyl region of the 2D NOESY spectrum of 12,14-ditbutylbenzo[g]chrysene
The carbon spectrum (fig. 10) shows 26 signals in addition to the THF solvent multiplets (at 24.18 and 66.36 ppm). Under usual acquisition (not overlong relaxation delay) and processing (exponential apodization with 1 Hz line broadening) conditions as shown in fig. 10, the carbons attached to protons appear higher than those not attached to protons.
Fig. 10. 13C-NMR spectrum of 12,14-ditbutylbenzo[g]chrysene
However, in this case the spectrum can be processed with 0.1 Hz line broadening because the signals have long relaxation times, being under vacuum (no oxygen to cause paramagnetic reaction) in a low viscosity solvent. Under these conditions, the carbons not attached to protons do not necessarily appear lower than those that are but do appear much narrower due to slower relaxation (fig. 11).
Fig. 11. Part of the 13C-NMR spectrum of 12,14-ditbutylbenzo[g]chrysene showing the difference between proton attached and non-proton attached carbons
The tbutyls appear in the aliphatic region (below 100 ppm). The two taller peaks at 30.56 and 32.62 ppm correspond to the methyls and the other two peaks at 34.24 and 28.94 correspond to the quaternary carbons. The effect of steric hindrance that affects the proton chemical shift does not make such a large difference to the carbon chemical shift relative to the larger 13C chemical shift range so it cannot be relied on as an indicator. However, the faster relaxation rate manifests itself in a broader carbon line of 0.7 Hz for tbuMe14 at 32.61 ppm as compared to 0.5 Hz for tbuMe12 at 30.56 ppm. The quaternary tbutyl signals at 34.24 and 38.94 ppm are not affected significantly by steric hindrance and cannot be assigned from their relaxations.
The remaining signals between 119 and 150 ppm are all aromatic and can be separated between those that are attached to proton and those that are not (fig. 12). No further reliable assignment can be achieved in the 1D carbon spectrum. (Note that the peak at 125.80 ppm is actually two peaks, one at 125.78 ppm that were not separated by the peak picker.)
Fig. 12. Aromatic Part of the 13C-NMR spectrum of 12,14-ditbutylbenzo[g]chrysene
The short-range (one-bond) 1H-13C correlation (HSQCSI) (fig. 13) spectrum clearly separates the aliphatic and aromatic regions. From the 1H spectrum, the proton attached carbons in the 13C spectrum are assigned, starting with the tbutyls. Their assignment confirms the assumption that tbu14 yields a wider peak than tbu12.
Fig. 13. 2D HSQCSI spectrum of 12,14-ditbutylbenzo[g]chrysene
The 13C resolution is a limiting factor in the spectrum above (fig. 13). This can be resolved by acquiring the spectrum over a restricted 13C range (fig. 14). However, signals falling outside the region of interest fold, are out of phase, may not be completely decoupled in the horizontal direction and have reduced sensitivity. Nonetheless, the resolution in the vertical (f1) direction is dramatically improved without increasing the acquisition time.
Fig. 14. 2D HSQCSI spectrum of 12,14-ditbutylbenzo[g]chrysene acquired over a restricted carbon region
The aromatic region (fig. 15) can then be used to assign all the proton-attached carbons. The one-bond 1H-13C coupling constants can be measured using a proton coupled HSQC (fig. 16)
Fig. 15. Aromatic region of the 2D HSQCSI spectrum of 12,14-ditbutylbenzo[g]chrysene
Fig. 16. Proton coupled 2D HSQCSI spectrum of 12,14-ditbutylbenzo[g]chrysene
This leaves the carbons that are not directly attached to protons unassigned. For this, the long-range 1H-13C correlation (HMBC) (fig. 17) is used.
Fig. 17. 2D HMBC spectrum of 12,14-ditbutylbenzo[g]chrysene
The spectrum (fig. 18) shows mainly three-bond correlations along with mostly weaker two and four-bond correlations. One-bond correlations are mostly suppressed but can be seen as doublets on the tbutyl methyls. The likely number of bonds is one of the parameters used to assign the spectrum. Two-bond correlations show the correct assignment of tbu12q at 34.24 ppm and tbu14q at 38.94 ppm while three-bond correlations assign C12 and 14 to 149.20 and 147.27 ppm, respectively. The remainder of the correlations are shown in table 1 and assignment made so that the most three-bond correlations are observed strongly while the fewest three-bond correlations are missing.
Fig. 18. Aromatic region of the 2D HMBC spectrum of 12,14-ditbutylbenzo[g]chrysene
Finally, we have the full assignment of 12,14-ditbutylbenzo[g]chrysene (table 2).
Table 2. Chemical shifts and coupling constants of 12,14-ditbutylbenzo[g]chrysene derived from HSQC, HMBC and the 1H chemical shifts