Ken Chiacchia, PSC Communications, contributed to this story
Since its discovery in the 1600s, spectroscopy has become an invaluable tool within the scientific community. This groundbreaking field has made revolutionary contributions to physics, chemistry and astronomy through studies regarding matter, light and radiation. A team of researchers from Tennessee Technological University – led by Wilson Gichuhi – recently conducted one such study with the help of an ACCESS allocation on Bridges-2 at the Pittsburgh Supercomputing Center.
The study results were recently published in The Journal of Physical Chemistry A.
The scientists used high-level quantum simulations of polyaromatic hydrocarbons (PAHs) on Bridges-2 to help predict the negative ion photoelectron spectra (NIPES) of cyanofluorene and cyanoindene radical anions.
PAHs are of interest to researchers for a number of reasons. A few are used as medicines, dyes, plastics and pesticides. The Tennessee Tech team studied these particular PAHs, though, because similar molecules have been detected in the interstellar medium. A deeper understanding of their properties can aid in future identification of similar-sized molecules, in turn explaining the chemical composition of distant astronomical objects.
The researchers used the output from the Gaussian 16, a quantum-chemical computational package provided by ACCESS to compute Franck-Condon (FC) factors of various cyanoindene and cynafluorene radical anion isomers. The output from Gaussian provides the electronic energies, vibrational frequencies, and geometric parameters of the negatively charged ions and their corresponding neutral forms. These outputs are then utilized to compute the NIPES under a double-harmonic oscillator model.
“We used harmonic vibrational frequencies and normal mode vectors derived from density functional theory while the FC calculations were used to simulate the NIPES to aid in future analysis of experimental spectra obtained in NIPE spectroscopic techniques,” Gichuhi said. “In other words, our ACCESS allocation allowed us to obtain high-level quantum chemical data that aided in the computation of the FCFs, allowing us to obtain isomer-specific vibrational spectra of the neutral molecules.”
Gichuhi and colleagues examined the NIPES of two PAH radical anionic (negatively charged) isomers – molecules with the same molecular formula but different arrangements of the same atoms – of cyanoindene and cyanofluorene. NIPES is commonly used to measure the energy differences between the initial anionic states of a molecule and its final, neutral state, to yield adiabatic electron affinities and vibrational profile of the neutral molecules. By analyzing the NIPES data, the group was able to determine each compound’s most and least stable isomers. They were also able to determine the adiabatic binding energies of the various isomers, a very important physical property that is a signature of each and every isomer.
Our ACCESS allocation allowed us to obtain high-level quantum chemical data…allowing us to obtain isomer-specific vibrational spectra of the neutral molecules.
–Tennessee Tech Associate Professor of Chemistry Wilson Gichuhi
He said that the team examined two key components in the spectra: the electron affinity values and the vibrational spectra.
“Scientists use the electron affinity values of different states to predict energy differences at specific sites in the compounds,” Gichuhi explained. “We specifically used Bridges-2 to calculate the electron affinity values of the ground singlet and the lowest-lying triplet states, which allowed us to determine the stability of the different isomers.”
He said that vibrational spectra analysis of the various isomers was useful since they act as signatures of these isomers – allowing scientists to use these spectroscopic data for future astrochemical searches and analysis.
“Our vibrational spectra findings can now serve as a guide for future gas-phase ion spectroscopic studies,” Gichuhi said.
Resource Provider Institution(s): Pittsburgh Supercomputing Center (PSC)
Affiliations: Tennessee Technological University
Funding Agency: NSF
Grant or Allocation Number(s): CHE230018
The science story featured here was enabled by the U.S. National Science Foundation’s ACCESS program, which is supported by National Science Foundation grants #2138259, #2138286, #2138307, #2137603, and #2138296.