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) m 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 H-bonded methanol chains and benzene−(methanol) 4 - 6 H-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 H-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 ν 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 π donor (π) (−6 to −9 cm -1 ), and (iii) OH ... O acceptor/π donor (Aπ, −4 to +2 cm -1 ). Calculations on (methanol) m 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, CDCl 3, and D 2 O indicate that methanol's CH stretch frequency shifts reflect methanol's H-bonding environment in solution as well.