CHARACTERIZATION OF A 1:1 METHANOL-BENZENE COMPLEX USING MATRIX ISOLATION INFRARED SPECTROSCOPY

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2013

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Ohio State University

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Matrix isolation infrared spectroscopy was used to characterize a 1:1 complex of methanol (CH$_{3}$OH) with benzene (C$_{6}$H$_{6}$). Co-deposition experiments with CH$_{3}$OH and C$_{6}$H$_{6}$ were performed at 17 - 20 K using nitrogen and argon as the matrix gases. New infrared bands attributable to the CH$_{3}$OH-C$_{6}$H$_{6}$ complex were observed near the O-H and C-O stretching vibrations of CH$_{3}$OH and near the hydrogen out-of-plane bending vibration of C$_{6}$H$_{6}$. 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$_{3}$OD) and benzene (C$_{6}$D$_{6}$). Quantum chemical calculations were also performed for the CH$_{3}$OH-C$_{6}$H$_{6}$ complex using density functional theory and ab initio methods. Two stable minima were found for the complex: one in which the CH$_{3}$OH is above the C$_{6}$H$_{6}$ ring with the hydroxyl hydrogen interacting with the $\pi$ cloud of the ring (H-$\pi$ complex) and the other in which the CH$_{3}$OH is in the plane of the C$_{6}$H$_{6}$ 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$_{3}$OH-C$_{6}$H$_{6}$ complex observed in the matrix isolation experiments is the H-$\pi$ complex.

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Author Institution: School of Science, Penn State Erie, Erie, PA 16563

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