Single-Conformation Spectroscopy and Dynamics on Multiple Potential Energy Surfaces: Flexible N-Heterocycle Chromophores and Complexes in Aerosols

In this project funded by the Chemical Structure Dynamics and Mechanisms-A (CSDM-A) Program of the Chemistry Division, Nathanael Kidwell and his research team at the College of William and Mary will use an array of modern laser techniques to study the delicate balance between the shape of flexible nitrogen-containing molecules (N-heterocycles) found in atmospheric aerosols and their interactions with light. The shape and interactions of N-heterocycles with light are strongly influenced by the presence of solvent molecules.

Understanding the intramolecular interactions of N-heterocycles and their intermolecular interactions with surrounding solvent molecules reveals the different pathways the molecules follow after absorption of solar radiation. Within aerosols, N-heterocycles enhance light absorption and water evaporation, key properties that are important to understand given the intricate relationships among aerosols, clouds and climate.

To disentangle the intermolecular interactions with the solvent, the team will examine the molecular structure and absorption processes of flexible N-heterocycles with a well-defined number of solvent molecules. This approach will facilitate the systematic investigation of molecular solar absorption from isolated to solvated environments to approach the complexity of liquid aerosol particles.

The unique properties of each molecular conformation in the absence or presence of solvent molecules will be examined with laser spectroscopic tools and compared to several theoretical models that attempt to predict the shape and absorption outcomes. The PI is dedicated to engaging high school students as citizen scientists by partnering with local schools to construct an Internet-of-Things (IoT) network of Arduino multisensors to record crowd-sourced atmospheric measurements.

This project focuses on the structure, spectroscopic signatures, and dynamical outcomes of flexible N-heterocycles and water-solvated complexes with substituents that influence the electronic character of the base imidazole and pyrrole units. Conformationally-flexible heterocycles will be isolated in a supersonic jet expansion and probed using single-conformation ultraviolet and infrared spectroscopy methods to determine the interplay between chemical structure and the correlated photophysics.

Upon sequential water molecule addition to the heterocycle chromophores, the evolving infrared and electronic spectra will be examined to monitor the local solvation environment effects on the isolated molecules and to explore the topology of extended hydrogen-bonded networks. In tandem with the single-conformation spectroscopy experiments, stimulated emission pumping and velocity map imaging studies will serve to report on competing photochemical channels and mechanisms.

The experimental observables will be compared simultaneously to the computed results from multiple theoretical approaches, including electronic structure methods, ab initio molecular dynamics and machine-learning techniques. Students mentored in the Kidwell group will gain experience in advanced experimental techniques and high-level theoretical methods that probe the physical properties of N-heterocycles on multiple potential energy surfaces.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.