Vibrational Spectra in Bulk Liquids

What are the molecular-level mechanisms of vibrational frequency shifts in solution?

Infrared and Raman spectra can provide important information about the environment; (both static and dynamic) of a chromophore in a condensed phase environment (e.g., a liquid or nanostructured material). We are working to better understand how spectral features reflect the molecular-level environment. To that end, we are using mixed quantum-classical and traditional molecular dynamics simulations to answer questions such as: How many solvent molecules contribute to the instantaneous frequency shift? Where are the solvent molecules with the biggest contribution to the frequency shift? To address these issues, we have developed a straightforward analysis that allows us to assign the contribution to the instantaneous frequency shift of each atom in the solvent around a chromophore. This gives an incredibly detailed picture of the origin of the frequency shift.

As an example, we have applied this approach to I2 and ICl in liquid Xe and some results are shown in the figure below. Specifically, the location of the Xe atom giving the largest contribution to the frequency shift is shown for two cases: when the total frequency shift is positive, or blueshifted (top panels), and when the total

We can see that the Xe atoms effect a blueshift by strongly colliding with the ends of the molecule (or, in the case of ICl, the Cl end of the molecule), thereby pushing the atoms together and increasing the vibrational frequency. On the other hand, the Xe atoms effect a redshift by colliding in a T-shaped geometry, pushing the atoms apart and thereby decreasing the vibrational frequency. This is just one example of the kind of detailed information that can be obtained.

We have extended this approach by showing how umbrella sampling can be used to efficiently sample rarely accessed frequency shifts. We are currently applying these methods to solutes in polar solvents as well as nanoconfined liquids.

Relevant References:

Christine M. Morales and Ward H. Thompson,
Journal of Physical Chemistry B 115, 7597-7605 (2011). Abstract
"Molecular-level Mechanisms of Vibrational Frequency Shifts in a Polar Liquid"

Christine M. Morales and Ward H. Thompson,
Journal of Physical Chemistry B 112, 313-320 (2008). Abstract
"Umbrella Sampling of Solute Vibrational Lineshifts in Mixed Quantum-Classical Molecular Dynamics Simulations"

Christine M. Morales and Ward H. Thompson,
Journal of Physical Chemistry A 111, 5422 - 5430 (2007). Abstract
"Mixed Quantum-Classical Molecular Dynamics Analysis of the Molecular-Level Mechanisms of Vibrational Frequency Shifts"

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