group can be investigated with this method. As an advantage of IR spectroscopy for
structure determination, the observed bands are rather sharp and defined, whereas
THz and electronic resonances often appear broad and featureless. Very recently
also methods have been realized that allowed the investigation of correlated
vibrational (V) and electronic (E) resonances. These techniques are consequently
termed 2D EV and 2D VE spectroscopy, and they are especially promising for the
elucidation of bio-molecular vibrational dynamics. Therefore, the spectral region of
electronic excitations ([ 8000 cm
-1 ) is also briefly covered in this chapter.
2D IR spectra are frequency-frequency correlation maps as schematically shown
in the right part of Fig. 1. To construct the spectra, an initial excitation with IR light
is used as a perturbation of a sample system, and its response is correlated to this
initial event. The two corresponding frequency axes are, therefore, generally
referred to as ‘‘excitation’’ and ‘‘detection frequencies’’ and the IR signals are
spread in both dimensions. This spreading of the signals can generate complex
shapes from the observed resonances (e.g. ellipses, circles or others) [10]. The
shapes allow a more sophisticated interpretation of the properties of the resonance
compared to linear absorption spectra. Many of the signals fall on the diagonal line
in a 2D IR spectrum and are thus related to linear IR absorbance (right panel). The
induced 2D IR signals come in pairs (blue/red) due to the anharmonicity of the
vibrational potential under study (D anh ) and the induced nature of the differential
signal (excitation). The ellipticity (E inh ) moreover reports on the degree of
correlation in the resonance and an underlying spectral inhomogeneity. Importantly,
also off-diagonal contributions can exist in a 2D IR spectrum, which may report on
interactions between different resonances. Functional groups of molecules can
interact in many different ways, e.g. via dipolar vibrational coupling (associated
with a coupling constant b AB ). This coupling strongly depends on the distance of the
interacting modes and is thus very sensitive to molecular structure (see Sect. 3.1.1).
As a particularly powerful approach, a combination of experimental data and
theoretical predictions can be used for almost quantitative structure modelling
[24, 25].
Adding a time delay between excitation and detection events in 2D IR allows one
to resolve different types of dynamics from the sample in a thermally equilibrated
electronic ground state (see Sect. 2). The most straightforward information that can
be obtained is the dynamics of vibrational relaxation (s VR ), or the dynamics of
spectral correlation via for instance spectral diffusion (s SD ). Moreover, once excited,
an excess vibrational energy in a vibrational bond can be transferred to another
oscillator with a rate constant k ET , which is characteristic of the sample system
under study. Additionally, it is possible that the sample undergoes a chemical
reaction following the initial excitation. Such a reaction can influence the
vibrational properties of certain functional groups and the dynamics of so-called
chemical exchange can be investigated by looking at the rate constants (k Ex ), with
which off-diagonal signals evolve. These signals also strongly depend on the
molecular structure of the sample and can, therefore, report on the dynamic
evolution of sample structure. Finally, 2D IR spectroscopy can be combined with
different types of pre-excitation methods (see Sect. 4.4). This allows one to address
non-equilibrium dynamics, e.g. photo-induced dynamics of chemical bonds or
Top Curr Chem (Z) (2017) 375:86
123
116
Reprinted from the journal
structure determination, the observed bands are rather sharp and defined, whereas
THz and electronic resonances often appear broad and featureless. Very recently
also methods have been realized that allowed the investigation of correlated
vibrational (V) and electronic (E) resonances. These techniques are consequently
termed 2D EV and 2D VE spectroscopy, and they are especially promising for the
elucidation of bio-molecular vibrational dynamics. Therefore, the spectral region of
electronic excitations ([ 8000 cm
-1 ) is also briefly covered in this chapter.
2D IR spectra are frequency-frequency correlation maps as schematically shown
in the right part of Fig. 1. To construct the spectra, an initial excitation with IR light
is used as a perturbation of a sample system, and its response is correlated to this
initial event. The two corresponding frequency axes are, therefore, generally
referred to as ‘‘excitation’’ and ‘‘detection frequencies’’ and the IR signals are
spread in both dimensions. This spreading of the signals can generate complex
shapes from the observed resonances (e.g. ellipses, circles or others) [10]. The
shapes allow a more sophisticated interpretation of the properties of the resonance
compared to linear absorption spectra. Many of the signals fall on the diagonal line
in a 2D IR spectrum and are thus related to linear IR absorbance (right panel). The
induced 2D IR signals come in pairs (blue/red) due to the anharmonicity of the
vibrational potential under study (D anh ) and the induced nature of the differential
signal (excitation). The ellipticity (E inh ) moreover reports on the degree of
correlation in the resonance and an underlying spectral inhomogeneity. Importantly,
also off-diagonal contributions can exist in a 2D IR spectrum, which may report on
interactions between different resonances. Functional groups of molecules can
interact in many different ways, e.g. via dipolar vibrational coupling (associated
with a coupling constant b AB ). This coupling strongly depends on the distance of the
interacting modes and is thus very sensitive to molecular structure (see Sect. 3.1.1).
As a particularly powerful approach, a combination of experimental data and
theoretical predictions can be used for almost quantitative structure modelling
[24, 25].
Adding a time delay between excitation and detection events in 2D IR allows one
to resolve different types of dynamics from the sample in a thermally equilibrated
electronic ground state (see Sect. 2). The most straightforward information that can
be obtained is the dynamics of vibrational relaxation (s VR ), or the dynamics of
spectral correlation via for instance spectral diffusion (s SD ). Moreover, once excited,
an excess vibrational energy in a vibrational bond can be transferred to another
oscillator with a rate constant k ET , which is characteristic of the sample system
under study. Additionally, it is possible that the sample undergoes a chemical
reaction following the initial excitation. Such a reaction can influence the
vibrational properties of certain functional groups and the dynamics of so-called
chemical exchange can be investigated by looking at the rate constants (k Ex ), with
which off-diagonal signals evolve. These signals also strongly depend on the
molecular structure of the sample and can, therefore, report on the dynamic
evolution of sample structure. Finally, 2D IR spectroscopy can be combined with
different types of pre-excitation methods (see Sect. 4.4). This allows one to address
non-equilibrium dynamics, e.g. photo-induced dynamics of chemical bonds or
Top Curr Chem (Z) (2017) 375:86
123
116
Reprinted from the journal