Monday, May 13, 2013

Validating a diabatic state picture for strong hydrogen bonds

In a recent paper I argued that a diabatic state picture can give a nice description of hydrogen bonding, spanning from strong symmetric bonds to weak asymmetric bonds.

A key implication/prediction of this picture is the existence of a "twin state" to the ground electronic state. The excited (ground) state is an antisymmetric (symmetric) combination of the two diabatic states.
This state should be in the UV region. I suggested it has a large photo-absorption cross-section and should lead to photo-dissociation of the complex.
The excited state should also be "visible" in quantum chemistry calculations, but may be mixed with other excited states (Rydberg states).

Recently I came across a paper (published in the Ukrainian Journal of Physics!) which supports this view for the H5O2+ complex (Zundel cation)
Pseudo Jahn−Teller Origin of the Proton Tunneling in Zundel Cation Containing Water Clusters
Geru I., Gorinchoy N., Balan I.

The Zundel cation consists of a proton shared by two water molecules and is believed to be important for understanding proton solvation (i.e. acids!) and transport in water. It is a classic example of a strong symmetric hydrogen bond.

The authors fit the ground and excited state potential energy surfaces to a two state effective Hamiltonian (pseudo Jahn-Teller), reminiscent of the one used in my paper (and many other peoples too!).

The matrix element coupling the two diabatic states, Delta = 7 eV, showing a very strong coupling.

The calculated energies for the lowest lying electronic states are shown below
The quantum chemistry calculations are
the electronic correlation was taken into account via the configuration interaction (CI) with single and double excitations (CISD). In calculating the potential energy curves, the full valence CI was employed. The active space of CI ...included six occupied and five lower unoccupied molecular orbitals (more than 40000 configurations with S = 0).
n.b. The low-lying excitations 1E and 3E are essentially intra-molecular excitations within one of the two water molecules.
In contrast, the 1B excitations are collective excitations of the whole complex.
The twin state of interest is the 1B2 state.
It is higher in the UV than I expected; but it is there.

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