C-H stretch modes as a probe of H-bonding in methanol-containing clusters

Resonant ion-dip infrared spectroscopy has been used to record infrared spectra of a series of benzene(methanol),, clusters with m = 1-5 in the O-H and C-H stretch regions. Previous work has used the O-H stretch region as a probe of the H-bonding topologies of these clusters, from which it was deduced that benzene-(methanol)(1-3) contain Ii-bonded methanol chains and benzene-(methanol)(4-6) II-bonded methanol cycles. In the present work, the C-H stretch fundamentals of the methyl group of methanol and the aryl C-H groups of benzene are studied. While benzene's C-H stretch Fermi triad is virtually unchanged in frequency from one cluster to the next, the methyl C-H stretch vibrations undergo systematic wavenumber shifts characteristic of the ii-bonding arrangement for each methanol in the cluster. Density functional theory calculations on the pure methanol and benzene-(methanol)(m) clusters faithfully reproduce the directions and approximate magnitudes of the observed shifts and provide a basis for assignment of the observed transitions to acceptor, donor, and acceptor-donor methanol subunits. The experimental results on the v(2) fundamental of methanol in benzene-(methanol)(1-5) show characteristic frequency shifts due to (i) donor (D, -20 to -15 cm(-1)), (ii) acceptor-donor (AD) and pi donor (pi) (-6 to -9 cm(-1)), and (iii) OH ... O acceptor/pi donor (A pi, -4 to +2 cm(-1)). Calculations on (methanol), and benzene-(methanol)(m) clusters extend the predictions to include characteristic shifts for (iv) double-acceptor/single-donor (AAD, +5 to +15 cm(-1)), (v) single-acceptor (A, +15 to +30 cm(-1)), and (vi) double-acceptor (AA) (+20 to +30 cm(-1)). FTIR spectra of liquid methanol and of binary solutions of methanol with acetone-d(6), CDCl3, and D2O indicate that methanol's CH stretch frequency shifts reflect methanol's II-bonding environment in solution as well.