coupling, also because the transition dipole moments for electronic transitions are
generally much larger as for vibrational transitions.
3.1.5.2 Intramolecular Energy Transfer The flow of energy within a given sample
molecule after vibrational excitation is another important mechanism, which can be
used to elucidate molecular structure, dynamics and intramolecular interactions
[10]. This is of particular importance for complex organic molecules in a solution
environment with a large range of functional groups as well as inter-group
distances. The observation of energy flow can help to understand in detail the
mechanisms, which underlie conformational fluctuations, vibrational relaxation and
IVR. As in the case of spectral diffusion (Sect. 3.1.3), such processes influence
time-dependent contributions in the Hamiltonian of the system. The typical
timescale for the fluctuations that determine these processes is approximately
picoseconds or faster and is thus very well accessible with ultrafast 2D IR
spectroscopy.
As stated in the introduction, 2D IR spectroscopy is often invoked as being the
optical analogue of 2D NMR spectroscopy, where intramolecular distances and
fluctuations are measured using for instance the nuclear Overhauser effect (NOESY)
[10, 25, 46, 66, 74, 77, 167]. Indeed, 2D IR spectroscopy has been used to
demonstrate incoherent energy transfer between amide-I modes in small peptides
(e.g. trialanine) by the observation of dynamic evolution of cross peak intensities
together with a detailed comparison of experimental results with MD simulations
[77]. As a consequence of population transfer from an initially excited oscillator to
an acceptor mode, both of which may be coupled to some extent, the intensity of the
observable cross peaks relative to the diagonal peaks grows with increasing
population delays, similar as in Fig. 10. Importantly, such relaxation rates depend
on the energy difference between the coupled states and slow down with increasing
energy separation. The determination of such variations in cross relaxation rates due
to increased spectral separation based on isotope-substituted samples, along with
polarization-dependent 2D IR measurements that reveal the time-averaged relative
orientations and coupling strengths of different vibrational modes, have overall
allowed revealing a detailed picture of structural fluctuations in such short peptide
chains in the amide-I spectral range [77].
As demonstrated in Sects. 3.1.2 and 3.1.3 the amide-I band is a very useful
marker for the determination of molecular structure; however, its occurrence is
largely restricted to peptides and proteins. It is highly desirable to follow energy
transport in molecules over a much larger spectral range, also covering different
other types of functional groups with varying absorption coefficients and varying
vibrational lifetimes. To resolve such intramolecular energy transport in different
types of molecules and to study the energy transport based on different mechanisms
(e.g. transition-dipole coupling, anharmonic coupling, mechanical coupling, or heat
diffusion) dependent on distances of functional groups, Rubtsov et al. have
developed a multi-color approach for 2D IR spectroscopy [74, 75, 168–170]. In their
so-called relaxation-assisted (RA) 2D IR method, a high frequency mode (a tag,
Fig. 11a) is initially excited and the influence of the initial excitation on other
Top Curr Chem (Z) (2017) 375:86
123
143
Reprinted from the journal
generally much larger as for vibrational transitions.
3.1.5.2 Intramolecular Energy Transfer The flow of energy within a given sample
molecule after vibrational excitation is another important mechanism, which can be
used to elucidate molecular structure, dynamics and intramolecular interactions
[10]. This is of particular importance for complex organic molecules in a solution
environment with a large range of functional groups as well as inter-group
distances. The observation of energy flow can help to understand in detail the
mechanisms, which underlie conformational fluctuations, vibrational relaxation and
IVR. As in the case of spectral diffusion (Sect. 3.1.3), such processes influence
time-dependent contributions in the Hamiltonian of the system. The typical
timescale for the fluctuations that determine these processes is approximately
picoseconds or faster and is thus very well accessible with ultrafast 2D IR
spectroscopy.
As stated in the introduction, 2D IR spectroscopy is often invoked as being the
optical analogue of 2D NMR spectroscopy, where intramolecular distances and
fluctuations are measured using for instance the nuclear Overhauser effect (NOESY)
[10, 25, 46, 66, 74, 77, 167]. Indeed, 2D IR spectroscopy has been used to
demonstrate incoherent energy transfer between amide-I modes in small peptides
(e.g. trialanine) by the observation of dynamic evolution of cross peak intensities
together with a detailed comparison of experimental results with MD simulations
[77]. As a consequence of population transfer from an initially excited oscillator to
an acceptor mode, both of which may be coupled to some extent, the intensity of the
observable cross peaks relative to the diagonal peaks grows with increasing
population delays, similar as in Fig. 10. Importantly, such relaxation rates depend
on the energy difference between the coupled states and slow down with increasing
energy separation. The determination of such variations in cross relaxation rates due
to increased spectral separation based on isotope-substituted samples, along with
polarization-dependent 2D IR measurements that reveal the time-averaged relative
orientations and coupling strengths of different vibrational modes, have overall
allowed revealing a detailed picture of structural fluctuations in such short peptide
chains in the amide-I spectral range [77].
As demonstrated in Sects. 3.1.2 and 3.1.3 the amide-I band is a very useful
marker for the determination of molecular structure; however, its occurrence is
largely restricted to peptides and proteins. It is highly desirable to follow energy
transport in molecules over a much larger spectral range, also covering different
other types of functional groups with varying absorption coefficients and varying
vibrational lifetimes. To resolve such intramolecular energy transport in different
types of molecules and to study the energy transport based on different mechanisms
(e.g. transition-dipole coupling, anharmonic coupling, mechanical coupling, or heat
diffusion) dependent on distances of functional groups, Rubtsov et al. have
developed a multi-color approach for 2D IR spectroscopy [74, 75, 168–170]. In their
so-called relaxation-assisted (RA) 2D IR method, a high frequency mode (a tag,
Fig. 11a) is initially excited and the influence of the initial excitation on other
Top Curr Chem (Z) (2017) 375:86
123
143
Reprinted from the journal