lead to higher acidity as was also discussed in Section V. Compared to these imidazolium ions, TetraMe has a lower positive charge at C2 and the C2 hydrogen atom, indicating a lower acidity for this more electron-rich imidazolium ion. The acidity order based on VDD atomic charge calculations therefore is DiCl > DiPh MM > TetraMe. 13C NMR spectroscopy is known to be a valuable tool for investigating charge density distributions in conjugated systems. In Chapter 3, we explored the influence of alkyl substituents at the N1 and N3 ring positions of imidazolium ions on the electron density at C2 by comparing 13C NMR chemical shifts of this atom. Substituents ranging from methyl to tert-butyl led to C2 chemical shifts of 137.85 ppm (MM) to 132.88 ppm (t-Bu t-Bu), and thus to a range of 5 ppm (Figure S4.20). When introducing substituents at C4 and C5, however, the observed range in chemical shifts of the C2 carbon atom is relatively narrow (2.4 ppm), namely from 137.85 ppm (MM) to 135.48 ppm (TetraMe). This indicates that the more remotely located substituents at C4 and C5 do not have as large an influence on the 13C chemical shift of C2 as substituents at N1 and N3. Within the same family of substituents, such as phenyl groups, adding a para substituent like methoxy should lead to C2 shift changes that can be rationalized. This is the case, but even here the C2 chemical shift difference induced by the probably weakly withdrawing phenyl- and the strongly donating methoxyphenyl-groups in the imidazolium ions is minimal (0.46 ppm). It must be noted that due to their size, the phenyl groups are not fully in plane with the imidazolium ring, which may limit their influence on its charge distribution. Expectations about C2–H acidity can partly be correlated with the results of the 13C NMR measurements. For example, the electronpoor DiCl is expected to be more acidic than the electron-rich TetraMe, which matches with the higher C2 shift value of DiCl. The higher chemical shift value of the DiPh C2 compared to that of DiMeOPh also is in accord with expectations. However, DiCl and DiPh are both expected to be more electron-poor than MM, which is not evident from the 13C NMR results. Therefore, additional work is needed to establish the (order of) acidity of these imidazolium
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