Noncovalently bound molecular dimers with more than four atoms: Inter- and intramolecular rovibrational states and frequency shifts from full-dimensional and fully coupled quantum calculations

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• Professor Zlatko Bačić, New York University
• Wednesday 26 May 2021, 14:30-15:30
• Department of Chemistry, Cambridge, Unilever lecture theatre.

If you have a question about this talk, please contact Lisa Masters.

Fluxional molecular dimers bound by noncovalent interactions have been the subject of intense research activity by experimentalists and theorists alike for decades. Scientists have been drawn to these systems to gain quantitative understanding of their vibration-rotation-tunneling dynamics dominated by strong nuclear quantum effects, and account for them in terms of accurate multidimensional potential energy surfaces. Crucial for achieving these goals is the ability to compute rigorously, as fully coupled and full-dimensional, the (ro)vibrational levels of floppy molecular dimers. For complexes having four or five atoms, such calculations in full dimensionality, 6D and 9D, respectively, are very demanding already when the monomers are in their ground intramolecular vibrational states. The difficulty of the task increases sharply when the calculations are extended to excited intramolecular vibrational states. Because of the order-of-magnitude disparity between their frequencies and those of the intermolecular vibrational modes, typical for noncovalently bound molecular dimers, hundreds of highly excited intermolecular vibrational states have energies below those of the monomer intramolecular vibrational excitations considered. For a long time, the prevailing view was that all of them have to be converged if accurate excited intramolecular vibrational eigenstates are desired, making the calculations prohibitively costly. This is the main reason why prior to our recent methodological developments that I will discuss in this talk, there was not a single full-dimensional and fully coupled quantum bound-state calculation in the literature involving weakly bound molecular dimers with more than four atoms that yielded converged energies of all intramolecular vibrational fundamentals, together with excited intermolecular vibrational states. The turning point came when we realized [1,2] that such calculations can be accomplished by including in the final basis only a small number of low-lying intermolecular eigenstates with energies far below those of the intramolecular vibrational eigenstates, drastically reducing the dimensionality of the problem. I will describe the newly developed computational approach particularly well suited to take advantage of this insight [2,3], and demonstrate its effectiveness by reviewing our full-dimensional (9D) and fully coupled quantum bound-state calculations of H2O /D2O-CO [3], HDO —CO [4], and HCl-H2O [5] dimers. In all instances, intramolecular (ro)vibrational excitations of the monomers and their frequency shifts are computed, together with the low-energy intermolecular (ro)vibrational states of the dimers within intramolecular vibrational manifold. Finally, if time permits, I will touch upon our 9D quantum calculations of the inter- and intramolecular vibrational states of benzene-H2O/HDO dimers, for flexible water and rigid-benzene [6].

References

[1] D. Lauvergnat, P. M. Felker, Y. Scribano, D. M. Benoit, and Z. Bačić, J. Chem. Phys. 150, 154303 (2019). [2] P. M. Felker and Z. Bačić, J. Chem. Phys. 151, 024305 (2019). [3] P. M. Felker and Z. Bačić, J. Chem. Phys. 153, 074107 (2020). [4] P. M. Felker and Z. Bačić, J. Phys. Chem. A, 125, 980 (2021). [5] Y. Liu, J. Li, P. M. Felker and Z. Bačić, Phys. Chem. Chem. Phys. 23, 7101 (2021). [6] P. M. Felker and Z. Bačić, J. Chem. Phys. 152, 124103 (2020).

This talk is part of the Theory - Chemistry Research Interest Group series.

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