Exploring Non-covalent Interactions by Jet-Cooled Electronic …
77
3.2 Unconventional Hydrogen Bond
Although the conventional hydrogen bonds are well-understood in the literature,
unconventional hydrogen bonds are yet to be explored in terms of their nature, physical origin, and strength [16, 20, 22, 93]. In the unconventional hydrogen bond, the
hydrogen bond donor and acceptor atoms are weakly electronegative. C-H…Y (Y =
O, N) is a relatively well-studied unconventional hydrogen bond, where the hydrogen
bond donor atom (C) is very weak in electronegativity [10, 80]. It has been reported
that C-H…O or C-H…N hydrogen bond is very weak in strength [80]. Desiraju and
co-workers have demonstrated the significance of the C-H…Y hydrogen bonds in
crystal engineering and supramolecular assemblies [10, 12, 16, 18]. In recent times,
there are reports on spectroscopic studies of various unconventional hydrogen bonds
having different unconventional atoms in the periodic table such as P, S, and Se as
hydrogen bond acceptors [80, 86–88, 91–93, 145, 193, 194]. Surprisingly, it has
been found from various gas-phase spectroscopy studies that these unconventional
hydrogen bonds, i.e., N-H…S, N-H…Se, N-H…P, etc. are as strong as any conventional hydrogen bonds such as N-H…O, O–H…O, etc. [80, 86–88, 93, 145, 193,
194]. Sulfur centered hydrogen bonds are also extensively present in proteins and
many other supramolecular assemblies [18, 20, 22, 88, 93, 111, 194].
Biswal et al. have made a seminal contribution to understanding the physical nature
and strength of sulfur centered hydrogen bonds by studying various model complexes
in a supersonic jet using different laser-based spectroscopic techniques and quantum
chemistry calculations [86–88, 93]. They have studied several dimeric complexes
of indole, p-cresol, p-fluorophenol with various solvents such as Me 2 S, Et 2 S, Et 2 O,
Me 2 O, H 2 S, etc. employing LIF excitation, R2PI, FDIRS, and RIDIRS spectroscopy
techniques [20, 87]. It has been found that the IR red-shift in the N-H and O-H
stretch frequencies in the N-H…S and O-H…S hydrogen bonds, respectively, are
quite similar to that in any conventional hydrogen bond [20, 22, 86–88, 93, 195]. They
have found from the energy decomposition analysis that dispersion interaction plays
a significant role in the stability of these unconventional hydrogen bonds, and hence
these non-covalent interactions are termed as dispersion stabilized hydrogen bonds
[20, 22, 86–88, 93, 194, 195]. Observation of unusually strong N-H…S hydrogen
bond has also been reported using room-temperature gas-phase Fourier transform
infrared (FTIR) spectroscopy by Kjaergaard and co-workers [89, 90]. They have
further demonstrated that phosphorous (P) can act as a strong hydrogen bond acceptor
by studying the O-H
… P hydrogen bond present in 1:1 complexes of trimethyl phosphine and various alcohols using matrix isolation as well as gas-phase FTIR spectroscopy [91, 92]. Later, Biswal et al. provided the spectroscopic evidence of the
formation of strong N-H…Se hydrogen bonds by investigating the complexes of
N-phenyl acetamide and 2-pyridone with dimethyl selenide [194].
Recently, Das and co-workers have performed a thorough investigation of the
nature of the selenium hydrogen bonding by studying both N-H…Se and OH…Se hydrogen bonds using gas-phase laser spectroscopy and detailed theoretical calculations [145]. Figure 10A shows mass-selected conformation-specific IR
77
3.2 Unconventional Hydrogen Bond
Although the conventional hydrogen bonds are well-understood in the literature,
unconventional hydrogen bonds are yet to be explored in terms of their nature, physical origin, and strength [16, 20, 22, 93]. In the unconventional hydrogen bond, the
hydrogen bond donor and acceptor atoms are weakly electronegative. C-H…Y (Y =
O, N) is a relatively well-studied unconventional hydrogen bond, where the hydrogen
bond donor atom (C) is very weak in electronegativity [10, 80]. It has been reported
that C-H…O or C-H…N hydrogen bond is very weak in strength [80]. Desiraju and
co-workers have demonstrated the significance of the C-H…Y hydrogen bonds in
crystal engineering and supramolecular assemblies [10, 12, 16, 18]. In recent times,
there are reports on spectroscopic studies of various unconventional hydrogen bonds
having different unconventional atoms in the periodic table such as P, S, and Se as
hydrogen bond acceptors [80, 86–88, 91–93, 145, 193, 194]. Surprisingly, it has
been found from various gas-phase spectroscopy studies that these unconventional
hydrogen bonds, i.e., N-H…S, N-H…Se, N-H…P, etc. are as strong as any conventional hydrogen bonds such as N-H…O, O–H…O, etc. [80, 86–88, 93, 145, 193,
194]. Sulfur centered hydrogen bonds are also extensively present in proteins and
many other supramolecular assemblies [18, 20, 22, 88, 93, 111, 194].
Biswal et al. have made a seminal contribution to understanding the physical nature
and strength of sulfur centered hydrogen bonds by studying various model complexes
in a supersonic jet using different laser-based spectroscopic techniques and quantum
chemistry calculations [86–88, 93]. They have studied several dimeric complexes
of indole, p-cresol, p-fluorophenol with various solvents such as Me 2 S, Et 2 S, Et 2 O,
Me 2 O, H 2 S, etc. employing LIF excitation, R2PI, FDIRS, and RIDIRS spectroscopy
techniques [20, 87]. It has been found that the IR red-shift in the N-H and O-H
stretch frequencies in the N-H…S and O-H…S hydrogen bonds, respectively, are
quite similar to that in any conventional hydrogen bond [20, 22, 86–88, 93, 195]. They
have found from the energy decomposition analysis that dispersion interaction plays
a significant role in the stability of these unconventional hydrogen bonds, and hence
these non-covalent interactions are termed as dispersion stabilized hydrogen bonds
[20, 22, 86–88, 93, 194, 195]. Observation of unusually strong N-H…S hydrogen
bond has also been reported using room-temperature gas-phase Fourier transform
infrared (FTIR) spectroscopy by Kjaergaard and co-workers [89, 90]. They have
further demonstrated that phosphorous (P) can act as a strong hydrogen bond acceptor
by studying the O-H
… P hydrogen bond present in 1:1 complexes of trimethyl phosphine and various alcohols using matrix isolation as well as gas-phase FTIR spectroscopy [91, 92]. Later, Biswal et al. provided the spectroscopic evidence of the
formation of strong N-H…Se hydrogen bonds by investigating the complexes of
N-phenyl acetamide and 2-pyridone with dimethyl selenide [194].
Recently, Das and co-workers have performed a thorough investigation of the
nature of the selenium hydrogen bonding by studying both N-H…Se and OH…Se hydrogen bonds using gas-phase laser spectroscopy and detailed theoretical calculations [145]. Figure 10A shows mass-selected conformation-specific IR