Substituent effects on the nonadiabatic dynamics of ethylene: [formula omitted]-donors and [formula omitted]-acceptors

Abstract

The energetic and structural trends in the minimum energy conical intersections of a series of substituted ethylenes are explained by the degree to which the chemical substituents polarize electron density across the C=C double bond. The addition of a substituent that significantly polarizes the electron density also reduces the magnitude of the large-amplitude pyramidalization motions required to reach a conical intersection and can thus very effectively “direct” these motions to occur at a specific carbon atom. These observations are summarized via the construction of a reduced dimensionality representation of the relevant potential energy surfaces inspired by a previously reported 3-state model for biradical systems. On-the-fly nonadiabatic dynamics simulations reveal that this effect dictates the dominant excited state decay pathway for acrylonitrile (π-acceptor CN substituent) and vinylamine (π-donor NH2 substituent). The ab initio multiple spawning simulations show that population transfer to the ground electronic state for both molecules occurs primarily in the regions around the substituent-altered minimum energy conical intersections, demonstrating that chemical substitution may be used to selectively alter photochemical pathways.