Hydrogen Bonds in Crystalline Imidazoles Studied by ¹⁵N NMR and Ab initio MO Calculations

Intermolecular hydrogen bonds of the type N-H...N in crystals of imidazole and its 4-substituted and 4,5-disubstituted derivatives were studied by ¹⁵N CP/MAS NMR and ab initio molecular orbital (MO) calculation. In the ¹⁵N CP/MAS NMR spectrum of each of the imidazole derivatives, two peaks due to the two different functional groups, >N-H and =N-, were observed, as shown in Fig. 1. The value of the ¹⁵N isotropic chemical shift for each nitrogen atom depends on both the length of the intermolecular hydrogen bond and the kind of the substituent or substituents. It was found that the difference between the experimental chemical shifts of >NH and =N- varies predominantly with the hydrogen bond length but does not show any systematic dependence on the kind of substituent. The ab initio MO calculations suggest that the hydrogen bond formation influences the ¹⁵N isotropic chemical shift predominantly, and that the difference between the 15N isotropic chemical shifts of >NH and =N- varies linearly with the hydrogen bond length.

Fig. 1. ¹⁵N CP/MAS NMR spectra of 4-substituted imidazoles with reference to ¹⁵N in nitromethane as an external reference at room temperature. Asterisks indicate the spinning sidebands.