The di-propylnitrile calixarene 14 was characterised by 1H NMR experiment in deuterated chloroform (Figure 2.8) with the assistance from a 2D COSY experiment for the assignments of the peaks. Since this is the first 1H NMR spectrum of a calixarene presented in the thesis, it will be elaborated in more detail for increased clarity. Unlabelled peaks in the spectrum are solvent peaks and all spectra are in CDCl3 unless otherwise stated.
Figure 2.8: 1H NMR spectrum of the di-propylnitrile calixarene 14.
The symmetry of the di-substituted calixarene can be verified by the 2 hydrogen environments in the aromatic region, singlet peaks b and c at 7.06 and 6.85 ppm respectively, as well as the alkyl region, singlet peaks i and j at 1.28 and 0.99 ppm respectively. The integration ratios of peak b and c is about 4 each, which represents the 2 hydrogens in the meta position on each of the 2 aromatic rings of their respective hydrogen environments. The integration ratio of peak i and j in the alkyl region is about 18 each, which represents the 9 hydrogens on each of the two tert-
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butyl groups in the para position of the aromatic rings of their respective hydrogen environments. All of these hydrogens do not couple with any other hydrogen nuclei in the molecule, and thus appear as singlets.
The methylene H atoms appear as an AB doublet of doublets, similar to that observed in the unsubstituted calixarenes, but have a slightly different chemical shift of 4.17 (doublet peak d) and 3.37 ppm (doublet peak f). The integration ratios of peaks d and f are both 4, which represent the axial and equatorial hydrogens in the four methylene bridges each. The doublet splitting is attributed to the coupling of the axial and equatorial hydrogens in the methylene bridges that denote peak d and f. The difference in the hydrogen environment and the splitting pattern of the hydrogens in the methylene bridges are typical for calix[4]arenes in the cone conformation, as the hydrogens pointing away (outwards) and closer (inwards) have their own independent hydrogen environment.
The hydrogen assignments of the elongated alkyl arms are peak e at 4.08 ppm, peak g at 3.03 ppm and peak h 2.34 ppm. The integration ratios of each of these peaks are 4, which represent the 2 hydrogens on each of the two arms in their respective hydrogen environments. The splitting patterns of peak e and g are both triplets as the hydrogens from both environments each couples with the 2 hydrogens of peak h that has a multiplet splitting pattern for that reason. Peak h is more upfield (right shifted) due to not having a nearby electronegative group like the other 2 environments, peak e and g, that has a neighbouring ether or nitrile group respectively.
The di-propylnitrile calixarene 14 was also characterised by 13C NMR spectroscopy (Figure 2.9) with assistance from DEPT-Q 13C and 2D HSQC experiments for the assignments of the peaks. All 17 of the carbon environments were accounted for and assigned as shown.
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The tetrazolisation reaction of the di-propylnitrile calixarene 14 yielded an unknown product that was definitely not the desired calixarene upon analysing the 1H NMR of the purified sample. One apparent observation is the loss of symmetry by looking closely in the hydrogen environments of the aromatic and alkyl regions (Figure 2.10).
The aromatic region has 4 hydrogen environments, while the tert-butyl groups in the alkyl region has 3 hydrogen environments. In the aromatic region, the splitting pattern of the hydrogens of peak c and e are singlet, while peak d and f are doublet. Each hydrogen of the 2 doublets couples with the other aromatic hydrogen in the meta position of the same aryl ring. The splitting pattern of the tert-butyl groups are all singlets due to the single bond rotation of the tert-butyl carbon-aromatic carbon.
There is also a noticeable increase of the hydrogen environments from 2 to 4 in the methylene bridges. The additional hydrogen environments in both the axial (peak g) and equatorial hydrogens (peak i) in the methylene bridges are indicative of the loss of symmetry as well. This is however not that apparent due to the very insignificant differences in the value of the chemical shifts, thus making it looks like a splitting instead (Figure 2.11).
Figure 2.10: Close up 1H NMR spectrum of the aromatic and alkyl regions of the purified calixarene sample.
The peaks are assigned as the mono-substituted calixarene with the hydrogens of peak c and l belonging to the aryl component attached to the electronegative substituted group, thus are more downfield (left shifted). However, more information is needed to confirm that it is the mono-substituted calixarenes instead of a 1-nitrile- 3-tetrazole di-substituted calixarene or even a tri-substituted calixarene, as they all share identical symmetry and thus have the same hydrogen environments. From the synthesis perspective, the 1-nitrile-3-tetrazole di-substituted calixarene makes the most sense due to the rationale of partial tetrazolisation of the di-propylnitrile
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calixarene reactant. However by analysing the other components of the 1H NMR, there is evidence pointing towards the mono-substituted calixarene instead (Figure 2.11).
Looking at Figure 2.11, the two singlet peaks a and b are the phenol hydrogens corresponding to 2 different hydrogen environments illustrating the symmetry and also matching integration ratios of a mono-substituted calixarene. The diametrical phenol hydrogen furthest away from the substituted group has an integration ratio of 1, while the 2 adjacent phenol hydrogens have an integration of ratio of 2.
The next indication is the integration ratio of the elongated alkyl arm that illustrates the presence of only one substituent on the lower rim. The integration ratio of the respective peak h, j and k are all 2, which represent the 2 hydrogens in their respective hydrogen environments of a single substituent.
Figure 2.11: 1H NMR spectrum of the purified sample.
Though it is now evident that the product is a mono-substituted calixarene, it is unclear whether the substituent is a nitrile or a tetrazole group based on 1H NMR. Thus far, characterisation via IR has not proven to be useful for this aspect, as the nitrile stretch that occurs around 2300-2200 cm-1 is not present in any of the nitrile functionalised calixarene in this project. However, it could be assumed that the product is the nitrile substituent on the basis that the chemical shift of peak j that is closest to the group stays relatively the same at around 3.05 ppm. This can be supported by the peak of the methoxy nitrile that moves downfield (left shifted) upon tetrazolisation to methoxy tetrazole, such as in the example of the tetrazolisation of the p-tert-butyl di-nitrile calixarene 7, which shifts from 4.81 ppm to 5.50 ppm (See experimental).
The mono-propylnitrile calixarene 15 was also characterised by 13C NMR spectroscopy (Figure 2.12) with assistance from DEPT-Q 13C and 2D HSQC experiments for the assignments of the peaks. All 26 of the carbon environments were accounted for and assigned as shown.
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Figure 2.12: 13C NMR spectrum of the mono-propylnitrile calixarene 15.
Likewise, the peaks in the 13C NMR spectrum showed the symmetry of a mono- substituted calixarene due to the additional carbon environments; peak k, l, m and n for the aromatic hydrocarbons, peak i, j and o as well as peak b, c, and d for the aromatic tertiary carbons, peak q, r and s for the tert-butyl carbons, and peak v, w and x for the tert-butyl hydrocarbons. Peak a was determined to be the nitrile carbon due to the similar chemical shift of around 150 ppm with other 13C NMR spectra of nitrile functionalised calixarenes in this project.