Abstract
The site-specific first microsolvation step of furan and some of its derivatives with methanol is explored to benchmark the ability of quantum-chemical methods to describe the structure, energetics, and vibrational spectrum at low temperature. Infrared and microwave spectra in supersonic jet expansions are used to quantify the docking preference and some relevant quantum states of the model complexes. Microwave spectroscopy strictly rules out in-plane docking of methanol as opposed to the top coordination of the aromatic ring. Contrasting comparison strategies, which emphasize either the experimental or the theoretical input, are explored. Within the harmonic approximation, only a few composite computational approaches are able to achieve a satisfactory performance. Deuteration experiments suggest that the harmonic treatment itself is largely justified for the zero-point energy, likely and by design due to the systematic cancellation of important anharmonic contributions between the docking variants. Therefore, discrepancies between experiment and theory for the isomer abundance are tentatively assigned to electronic structure deficiencies, but uncertainties remain on the nuclear dynamics side. Attempts to include anharmonic contributions indicate that for systems of this size, a uniform treatment of anharmonicity with systematically improved performance is not yet in sight.
Highlights
Any rigorous comparison of electronic structure theory and experiment in the field of solvation has to cope with zero-point motion and thermal motion of the nuclei, before firm conclusions about the underlying electronic structure theory are possible
The first round of the blind challenge5 led to an ambiguity between the majority of harmonically evaluated predictions, which favor a π or top coordination of the ether oxygen by methanol (Ot), and a single anharmonic calculation, which favors a more planeparallel and more σ hydrogen-bonded coordination (Op)
Microwave spectroscopy can distinguish between such isomers, whereas dynamics that is much faster than nanoseconds, such as the question of true vs effective symmetry in low barrier situations, can be more challenging
Summary
Any rigorous comparison of electronic structure theory and experiment in the field of solvation has to cope with zero-point motion and thermal motion of the nuclei, before firm conclusions about the underlying electronic structure theory are possible. Zero-point motion effects can often be approximated harmonically, in particular, when different solvation sites for the same species are compared such that most of the anharmonic contributions cancel out to a significant degree.. Zero-point motion effects can often be approximated harmonically, in particular, when different solvation sites for the same species are compared such that most of the anharmonic contributions cancel out to a significant degree.3 This approximation needs careful verification, e.g., by isotope substitution, because the non-covalent binding is soft and of large amplitude. Several theory groups predicted the docking preference of the first methanol molecule on furan (Fu), 2-methylfuran (MFu), and 2,5-dimethylfuran (DMFu), while the corresponding mixed complexes were independently characterized by Fourier transform infrared (FTIR) jet spectroscopy. The initial results were published in Ref. 5, but the comparison between theory and experiment raised several challenging questions on both sides, which the present contribution addresses.
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