dynamics from excited electronic states. It is clear that 2D IR spectroscopy can be
used to obtain all the described information, which is often difficult to obtain by
other experimental or analytical methods.
1.3 Scientific Impact of 2D IR Spectroscopy
2D IR spectroscopy has become a versatile analytical tool for bond-specific
molecular structure determination on ultrafast timescales. Its inherent ability to
resolve molecular dynamics of often less than a picosecond allows obtaining an
unprecedented view of structural changes caused by direct rearrangements of
chemical bonds. This advantage is based on the comparatively fast fluctuations that
govern IR transitions, often resulting in dephasing times of about picoseconds or
less. Similar to electronic spectroscopy of condensed phase systems, such fast
dephasing times broaden the involved resonances at the expense of inherent spectral
resolution. 2D IR can thus be viewed a complementary approach to the standard and
widespread analytical techniques for structure determination such as X-ray
crystallography and different forms of NMR spectroscopy. Especially the latter
method exhibits dephasing times that are orders of magnitude slower (milliseconds)
than those of IR transitions, which intrinsically limits the possible temporal
resolution, at least regarding non-equilibrium dynamics.
To put 2D IR in a context for analytical chemistry methods, the technique can in
principle be applied in two different ways. One variant uses a single 2D IR
spectrum, generally at one very initial delay of vibrational relaxation, to correlate
several vibrational resonances in a sample, just in the same way as different forms
of 2D NMR do. Coupling between vibrational modes, evidenced by cross peaks in
the spectrum, and the corresponding frequencies of diagonal and cross peaks are
then the primary observables, from which molecular structure can be deduced.
Couplings can exist as intra- and intermolecular interactions, which can occur
‘‘through bond’’ or ‘‘through space’’ [10]. One must keep in mind, however, that
even in case of intramolecular couplings, these interactions are comparatively shortranged, i.e. they occur on sub-nm distances. Although this is a typical value also
often encountered for interactions in case 2D NMR spectroscopy, the difference is
that in case of 2D IR cross peaks, the couplings drastically depend also on the
strength transition dipole moments, which are generally low for IR transitions.
Therefore, such couplings are often weak and difficult to resolve. In addition to
measuring one isolated 2D IR spectrum, systematic variation of sample parameters
at macroscopic time scales (milliseconds to hours or even days and weeks) then
report on structural changes in the sample that are significantly slower compared to
vibrational relaxation (picoseconds to nanoseconds).
In the second way of performing 2D IR spectroscopy for structure determination,
a full series of 2D IR spectra is recorded throughout the vibrational lifetime of the
sample. This variant fully exploits the striking advantage of laser-based optical
spectroscopy methods in that they can operate on time scales down to the
femtosecond regime. The ultrafast temporal resolution allows the direct observation
of time-dependent structure and real time molecular dynamics, such as vibrations or
the formation and breaking of a chemical bond. In that way, one determines the
Top Curr Chem (Z) (2017) 375:86
123
117
Reprinted from the journal
used to obtain all the described information, which is often difficult to obtain by
other experimental or analytical methods.
1.3 Scientific Impact of 2D IR Spectroscopy
2D IR spectroscopy has become a versatile analytical tool for bond-specific
molecular structure determination on ultrafast timescales. Its inherent ability to
resolve molecular dynamics of often less than a picosecond allows obtaining an
unprecedented view of structural changes caused by direct rearrangements of
chemical bonds. This advantage is based on the comparatively fast fluctuations that
govern IR transitions, often resulting in dephasing times of about picoseconds or
less. Similar to electronic spectroscopy of condensed phase systems, such fast
dephasing times broaden the involved resonances at the expense of inherent spectral
resolution. 2D IR can thus be viewed a complementary approach to the standard and
widespread analytical techniques for structure determination such as X-ray
crystallography and different forms of NMR spectroscopy. Especially the latter
method exhibits dephasing times that are orders of magnitude slower (milliseconds)
than those of IR transitions, which intrinsically limits the possible temporal
resolution, at least regarding non-equilibrium dynamics.
To put 2D IR in a context for analytical chemistry methods, the technique can in
principle be applied in two different ways. One variant uses a single 2D IR
spectrum, generally at one very initial delay of vibrational relaxation, to correlate
several vibrational resonances in a sample, just in the same way as different forms
of 2D NMR do. Coupling between vibrational modes, evidenced by cross peaks in
the spectrum, and the corresponding frequencies of diagonal and cross peaks are
then the primary observables, from which molecular structure can be deduced.
Couplings can exist as intra- and intermolecular interactions, which can occur
‘‘through bond’’ or ‘‘through space’’ [10]. One must keep in mind, however, that
even in case of intramolecular couplings, these interactions are comparatively shortranged, i.e. they occur on sub-nm distances. Although this is a typical value also
often encountered for interactions in case 2D NMR spectroscopy, the difference is
that in case of 2D IR cross peaks, the couplings drastically depend also on the
strength transition dipole moments, which are generally low for IR transitions.
Therefore, such couplings are often weak and difficult to resolve. In addition to
measuring one isolated 2D IR spectrum, systematic variation of sample parameters
at macroscopic time scales (milliseconds to hours or even days and weeks) then
report on structural changes in the sample that are significantly slower compared to
vibrational relaxation (picoseconds to nanoseconds).
In the second way of performing 2D IR spectroscopy for structure determination,
a full series of 2D IR spectra is recorded throughout the vibrational lifetime of the
sample. This variant fully exploits the striking advantage of laser-based optical
spectroscopy methods in that they can operate on time scales down to the
femtosecond regime. The ultrafast temporal resolution allows the direct observation
of time-dependent structure and real time molecular dynamics, such as vibrations or
the formation and breaking of a chemical bond. In that way, one determines the
Top Curr Chem (Z) (2017) 375:86
123
117
Reprinted from the journal