Electronic Transitions of Molecules: Vibrating Lewis Structures

In this work we demonstrate a simple and intuitive description of electronic resonances in terms of localized electron vibrations. By partitioning the 3N-dimensional space of a many-electron wavefunction into hyper-regions related by permutation symmetry, chemical structures naturally result which correspond closely to Lewis structures, with identifiable single and double bonds, and lone pairs. Here we demonstrate how this picture of electronic structure develops upon the admixture of electronic wavefunctions, in the spirit of coherent electronic transitions. We show that pi-pi* transitions correspond to double-bonding electrons oscillating along the bond axis, and n-pi* transitions reveal lone-pairs vibrating out of plane. In butadiene and hexatriene, the double-bond oscillations combine with in- and out-of-phase combinations, revealing the correspondence between electronic transitions, molecular normal mode vibrations, and molecular plasmonics. This analysis allows electronic excitations to be described by building upon ground state electronic structures, without the need for molecular orbitals.