(2625 cm
-1 ) from the frequency of the free phenol (2660 cm
-1 ) form, both
resulting in ground state bleach signals at early waiting time (200 fs), located on the
diagonal line of a 2D IR spectrum (red, Fig. 9b). The associated excited state
absorption signals (outside the observation window) are additionally spectrally
strongly red-shifted for the bleach contributions, which, for this particular system,
makes the characterization of chemical exchange advantageous due to essentially
non-existing spectral overlap between oppositely signed ground- and excited-state
absorption features. At initial delays (200 fs), only diagonal peaks exist, which
indicates that both forms have not yet interconverted. However, when the
population waiting time is increased to several picoseconds [e.g. 14 ps (Fig. 9c)],
cross peaks start to appear between the complex and free phenol bands for both the
ground state, as well as the excited state absorption bands. A detailed kinetic
analysis of a full population time series of spectra based on an equilibrium model
for the complex and the free form of the phenol yielded a dissociation time constant
of about 8 ps, which is well resolvable within the vibrational lifetime of the
complex and the free phenol (ca. 10–13 ps) [131]. By the observation of the same
exchange kinetics for the ground state and the excited state absorption signals, it
was furthermore revealed that the excitation and probing of the complex by the help
of the 2D IR pulse sequence does not perturb the equilibrium and thus reveals a
direct insight into the dynamics behind the chemical equilibrium. Such expectations
might not always be encountered, especially when highly excited vibrational levels
are populated (see Sect. 5.2).
The formation of hydrogen bonds is a good example of the applicability of 2D IR
spectroscopy to ultrafast dynamics also from another perspective: Once the
hydrogen bond gets much stronger, with consequently slower exchange rates,
chemical exchange can no longer be observed within the vibrational lifetime of the
sample. This is illustrated in Fig. 9d–f, for the case of hydrogen bonding between
phenol-OD and acetonitrile. The nitrile functional group acts as a comparatively
strong hydrogen bond acceptor that down shifts the complex IR absorption band by
almost 100 cm
-1 . For this particular system, the hydrogen bond is too strong and
does not break on the timescale of the vibrational lifetime as demonstrated by the
absence of cross peaks in the 2D IR spectra.
The benzene-phenol complex is also a good example for studying influences of
the electronic structure of the hydrogen bonding partners. Further investigations by
Fayer’s group have looked at the impact of electron-donating/withdrawing
functional groups [132, 133], solvent compositions [134], or the presence of
multiple acceptor sites in the p-base [135]. As one key result, it could be established
that the dissociation rates of the complexes are strongly correlated with the
corresponding formation enthalpies, what was argued to be describable in an
Arrhenius-like manner [132].
The example from Fig. 9 has served as a paradigm for chemical exchange
dynamics during solute-solvent complexation. Further examples have been investigated, which demonstrated bond formation/breaking in the context of chemical
exchange also in simpler, as well as much more complicated sample systems.
Hydrogen bonding was further studied between methanol and organic nitriles
[68, 136], methanol and N-methylacetamide [137], methanol and different esters
Top Curr Chem (Z) (2017) 375:86
123
138
Reprinted from the journal
-1 ) from the frequency of the free phenol (2660 cm
-1 ) form, both
resulting in ground state bleach signals at early waiting time (200 fs), located on the
diagonal line of a 2D IR spectrum (red, Fig. 9b). The associated excited state
absorption signals (outside the observation window) are additionally spectrally
strongly red-shifted for the bleach contributions, which, for this particular system,
makes the characterization of chemical exchange advantageous due to essentially
non-existing spectral overlap between oppositely signed ground- and excited-state
absorption features. At initial delays (200 fs), only diagonal peaks exist, which
indicates that both forms have not yet interconverted. However, when the
population waiting time is increased to several picoseconds [e.g. 14 ps (Fig. 9c)],
cross peaks start to appear between the complex and free phenol bands for both the
ground state, as well as the excited state absorption bands. A detailed kinetic
analysis of a full population time series of spectra based on an equilibrium model
for the complex and the free form of the phenol yielded a dissociation time constant
of about 8 ps, which is well resolvable within the vibrational lifetime of the
complex and the free phenol (ca. 10–13 ps) [131]. By the observation of the same
exchange kinetics for the ground state and the excited state absorption signals, it
was furthermore revealed that the excitation and probing of the complex by the help
of the 2D IR pulse sequence does not perturb the equilibrium and thus reveals a
direct insight into the dynamics behind the chemical equilibrium. Such expectations
might not always be encountered, especially when highly excited vibrational levels
are populated (see Sect. 5.2).
The formation of hydrogen bonds is a good example of the applicability of 2D IR
spectroscopy to ultrafast dynamics also from another perspective: Once the
hydrogen bond gets much stronger, with consequently slower exchange rates,
chemical exchange can no longer be observed within the vibrational lifetime of the
sample. This is illustrated in Fig. 9d–f, for the case of hydrogen bonding between
phenol-OD and acetonitrile. The nitrile functional group acts as a comparatively
strong hydrogen bond acceptor that down shifts the complex IR absorption band by
almost 100 cm
-1 . For this particular system, the hydrogen bond is too strong and
does not break on the timescale of the vibrational lifetime as demonstrated by the
absence of cross peaks in the 2D IR spectra.
The benzene-phenol complex is also a good example for studying influences of
the electronic structure of the hydrogen bonding partners. Further investigations by
Fayer’s group have looked at the impact of electron-donating/withdrawing
functional groups [132, 133], solvent compositions [134], or the presence of
multiple acceptor sites in the p-base [135]. As one key result, it could be established
that the dissociation rates of the complexes are strongly correlated with the
corresponding formation enthalpies, what was argued to be describable in an
Arrhenius-like manner [132].
The example from Fig. 9 has served as a paradigm for chemical exchange
dynamics during solute-solvent complexation. Further examples have been investigated, which demonstrated bond formation/breaking in the context of chemical
exchange also in simpler, as well as much more complicated sample systems.
Hydrogen bonding was further studied between methanol and organic nitriles
[68, 136], methanol and N-methylacetamide [137], methanol and different esters
Top Curr Chem (Z) (2017) 375:86
123
138
Reprinted from the journal