On the existence of distinct equilibrium configurations under orienting external electric fields

Abstract

Oriented external electric fields (OEEFs) are ubiquitous in chemistry; however, the effects of fields applied in different directions on molecular systems remain underexplored. A major challenge is that an applied field exerts a torque on a molecule, reorienting the molecular frame and complicating the interpretation of orientation-dependent electric-field effects. Thus, free polar molecules experience orienting rather than oriented fields, such that the field response drives the molecular orientation rather than the other way around. In this work, we explore a new regime of distinct molecular equilibrium configurations in the presence of OEEFs, differing in the relative direction of the external field and the molecular frame. While static dipole moments typically dominate molecular responses to external electric fields, these equilibria are enabled by exploiting molecular polarizability. These distinct "directomers" exhibit unique electronic and nuclear configurations, particularly in their low-lying excited states. We employ oriented electric field vectors referenced to a molecule-fixed principal axis frame along with hybrid analytical-numerical geometry optimization in order to explore the rotational potential energy surface (rPES), as well as a simply analytic model based on equilibrium electrical properties which captures the double-well character of the rPES, including some geometry relaxation effects.