Understanding the Impact of Conformational Isomerism on Ultrafast Molecular Dynamics

With support from the Chemical Structure and Dynamics (CSD) program in the Division of Chemistry, Professors Katharine Tibbetts and Ka Un Lao of Virginia Commonwealth University will combine advanced laser techniques and molecular-level simulations to study the motions of complex molecules that take on multiple shapes, or conformations. Little is known about how specific molecular conformations determine chemical reaction pathways under ambient conditions because molecules at room temperature can interconvert between different conformations at least 100 billion times per second.

To probe these fast molecular motions, Dr. Tibbetts and her group create an ultrafast camera using two laser pulses, each lasting less than 30 femtoseconds (a femtosecond is one billionth of one trillionth of a second). These time-resolved measurements will be combined with quantum chemical and dynamics simulations by Dr.

Lao’s group to provide a molecular-level description of how different conformations impact the electronic excited state dynamics and vibrational motions in the sugar-phosphate backbone of DNA. Their discoveries could lead to a better understanding of photodegradation pathways in molecules important to life. In addition to the graduate and undergraduate students who will directly participate, the Tibbetts and Lao groups will mentor economically disadvantaged high school students through summer internships to introduce them to this collaborative research.

This work will investigate the femtosecond-picosecond time scale dynamics of conformational isomers, or conformers, of molecules that model the DNA sugar-phosphate backbone. The femtosecond time-resolved mass spectrometry (FTRMS) pump-probe technique developed in the Tibbetts group enables direct probing of dynamics in molecular cations with resolution of about 30 femtoseconds.

FTRMS will be used to probe coherent vibrational motions, electronic excitation and relaxation pathways, and molecular rearrangement and dissociation time scales. The results of quantum chemical and nonadiabatic molecular dynamics calculations will be used to interpret the FTRMS measurements, enabling a detailed understanding of both conformer-specific nuclear motions and how different conformers undergo nonadiabatic relaxation, rearrangement, and dissociation pathways.

The outcomes of these investigations will advance understanding of how conformer-specific dynamics impact photochemical degradation pathways in biochemically important processes such as radiation-induced DNA damage and establish robust experimental-theoretical approaches for investigating the dynamics of molecular conformers in other scientific contexts. The project provides training opportunities for graduate and undergraduate students in advanced laser spectroscopy techniques and quantum chemistry calculations, as well as experience in contributing to a collaborative research effort.

Additionally, the project will introduce economically disadvantaged high school students to scientific research through summer internships that give them the opportunity to participate in this project and prepare them for matriculation into STEM degree programs.

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.