Collaborative Research: Noncovalently Bound Molecular Trimers: High-dimensional and Fully Coupled Quantum Calculations of their Vibrational Levels

Bacic (NYU) and Felker (UCLA) are supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry to develop and implement a novel and powerful methodology for accurate high-dimensional and fully coupled quantum calculations of the vibrational eigenstates of hydrogen-bonded and van der Waals molecular trimers. They will focus on realistic molecular complexes of considerable practical and fundamental importance and amenable to spectroscopic investigations, allowing for direct comparison between theory and experiment.

Their studies will lead to more accurate description of the noncovalent intermolecular interactions, which play a major role in bulk liquids, molecular solids, and atmospheric chemistry, and the processes that occur in these environments. Bacic and Felker will continue to foster inclusivity by mentoring diverse and gender-balanced groups of graduate and undergraduate students, as well as postdoctoral researchers.

They will engage in a variety of outreach activities, including popular science presentations, to underrepresented high-school minority students.

Bacic and Felker will introduce a general approach that will enable, for the first time, high-dimensional fully coupled quantum calculations of the vibrational levels of noncovalently bound molecular trimers, for either flexible or rigid monomers. They will develop rigorous quantum bound-state methodologies aimed at the following three classes of paradigmatic hydrogen-bonded molecular trimers. (1) Trimers of flexible diatomic molecules, e.g., (HF)3 and (HCl)3, in 12D. (2) Trimers of rigid small polyatomic molecules, e.g., (H2O)3, in 12D. (3) Two types of mixed trimers of rigid molecules, exemplified by (HCl)(H2O)2 and the halide dihydrate complexes X-(H2O)2 (X = Cl, Br, I), in 11D and 9D, respectively.

Through comparison between the computed vibrational levels and the transitions observed in the infrared (IR) spectra of the trimers, they will assess the accuracy of the ab initio calculated nonadditive three-body interactions incorporated in the trimer potential energy surfaces (PESs). Moreover, such a comparison will help them greatly with the assignment of the measured IR spectra, which for fluxional complexes is heavily reliant on theory.

The intermolecular PESs of many molecular trimers, e.g., (H2O)3 and X-(H2O)2 (X = Cl, Br, I), exhibit multiple low-barrier tunneling pathways connecting the degenerate minima, which give rise to intricate patterns of tunneling splittings, observable in the measured spectra. The fully coupled quantum calculations by Bacic and Felker of the intermolecular vibrational states of the trimers will for the first time yield the tunneling splittings in the excited vibrational states.

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.