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P. Panwaria and A. Das
3.1.2 Indole…Imidazole and Indole…(Pyrrole) 2 Complexes
Indole…imidazole and indole…pyrrole complexes have been studied further by Das
and co-workers not only to explore the similar interplay between multiple noncovalent interactions present there but also to mimic aromatic-aromatic interactions
present in the side-chains of the aromatic amino acid residues in proteins [173,
174]. It has been demonstrated from extensive PDB analysis that aromatic-aromatic
interaction exists in almost 50% of the proteins deposited there.
Figure 6Aa shows the electronic spectrum measured in the mass channel
of indole…imidazole dimeric complex. The electronic origin band (0
0
0 ) of the
indole…imidazole complex appears at 34,933 cm
−1 [173]. The spectrum exhibits
many low-frequency transitions within 100 cm
−1 blue side from the origin band. The
UV-UV hole-burning spectrum (Fig. 6Ab) by probing the origin band demonstrates
that all the bands in the electronic spectrum originate due to a single conformer of
the dimer. Thus, the low-frequency bands riding on the origin bands are assigned as
intermolecular vibrations for the stretching and bending of the N-H…N hydrogen
bonds present there.
The IR spectrum of indole…imidazole measured in the N-H stretching region
using RIDIR spectroscopy is provided in Fig. 6Ba. The theoretical IR spectrum of
the most stable conformer of the dimer calculated at the B97-D/6–311 + G(3df, 3pd)
level of theory is presented in Fig. 6Bb. A reasonable agreement between the experimental and theoretical IR spectra bespeaks that the observed conformer of the dimer
has a V-shaped structure stabilized primarily by a strong N-H…N hydrogen bond with
secondary C-H…π and π-stacking interactions. The bands at 3270 and 3516 cm
−1
Fig. 6 A (a) Electronic spectrum of indole…imidazole dimer, (b) UV-UV hole-burning spectrum
by probing the 0 0
0 band of the dimer; B (a) IR spectrum in the N-H stretching region by probing
the origin band of indole…imidazole dimer, (b) theoretical IR spectrum of the dimer calculated at
the B97-D/6-311 + G(3df, 3pd) level of theory. Adapted with permission from Ref. [173], copyright
2012, American Chemical Society
P. Panwaria and A. Das
3.1.2 Indole…Imidazole and Indole…(Pyrrole) 2 Complexes
Indole…imidazole and indole…pyrrole complexes have been studied further by Das
and co-workers not only to explore the similar interplay between multiple noncovalent interactions present there but also to mimic aromatic-aromatic interactions
present in the side-chains of the aromatic amino acid residues in proteins [173,
174]. It has been demonstrated from extensive PDB analysis that aromatic-aromatic
interaction exists in almost 50% of the proteins deposited there.
Figure 6Aa shows the electronic spectrum measured in the mass channel
of indole…imidazole dimeric complex. The electronic origin band (0
0
0 ) of the
indole…imidazole complex appears at 34,933 cm
−1 [173]. The spectrum exhibits
many low-frequency transitions within 100 cm
−1 blue side from the origin band. The
UV-UV hole-burning spectrum (Fig. 6Ab) by probing the origin band demonstrates
that all the bands in the electronic spectrum originate due to a single conformer of
the dimer. Thus, the low-frequency bands riding on the origin bands are assigned as
intermolecular vibrations for the stretching and bending of the N-H…N hydrogen
bonds present there.
The IR spectrum of indole…imidazole measured in the N-H stretching region
using RIDIR spectroscopy is provided in Fig. 6Ba. The theoretical IR spectrum of
the most stable conformer of the dimer calculated at the B97-D/6–311 + G(3df, 3pd)
level of theory is presented in Fig. 6Bb. A reasonable agreement between the experimental and theoretical IR spectra bespeaks that the observed conformer of the dimer
has a V-shaped structure stabilized primarily by a strong N-H…N hydrogen bond with
secondary C-H…π and π-stacking interactions. The bands at 3270 and 3516 cm
−1
Fig. 6 A (a) Electronic spectrum of indole…imidazole dimer, (b) UV-UV hole-burning spectrum
by probing the 0 0
0 band of the dimer; B (a) IR spectrum in the N-H stretching region by probing
the origin band of indole…imidazole dimer, (b) theoretical IR spectrum of the dimer calculated at
the B97-D/6-311 + G(3df, 3pd) level of theory. Adapted with permission from Ref. [173], copyright
2012, American Chemical Society