[138], between water and small ions at high concentrations [139], or intramolecular
hydrogen bonding in diols [140]. All the obtained results highlighted the importance
of picosecond formation/dissociation of intermolecular hydrogen bonds and
discussed its impact on systems that are more complex. The concept of chemical
exchange is, however, much more general, and since its advent, many examples
other than hydrogen bonding have been reported. Fayer et al. have demonstrated
picosecond exchange in carbon-carbon single-bond isomerization [141]. For a
particular system, isomerization time constants around 43 ps have been determined
and estimations have been made regarding non-substituted ethane and n-butane.
These experiments thus give direct insight into the barrier height of intramolecular
bond rotation. Other experiments looked at conformational switching in mutants of
myoglobin and demonstrated that ultrafast chemical exchange dynamics play a role
even for such complex bio-molecular systems [142]. Harris et al. achieved to
determine transition state geometries by 2D IR chemical exchange spectroscopy in
fluxional rearrangements of Fe(CO) 5 . They were able to demonstrate a pseudorotation mechanism for the rearrangement and to rule out IVR between the different
involved modes, which is another possibility for growing cross peaks in 2D IR
spectra (Sect. 3.1.5). Finally, Gaffney et al. studied ligand exchange dynamics in the
coordination sphere of small cations [143, 144]. They demonstrated the preferential
contact ion-pair formation between soft Lewis acids and hard Lewis bases, which
cannot be easily explained by Pearson’s acid-base concept. Such experiments are
highly relevant for biological systems, since insights into solvation and coordination-shell dynamics by ultrafast exchange dynamics with naturally abundant ions
are of pivotal relevance for instance in biological signal transduction.
3.1.5 Intermolecular and Intramolecular Vibrational Energy Transfer Observed
with 2D IR
Another possibility for cross peaks to show up in a 2D IR spectrum is from
vibrational energy transfer between different oscillators in the sample [10]. Ultrafast
2D IR spectroscopy is, therefore, the ideal method to characterize such processes
since the underlying dynamics intrinsically occur on the femto- to picosecond
timescale. As an important point, vibrational energy transfer is strongly sensitive to
molecular structure, thereby allowing obtaining direct information about the sample
constitution. Moreover, energy transfer can take place in different ways, e.g. in an
inter- or intramolecular manner. In the former case, the donor and acceptor
molecules need not be of the same type, i.e. they can be a solvent and a solute
molecule. Similarly, in the latter case, the donor and acceptor modes need not be the
same functional groups, but can be any two groups within a sample molecule. As
such, the dynamics of intramolecular cross peaks from energy transfer in a 2D IR
spectrum can under some circumstances be taken as a measure for intramolecular
vibrational energy redistribution (IVR). Energy transfer can moreover occur in
either a resonant or a non-resonant way. The access energy of a certain mode after
excitation can be transferred to another oscillator that at least partly spectrally
overlaps with the transition frequency of the donor. In such a case spectral diffusion
of the donor and the acceptor bands plays an important role since the transition
Top Curr Chem (Z) (2017) 375:86
123
139
Reprinted from the journal
hydrogen bonding in diols [140]. All the obtained results highlighted the importance
of picosecond formation/dissociation of intermolecular hydrogen bonds and
discussed its impact on systems that are more complex. The concept of chemical
exchange is, however, much more general, and since its advent, many examples
other than hydrogen bonding have been reported. Fayer et al. have demonstrated
picosecond exchange in carbon-carbon single-bond isomerization [141]. For a
particular system, isomerization time constants around 43 ps have been determined
and estimations have been made regarding non-substituted ethane and n-butane.
These experiments thus give direct insight into the barrier height of intramolecular
bond rotation. Other experiments looked at conformational switching in mutants of
myoglobin and demonstrated that ultrafast chemical exchange dynamics play a role
even for such complex bio-molecular systems [142]. Harris et al. achieved to
determine transition state geometries by 2D IR chemical exchange spectroscopy in
fluxional rearrangements of Fe(CO) 5 . They were able to demonstrate a pseudorotation mechanism for the rearrangement and to rule out IVR between the different
involved modes, which is another possibility for growing cross peaks in 2D IR
spectra (Sect. 3.1.5). Finally, Gaffney et al. studied ligand exchange dynamics in the
coordination sphere of small cations [143, 144]. They demonstrated the preferential
contact ion-pair formation between soft Lewis acids and hard Lewis bases, which
cannot be easily explained by Pearson’s acid-base concept. Such experiments are
highly relevant for biological systems, since insights into solvation and coordination-shell dynamics by ultrafast exchange dynamics with naturally abundant ions
are of pivotal relevance for instance in biological signal transduction.
3.1.5 Intermolecular and Intramolecular Vibrational Energy Transfer Observed
with 2D IR
Another possibility for cross peaks to show up in a 2D IR spectrum is from
vibrational energy transfer between different oscillators in the sample [10]. Ultrafast
2D IR spectroscopy is, therefore, the ideal method to characterize such processes
since the underlying dynamics intrinsically occur on the femto- to picosecond
timescale. As an important point, vibrational energy transfer is strongly sensitive to
molecular structure, thereby allowing obtaining direct information about the sample
constitution. Moreover, energy transfer can take place in different ways, e.g. in an
inter- or intramolecular manner. In the former case, the donor and acceptor
molecules need not be of the same type, i.e. they can be a solvent and a solute
molecule. Similarly, in the latter case, the donor and acceptor modes need not be the
same functional groups, but can be any two groups within a sample molecule. As
such, the dynamics of intramolecular cross peaks from energy transfer in a 2D IR
spectrum can under some circumstances be taken as a measure for intramolecular
vibrational energy redistribution (IVR). Energy transfer can moreover occur in
either a resonant or a non-resonant way. The access energy of a certain mode after
excitation can be transferred to another oscillator that at least partly spectrally
overlaps with the transition frequency of the donor. In such a case spectral diffusion
of the donor and the acceptor bands plays an important role since the transition
Top Curr Chem (Z) (2017) 375:86
123
139
Reprinted from the journal