R. C. Lord

The Vibrational Spectra of Pyridine and Pyridine‐d5

Pyridine‐d5 has been prepared by exchange between pyridine vapor and heavy water in the presence of a palladium catalyst. Infrared and Raman spectra are reported for pyridine and pyridine‐d5 over the spectral range 300–4000 cm−1. Interpretation of the spectra with the help of the product rule and by analogy with the fundamental frequencies of benzene and benzene‐d6 leads to complete sets of frequencies for the 27 vibrational degrees of freedom of pyridine and of pyridine‐d5. The frequency assignment for pyridine does not differ greatly from previous ones, but is believed to be more reliable because of the additional spectroscopic data.

Far‐Infrared Spectra of Ring Compounds. II. The Spectrum and Ring‐Puckering Potential Function of Cyclopentene

The infrared spectrum of cyclopentene has been recorded in the range 18–300 cm−1. Two series of thirteen and nine Q branches, respectively, were observed, both of which arise from the strongly anharmonic energy levels of the B1 ring‐puckering vibration. The longer, more intense series is assigned to molecules in the ground vibrational state of all the other vibrations, and the shorter, weaker series to molecules in the first excited state of the A2 ring‐twisting vibration. Each set of Q branches can be fitted to transitions between the energy levels of a double‐minimum potential function of the form V=a(x4—bx2), where x is the B1 ring‐puckering coordinate. The height of the barrier opposing planarity is calculated to be 232±5 cm−1, and the equilibrium angle between the two dihedral planes of the puckered ring to be 23.3°±1°.

Rotation‐Vibration Spectra of Methyl Amine and Its Deuterium Derivatives

Methyl amine, methyl amine‐d2, methyl‐d3 amine and methyl‐d3 amine‐d2 of high chemical and isotopic purity have been synthesized and their infrared spectra studied at both prism and grating resolution. The rotation‐vibration band centers of methyl amine have been measured with improved accuracy over previous investigations and with the aid of the spectra of the deuterium analogs a vibrational assignment differing in some important respects from those hitherto suggested has been made.The grating spectra in some cases show irregularities in the rotational fine structure but none which can be associated with effects due to hindered internal rotation. Recent studies in the microwave region have shown that these effects should not be observed within the resolution limits of the grating spectrometers used in this work. However, the first overtone of the torsional frequency in methyl amine, methyl amine‐d2 and methyl‐d3 amine has been observed in a position consistent with calculations based on a barrier height ...

Far‐Infrared Spectra of Ring Compounds. III. Spectrum, Structure, and Ring‐Puckering Potential of Silacyclobutane

The infrared spectra of gaseous silacyclobutane and silacyclobutane‐1, 1‐d2 have been recorded in the range 24‐300 cm−1. Both molecules show the complicated vibrational fine structure expected from a double‐minimum potential for the ring‐puckering vibration. The absorption maxima of silacyclobutane were fitted to a potential of the form V = a (x4 − bx2), where x is the ring‐puckering coordinate. The potential barrier is calculated as 440 ± 3 cm−1, and the dihedral angle of the puckered ring as 35.9 ± 2°. The spectrum of silacyclobutane‐1, 1‐d2 can be interpreted with the same potential function but the quantitative agreement with observed levels above the barrier is less accurate. The discrepancy is ascribed to interaction between the ring‐puckering mode and the SiD2 rocking vibration.

Far‐Infrared Spectra of Ring Compounds. V. Ring‐Puckering Potential Functions of Some Oxygen‐Containing Molecules

The infrared absorption spectra of gaseous 2,5‐dihydrofuran, 3‐oxetanone, 2‐methyl‐4,5‐dihydrofuran, diketene, cyclopentanone, and cyclohexanone have been observed in the range 250–25 cm−1. Under high resolution the spectrum of 2,5‐dihydrofuran shows a satellite series of Q branches in addition to those observed by Shimanouchi and Ueda. The satellites arise from transitions between ring‐puckering energy levels in an excited ring‐twisting state with altered spacing due to higher‐order cross terms in the potential energy. The spectrum of 3‐oxetanone is that of a planar molecule, and the energy levels of the ring‐puckering mode fit a potential function of the form V(x) = ax4 + bx2. The absorption of 2‐methyl‐4,5‐dihydrofuran is complicated by several low‐frequency modes, but the spectrum can be fairly well interpreted with a double‐minimum potential function of the form V(x) = ax4−bx2 with a barrier height of 98 cm−1. Seven sharp Q branches were observed for cyclopentanone, which were fitted to a hindered ps...

