Two Dimensional Infrared (2D-IR) Spectroscopy 

Development of ultrafast multi-dimensional spectroscopy with multiple infrared (IR) pulses generated using high power laser technology has enabled us to measure the ultrafast dynamics of complex molecular systems that previously could not be studied by conventional spectroscopic methods. Ultrafast molecular dynamics and transient intermolecular structures and interactions happening on subpicosecond and picosecond timescales can be measured in 2D-IR experiments (with a time resolution of <100 picoseconds) by manipulating the sequences of ultrashort femtosecond pulses and examining the response of the system as a function of the timing between the pulses.

Three femtosecond IR pulses tuned to a particular frequency interact with the system, sequencially exciting a molecular bond (see the figure below on the left). The correlation between the varying bond vibration frequencies as a function of the time delay between the IR pulses provide information on the ultrafast fluctuations and conformational dynamics of the molecular system. A typical 2D-IR spectrum spreads the spectral information in two frequency axes (compared to one frequency axis for a conventional FTIR spectrum, see the figure below on the right). The lineshapes of the diagonal peaks provide structural fluctuation timescales. The off-diagonal peaks can either provide information of vibrational couplings or directly estimate the timescale chemical exchange (e.g. making and breaking of hydrogen bonds, conformational switching from one state to another).

Ultrafast dynamics measured using 2D-IR can when compared with molecular dynamics simulations can provide us molecular level information about how these ultrafast motions are influencing the molecular functions. 2D-IR has been successfully used to study enzymatic activity, neurodegenerative disease related proteins, drug dynamics, hetetrogenous catalysis, dynamics of ionic liquids and aqueous binary mixtures, etc. 

 Selected Publications:

       1. Cosolvent Effects on Solute-Solvent Hydrogen-Bond Dynamics: Ultrafast 2D IR Investigations

            J. Phys. Chem. B, 2015, 119, 15334 - 15343 (DOI: 10.1021/acs.jpcb.5b08643)

      2. Ribonuclease S Dynamics USing a Nitrile Label with 2D IR Vibrational Echo Spectroscopy

     3. Dynamics of a Myoglobin Mutant Enzyme: 2D IR Vibrational Echo Experiments and Simulations