Switching a HO···π Interaction to a Nonconventional OH···π Hydrogen Bond: A Completed Crystallographic Puzzle.

In this article, we present crystallographic and spectroscopic evidence of a tunable system wherein a HO center dot center dot center dot pi interaction switches incrementally to a nonconventional OH center dot center dot center dot pi hydrogen bonding (FIB) interaction. More specifically, we report the synthesis of substituted forms of model system 1 to study the effects of aryl ring electronic density on the qualitative characteristics of OH center dot center dot center dot pi hydrogen bonds therein. The OH stretch in experimental infrared data, in agreement with density-functional theory (DFT) calculations, shows continuous red-shifts as the adjacent ring becomes more electron rich. For example, the OH stretch of an amino-substituted analogue is red-shifted by roughly 50 cm(-1) compared to the same stretch in the CF3 analogue, indicating a significantly stronger HB interaction in the former. Moreover, DFT calculations (omega B97XD/6-311+G**) predict that increasing electronic density on the adjacent top ring reduces the aryl pi-OH sigma* energy gap with a concomitant enhancement of the OH n-pi* energy gap. Consequently, a dominant pi-sigma* interaction in the amino substituted analogue locks the system in the in-form while a favorable n-pi* interaction reverses the orientation of the oxygen-bound hydrogen in its protonated form. Additionally, the( 1)H NMR data of various analogues reveal that stronger OH center dot center dot center dot pi interactions in systems with electron-rich aromatic rings slow exchange of the alcoholic proton, thereby revealing coupling with the geminal proton. Finally, X-ray crystallographic analyses of a spectrum of analogues clearly visualize the three distinct stages of "switch"-starting with exdusive HO center dot center dot center dot pi, to partitioned HO center dot center dot center dot pi/OH center dot center dot center dot pi; and finally to achieving exclusive OH center dot center dot center dot pi forms. Furthermore, the crystal structure of the amino analogue reveals an interesting feature in which an extended HB network, involving two conventional NH center dot center dot center dot O) and two nonconventional (OH center dot center dot center dot pi) HBs, dimerizes and anchors the molecule in the unit cell.