I blogged on Bickelhaput’s examination of the stability of kinked vs. linear polycyclic aromatics1 in this post. Bickelhaupt argued against any H…H stabilization across the bay region, a stabilization that Matta and Bader2 argued is present based on the fact that there is a bond path linking the two hydrogens.
Grimme and Erker have now added to this story.3 They prepared the dideuterated phenanthrene 1 and obtained its IR and Raman spectra. The splitting of the symmetric (a1) and asymmetric (b1) vibrational frequencies is very small 9-12 cm-1. The computed splitting are in the same range, with very small variation with the computational methodology employed. The small splitting argues against any significant interaction between the two hydrogen (deuterium) atoms. Further, the sign of the coupling between the two vibrations indicates a repulsive interaction between the two atoms. These authors argue that the vibrational splitting is almost entirely due to conventional weak van der Waals interactions, and that there is no “bond” between the two atoms, despite the fact that a bond path connects them. This bond path results simply from two (electron density) basins forced to butt against each other by the geometry of the molecule as a whole.
(1) Poater, J.; Visser, R.; Sola, M.; Bickelhaupt, F. M., "Polycyclic Benzenoids: Why Kinked is More Stable than Straight," J. Org. Chem. 2007, 72, 1134-1142, DOI: 10.1021/jo061637p
(2) Matta, C. F.; Hernández-Trujillo, J.; Tang, T.-H.; Bader, R. F. W., "Hydrogen-Hydrogen Bonding: A Stabilizing Interaction in Molecules and Crystals," Chem. Eur. J. 2003, 9, 1940-1951, DOI: 10.1002/chem.200204626
(3) Grimme, S.; Mück-Lichtenfeld, C.; Erker, G.; Kehr, G.; Wang, H.; Beckers, H. W., H., "When Do Interacting Atoms Form a Chemical Bond? Spectroscopic Measurements and Theoretical Analyses of Dideuteriophenanthrene," Angew. Chem. Int. Ed. 2009, 48, 2592-2595, DOI: 10.1002/anie.200805751