Showing papers by "Willis B. Person published in 1976"

Dipole moment derivatives and infrared intensities. II. Polar tensors in methyl halide molecules

Abstract: Atomic polar tensors have been determined for H, C, and X atoms of the methyl halide molecules (CH3X), using infrared intensity data reported by Russell, Needham, and Overend. The signs for the ∂p/∂Qi’s have been chosen by considering Coriolis coupling constants, agreement between intensity parameter values for CH3X and CD3X, and by considering the values of the atomic polar tensors calculated by CNDO methods for CH3F and CH3Cl. The resulting atomic polar tensors show systematic trends for the CH3X molecules as X change from F to I. The possibility that atomic polar tensors can be transferred from one molecule to another (with some allowance for expected trends) cannot be ignored. Atomic effective charges for each atom have been calculated, and ξH is found to be nearly constant for the four molecules. Additional properties of the atomic polar tensor are examined, including the anisotropy. A ''modified bond moment hypothesis'' appears to be reasonably successful in modeling the infrared intensities of the methyl halides.

Transition moments in infrared‐active fundamentals of spherical‐top molecules

Abstract: The theoretical expression which relates the total strength of a vibration–rotation band of a tetrahedral or octahedral molecule to an individual line strength and to an appropriate transition electric‐dipole moment is re‐examined. Values of these transition moments are inferred from experimental band and line strengths. For the few molecules where direct comparisons can be made, the agreement between the values of transition moments obtained by the two methods is satisfactory. Remarkable systematics in the results are found for a variety of spherical‐top molecules, which suggests that transition electric‐dipole moments may be transferable from molecule to molecule for similar vibrations.

The predicted infrared spectrum and the structure of the isolated UF5 molecule

Abstract: The infrared absorption spectrum by the fundamental modes of UF5 is calculated using force constants transferred from UF6 to predict the wavenumbers and using the F atom polar tensor from CH3F to predict the intensities. Calculations are made for several assumed geometrical configurations of UF5. Comparison of the predicted spectra with the recently observed spectrum, in the UF‐stretching region, of a photolysis product of UF6 isolated in an Ar matrix suggests strongly that the structure is square pyramidal (C4v) with the U atom above the F atom equatorial plane. Although the model for the intensities is simple, the calculated spectrum is expected to predict the correct order of magnitude of the intensities of the fundamentals, first overtones, and binary combination bands of UF5, and the approximate wavenumbers for the expected absorption. Some comparisons are given between the calculations for the several structures of UF5 and observed spectra of other XF5 molecules, including PF5, BrF5, and IF5.

Preliminary interpretation of the infrared intensities of combination and difference bands of N2O and CO2.

Abstract: Expressions derived by Overend for the infrared absorption of combination and difference bands are tested against experimental data for N2O and CO2(ν3±ν1, ν3±ν2, and ν1±ν2, where data are available). From these comparisons, values of the electrical anharmonicities Pαss′(=∂2pα /∂qs∂qs′) are obtained. Overend’s prediction of an anomalously higher value for the transition moment of certain difference bands than for the corresponding combination bands is verified. For CO2, mechanical anharmonicity dominates the intensities perdicted for combination and difference bands; for N2O, electrical anharmonicity is comparable in its contribution to the intensities of combination and difference bands.