46
D. Ghosh et al.
distinct peak pairs in the 2D IR spectrum [15, 16]. A similar spectrum containing
multiple diagonal peak pairs can also be obtained in case a certain vibrational mode
exists in multiple conformations, thereby leading to distinct spectrally resolved population distributions. For example, vibrational mode shows distinct spectral signatures
in the presence and absence of hydrogen bonding [17–19]. Similar spectral signature
can be obtained when the vibrational probe exists in two different conformations
with the protein environment [20, 21].
In addition, cross peaks between the diagonal peak pairs can be observed in
the 2D IR spectrum. The cross peaks in a 2D IR spectrum are the signatures of
vibrational couplings [22], chemical exchange [23], and energy transfer [24] between
the vibrational modes. The characteristics of the cross peaks will depend on either the
orientation between two the vibrational modes or the interconversion rate between the
two different populations of the vibration. Energy and population transfer being time
dependent, cross peaks arising from these processes are absent at T w = 0 and become
more prominent with increasing waiting times. Whereas, off-diagonal peaks linking
the vibrational modes through coupling are waiting time independent and are clearly
visible at T w = 0. Hence, origin of cross peaks can easily be separated. Structural
information can be generated from off-diagonal peaks of vibrational coupling. The
angle between the transition dipole moments of the two coupled vibrational modes
can be extracted from the relative amplitudes of the diagonal and cross peak pairs as
a function of input pulse polarization [25, 26].
5 Applications of 2D IR
In the last two decades, 2D IR has been utilized to obtain structural information through dipolar couplings in small model compounds and to study fundamental processes in the fields of chemistry, materials, and biology. The fundamental
processes include energy transfer and relaxation, chemical exchange, hydrogen bond
making and breaking, interfacial dynamics, and charge transport [27–30]. Here, we
review some of the important 2D IR results which allowed us to obtain interesting
insights about the above mentioned fundamental processes. It has been revealed that
vibrational energy transfer in silicon nanoparticles, occurring in tens of picoseconds
timescale, is diminished when doped with small amounts of boron and phosphorous [31]. Berry pseudo-rotation, the pairwise exchange between axial and equatorial ligands, was experimentally validated in five coordinated ruthenium complex
[32]. Very fast making and breaking of intermolecular and intramolecular hydrogen
bonding as well as interconversion between free ion and contact ion pair have
key importance in biological systems. Structural interconversions of acetonitrile
in methanol, methyl and ethyl acetate in methanol, 2-methoxyphenol in toluene
(Fig. 6), hydrogen bonding delocalisation in liquid NMA and β-isocyanoalanine in
fluorinated alcohols, and ion pair dynamics of LiNCS in benzonitrile, Li
+ and SCN
−
in dimethylformamide, SeCN
− ion in N,N-dimethyl formamide (DMF) have been
D. Ghosh et al.
distinct peak pairs in the 2D IR spectrum [15, 16]. A similar spectrum containing
multiple diagonal peak pairs can also be obtained in case a certain vibrational mode
exists in multiple conformations, thereby leading to distinct spectrally resolved population distributions. For example, vibrational mode shows distinct spectral signatures
in the presence and absence of hydrogen bonding [17–19]. Similar spectral signature
can be obtained when the vibrational probe exists in two different conformations
with the protein environment [20, 21].
In addition, cross peaks between the diagonal peak pairs can be observed in
the 2D IR spectrum. The cross peaks in a 2D IR spectrum are the signatures of
vibrational couplings [22], chemical exchange [23], and energy transfer [24] between
the vibrational modes. The characteristics of the cross peaks will depend on either the
orientation between two the vibrational modes or the interconversion rate between the
two different populations of the vibration. Energy and population transfer being time
dependent, cross peaks arising from these processes are absent at T w = 0 and become
more prominent with increasing waiting times. Whereas, off-diagonal peaks linking
the vibrational modes through coupling are waiting time independent and are clearly
visible at T w = 0. Hence, origin of cross peaks can easily be separated. Structural
information can be generated from off-diagonal peaks of vibrational coupling. The
angle between the transition dipole moments of the two coupled vibrational modes
can be extracted from the relative amplitudes of the diagonal and cross peak pairs as
a function of input pulse polarization [25, 26].
5 Applications of 2D IR
In the last two decades, 2D IR has been utilized to obtain structural information through dipolar couplings in small model compounds and to study fundamental processes in the fields of chemistry, materials, and biology. The fundamental
processes include energy transfer and relaxation, chemical exchange, hydrogen bond
making and breaking, interfacial dynamics, and charge transport [27–30]. Here, we
review some of the important 2D IR results which allowed us to obtain interesting
insights about the above mentioned fundamental processes. It has been revealed that
vibrational energy transfer in silicon nanoparticles, occurring in tens of picoseconds
timescale, is diminished when doped with small amounts of boron and phosphorous [31]. Berry pseudo-rotation, the pairwise exchange between axial and equatorial ligands, was experimentally validated in five coordinated ruthenium complex
[32]. Very fast making and breaking of intermolecular and intramolecular hydrogen
bonding as well as interconversion between free ion and contact ion pair have
key importance in biological systems. Structural interconversions of acetonitrile
in methanol, methyl and ethyl acetate in methanol, 2-methoxyphenol in toluene
(Fig. 6), hydrogen bonding delocalisation in liquid NMA and β-isocyanoalanine in
fluorinated alcohols, and ion pair dynamics of LiNCS in benzonitrile, Li
+ and SCN
−
in dimethylformamide, SeCN
− ion in N,N-dimethyl formamide (DMF) have been