Exploring Non-covalent Interactions by Jet-Cooled Electronic …
75
Fig. 8 A Mass-selected electronic spectra measured in the (a) indole, (b) indole…furan dimer, and
(c) indole…(furan) 2 trimer mass channels using 1C-R2PI spectroscopy; B IR spectra obtained in
the N–H stretching region by probing the origin bands of (a) indole and (b) indole…furan dimer
using RIDIR spectroscopy. Adapted with permission from Ref. [144], copyright 2012, American
Chemical Society
stability due to the presence of the C-H…π and π…π interactions. In the SynPD
/ structure, the indole N-H group and the oxygen atom of furan are positioned
on the same side, having a tilted parallel-displaced π-stacked structure. The N-H
group and oxygen atom are on the opposite side with a parallel-displaced π-stacked
geometry in the Anti-PD structure. A comparison of the experimental IR spectrum
with the theoretical IR spectra of the four conformers of indole…furan (Fig. 8B)
reveals that the observed dimer has a tilted T-shaped structure, i.e., T
/ (N-H…π),
which provides 40 cm
−1 red-shift in the N-H stretch frequency with respect to that in
the indole monomer. A remarkable point to note here is that the π-hydrogen-bonded
(unconventional hydrogen bond) structure dominates over the conventional N–H…O
hydrogen-bonded structure.
Das and co-workers further extended their spectroscopic investigation on N-H…π
interaction by replacing the heteroatom in the π-hydrogen bond acceptor aromatic
ring by sulfur (S) atom [188]. Figure 9Ab shows the mass-selected IR spectrum of the
single conformer of indole…thiophene complex measured by RIDIR spectroscopy.
The theoretical IR spectrum of the dimeric complex calculated at the M05-2X/6–
311++G(2df,2pd) level of theory is provided in Fig. 9Ac. The observed conformer
of the indole…thiophene complex has a tilted T-shaped structure bound by N-H…π
hydrogen bonding interaction similar to that of the indole…furan complex. However,
the tilt angle in the T-shaped structure of the indole…thiophene complex is larger
than that of the indole…furan complex [144]. The red-shift of 56 cm
−1 in the NH stretching frequency of the indole…thiophene complex with respect to that of
75
Fig. 8 A Mass-selected electronic spectra measured in the (a) indole, (b) indole…furan dimer, and
(c) indole…(furan) 2 trimer mass channels using 1C-R2PI spectroscopy; B IR spectra obtained in
the N–H stretching region by probing the origin bands of (a) indole and (b) indole…furan dimer
using RIDIR spectroscopy. Adapted with permission from Ref. [144], copyright 2012, American
Chemical Society
stability due to the presence of the C-H…π and π…π interactions. In the SynPD
/ structure, the indole N-H group and the oxygen atom of furan are positioned
on the same side, having a tilted parallel-displaced π-stacked structure. The N-H
group and oxygen atom are on the opposite side with a parallel-displaced π-stacked
geometry in the Anti-PD structure. A comparison of the experimental IR spectrum
with the theoretical IR spectra of the four conformers of indole…furan (Fig. 8B)
reveals that the observed dimer has a tilted T-shaped structure, i.e., T
/ (N-H…π),
which provides 40 cm
−1 red-shift in the N-H stretch frequency with respect to that in
the indole monomer. A remarkable point to note here is that the π-hydrogen-bonded
(unconventional hydrogen bond) structure dominates over the conventional N–H…O
hydrogen-bonded structure.
Das and co-workers further extended their spectroscopic investigation on N-H…π
interaction by replacing the heteroatom in the π-hydrogen bond acceptor aromatic
ring by sulfur (S) atom [188]. Figure 9Ab shows the mass-selected IR spectrum of the
single conformer of indole…thiophene complex measured by RIDIR spectroscopy.
The theoretical IR spectrum of the dimeric complex calculated at the M05-2X/6–
311++G(2df,2pd) level of theory is provided in Fig. 9Ac. The observed conformer
of the indole…thiophene complex has a tilted T-shaped structure bound by N-H…π
hydrogen bonding interaction similar to that of the indole…furan complex. However,
the tilt angle in the T-shaped structure of the indole…thiophene complex is larger
than that of the indole…furan complex [144]. The red-shift of 56 cm
−1 in the NH stretching frequency of the indole…thiophene complex with respect to that of