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Research in Biomedical Optics

MIT Laser Research Facility

Hydrogen Bond Dynamics of Water

Investigators: Andrei Tokmakoff

Andrei Tokmakoff Research Group

The Tokmakoff group studies molecular dynamics in solution, including the structure and dynamics of liquids, the conformational dynamics of proteins and peptides, and solute-solvent interactions. We wish to understand these fundamental phenomena both in equilibrium systems and during chemical reactions and biophysical processes. Our experimental approach involves development of new structurally sensitive femtosecond vibrational spectroscopies that can be used to follow molecular dynamics in solution. The most sensitive and broadly applicable tool is two-dimensional infrared spectroscopy, which we have been developing to capture information on transient molecular structure and characterize structural variation. Our experimental work is complemented by theoretical work on nonlinear spectroscopy, statistical mechanical modeling of dynamics and relaxation, and molecular dynamics computer simulations.

Our work in the Harrison Spectroscopy Lab includes the following :

Hydrogen Bond Dynamics of Water: Water is composed of an ever-changing network of hydrogen bonds. The structural evolution of this network plays an active role in aqueous chemistry, governing aqueous solvation and the transport of protons. Vibrational spectroscopy is a powerful technique to study hydrogen bonded systems because the frequency of the intramolecular OH stretching vibration is particularly sensitive to a molecule's hydrogen bonding environment. We are using nonlinear infrared spectroscopy to follow time dependent changes in the OH stretching frequency of HOD in D 2 O. Molecular dynamics simulations serve to help connect measured spectroscopic observables with the ultrafast microscopic dynamics of liquid water. To date we have used vibrational echo measurements along with polarization selective pump-probe measurements to provide a unified description of translational and reorientational fluctuations of the hydrogen bonded network. Recent work has focused on using two-dimensional infrared spectroscopy to follow the interchange between hydrogen bonded and non-bonded molecules.

Development of Nonlinear Infrared Spectroscopy: Nonlinear IR spectroscopy is rapidly becoming one of the leading methods of determining transient structures of complex molecules in solution. Currently, nonlinear IR spectroscopy requires expensive detectors and complex equipment. Recent work has focused on developing alternate methods of performing multi-dimensional spectroscopy, reducing both its cost and complexity. We have recently developed a method of measuring pulsed IR spectra with silicon CCD technology. Typically, standard CCD arrays are insensitive to wavelengths longer than 1 micron, making them useless for mid-IR detection. To overcome this technical limitation, we have developed an infrared spectrometer that uses ultrafast sum frequency generation (SFG) techniques in combination with CCD technology to image mid-IR beams. This technical advance may help reduce the cost and complexity of many fields of IR spectroscopy; including linear IR spectroscopy, multi-dimensional IR spectroscopy, and IR imaging.

Selected References

  1. Ultrafast Hydrogen Bond Dynamics in the Infrared Spectroscopy of Water Fecko, C.J.; Eaves, J.D.; Loparo, J.J.; Tokmakoff, A.; Geissler, P.L., Science 301 (2003) 1698-1702.
  2. Local Hydrogen Bond Dynamics and Collective Reorganization of Water: Ultrafast Infrared Spectroscopy of HOD in D2O Fecko, C.J.; Loparo, J.J.; Roberts, S.T.; Tokmakoff, A., J.Chem. Phys. 122 (2005).
  3. Upconversion Multichannel Infrared Spectrometer: DeCamp, M.F. and Tokmakoff, A. Opt. Lett. In Press (2005).
  4. Upconversion Infrared Spectrometer: DeCamp, M.F. and Tokmakoff, A. USA Patent pending