Resonant two-photon ionization, ultraviolet hole-burning, and resonant ion-dip infrared (RIDIR) spectroscopy were used to assign and characterize the hydrogen-bonding topology of two conformers of the benzene-(water) 8 cluster. In both clusters, the eight water molecules form a hydrogen-bonded cube to which benzene is surface-attached. Comparison of the RIDIR spectra with density functional theory calculations is used to assign the two (water) 8 structures in benzene-(water) 8 as cubic octamers of D 2 d and S 4 symmetry, which differ in the configuration of the hydrogen bonds within the cube. OH stretch vibrational fundamentals near 3550 wave numbers provide unique spectral signatures for these “molecular ice cubes.”

Density functional theory (DFT) calculations of the structures, binding energies, vibrational frequencies and infrared intensities of methanol clusters containing two to five molecules have been carried out using the Becke3LYP functional. Thirteen representative H-bonded structures have been studied including cyclic, chain, branched-cyclic and branched-chain hydrogen bond structures. In the methanol trimer, tetramer, and pentamer, the cyclic structure is more stable by 3.5, 8.3, and 3.6 kcal/mol over the next most strongly bound minimum. In the tetramer and pentamer, the second-most stable minimum corresponds to a branched cycle. Chain structures are destabilized from the cyclic minimum by the loss of a hydrogen bond and from a smaller cooperative strengthening of the H-bonds that remain. In all branched structures studied, the formation of the branch H-bond strengthens the "branch-point" methanol's H-bond donation to its neighbor, but weakens its two acceptor H-bonds, leading to largely compensating effects on the total binding energy. The computed OH stretch vibrational frequency shifts (relative to the monomer at the same level of calculation) are used as points of comparison with recent experimental work on gas-phase (methanol) m and benzene-(methanol) m clusters and matrix-isolated (methanol) m clusters.

Resonant ion-dip infrared spectroscopy of benzene-(water) 9 : Expanding the cube The techniques of resonant two-photon ionization ͑R2PI͒, UV-UV ͑ultraviolet͒ hole-burning, and resonant ion-dip infrared ͑RIDIR͒ spectroscopies have been employed along with density functional theory ͑DFT͒ calculations to assign and characterize the hydrogen-bonding topologies of two isomers each of the benzene-͑water) 8 and (benzene͒ 2 ͑water) 8 gas-phase clusters. The BW 8 isomers ͑Bϭbenzene, Wϭwater͒ have R2PI spectra which are nearly identical to one another, but shifted by about 5 cm Ϫ1 from one another. This difference is sufficient to enable interference-free RIDIR spectra to be recorded. As with smaller BW n clusters, the BW 8 clusters fragment following photoionization by loss of either one or two water molecules. The OH stretch IR spectra of the two BW 8 isomers bear a close resemblance to one another, but differ most noticeably in the double-donor OH stretch transitions near 3550 cm Ϫ1 . Comparison to DFT calculated minimum energy structures, vibrational frequencies, and infrared intensities leads to an assignment of the H-bonding topology of the BW 8 isomers as nominally cubic water octamers of S 4 and D 2d symmetry surface attached to benzene through a H-bond. A series of arguments based on the R2PI and hole-burning spectra leads to an assignment of additional features in the R2PI spectra to two isomers of B 2 W 8 . The OH stretch RIDIR spectra of these isomers show them to be the corresponding S 4 and D 2d analogs of B 2 W 8 in which the benzene molecules each form a H-bond with a different dangling OH group on the W 8 sub-cluster.

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.