Experimental and Computational Maps of Electron Density Distributions in Molecules: New Benchmarks for Quantum Chemistry

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In this project funded by the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program of the Chemistry Division, Professor Peter M. Weber and his students at Brown University are using an X-Ray Free Electron Laser to explore how electrons are arranged within molecules. Molecules can be roughly described as positively charged atomic nuclei surrounded by a cloud of electrons. Bonding between adjacent atoms occurs when the density of negatively charged electrons between the positive atomic nuclei is high. The Weber research group seeks to generate maps of electron density within molecules. These electron density distributions determine all molecular properties and, by implication, the properties of all materials. In addition, electron density distributions determine how chemical reactions proceed, and the quantities and types of reaction products that are generated. Measuring electron density distributions in molecules for reacting states is therefore important to our understanding of fundamental chemistry and has myriad implications in industrial chemistry. A unique aspect of Prof. Weber’s research is that electron density distributions are being measured not only for molecules in their lowest “relaxed” energy states, but also in their excited states, where the electron densities have been altered by laser pulses. By directly comparing the experimentally determined density maps to those generated by computational models, advances in computational techniques can be made. The project is providing training for students in advanced experimental methodologies, offers them experience at a National facility (SLAC), and also gives them experience in complex computer simulations.The project focuses first on the ground state density distributions in a range of model systems. They include sulfur hexafluoride, aromatic and conjugated systems such as phenol and 1,3 cyclohexadiene, and heterocyclic molecules such as pyrazine. Absolute scattering intensities are obtained by calibrating with atomic systems such as argon and neon. In a second phase of the project, excited state electron density distributions are obtained for excitation to valence states or Rydberg states. Model systems for this part include 1,3 cyclohexadiene as well as di-amines, such as diazabicyclo[2,2,2]octane. Technical innovations of the scattering experiment develop an absolute calibration of the scattering intensity, measurement of the shot-by shot pulse energies of the laser pulse that brings the molecules to the electronically excited state and the x-ray pulse that casts the scattering pattern, and an improvement in the time resolution. The latter is achieved by implementing a special set of up-conversion optics with very short harmonic generation crystals. Finally, innovations in the analysis of the data focus on the iteration between experimental data and computational models with the aim to determine the three-dimensional electron density distributions. Data codes are developed to calculate excited-state scattering patterns based on high-level ab-initio wave functions. The broader impact of the research includes the improvement of computational codes that are widely used in chemistry and related disciplines.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.