Spectroscopic Characterization of Transition States

The central paradigm of transition state theory is based on the idea that the reaction rate depends primarily on the highest point along the lowest energy path from reactants to products. The transition state is a crucial concept of the models that chemists use to understand chemical reactions. Despite its importance to chemistry we could never ``see'' a transition state and it is almost impossible to characterize a transition state experimentally. In the past there has been considerable effort to develop methods that may allow us a characterization of transition states based on spectroscopic measurements.

The most straightforward experimental characterization of a reacting molecular system is the frequency domain spectroscopy of the rovibrational eigenenergies. There are two main challenges that we have to solve if we want to use frequency domain spectroscopy to characterize the transition state of a reacting molecular system. First, we need a simple experimental method that allows us to sample the eigenenergies of the molecular system relevant for a chemical reaction at excitation energies exceeding 15000 cm^-1. The second task is to understand and model the structure of the eigenstates at the transition state of the molecular system. The first full dimensional analysis [1] of the rovibrational eigenstates at the transition state for the HCN↔HNC hydrogen shift isomerization reaction showed that for such a linear system there is a vibrational angular momentum dependent decrease of the vibrational level spacing near the barrier to isomerization. The observed decrease (we may call it "barrier anharmonicity") appears four bending excitations below the isomerization barrier in addition to the overall decrease of the vibrational spacing due to the overall anharmonicity of the vibrational states.

A simple method to connect the barrier anharmonicity described in [1] with the isomerization barrier is presented in a current work [2]. In this publication we demonstrate how it is possible to extract chemically relevant saddle point energies from spectroscopically measured eigenenergies. By fitting the energy level spacings to a "dip formula" presented in this paper we can determine the activation energy for isomerization from spectroscopic data. In general the pattern of the eigenenergy level spacing near the barrier for a given vibrational mode gives an indirect visual representation of the transition state regarding the effects of the selected normal mode. Using such a representation one can easily determine if the mode is involved in the isomerization or if it is a perpendicular inactive mode. The method presented allows to elucidate the mechanism of the isomerization if all eigenenergies of the system at the barrier have been measured. The curvature in the saddle-point region characterized through the harmonic frequencies of the normal modes found to be active in the reaction can be determined in the presented analysis and can be used in kinetic models including RRKM and transition state theories.    

My main research topic is related to the spectroscopic characterization of transition states for isomerization reactions of small but chemically very important molecular systems. I intend to extend my theoretical method of spectroscopically assigned ab initio eigenenergy lists to other systems. Currently I am working in a collaboration with the Tennyson/Polyansky group (UCL London/IAP Nizhny Novgorod) to calculate and understand the complete [H,C,N] rovibrational eigenenergy spectrum/wavefunctions for a full dimensional ab initio calculation with stored wavefunctions. Another current project is the construction of a new experimental setup for my hot gas emission experiments with increased sensitivity and higher temperature (Fa. Nabertherm/Bremen) to sample the transition state eigenenergies directly at very high excitation energies.

[1] G. C. Mellau

Complete experimental rovibrational eigenenergies of HCN up to 6880 cm^-1 above the ground state

J. Chem. Phys. 134, 234303 (2011).

[2] J. H. Baraban, P. B. Changala, G. C. Mellau, J. F. Stanton, A. J. Merer, and R. W. Field

Spectroscopic Characterization of Transition States

submitted (2015).