INTRAMOLECULAR HYDROGEN BOND IN ACETYLACETONE: INFRARED SPECTROSCOPY AND TUNNELING DYNAMICS

N. Doslic and I. Matanovic
R. Boskovic Institute, Zagreb, Croatia

The portion of the potential energy surface (PES) of acetylacetone (ACAC) relevant for the intramolecular proton transfer reaction is analyzed using ab initio and DFT methods. Six stationary points on the PES were characterized as well at the reaction paths connecting these points. Our study reveals that the intramolecular proton transfer is not synchronized with the internal rotation of the distal methyl group.

The infrared spectrum of the O-H-O fragment of ACAC is studied using a four-dimensional model. This comprises the OH stretching, the in-plane OH bending and two O-O ring deformation modes. The full anharmonic PES and dipole moment surface are calculated using DFT/B1LYP. We have shown that the strong mode mixing in the OH stretching region which goes beyond the adiabatic two mode model 1, ν OH and ν OO, makes an assignment in terms of combination transitions impossible. 

Further, the influence of the H-atom tunneling on the spectrum is treated by using large amplitude, symmetry adapted coordinates. The three-dimensional vibrational eigenvalue problem is solved by combining the generalized Fourier grid method 2 and an iterative Lanczos diagonalization. In ACAC the G-matrix elements vary markedly along the selected coordinates, and our best estimate of the tunneling splitting of 116 cm -1 is obtained by explicitly taking the G sr(q) coordinate dependence into account.    

References

1. O. Henri-Rousseau, P. Blaise, Adv. Chem. Phys. 103, 1 (1998).
2. J. Stare, G. G. Balint-Kurti, J. Chem. Phys. A 107, 7204 (2003).