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Title: When Experimental and Theoretical Geometries are Truly Different
computational models are validated against various experimental data in order to assess overall accuracy and identify the method most apt for a given situation. In my research group, we have often validated quantum-chemical structure predictions against experimental gas-phase geometries. However, these structures are formally different; the experimental results reflect average distances that result upon ground-state vibrational averaging, whereas the theory predicts equilibrium geometries. Almost always, this distinction is trivial, and beyond the precision afforded by either experiment or theory. In one rather noteworthy instance, however, the CH3CN–BF3 complex, we demonstrated that there was a genuine, and rather significant difference between the experimental and theoretical geometries; the experimental B-N distance that was nearly 0.2 Å longer than that in the theoretically-determined equilibrium geometry. This resulted from an extreme anharmonicity in the donor-acceptor (B-N) potential, which manifested a significant asymmetry in ground state vibrational wavefunction. In our recent work, we have a similar, albeit more subtle trend in several other donor-acceptor systems including H2O–SO3, H3N–SiF4, C6H5N–SO2, and H3N–SO3. What we have noted specifically, is that the predicted donor-accptor bond lengths among several DFT and post-HF methods (with the aug-cc-pVTZ and aug-cc-pVQZ basis sets) are incredibly consistent, yet they differ from the experimental bond lengths to an extent that exceeds the quoted uncertainty. For pyridine-SO2, these differences rival CH3CN–BF3 (about 0.2) and in fact, the shape of the donor-acceptor potentials are quite similar. The other cases are more subtle, with experiment-theory differences of several hundredths of an angstrom. Yet the donor-acceptor potential curves in these systems are significantly anharmonic. The extent of the asymmetry in the ground vibrational wavefunction for the donor-acceptor stretching mode will be explored explicitly, in order to quantitatively assess the differences between the equilibrium and vibrationally averaged structures.  more » « less
Award ID(s):
2154736
PAR ID:
10537466
Author(s) / Creator(s):
Publisher / Repository:
American Chemical Society
Date Published:
Format(s):
Medium: X
Location:
Denver, CO
Sponsoring Org:
National Science Foundation
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