Knowledge Bank

Matrix isolation infrared spectroscopy was used to characterize a 1:1 complex of methanol (CH$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$OH) with benzene (C$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$). Co-deposition experiments with CH$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$OH and C$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$ were performed at 17 - 20 K using nitrogen and argon as the matrix gases. New infrared bands attributable to the CH$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$OH-C$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$ complex were observed near the O-H and C-O stretching vibrations of CH$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$OH and near the hydrogen out-of-plane bending vibration of C$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$. The initial identification of the new infrared bands observed was established by performing a conentration study (1:200 to 1:2000 S:M ratios), by comparing the co-deposition spectra with the spectra of the individual monomers, by matrix annealing experiments, and by performing experiments using isotopically labeled methanol (CD$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$OD) and benzene (C$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$D$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$). Quantum chemical calculations were also performed for the CH$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$OH-C$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$ complex using density functional theory and ab initio methods. Two stable minima were found for the complex: one in which the CH$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$OH is above the C$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$ ring with the hydroxyl hydrogen interacting with the $\pi$ cloud of the ring (H-$\pi$ complex) and the other in which the CH$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$OH is in the plane of the C$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$ ring with the hydroxyl oxygen interacting with one of the C-H bonds of the ring (CH-O complex). Comparing the calculated shifts of the vibrational frequencies for both complexes to the observed experimental frequency shifts, it is found that the H-$\pi$ complex is in best agreement with the experimental shifts in both magnitude and direction. Therefore, it is concluded that the geometry of the CH$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$OH-C$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}$ complex observed in the matrix isolation experiments is the H-$\pi$ complex.