Infrared Spectroscopy from 5 to 200 Microns with a Small Grating Spectrometer

High-resolution infrared spectroscopy with a small grating instrument has now been extended to wavelengths of nearly 200 microns. In the range 5–50 microns the achieved resolution was about 0.3 cm−1 or slightly better. Beyond 50 microns the resolution varied from 0.5 to 1.0 cm−1. No predispersive device was used, but filters, residual-ray reflections, selective chopping of radiation, and “off-blaze” use of the doubly passed gratings were employed to remove higher orders from the spectra. These devices are discussed briefly and their characteristics for the various spectral regions have been tabulated. Calibration has been carried out with higher orders of atomic emission lines and absorption lines of previously measured molecular spectra. Wave-number precision is estimated as about 0.04–0.05 cm−1 in most spectral regions. Performance of the spectrometers is illustrated by reproductions of the actual recorded spectra of water vapor at 1600, 300, and 200–55 cm−1, of ethylene and ammonia at 950 cm−1, of benzene at 673 cm−1, and of carbon disulfide at 400 cm−1.

The Vibrational Spectra of Diborane and Some of its Isotopic Derivatives

The infrared spectra of ordinary diborane, of B210H6, and of B210D6 have been determined in the vapor state over the spectral range 250–3800 cm−1. The Raman spectra of ordinary diborane and B210D6 have been measured in the liquid at −130°C. Qualitative depolarization measurements have been made on nearly all observed Raman lines. The vibrational spectra have been interpreted with the help of the various selection rules and the product rule for isotope shifts. The fundamental frequencies obtained from this interpretation differ considerably from those previously published, but are in agreement with a large amount of spectroscopic data, including much not hitherto available. It is concluded that the nonplanar bridge model (D2h symmetry) fits the spectroscopic data best, although the unsymmetrical C2h form cannot be definitely excluded. The theoretical and observed vapor heat capacities agree satisfactorily over the temperature range 100–300°K.

Far‐Infrared Spectra of Ring Compounds. IV. Spectra of Compounds with an Unsymmetrical Potential Function for Ring Inversion

The infrared spectra of gaseous trimethylenimine, trimethylenimine‐N‐d, 2,5‐dihydropyrrole, 2,5‐dihydropyrrole‐N‐d, and cyclopentene oxide have been observed in the range 300–25 cm−1. The N–H bonds of trimethylenimine and 2,5‐dihydropyrrole and the oxygen atom of cyclopentene oxide introduce asymmetry into the potential function for inversion of the ring in these molecules. In consequence the eigenvalues of the inversion vibrations are quite irregular but can be inferred from the spectra and fitted to a potential function of the form V(x) = ax4 − bx2 + cx3, where x is the ring‐inversion coordinate. Interpretation of the far‐infrared spectrum also enables assignment of the satellite fine structure of certain of the mid‐infrared fundamentals as difference bands involving the inversion vibration.

Far‐Infrared Spectra of Four‐Membered‐Ring Compounds. I. Spectra and Structure of Cyclobutanone, Cyclobutanone‐d4, Trimethylene Sulfide, and Perfluorocyclobutanone

Two series of very sharp Q branches have been observed in the far‐infrared spectra of cyclobutanone and cyclobutanone‐d4 vapor. For cyclobutanone, these Q branches have wavenumbers of 35.3, 57.03, 64.99, 72.17, 77.77, 81.85, and 85.33 cm−1, and for cyclobutanone‐d4 the wavenumbers are (∼30), 47.1, 54.2, 60.6, 65.6, 69.8, 73.2, and 78.9 cm−1. Relative frequency values were calculated with the energy levels of a pure quartic oscillator which fitted rather accurately all the transitions except the first observed one. This suggests that the four‐membered ring of cyclobutanone is planar. The far‐infrared spectrum of trimethylene sulfide revealed at least six Q branches of varying intensity and spacing. Their wavenumbers were 62.27, 84.50, 92.14, 100.18, 105.75, and 139.1 cm−1. It is impossible to fit the series to the energy levels of either a pure quartic oscillator or one perturbed by a quadratic term. Comparison of the statistical entropy with the measured calorimetric entropy indicated a double‐minimum pot...

Vibrational Spectra of PF5 and AsF5: Height of the Barrier to Internal Exchange of Fluorine Nuclei

The infrared spectra of gaseous PF5 and AsF5 have been obtained over the spectral range 30–2000 cm−1, and Raman spectra have been observed for the liquid state. The observed frequencies for both molecules are satisfactorily assigned on the basis of a D3h structure. A normal‐coordinate calculation is made with the help of a valence‐force system and simplifying assumptions about off‐diagonal elements in the potential‐energy matrix. An approximate potential diagram for internal exchange of axial and equatorial fluorine nuclei is evaluated from the potential constants of species E′. From the calculated exchange rates it is concluded that it is unlikely that the axial—equatorial chemical shifts in the NMR spectra of PF5 and AsF5 will be resolved at low temperatures.