constituents in functionalized monolayers is most likely too large for intermolecular
energy transfer to occur or that the relaxation rates are too fast compared to a
possible transfer rate. The factors, which determine the energy transfer rates, are the
distance and orientation between the molecules, as well as the magnitudes of the
transition dipole moments of donor and acceptor molecules (Eq. 5). Taken into
account that the often employed carbonyl stretch vibrations exhibit transition dipole
moments [173, 206] that by far exceed that of OH stretches from liquid water [207],
it is very likely that a dominant factor is the r
-6 dependence of transfer rate (Eq. 5),
which inhibits the observation of cross peaks in the 2D IR experiments.
3.2.2 2D IR Spectroscopy of Three-Dimensionally Confined Molecules
Next to the two-dimensional confinement discussed in Sect. 3.2.1, additional
important cases exist in chemistry and biology where a confinement of molecules in
three dimensions plays a crucial role for the dynamics and molecular interactions.
Consider water molecules that take part in the hydration of minerals [208], protein
hydration and molecules in micelles [209, 210], or molecules and ions in channels
across biological membranes [95, 211]. Other important cases are organic molecules
in crystals of functional materials [212], molecules at interfaces of layered
heterogeneous catalysts [213], or molecules caged in defined synthetic environments such as metal-organic frameworks [214–216]. In such systems, molecular
motions are considerably restricted and the electrostatic environment around a local
probe can be significantly different as compared to an isotropic case. This can have
a profound impact on the stability and the particular function of materials. It is,
therefore, important to understand in detail, by which mechanisms threedimensional confinement influences molecular dynamics. 2D IR spectroscopy has
been shown to yield manifold information about this effect.
To demonstrate how 2D IR spectroscopy can to this, Fig. 15 shows an example
of the ultrafast dynamics of confined water in structurally different environments. In
that particular case, two minerals, gypsum and basanite, have been chosen for the
investigations due to their similar chemical constitution (CaSO 4 Án H 2 O), but
different three-dimensional structure (top panels in Fig. 15a and b, respectively).
The number of water molecules thus plays an important role in the structural
framework of the crystals. As a key point regarding the dynamics, the containing
water molecules do not form a similarly branched hydrogen bonding network as in
bulk water, but rather interact with the ions in the crystal structure. Thus, their
orientational, structural and hydrogen bond dynamics and contributions from energy
transfer can be expected to drastically differ from, e.g. bulk water.
Looking at the OD-stretch in isotope-diluted water (HOD), Fayer et al. were able to
observe directly the structural dynamics of water molecules intercalated in inorganic
crystals. The authors chose gypsum and basanite crystals as the samples, which exhibit
similar chemical constitution, only differing by the amount of intercalated water
molecules (Fig. 15) [208]. As in many studies on water dynamics, HOD was used
instead of pure H 2 O or D 2 O to avoid intra- and intermolecular energy transfer.
Measuring 2D IR signals of gypsum from freshly prepared and annealed samples
(middle and lower panel in Fig. 15a, respectively, at T = 0.5 ps), it could be shown
Top Curr Chem (Z) (2017) 375:86
123
152
Reprinted from the journal
energy transfer to occur or that the relaxation rates are too fast compared to a
possible transfer rate. The factors, which determine the energy transfer rates, are the
distance and orientation between the molecules, as well as the magnitudes of the
transition dipole moments of donor and acceptor molecules (Eq. 5). Taken into
account that the often employed carbonyl stretch vibrations exhibit transition dipole
moments [173, 206] that by far exceed that of OH stretches from liquid water [207],
it is very likely that a dominant factor is the r
-6 dependence of transfer rate (Eq. 5),
which inhibits the observation of cross peaks in the 2D IR experiments.
3.2.2 2D IR Spectroscopy of Three-Dimensionally Confined Molecules
Next to the two-dimensional confinement discussed in Sect. 3.2.1, additional
important cases exist in chemistry and biology where a confinement of molecules in
three dimensions plays a crucial role for the dynamics and molecular interactions.
Consider water molecules that take part in the hydration of minerals [208], protein
hydration and molecules in micelles [209, 210], or molecules and ions in channels
across biological membranes [95, 211]. Other important cases are organic molecules
in crystals of functional materials [212], molecules at interfaces of layered
heterogeneous catalysts [213], or molecules caged in defined synthetic environments such as metal-organic frameworks [214–216]. In such systems, molecular
motions are considerably restricted and the electrostatic environment around a local
probe can be significantly different as compared to an isotropic case. This can have
a profound impact on the stability and the particular function of materials. It is,
therefore, important to understand in detail, by which mechanisms threedimensional confinement influences molecular dynamics. 2D IR spectroscopy has
been shown to yield manifold information about this effect.
To demonstrate how 2D IR spectroscopy can to this, Fig. 15 shows an example
of the ultrafast dynamics of confined water in structurally different environments. In
that particular case, two minerals, gypsum and basanite, have been chosen for the
investigations due to their similar chemical constitution (CaSO 4 Án H 2 O), but
different three-dimensional structure (top panels in Fig. 15a and b, respectively).
The number of water molecules thus plays an important role in the structural
framework of the crystals. As a key point regarding the dynamics, the containing
water molecules do not form a similarly branched hydrogen bonding network as in
bulk water, but rather interact with the ions in the crystal structure. Thus, their
orientational, structural and hydrogen bond dynamics and contributions from energy
transfer can be expected to drastically differ from, e.g. bulk water.
Looking at the OD-stretch in isotope-diluted water (HOD), Fayer et al. were able to
observe directly the structural dynamics of water molecules intercalated in inorganic
crystals. The authors chose gypsum and basanite crystals as the samples, which exhibit
similar chemical constitution, only differing by the amount of intercalated water
molecules (Fig. 15) [208]. As in many studies on water dynamics, HOD was used
instead of pure H 2 O or D 2 O to avoid intra- and intermolecular energy transfer.
Measuring 2D IR signals of gypsum from freshly prepared and annealed samples
(middle and lower panel in Fig. 15a, respectively, at T = 0.5 ps), it could be shown
Top Curr Chem (Z) (2017) 375:86
123
152
Reprinted from the journal