300
M. A. Czarnecki et al.
NIR imaging spectroscopy should be mentioned as well since it demonstrates a strong
potential in this area as well, e.g. when investigating processes involving solvent
molecules or molecular interaction in polymers [13].
NIR spectroscopy has frequently been used when studying solution phase and
various solvent effects [5]. Certain vibrations, e.g. ν(OH), undergo distinct spectral
changes in response to change in the chemical environment (for instance change
in the concentration level of solvent) [5]. Similar observations have been made for
hydrogen-bonded complexes [14]. It has been noticed that the underlying mechanisms are non-trivial and involve an interplay of the anharmonicity in the vibrational potential and the nonlinearity of the transition dipole moments [2, 9]. NIR
spectroscopy is indispensable in investigation of this phenomenon, as data on fundamental, first, second, and often third overtone bands are necessary. At the same time,
conventional methods of spectral analysis fail to deliver decisive insights in this case
and advanced tools of computational chemistry proofed necessary to reproduce and
explain the changes occurring in mechanical and electrical anharmonicity in response
to the changing environment [2, 9].
13.2 Hydrogen Bonding Studies
Vibrational spectra are sensitive markers of hydrogen bond interactions. The most
evident proof of the presence of hydrogen bonding is a shift in the position of IR
peaks originating from the groups involved in this interaction. The shift is often
easily observable in vibrational spectra and enables a monitoring of hydrogen bond
properties. The origin of this shift can be explained by using a relatively simple model
of molecular vibrations, namely the classical harmonic oscillator. For a diatomic
molecule with masses m and M, the reduced mass μ is given as:
μ =
Mm
M + m
(13.1)
This oscillator has a single vibration with the frequency ν osc expressed as:
ν osc =
1
2π
k
μ
(13.2)
with k being the associated force constant. The frequency of vibration is proportional to the square root of the force constant specific to the bond. Hydrogen bond
formation leads to changes in the effective force constant of the vibration, which is
manifested as a shift of the corresponding band. Upon formation of a hydrogen bond,
the oscillator may be subjected to two kinds of changes, depending on the type of
vibration and the geometry of the bond (Fig. 13.3). Typically, the force constant of
an X–H stretching vibration is red-shifted upon the formation of a hydrogen bond
M. A. Czarnecki et al.
NIR imaging spectroscopy should be mentioned as well since it demonstrates a strong
potential in this area as well, e.g. when investigating processes involving solvent
molecules or molecular interaction in polymers [13].
NIR spectroscopy has frequently been used when studying solution phase and
various solvent effects [5]. Certain vibrations, e.g. ν(OH), undergo distinct spectral
changes in response to change in the chemical environment (for instance change
in the concentration level of solvent) [5]. Similar observations have been made for
hydrogen-bonded complexes [14]. It has been noticed that the underlying mechanisms are non-trivial and involve an interplay of the anharmonicity in the vibrational potential and the nonlinearity of the transition dipole moments [2, 9]. NIR
spectroscopy is indispensable in investigation of this phenomenon, as data on fundamental, first, second, and often third overtone bands are necessary. At the same time,
conventional methods of spectral analysis fail to deliver decisive insights in this case
and advanced tools of computational chemistry proofed necessary to reproduce and
explain the changes occurring in mechanical and electrical anharmonicity in response
to the changing environment [2, 9].
13.2 Hydrogen Bonding Studies
Vibrational spectra are sensitive markers of hydrogen bond interactions. The most
evident proof of the presence of hydrogen bonding is a shift in the position of IR
peaks originating from the groups involved in this interaction. The shift is often
easily observable in vibrational spectra and enables a monitoring of hydrogen bond
properties. The origin of this shift can be explained by using a relatively simple model
of molecular vibrations, namely the classical harmonic oscillator. For a diatomic
molecule with masses m and M, the reduced mass μ is given as:
μ =
Mm
M + m
(13.1)
This oscillator has a single vibration with the frequency ν osc expressed as:
ν osc =
1
2π
k
μ
(13.2)
with k being the associated force constant. The frequency of vibration is proportional to the square root of the force constant specific to the bond. Hydrogen bond
formation leads to changes in the effective force constant of the vibration, which is
manifested as a shift of the corresponding band. Upon formation of a hydrogen bond,
the oscillator may be subjected to two kinds of changes, depending on the type of
vibration and the geometry of the bond (Fig. 13.3). Typically, the force constant of
an X–H stretching vibration is red-shifted upon the formation of a hydrogen bond