levels, which gives rise to distinct frequency separations in the 2D IR spectra. In
addition to the expected single (e.g. h0jj) and doubly (e.g. h02jj) excited states of
each mode exist in this description, which give rise to the pairs of diagonal peaks (1/
3 and 2/4), a combination state (hijhjj) also contributes, which influences the spectral
positions for the cross peaks (2/5 and 1/6). Note that the coupling directly manifests
itself by the existence of a common ground state of the two modes, which gives rise
to ground state bleach signals at cross peak positions for both modes simultaneously, even if only one of them is initially excited (1/2). The 2D IR spectrum
contains information not only on the frequencies of the two modes, but also the
frequency differences between ground state bleach (blue) and excited state
absorption (red) transitions, which relate to the diagonal anharmonicities (D i , D j ).
Additional information is contained in the off-diagonal anharmonicity (D ij ), which
is directly related to the magnitude of the coupling constant b ij . Especially for the
latter parameter, strong and weak coupling regimes can be directly distinguished
[10]. This is because in the case of weak coupling, the frequency-separation of the
oppositely signed signals (2/5 and 1/6) is much smaller than in the strong coupling
case, which leads to mutual partial cancellation. Thus, the intensity of the cross
peaks is a direct measure of the strength of coupling. Moreover, the magnitude of
the coupling constant contains information about the distance and angles of the
contributing modes [1, 10, 18, 42, 76], i.e. the molecular geometry. These quantities
may be retrieved from polarization-resolved 2D IR spectra using an appropriate
model to account for the coupling (Sect. 3.1.1). In fact, it is noted that the coupling
also slightly changes the appearance of the diagonal peaks with respect to an
uncoupled case, due to its influence on the shape of the potential energy surface.
The described picture is static and does not include any time-dependence of the
coupling constant. As a consequence, the cross peaks can already be expected
directly after excitation at zero population delay, but may show some dynamics
according to the molecular system under study. Moreover, as the samples molecular
structure generally fluctuates in solution, also the coupling constant changes its
magnitude. This results in dynamic effects regarding the cross peak shapes and
amplitudes, which has been characterized experimentally and theoretically in detail
[10, 37, 77].
The situation for the observation of cross peaks is different if the signals arise
from other processes in the sample, e.g. chemical exchange (Fig. 5b). The term
chemical exchange generally refers to a reaction that takes place in the sample,
which can be either intra- or intermolecular. This reaction can involve either
intramolecular conformational changes of the sample, as well as formation and
breaking of chemical bonds. The details of the mechanism and the dynamics can
in fact be quite complex, and different examples for chemical exchange have been
reported for 2D IR spectroscopy (Sect. 3.1.4). The power of 2D IR spectroscopy is
that the dynamics of the reaction can be followed in real time, i.e. on the femto- to
picosecond timescale. In general, one considers a reaction, which contains two
species that exist in chemical equilibrium, here: A and B. That is, A can convert to
B and vice versa. Necessary conditions for chemical exchange to be observed with
2D IR are again the spectral separation of the modes that contribute to the species
A and B, along with the IR-activity, a non-zero anharmonicity for each mode
Top Curr Chem (Z) (2017) 375:86
123
127
Reprinted from the journal
addition to the expected single (e.g. h0jj) and doubly (e.g. h02jj) excited states of
each mode exist in this description, which give rise to the pairs of diagonal peaks (1/
3 and 2/4), a combination state (hijhjj) also contributes, which influences the spectral
positions for the cross peaks (2/5 and 1/6). Note that the coupling directly manifests
itself by the existence of a common ground state of the two modes, which gives rise
to ground state bleach signals at cross peak positions for both modes simultaneously, even if only one of them is initially excited (1/2). The 2D IR spectrum
contains information not only on the frequencies of the two modes, but also the
frequency differences between ground state bleach (blue) and excited state
absorption (red) transitions, which relate to the diagonal anharmonicities (D i , D j ).
Additional information is contained in the off-diagonal anharmonicity (D ij ), which
is directly related to the magnitude of the coupling constant b ij . Especially for the
latter parameter, strong and weak coupling regimes can be directly distinguished
[10]. This is because in the case of weak coupling, the frequency-separation of the
oppositely signed signals (2/5 and 1/6) is much smaller than in the strong coupling
case, which leads to mutual partial cancellation. Thus, the intensity of the cross
peaks is a direct measure of the strength of coupling. Moreover, the magnitude of
the coupling constant contains information about the distance and angles of the
contributing modes [1, 10, 18, 42, 76], i.e. the molecular geometry. These quantities
may be retrieved from polarization-resolved 2D IR spectra using an appropriate
model to account for the coupling (Sect. 3.1.1). In fact, it is noted that the coupling
also slightly changes the appearance of the diagonal peaks with respect to an
uncoupled case, due to its influence on the shape of the potential energy surface.
The described picture is static and does not include any time-dependence of the
coupling constant. As a consequence, the cross peaks can already be expected
directly after excitation at zero population delay, but may show some dynamics
according to the molecular system under study. Moreover, as the samples molecular
structure generally fluctuates in solution, also the coupling constant changes its
magnitude. This results in dynamic effects regarding the cross peak shapes and
amplitudes, which has been characterized experimentally and theoretically in detail
[10, 37, 77].
The situation for the observation of cross peaks is different if the signals arise
from other processes in the sample, e.g. chemical exchange (Fig. 5b). The term
chemical exchange generally refers to a reaction that takes place in the sample,
which can be either intra- or intermolecular. This reaction can involve either
intramolecular conformational changes of the sample, as well as formation and
breaking of chemical bonds. The details of the mechanism and the dynamics can
in fact be quite complex, and different examples for chemical exchange have been
reported for 2D IR spectroscopy (Sect. 3.1.4). The power of 2D IR spectroscopy is
that the dynamics of the reaction can be followed in real time, i.e. on the femto- to
picosecond timescale. In general, one considers a reaction, which contains two
species that exist in chemical equilibrium, here: A and B. That is, A can convert to
B and vice versa. Necessary conditions for chemical exchange to be observed with
2D IR are again the spectral separation of the modes that contribute to the species
A and B, along with the IR-activity, a non-zero anharmonicity for each mode
Top Curr Chem (Z) (2017) 375:86
123
127
Reprinted from the journal