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Resonant ion-dip infrared spectroscopy has been employed to record cluster-size-specific spectra of C6H6–(CH3OH)m with m=1–6 in the OH stretch fundamental region. The comparison of the spectra with the results of ab initio calculations on the pure methanol clusters enables the assignment of the hydrogen-bonding architecture in the clusters. In all cases, the methanol molecules aggregate together in a single subcluster. With m=1, a single infrared transition is observed, redshifted from that of a free methanol momomer by 42 cm−1 due to π hydrogen bonding between benzene and methanol. The m=2 spectrum features two strong transitions at 3506 and 3605 cm−1. The lower frequency peak is redshifted from the free monomer value by 175 cm−1 and is assigned to the proton donor in the methanol dimer subcluster. The proton acceptor, which would be a free OH stretch in the absence of benzene, is redshifted by 76 cm−1 due to a strengthened π hydrogen bond. In benzene–(CH3OH)3, three sharp OH stretch transitions are obse...

Resonant ion‐dip infrared spectra of C6H6–H2O and C6H6–HOD have been recorded in the OH stretch fundamental region. The spectra provide further evidence for the unique, large‐amplitude motions present in these π hydrogen‐bonded complexes. In C6H6–H2O, transitions out of the lowest ortho (Π) and para (Σ) ground state levels are observed. A transition at 3634 cm−1 is assigned as an unresolved pair of parallel transitions (Σ→Σ and Π→Π) involving the symmetric stretch fundamental (at 3657 cm−1 in free H2O). In the antisymmetric stretch region, transitions at 3713, 3748, and 3774 cm−1 are assigned as Π→Σ, Σ→Π, and Π→Δ transitions, respectively. The spacing of the transitions is consistent with nearly free internal rotation of H2O about benzene’s sixfold axis in both ground and vibrationally excited states. The intensities of combination bands depends critically on the mixing of some local mode character into the symmetric and antisymmetric stretches at asymmetric positions of H2O on benzene. Surprisingly, in C6H6–HOD, five transitions are observed in the OH stretch region, all arising from the ground state zero point level. Even more unusual, the higher‐energy combination bands are many times stronger than the OH stretch fundamental. The local mode OH stretch has components both parallel and perpendicular to benzene’s sixfold axis, leading to strong parallel and perpendicular transitions in the spectrum. A two‐dimensional model involving free internal rotation and torsion of HOD in its plane is used to account for the qualitative appearance of the spectrum. The form of the OH(v=0) and OH(v=1) torsional potentials which reproduce the qualitative features of the spectrum are slightly asymmetric, double‐minimum potentials with large‐amplitude excursions for HOD over nearly 180°.

Resonant ion-dip infrared spectroscopy has been used to record size-specific infrared spectra of C(6)H(6)-(H(2)O)n clusters with n = 1 through 7 in the O-H stretch region. The O-H stretch spectra show a dramatic dependence on cluster size. For the n = 3 to 5 clusters, the transitions can be divided into three types-attributable to free, pi hydrogen-bonded, and single donor water-water O-H stretches-consistent with a C(6)H(6)-(H(2)O)n structure in which benzene is on the surface of a cyclic (H(2)O)n cluster. In n = 6 and 7 clusters, the spectra show distinct new transitions in the 3500 to 3600 wave number region. After comparison of these results with the predictions of ab initio calculations on (H(2)O)n clusters, these new transitions have been assigned to double donor O-H stretches associated with the formation of a more compact, noncyclic structure beginning with (H(2)O)(6). This is the same size cluster for which ab initio calculations predict that a changeover to noncyclic (H(2)O)n structures will occur.

Resonant ion–dip infrared spectroscopy (RIDIRS) is used to record clustersize-specific spectra of C6H6–(H2O)n(BWn) clusters with n= 1–7 in the C—H and O—H stretch regions (2900–4000 cm–1). The Fermi triad characteristic of benzene in the C—H stretch region is virtually unchanged by addition of as many as seven water molecules. By contrast, the O—H stretch spectra show a dramatic dependence on cluster size. In C6H6–H2O, the O—H stretch transitions are complicated by additional structure due to internal rotation of H2O in the complex. In C6H6–(H2O)2, the O—H stretch spectrum closely mimics that of the free water dimer. For the n= 3–5 clusters, the spectra can be divided roughly into three categories, due to free O—H (3715 cm–1), π H-bonded O—H (3650 cm–1), and single donor O—H stretches, consistent with a BWn structure composed of a cyclic Wn cluster in which one of the free O—H groups is used in π hydrogen bonding with benzene. At n= 6, the spectra show distinct new transitions in the 3500–3600 cm–1 region. By comparing with the predictions of ab initio calculations of Wn, these new transitions are assigned to double donor O—H stretches associated with the formation of a more compact, ice-like structure in W6. The transformation to double-donating waters continues to develop in BW7, where additional transitions in the double-donor O—H stretch region appear at the expense of free O—H transition intensity.

Resonant ion-dip infrared spectroscopy (RIDIRS) is used to record near-infrared spectra of several size-selected (benzene)m(solvent)n molecular clusters. In benzene-H2O and benzene-HOD, large-amplitude motions of the water molecule on the benzene ring produce unusual OH stretch infrared absorptions. In the former complex, seven resolved transitions appear in the OH stretch region, rather than the two expected of a rigid complex. In C6H6-HOD, several combination bands involving internal rotation and in-plane torsion of the HOD have intensities greater than the OH stretch fundamental. Similar spectra of the (C6H6)2-H2O complex show that such large-amplitude motions are effectively quenched by the presence of the second benzene molecule. RIDIR spectra of C6H6-(CH3OH)n with n equals 1 - 5 are also presented. In C6H6-(CH3OH)3, the OH stretch infrared spectrum provides clear evidence that the three methanol molecules attach to benzene as a hydrogen-bonded chain. This occurs despite the fact that the lowest energy structure for the free methanol trimer is a cyclic structure, indicating that the presence of benzene has caused a structural rearrangement of solvent molecules near it.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.