G. E. Leroi

Direct Observation of the Infrared Torsional Spectrum of C2H6, CH3CD3, and C2D6

The torsional spectra of gaseous C2H6, CH3CD3CD3, and C2D6 have been observed directly in the infrared under high‐pressure‐path‐length conditions (30‐70 atm·m). The fundamental was observed for all three species, at 289, 253, and 208 cm−1, respectively. Hot bands were observed for C2H6 (255 cm−1 [A], 258 cm−1[E]) and CH3CD3 (228 cm−1) as was the first overtone for CH3CD3 (482 cm−1). All the experimental frequencies can be very well accounted for by use of a single internal rotation potential function with a barrier height V3 = 2928 cal / mole. This implies that V6 (and higher terms) are negligibly small. Two alternative explanations for the occurrence of these spectra in the infrared, one in terms of Coriolis interaction with an infrared‐active vibration and one in terms of a ν4 + νx − νx vibration, are discussed. A more reliable determination of ν12, the “uncertain frequency” of C2H6, has been obtained from the ν12 − ν9 difference band, giving ν12 = 1206 cm−1.

Raman Spectra of Solid CO2, N2O, N2, and CO

The Raman spectra of solid CO2, N2O, N2, and CO have been observed. Each spectrum can be analyzed by assuming an approximately ordered structure isomorphous with that of CO2. The lattice mode intensities have been calculated on the basis of the oriented gas model; they are in good agreement with the measured values and lead to confident assignments of the observed bands. The frequencies of the Eg and two Tg librations are 73.5, 91.5, and 132 cm−1, respectively, for CO2(15°K) and 68, 82, and 124.5 cm−1, respectively, for N2O (15°K). In α‐N2 at 16°K two sharp bands are observed at 33.5 cm−1 (Eg) and 37.5 cm−1 (Tg). The other Tg mode may coincide with the Eg. Only one broad, asymmetric lattice band is observed for α‐CO, which is located at 47.5 cm−1 at 12°K. The peak frequencies of these bands decrease an average of ∼ 15% on warming from 15°K to the phase transition, and there is a considerable concommitant increase in their widths.

Raman Studies of Molecular Motion in Condensed Oxygen

Raman scattering has been observed from both the external and internal motions of the molecules in each of the four condensed phases of oxygen. The stretching vibration gives rise to a sharp band at 1552.5 cm−1 in each phase. Weak wings occur at the base of this band in the γ and liquid phases. Librational lattice modes are observed at 44 and 79 cm−1 in the α phase and at ∼ 50 cm−1 in the β phase. The intensities of these bands relative to that of the internal fundamental are in reasonable agreement with those expected from the oriented gas model. In γ‐ and liquid oxygen, the external motion gives rise to strong wings on the Rayleigh line. Estimates of the mean‐square torsional amplitude and the torque acting on the molecule have been made from the observed low‐frequency spectra, and indicate that the torsional motion is highly hindered in each phase. The root‐mean‐square torsional amplitude increases from an average of 9° in the α phase to 18° in the liquid.

Raman Spectra of Solid Chlorine and Bromine

Librational lattice modes have been observed at 15°K in solid Cl2 at 83, 100, and 118 cm−1, and at 55, 74, 86, and 101 cm−1 in solid Br2. Their intensities relative to one another can be qualitatively accounted for by the oriented gas model. The stretching frequencies [ν(1–0)] of Cl2 and Br2 are shifted −13 and −20 cm−1, respectively, from their gas phase values and show structure due to isotope splittings and inter‐molecular coupling. The spectra of the lattice modes, the Br2 stretching motion, and the Cl2 stretching motion indicate, respectively, strong, intermediate, and weak intermolecular coupling relative to the isotope splittings. Both lattice and internal frequencies indicate stronger intermolecular forces in solid Br2 than in solid Cl2.

Infrared and Raman Spectra of Carbon Subsulfide

Infrared and Raman spectra of carbon subsulfide (C3S2) in various phases have been obtained in the region 30–4000 cm−1. Mass and ultraviolet spectra are also presented. Analysis of the vibrational spectra indicates a D∞h structure. Of particular interest is the 94‐cm−1 band assigned as ν7, the central‐carbon bending frequency, which lies at a higher frequency than the analogous band in C3O2. A vibrational analysis has also been performed.

Infrared spectra and internal rotation in propane, isobutane and neopentane

Abstract : The infrared spectra of propane, isobutane and neopentane were recorded at high path-length x pressure conditions between 200 and 800 inverse cm. A large number of combination bands were observed and assigned, and values for the torsional frequencies of isobutane and neopentane were derived from the combination bands. Potential barriers to internal rotation were calculated from the torsional frequencies and are compared with other results available from the literature. (Author)

Pressure‐Induced Infrared Spectrum of Methane

Abstract : The pressure-induced infrared absorption spectrum of gaseous methane was observed using a 10m cell and 4-10 atm. pressure in the region 200-550 cm. The intensity can be expressed according to the relation: A(nu) = gamma(nu)(d sq), where A(nu) is the absorbance at frequency nu, and d is the gas density. The spectrum is interpreted in terms of rotational transitions in a pair of CH4 molecules possessing a transient dipole moment induced by the octopole moment of the molecule. (Author)

Raman Spectra of Polycrystalline H2S and D2S

The Raman spectra of H2S and D2S have been obtained in all three solid modifications, using both conventional and laser excitation. The spectra in the two higher‐temperature phases are consistent with their known disordered crystal structures. Previously undetected vibrational structure also has been obtained for both the internal and lattice vibrations in the lowest‐temperature phase (III); an assignment of the entire spectrum is proposed. Conclusions about the crystal structure of this phase, which has not been previously established, can be drawn from the combined results of x‐ray, electron diffraction, infrared, far‐infrared, and Raman investigations. It is shown that a C4h structure with eight molecules per unit cell on C1 sites fits all the data for Phase III of H2S and D2S.

CORRELATION OF ELECTRONIC STRUCTURE AND BENDING FORCE CONSTANTS IN SOME LINEAR MOLECULES.

A qualitative relationship between electron density in the outermost occupied orbital at the central carbon atom and ease of bending is described for several triatomic and pentatomic linear molecules, including CO2, C3, NCN, C3N2, C3O2, C3S2, and C3H4. Both valence bond and molecular orbital models are discussed.

Infrared Spectrum of Deuterium Sulfide

The bending fundamental (v2) of D2S has been investigated under high resolution. Centrifugal‐distortion corrections were applied in the rotational analysis, and transition intensities were computed in both the rigid‐rotor approximation and with the inclusion of classical centrifugal distortion. The inertial constants arising from this analysis for the ground state are A=5.484, B=4.508, C=2.444; for the excited state they are A′=5.616, B′=4.585, C′=2.4216 cm−1, and the v2 band center is 855.45 cm−1. These results together with information derived from previous investigations of D2S and H2S have been used to evaluate the constants in the quadratic expression for the vibrational energy levels and to compute some of the constants in the general expressions for the moments of inertia as a function of vibrational quantum numbers. The experimental values of the inertia defect for the ground state and the v1 and v2 excited states are in good agreement with those predicted theoretically.