Here is another fine example of the power of combining experiment and computation. Xu and co-worker has applied the FT microwave technique, which has been used in conjunction with computation by the Alonso group (especially) as described in these posts, to the trimer of 2-fluoroethanol.1 They computed a number of trimer structures at MP2/6-311++G(2d,p) in an attempt to match up the computed spectroscopic constants with the experimental constants. The two lowest energy structures are shown in Figure 1. The second lowest energy structure has nice symmetry, but it does not match up well with the experimental spectra. However, the lowest energy structure is in very good agreement with the experiments.
Table 1. MP2/6-311++G(2d,p) optimized structures and relative energies (kJ mol-1) of the two lowest energy structures of the trimer of 2-fluoroethanol. The added orange lines in the lowest energy structure denote the bifurcated hydrogen bonds identified by QTAIM.
Of particular note is that topological electron density analysis (also known as quantum theoretical atoms in a molecule, QTAIM) of the wavefunction of the lowest energy structure of the trimer identifies two hydrogen bond bifurcations. The authors suggest that these additional interactions are responsible, in part, for the stability of this lowest energy structure.
(1) Thomas, J.; Liu, X.; Jäger, W.; Xu, Y. "Unusual H-Bond Topology and Bifurcated H-bonds in the 2-Fluoroethanol Trimer," Angew. Chem. Int. Ed. 2015, 54, 11711-11715, DOI: 10.1002/anie.201505934.
2-fluoroethanol: InChI=1S/C2H5FO/c3-1-2-4/h4H,1-2H2, InChIKey=GGDYAKVUZMZKRV-UHFFFAOYSA-N