C–H···Y (Y=N, O, π) Hydrogen Bond: A Unique Unconventional Hydrogen Bond

Abstract

We present a spectroscopic overview of the C–H···Y
(Y = hydrogen bond acceptors) hydrogen bonded (HB or H-bond)
complexes in this article. Although C–H···Y interactions have been recognized
as H-bonding interactions for quite some time, they have not
been investigated spectroscopically until recently. Recent results indicated
that unlike the conventional hydrogen bond, C–H···Y H-bond has
interesting spectroscopic characteristics, i.e. it shows both red as well
as blue shift in C–H stretching frequency upon H-bond formation. This
review presents examples of red, blue, and zero shifted C–H···Y H-bonds
investigated in our laboratory that were characterized using laser-based
IR and UV spectroscopic techniques applied to the cold isolated molecular
complexes formed under supersonic expansion conditions. Along
with spectroscopic information, ab initio/DFT-predicted geometry optimized
structures of various conformers, harmonic frequency calculations
of the optimized structures, and a number of properties such as electron
densities at the bond critical points, orbital interaction energies, binding
energies of the C–H···Y bound complexes are also summarized for
better understanding of this type of H-bond. Not only the spectroscopic
shift in C–H stretching frequency, but also the role of C–H···O H-bonds
in microsolvation of several organic molecules has been highlighted. It
has been found that depending upon activation of C–H moiety, C–H···Y
H-bonds can provide primary or secondary stabilization for the growth of
the primary solvation shell around organic molecules.