I. A Reinvestigation of the Vibration Spectra of Several Triatomic Molecules. II. The Infrared Spectrum and Configuration of Hydrogen Persulfide. III. Some Observations on the Spectrum of "Sulfur Monoxide"

Citation

Wilson, Mathew Kent (1949) I. A Reinvestigation of the Vibration Spectra of Several Triatomic Molecules. II. The Infrared Spectrum and Configuration of Hydrogen Persulfide. III. Some Observations on the Spectrum of "Sulfur Monoxide". Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/dpf4-d440. https://resolver.caltech.edu/CaltechTHESIS:07092024-220418464

Abstract

The vibration spectra and molecular configuration of ozone and nitrogen dioxide have been the subject of considerable controversy. Part I of this thesis is a comparative study of the positions, relative intensities, and temperature dependence of the infrared absorption bands in the region 1.5-15 µ of ozone, nitrogen dioxide, oxygen fluoride, and sulfur dioxide.

The ozone band at 705 cm⁻¹ is not temperature sensitive and, therefore, is not a difference band as was once postulated. The relative intensity of the ozone band at 2100 cm⁻¹ is about that expected for a combination or overtone band and, therefore, this band need not be considered as a fundamental on the basis of intensity. A previously unreported band was found for ozone and has been shown to be the fundamental ν₁. A new assignment of fundamentals has been given which agrees with the positions, intensities, and contours of the observed ozone bands. In agreement with the electron diffraction results, this assignment requires an obtuse model for ozone. However, the rotational spacing observed in the perpendicular bands is only about one-half that calculated for the model found by electron diffraction.

The symmetric valence vibration, ν₁, for nitrogen dioxide previously had been unreported in the infrared, and conflicting assignments had been proposed for the observed nitrogen dioxide bands. By examining the spectrum of the nitrogen dioxide - nitrogen tetroxide system at 25° and 200° C, it was possible to locate the missing fundamental and to show that several of the bands previously ascribed to nitrogen dioxide are in reality nitrogen tetroxide bands. The symmetric valence vibration of nitrogen dioxide was found near 1306 cm⁻¹ which is in good agreement with the value of 1320 cm⁻¹ reported from measurements upon the temperature-sensitive ultra,-violet bands. The rotational. spacing observed in ν₄ for nitrogen dioxide is much smaller than that required by the molecular structure as determined by electron diffraction. If the existence of two identical overlapping bands is assumed, the spectroscopic and electron diffraction data can be reconciled. No explanation for the occurrence of two such overlapping bands can be given at this time.

Many of the bands previously assigned to oxygen fluoride have been shown to be due to an impurity, presumably carbon tetrafluoride. A new assignment of fundamentals has been given which accounts satisfactorily for the observed oxygen fluoride bands.

The apex angle decreases in the series, nitrogen dioxide - ozone - oxygen fluoride. It was observed that the values of ν₃/ν₁, and the bond constant, K₁, also decrease while the interaction constant, K₁₂, increases. The value for the ozone bond constant seemed remarkably low in comparison with the value obtained for sulfur dioxide, but has now been shown to be intermediate between the bond constants of the more closely similar molecules, nitrogen dioxide and oxygen fluoride.

Heretofore the evidence has not been conclusive in support of either the plane symmetric or symmetric 90° model for nitrogen tetroxide. By new considerations involving the occurrence and frequencies of the N-O vibrations in nitrogen dioxide and nitrogen tetroxide, it has been possible to indicate that very probably the planes determined by the two NO₂ groups in nitrogen tetroxide are at 90° to each other.

Part II of this thesis is an investigation of the vibration spectrum of hydrogen persulfide and the rotational. analysis of the band at 2µ. Since only one near-perpendicular type band and no near-parallel band could be investigated under high resolution, only the small moment of inertia was experimentally determined for the hydrogen persulfide molecule. Assuming a non-linear, non-planar (C₂) structure, it has been found that when the S-S distance, S-H distance, and S-S-H angle are allowed to vary within their range of probable values as inferred from the structures of similar molecules, the angle between the two S-S-H planes may be as small as 10° but no larger than 75° if the calculated least moment of inertia is to agree with observation. The maximum possible angle for hydrogen persulfide is somewhat smaller than the most probable angle in hydrogen peroxide.

The carrier of the absorption spectrum observed in a mixture of sulfur vapor and sulfur dioxide which has been passed through an electric discharge has variously been described as SO, S₂O₂ , and S₂≠. The final part of the thesis describes several experiments undertaken in an attempt to identify the substance responsible for this spectrum. It was shown conclusively that oxygen is needed for the production of the spectrum, but that the absorption spectrum was not observed under conditions which yielded the SO bands strongly in emission. Although the necessity for the presence of oxygen appears to rule out S₂≠ as the carrier, no decision could be made regarding the actual carrier of the spectrum.

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