Brian J. Howard

Vibrational predissociation, tunneling, and rotational saturation in the HF and DF dimers

The high‐resolution spectra of the intramolecular stretching bands of the HF and DF dimers have been recorded with a tunable difference‐frequency laser. These measurements yield considerable information about the dynamics of hydrogen bonding in these complexes. Vibrational predissociation is observed as a non‐pressure‐dependent excess linewidth for the ‘‘bound‐H’’stretching band of the HF dimer, but no excess linewidth is observed for the ‘‘free‐H’’ stretching band of the HF dimer or for either band of the DF dimer. An unusually large vibrational dependence to the interconversion tunneling frequency is observed for both species, with about a factor of three reduction from the ground state splitting upon excitation of any of the intramolecular stretches. The K subband origins obtained from the A/B hybrid free‐H stretching band of the HF dimer exhibit an irregular pattern indicating anomalous centrifugal distortion effects suggestive of rotational saturation of the angular orientation of the hydrogen bond.

The molecular beam spectrum and the structure of the hydrogen fluoride dimer

The molecular beam spectrum of the hydrogen fluoride dimer has been investigated in detail. Microwave and radiofrequency transitions of (HF)2, HFDF, and (DF)2 are reported for both the Ka =0 and Ka =1 rotational levels. The results provide information on the structure of the dimers and on the nature of the intermolecular potential energy surface. The average F–F distance is shown to be approximately 2.78 A but with a probably significantly shorter equilibrium distance. The nonhydrogen bonded hydrogen atom is bent 63°±6° from the F–F axis. In addition, analysis of the perpendicular electric dipole moment indicates a nonlinear hydrogen bond. The large centrifugal distortion effects and the unusual quantum mechanical tunneling provide a crude model for the potential surface associated with the hydrogen bond; the effective bond stretching force constant is (1.36±0.03)×104 dyn cm−1.

Anisotropic intermolecular forces

Anisotropic potential energy surfaces for Ne·HCl, Kr·HCl and Xe·HCl are obtained by simultaneous least squares fitting to molecular beam spectra and rotational line broadening cross sections. A revised potential surface for Ar·HCl is also presented. The potentials are all very similar in shape, with the absolute minimum at the linear rare gas—HCl geometry in each case. The absolute well depths and well depth anisotropies increase steadily as the size of the rare gas atom increases. The potentials should be reliable in the region of the absolute minimum and on the repulsive wall of the potential. The molecular beam spectra for Ne·HCl can be fitted only by a potential with a significant secondary minimum at the linear Ne·Cl-H geometry, but the existing data for the more anisotropic Ar, Kr and Xe systems are not sensitive to the presence of this potential feature. The potential surfaces for these systems have accordingly been constrained to have a similar secondary minimum near the linear rare gas-ClH geomet...

Pulsed molecular beam infrared absorption spectroscopy of CO2 dimer

Abstract The rotationally resolved spectrum of the carbon dioxide dimer has been obtained in the region of the ν 3 CO 2 monomer vibration by diode laser absorption spectroscopy of a pulsed molecular beam. Rotational and centrifugal distortion constants within a Watson S -reduced Hamiltonian are determined and interpreted in terms of a centrosymmetric slipped parallel structure. The ground-state geometrical parameters are R (CC) = 3.602 A and ∠ CCO = 57.9°. This effective structure is explained by a potential including distributed multipole and dispersion interactions and a cylindrical hard-core repulsion.

Molecular-beam infrared absorption studies of rare gas-ocs complexes

Abstract Resolved vibration-rotation spectra for Ne·OCS, Ar·OCS and Kr·OCS in the ν 3 region of the OCS monomer have been obtained by absorption spectroscopy of pulsed molecular beams. Rotational constants for the upper and lower vibrational states and the band origins were determined, and effective structures for these molecules were calculated.

The intermolecular potential energy surface of Ar · HC1

Anisotropic potential energy surfaces for Ar · HC1 are obtained by simultaneous least squares fitting to molecular beam spectra, rotational line broadening cross sections, second virial coefficients and molecular beam total differential cross sections. Several potentials are obtained which given good agreement with all these data and the sensitivity of the data to various aspects of the potential is investigated. For all potentials the equilibrium configuration is linear with the atomic arrangement Ar · H-C1. Several different ways of parameterizing the intermolecular potential are considered and it is concluded that the conventional multipole parameterization is not adequate for strongly anisotropic intermolecular potentials.

Anisotropic intermolecular forces: II. Rare gas-hydrogen fluoride systems

Anisotropic intermolecular potentials for Ar · HF, Kr · HF and Xe · HF are obtained by least squares fitting to molecular beam spectra of van der Waals complexes. The absolute minimum is at the lin...

Molecular beam electric resonance spectroscopy of the nitric oxide dimer

Microwave and radiofrequency spectra have been observed for (NO)2 using the technique of molecular beam electric resonance spectroscopy. Precise values have been determined for rotational, nuclear quadrupole coupling and spin-rotation constants of the ground electronic state treated as a singlet state. The values obtained are A = 25829·4803(20) MHz, B = 5614·3093(4) MHz, C = 4605·4396(12) MHz xaa = -4·0652(2) MHz, xbb - xcc = -8·5498(6) MHz caa = 10·4(5) kHz, cbb = 13·8(4) kHz, ccc = 0·8(4) kHz. The electric dipole moment has also been determined: μ = 0·17120(2) D. From these data (NO)2 is interpreted to have a symmetric cis-planar structure with the structural parameters r NN = 2·33(12) A, r NO = 1·15(1) A, ∠NNO = 95(5), 1/2(r NN + r NO) = 2·444(8) A. The quadrupole coupling constants of the nitrogen nuclei and the dipole moment demonstrate that little electron rearrangement occurs on dimer formation. In addition xaa and xbb - xcc indicate that the unpaired π electrons lie in the plane of the dimer imply...

The NO dimer: 15N isotopic infrared spectra, line-widths, and force field

The v 1 fundamental bands of (15N16O)2 and 14N16O-15N16O isotopes of the NO dimer have been studied using a long-path (200 m) low temperature (84 K) gas cell and Fourier transform infrared spectrometer. The spectra, obtained with a resolution of 0·005 cm-1, were analysed to give accurate rotational and quartic centrifugal distortion parameters for the ground and the v 1 = 1 vibrational states. The predissociation-limited v 1 band line width for (14N16O)2 was measured to be 0·0056(3) cm-1, which is slightly less than previous determinations. The (15N16O)2 width was found to be 0·0044(2) cm-1, and the significant isotope dependence will help to constrain models of the predissociation process. A much larger width of 0·014(2) cm-1 was measured for the mixed isotope, 14N16O-15N16O. This, and the mixed isotope vibrational frequency, may be understood on the basis of a simple coupled-oscillator model. The present results were used to derive a refined structure for the NO dimer: r(NO) = 1·1515(3) A, r(NN) = 2·263...

Molecular beam electric deflection study of ammonia polymers

The molecular beam electric deflection behavior of (NH3)n, n=1 to 6, has been determined. The ammonia dimer is found to be polar and presumably has a single hydrogen‐bond structure. The higher polymers are nonpolar, compatible with cyclic, hydrogen‐bonded ring structures.