A close competition between O-HO and O-Hπ hydrogen bonding: IR spectroscopy of anisole-methanol complex in helium nanodroplets.

Anisole is a multifunctional molecule that can form intermolecular complexes via its aromatic π-electron system as well as its methoxy group. We have studied the complexation of anisole with methanol. This serves as a prototype system to explore the competition between O-HO, O-Hπ, C-HO and C-Hπ hydrogen bonding. The anisolemethanol molecular complexes were formed in superfluid helium droplets and were detected using high-resolution laser-infrared spectroscopy, in the frequency range between 2630 and 2730 cm-1 covering the O-D stretches of methanol-d4 (CD3OD). Several bands assigned to (anisole)m(methanol)n complexes (where m = 1, and 2 and n = 1) were observed. The experimental results are complemented by the ab initio electronic structure calculations at the MP2/6-311++G(d,p) and B3LYP-D3/aug-cc-pVTZ levels of theory. Based on a comparison of the observed spectra with the ab initio theoretical spectra, we suggest that for the anisolemethanol complex, structures bound via O-HO and O-Hπ hydrogen bonding are almost equally preferred.