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D. Ghosh et al.
multidimensional techniques have proven to be powerful spectroscopic tools to interrogate the structure and dynamics across broad fields of science. Here, we will focus
on 2D IR spectroscopy. This chapter will include the experimental methodology of
2D IR, the basic principles of 2D IR, the technological advances in 2D IR, and the
applications of 2D IR spectroscopy in various fields of science.
2D IR spectroscopy began its development some twenty years ago [2] and since
then it has served as a powerful experimental method to address several aspects
of ultrafast structural and conformational dynamics in chemical and biological
systems. Although an infrared absorption spectrum contains the complete knowledge of structures and dynamics, the spectral responses to the electromagnetic fields
are interpreted based on statistical models, which get ever more challenging with the
increasing complexity of the systems. In NMR spectroscopy, a successful approach to
disentangle the overlapping features in a spectrum has been to increase the dimensionality of the spectroscopic technique. Based upon a sequence of infrared laser pulses,
2D IR is closely related to two dimensional NMR (2D NMR) method. Similar to 2D
NMR, 2D IR can spread the spectral information in two dimensions and serve as an
ideal experimental technique to identify interactions between vibrational modes and
to measure the temporal evolution of the vibrational frequencies. Unlike 2D NMR,
which is typically limited to detection of species interconverting on millisecond and
slower timescales, 2D IR spectroscopy has the advantage of an inherently fast, subpicosecond timescale that ensures detection of states that rapidly interconvert and
enables direct measurement of fast structural and environmental fluctuations. 2D IR
spectroscopy is well suitable to study condensed phase samples where conformational fluctuations occur on fast timescales. Based on the vibrational frequencies
that are highly sensitive to molecular environments, 2D IR spectroscopy provides
information about the local sites and their fluctuations within complex systems.
2D IR spectroscopy conveys rich information on molecular systems such as homogeneous and inhomogeneous spectral broadening effects, vibrational anharmonicity,
spectral diffusion, intermode coupling strength and its temporal variation, energy
relaxation, chemical exchange and conformational interconversions. Over the past
two decades, 2D IR spectroscopy has been extensively used to study the structure
and dynamics of small peptides, proteins, DNA, and lipid bilayers. Moreover, 2D
IR has been utilized to interrogate ultrafast energy transfer in materials and biology,
hydrogen-bond (H-bond) making and breaking in liquid water and in biomolecules,
and solvation dynamics arising from solute-solvent interactions.
The purpose of this article is to provide a comprehensive understanding of the
2D IR experimental technique to determine equilibrium structures and ultrafast
timescales of various chemical and biological processes. Mathematical expressions
and equations have been avoided throughout the article to provide a simple and
qualitative, yet detailed picture regarding the multiple light-matter interactions in
2D IR experiments. The spectral signatures of different ultrafast processes in the
2D IR spectrum have been illustrated. Several 2D IR works have been reviewed to
explain the important roles played by this nonlinear experimental technique towards
our understanding of the molecular structure, energy relaxations, ultrafast fluctuations, and conformational dynamics. The organization of this article is as follows: it
D. Ghosh et al.
multidimensional techniques have proven to be powerful spectroscopic tools to interrogate the structure and dynamics across broad fields of science. Here, we will focus
on 2D IR spectroscopy. This chapter will include the experimental methodology of
2D IR, the basic principles of 2D IR, the technological advances in 2D IR, and the
applications of 2D IR spectroscopy in various fields of science.
2D IR spectroscopy began its development some twenty years ago [2] and since
then it has served as a powerful experimental method to address several aspects
of ultrafast structural and conformational dynamics in chemical and biological
systems. Although an infrared absorption spectrum contains the complete knowledge of structures and dynamics, the spectral responses to the electromagnetic fields
are interpreted based on statistical models, which get ever more challenging with the
increasing complexity of the systems. In NMR spectroscopy, a successful approach to
disentangle the overlapping features in a spectrum has been to increase the dimensionality of the spectroscopic technique. Based upon a sequence of infrared laser pulses,
2D IR is closely related to two dimensional NMR (2D NMR) method. Similar to 2D
NMR, 2D IR can spread the spectral information in two dimensions and serve as an
ideal experimental technique to identify interactions between vibrational modes and
to measure the temporal evolution of the vibrational frequencies. Unlike 2D NMR,
which is typically limited to detection of species interconverting on millisecond and
slower timescales, 2D IR spectroscopy has the advantage of an inherently fast, subpicosecond timescale that ensures detection of states that rapidly interconvert and
enables direct measurement of fast structural and environmental fluctuations. 2D IR
spectroscopy is well suitable to study condensed phase samples where conformational fluctuations occur on fast timescales. Based on the vibrational frequencies
that are highly sensitive to molecular environments, 2D IR spectroscopy provides
information about the local sites and their fluctuations within complex systems.
2D IR spectroscopy conveys rich information on molecular systems such as homogeneous and inhomogeneous spectral broadening effects, vibrational anharmonicity,
spectral diffusion, intermode coupling strength and its temporal variation, energy
relaxation, chemical exchange and conformational interconversions. Over the past
two decades, 2D IR spectroscopy has been extensively used to study the structure
and dynamics of small peptides, proteins, DNA, and lipid bilayers. Moreover, 2D
IR has been utilized to interrogate ultrafast energy transfer in materials and biology,
hydrogen-bond (H-bond) making and breaking in liquid water and in biomolecules,
and solvation dynamics arising from solute-solvent interactions.
The purpose of this article is to provide a comprehensive understanding of the
2D IR experimental technique to determine equilibrium structures and ultrafast
timescales of various chemical and biological processes. Mathematical expressions
and equations have been avoided throughout the article to provide a simple and
qualitative, yet detailed picture regarding the multiple light-matter interactions in
2D IR experiments. The spectral signatures of different ultrafast processes in the
2D IR spectrum have been illustrated. Several 2D IR works have been reviewed to
explain the important roles played by this nonlinear experimental technique towards
our understanding of the molecular structure, energy relaxations, ultrafast fluctuations, and conformational dynamics. The organization of this article is as follows: it