KubarychGroup

 

Shared Common Ground State

 

Although treating molecular motions as a collection of harmonic oscillators has many practical advantages, there would be no chemistry if there were not anharmonicity. Anharmonicity tells us how bonds break and how energy can be transfered from one location to another in a molecule.

Coupling between motions in a molecule often reveals itself as split or new peaks in a linear spectrum, and with 2D spectroscopy we can access even finer detail due to the nonlinear interactions.

 

Coupling

When two transitions are coupled, they share a common ground state. For example, the two modes shown above correspond to two different modes of Mn2(CO)10. Indeed we observe cross peaks between these two transitions in the 2D spectrum in the upper left and lower right parts of the spectrum.

The equation below shows a simple two-mode potential with bilinear coupling and third order anharmonicity. We use perturbation theory and direct diagonalization methods to compute anharmonicities from quantum chemical results.

ωdetect

ωexcite

Local vs. Normal Modes

The cross peaks in a two-dimensional spectrum can be modeled in order to extract the parameters of the molecular hamiltonian that give rise to the coupled motions. In particular it is possible to deduce the relative directions of transition dipoles, providing orientational information.

Just as all chemists know that electrons delocalize to form molecular orbitals when there is favorable spatial and energy overlap, nuclei can do the same thing. Depending on symmetry, the solutions to the Schrödinger equation are eigenfunctions that can be excited using light.