Vibrational predissociation spectroscopy is used to obtain infrared spectra of the Cl−–C6H6, Br−–C6H6, and I−–C6H6 complexes in the region of the benzene CH stretch vibrations (2800–3200 cm−1). The infrared spectra of the three dimers are similar, each exhibiting several narrow bands (full width at half maximum <10 cm−1) that are only slightly redshifted from the absorptions of the free benzene molecule. Ab initio calculations predict that the most stable form of the three complexes is a planar C2v structure in which the halide is hydrogen bonded to two adjacent CH groups. The planar C2v structure in which the halide is linearly H bonded to a single CH group is predicted to be slightly less stable than the bifurcated form. Comparisons between experimental and theoretically predicted infrared spectra confirm that the bifurcated structure is indeed the most stable conformer for all three complexes. Ab initio calculations show that the electron density transfer from the halide to the benzene is not limited to the σ*(CH) orbitals adjacent to the halide, but extends to the σ domain of the benzene ring, consistent with the moderate shift of the CH stretch frequencies. The presence of weak satellite bands is explained in terms of Fermi resonances reminiscent of the benzene Fermi tetrad or hot bands involving the in-plane intermolecular bend vibration.

Infrared spectra of mass-selected F- -(CH4)n (n = 1-8) clusters are recorded in the CH stretching region (2500-3100 cm-1). Spectra for the n = 1-3 clusters are interpreted with the aid of ab initio calculations at the MP2/6-311++G(2df 2p) level, which suggest that the CH4 ligands bind to F- by equivalent, linear hydrogen bonds. Anharmonic frequencies for CH4 and F--CH4 are determined using the vibrational self-consistent field method with second-order perturbation theory correction. The n = 1 complex is predicted to have a C3v structure with a single CH group hydrogen bonded to F-. Its spectrum exhibits a parallel band associated with a stretching vibration of the hydrogen-bonded CH group that is red-shifted by 380 cm-1 from the nu1 band of free CH4 and a perpendicular band associated with the asymmetric stretching motion of the nonbonded CH groups, slightly red-shifted from the nu3 band of free CH4. As n increases, additional vibrational bands appear as a result of Fermi resonances between the hydrogen-bonded CH stretching vibrational mode and the 2nu4 overtone and nu2+nu4 combination levels of the methane solvent molecules. For clusters with n < or = 8, it appears that the CH4 molecules are accommodated in the first solvation shell, each being attached to the F- anion by equivalent hydrogen bonds.

Ab initio calculations are performed at the MP2/aug-cc-pVTZ level for F−-CH4 and Cl−-CH4, to show that the dimers have C3v symmetry with the CH4 sub-unit attached to the halide anion by a single hydrogen bond. This geometry is consistent with infrared spectra of F−-CH4 and Cl−-CH4 recorded in the CH-stretch region. The calculations also indicate substantial anharmonicity in the H-bonded CH stretch of F−-CH4. Infrared spectra of the F−-(CH4)2 and Cl−-(CH4)2 trimer clusters are consistent with structures that have two equivalent CH4 sub-units H-bonded to the halide core. Additional bands in the F−-(CH4)2 spectrum are assigned as transitions to CH4 bending overtone and combination levels, gaining infrared intensity from Fermi interaction with the H-bonded CH stretch.

In an effort to elucidate their structures, mass-selected Cl--(CH4)n (n = 1-10) clusters are probed using infrared spectroscopy in the CH stretch region (2800-3100 cm(-1)). Accompanying ab initio calculations at the MP2/6-311++G(2df,2p) level for the n = 1-3 clusters suggest that methane molecules prefer to attach to the chloride anion by single linear H-bonds and sit adjacent to one another. These conclusions are supported by the agreement between experimental and calculated vibrational band frequencies and intensities. Infrared spectra in the CH stretch region for Cl--(CH4)n clusters containing up to ten CH4 ligands are remarkably simple, each being dominated by a single narrow peak associated with stretching motion of hydrogen-bonded CH groups. The observations are consistent with cluster structures in which at least ten equivalent methane molecules can be accommodated in the first solvation shell about a chloride anion.

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Articles you may be interested inPotential energy surface and rovibrational calculations for the Mg + -H 2 and Mg + -D 2 complexes J. Chem. Phys. 134, 044310 (2011); 10.1063/1.3530800The Al + -H 2 cation complex: Rotationally resolved infrared spectrum, potential energy surface, and rovibrational calculations Ab initio potential energy and dipole moment surfaces, infrared spectra, and vibrational predissociation dynamics of the 35 Cl − H 2 / D 2 complexes Midinfrared spectra of the 81 Br Ϫ -H 2 and I Ϫ -H 2 anion complexes are measured in the H-H stretch region by monitoring the production of halide anion photofragments. The spectra, which are assigned to complexes containing ortho H 2 , exhibit rotationally resolved ͚-͚ bands whose origins are redshifted from the molecular hydrogen Q 1 (1) transition by 110.8 cm Ϫ1 (Br Ϫ -H 2 ) and 74.1 cm Ϫ1 (I Ϫ -H 2 ). The complexes are deduced to possess linear equilibrium structures, with vibrationally averaged intermolecular separations between the halide anion and H 2 center of mass of 3.461 Å (Br Ϫ -H 2 ) and 3.851 Å (I Ϫ -H 2 ). Vibrational excitation of the H 2 subunit causes the intermolecular bond to stiffen and contract by 0.115 Å (Br Ϫ -H 2 ) and 0.112 Å (I Ϫ -H 2 ). Rydberg-Klein-Rees inversion of the spectroscopic data is used to generate effective radial potential energy curves near the potential minimum that are joined to long-range potential energy curves describing the interaction between an H 2 molecule and a point negative charge. From these curves the dissociation energies of Br Ϫ -H 2 and I Ϫ -H 2 with respect to isolated H 2 ( jϭ1) and halide fragments are estimated as 365 and 253 cm Ϫ1 , respectively.

Cross sections for various reactions induced by beams of 'H and H ions on targets of natural Mg, Al, and Si were determined from observations of the radioactive product y rays. 'H energies between 14.5 and 27.0 MeV and 'H energies between 8.7 and 18.0 MeV from the University of Colorado cyclotron were used.NUCLEAR REACTIONS Observed nuclear y rays, obtained o(E, ),~(E2 ) on natural targets Mg, Al, and Si; E&"= 14.5-27.0 MeV, E2 "= 8.7 -18.0 MeV.

The mid-infrared spectrum of the Cl37−–H2 anion complex has been measured over the 3990–4050 cm−1 range (H–H stretch region) using infrared vibrational predissociation spectroscopy. The spectrum features a well resolved Σ–Σ transition red shifted by 156 cm−1 from the free H2 molecule stretch. Analysis of the P and R branch line positions using a linear molecule energy level expression yields ν0=4004.77±0.08 cm−1, B″=0.853±0.002 cm−1, D″=(9.3±1.0)×10−5cm−1, B′=0.919±0.002 cm−1, and D′=(9.0±1.0)×10−5 cm−1. The Cl−–H2 complex appears to have a linear equilibrium structure, with a vibrationally averaged separation of 3.19 Å between the Cl− and the H2 center-of-mass. Vibrational excitation of the H–H stretch induces a 0.12 Å contraction in the intermolecular bond.