Ab Initio Quantum Chemical Investigation of Electric Field Gradients in Molecules

RNDr. Rudolf Polák CSc.

The electric field gradient (EFG) tensor at the location of an atomic nucleus in a molecule is determined by the total environment of electric charges, stemming both from electrons and other nuclei. The EFG tensor can be calculated directly from high-level correlated molecular wave functions as a one-electron expectation value or by finite field approaches.

The components of the EFG tensor give evidence about the local distribution of the σ, π and lone-pair electrons and, therefore in certain circumstances, allow concepts like hybridization of orbitals, σ and π bonding, covalent and ionic character to be put on a quantitative basis. Experimentally, the EFG is manifested through its interaction with the electric nuclear quadrupole moment to produce a characteristic splitting of vibration-rotation levels, obtainable from microwave experiments.

The interaction is usually described in terms of nuclear quadrupole coupling constants (NQCC) attainable both by experiment and theory: This situation offers the possibility for interpretation and understanding of highly resolved experimental spectra. Thus, the computed EFGs are of interest for a number of contemporary questions in the theory of chemical bonding, high resolution spectroscopy etc.

Recently, the EFGs at the nitrogen nucleus and the 14N NQCCs in a series of nitrogen containing diatomic species (CN+, CN-, CN, N2+, N2, NO+, NO-, NO) were calculated* by using the multiconfiguration selfconsistent field and multireference configuration interaction methods. Accurate calculations of EFG's at various internuclear separations, and the deduced rovibrational dependences of the EFG's were used for gaining specific insight into the chemical bonding in the diatomics and for interpreting the hyperfine structure of some rovibronic states, respectively. Extension of the work with respect to the behavior of the EFG in systems with a larger number of atoms, particularly when the EFG undergoes electronic redistribution due to van der Waals bond formation [ e.g. (NO)2, (NO2)+, (NO2)-], is presently going on.

The work proceeds in collaboration with doc. Dr. J. Fišer, Department of Physical and Macromolecular Chemistry, Charles University.