R. E. Bumgarner

BENZENE FORMS HYDROGEN BONDS WITH WATER

Fully rotationally resolved spectra of three isotopic species of 1:1 clusters of benzene with water (H2O, D2O, and HDO) were fit to yield moments of inertia that demonstrate unambiguously that water is positioned above the benzene plane in nearly free internal rotation with both hydrogen atoms pointing toward the π cloud. Ab initio calculations (MP2 level of electron correlation and 6-31 G** basis set with basis set superposition error corrections) predict a binding energy De ≳ 1.78 kilocalories per mole. In both the experimental and theoretical structures, water is situated nearly 1 angstrom within the van der Waals contacts of the monomers, a clear manifestation of hydrogen bond formation in this simple model of aqueous-π electron interactions.

Direct measurement of the HCl dimer tunneling rate and Cl isotope dependence by far‐infrared laser sideband spectroscopy of planar supersonic jets

The large amplitude tunneling motion of the HCl dimer has been directly studied with a tunable far‐infrared laser sideband/two‐dimensional free jet expansion spectrometer at hyperfine resolution. Rotationless tunneling rates for the three common chlorine isotopic forms are v(35–35) =463 979.2(1) MHz, v(35–37)=463 357.7(1) MHz, and v(37–37)=462 733.7(3) MHz. Both the rotational constants and hyperfine parameters indicate that the vibrationally averaged structure shows little variation within a given tunneling state, with both HCl bond angles giving an average projection on the a‐axis of 47° in all states with resolved hyperfine patterns.

Microwave and tunable far-infrared laser spectroscopy of OCH2O: investigation of the water tunneling potential

The K_p=O → 1 rotation—tunneling bands of O^(12)C-H_2O and O^(12)C-D_2O have been measured in the region between 400 and 600 GHz. Two bands for each isotopomer were observed corresponding to ΔK_p = 1, b-type rotational transitions in the A and B water tunneling states. Each band was fit independently using a Watson A-reduced Hamiltonian yielding all three rotational and several distortion constants per band. The effective A-rotational constants, A^*, contain a contribution due to water tunneling. Assuming the tunneling splittings are the same in K_p=0 and K_p=1, A^*(A state) − A^*(B state) = 2ν_t, where ν_t is the tunneling splitting. We obtain tunneling splitting of 16.684 GHz for O^(12)C-H_2O and 1.012 GHz for O^(12)C-D_2O. These measurements are in good agreement with the predictions of Yaron (J. Chem. Phys. 92 (1990) 7095). Effective one-dimensional potentials have been employed to place constraints on the hydrogen bond geometry, to model the measured tunneling splittings, and to predict higher frequency vibration—rotation-tunneling transitions.

Microwave and tunable far-infrared laser spectroscopy of the ammonia- water dimer

bond is most consistent with the vibrationally averaged rotational constants; with the angle cos-‘[(A,)] d e t ermined from (A,), the projection of the ammonia’s angular momentum onto the framework; and with the nitrogen quadrupole coupling constants of Herbine and Dyke. The water tunneling splitting and observed selection rules place constraints on the barrier height for proton exchange of the water as well as the most feasible water tunneling path along the intermolecular potential energy surface. An estimated barrier of - 700 cm - ’ is derived for the water tunneling motion about its c axis.

Microwave direct absorption spectroscopy of weakly-bound clusters in a planar supersonic jet: Spectra of Ar-HCN and (HCN)2 from 17.4 to 76.7 GHz

A direct absorption microwave and millimeterwave spectrometer capable of observing the rotational transitions of weakly-bound complexes formed in a planar supersonic nozzle has been constructed. Both the technique for observing weakly-bound complexes and its initial application to Ar-HCN and (HCN)_2 are described. The Ar-HCN lines reported here show significant frequency discrepancies (up to −3187.2 MHz) from predictions using previously obtained constants with distortion treatments through sixth order in J, reflecting the strong angular-radial coupling and pervasive intermolecular vibrational state mixing in this complex.

Tunable far‐infrared laser spectroscopy of deuterated isotopomers of Ar–H2O

Several far‐infrared vibration‐rotation‐tunneling transitions have been measured in deuterated isotopomers of Ar–H2O for the first time. These experimental results will enable the generation of improved intermolecular potential energy surfaces for the Ar–H2O system when combined with existing microwave, far‐infrared, and infrared data.