Bryce Crawford

Vibrational Intensities. II. The Use of Isotopes

Two rules are presented which relate the intensities of vibrational fundamentals of different isotopic species. They are thus analogous to the Teller‐Redlich product rule which relates frequencies. They apply to either infrared or Raman intensities. One rule permits the calculation of dipole‐moment derivatives without the determination of normal coordinates. The application of the rules is illustrated.

Vibrational Intensities. VIII. CH3 and CD3 Chloride, Bromide, and Iodide

Absolute intensity measurements have been made on the fundamental vibrations of methyl chloride, bromide, and iodide, and their fully deuterated derivatives, by integrating the optical density over the absorption bands. The bands were fully pressure broadened by using up to 80 atmos of foreign gas. Band separations were made graphically. The results are analyzed in terms of the dipole moment derivatives with respect to symmetry coordinates in the molecule, (∂p/∂Si). The data on the different isotopic species are shown to yield consistent results, and this requirement of consistency has also been used as an aid in the analysis. In the E‐class vibrations the signs of the dipole moment derivatives have been determined unambiguously by assuming the permanent dipole to be directed CH3+–X—.

Normal Coordinates in the Methyl Halides

The potential‐energy functions found by Chang for the methyl halides have been put into valence‐type form and revised to eliminate inconsistencies and to accord with the true (nontetrahedral) geometry and the normal frequencies (corrected for Fermi resonance and anharmonicity). The resulting valence‐type force constants and normal coordinates are given for light (CH3) and heavy (CD3) chlorides, bromides, and iodides.

Vibrational Intensities. VI. Ethylene and Its Deuteroisotopes

Absolute intensity measurements have been made on the fundamental vibrations of ethylene and four of its deuteroisotopes. The bands were pressure broadened with nitrogen at 50 atmos, and the intensities were determined by the method of Wilson and Wells except that the observed optical density was integrated against logv rather than v. Normal coordinates have been calculated, and the intensities have been interpreted in terms of quantities (∂p/∂Si) giving the change in dipole moment with respect to each internal symmetry coordinate. Data from the different isotopic species have been used to eliminate ambiguities in the interpretation. Effective bond moments are calculated for each symmetry coordinate.

The Calibration of Infrared Prism Spectrometers

A search has been made for sharp absorption maxima suitable for calibrating infrared prism spectrometers at closely spaced intervals. A set of such points, corrected and tested, is presented together with traces showing the appearance of the calibration spectra, and the conditions for reproducing them. Suitable points are included for SiO2, LiF, CaF2, NaCl, KBr, and CsBr prisms.

Normal Coordinates of the Planar Vibrations of Pyridine and Its Deuteroisomers with a Modified Urey—Bradley Force Field

A Urey—Bradley potential function has been fitted to the observed vibrational frequencies of pyridine, pyridine‐2,6‐d2, pyridine‐3,5‐d2, pyridine‐4‐d1, and pyridine‐d5.As in the case of benzene, it was necessary to include an additional non‐Urey—Bradley force constant ρ to describe the aromatic character of the molecule. Our results suggest several minor changes in the vibrational assignment of some of these molecules.

Vibrational Intensities. X. Integration Theorems

The advantages of integrating absorption intensities against the logarithm of the frequency, rather than against the frequency itself, are presented.

Infrared Spectrum of Matrix‐Isolated NO2

The infrared spectrum of NO2, suspended in oxygen and argon matrices at helium temperature, is described. The spectra are similar to those previously reported, but many new absorptions are observed as well. The absorptions are assigned among the molecular species NO2, stable N2O4, and three unstable forms of N2O4.

The application of the Urey-Bradley force field to the in-plane vibrations of benzene

Abstract A normal-co-ordinate treatment of benzene is carried out using the Urey-Bradley force field, and also a slight modification of it. These calculations favor the assignment of v 14 to 1618 cm −1 , close to the original suggestion of I ngold , and in the absence of definitive experimental evidence this assignment is adopted.

Redundant vibrational coordinates. Part I. The harmonic potential function

Abstract Redundant coordinates, so useful in many problems in molecular dynamics, introduce certain new considerations regarding force constants. The general requirements on the potential function in such coordinates are discussed in the harmonic approximation, with special attention to the linear force constants, intramolecular tensions, and null coordinates (the coordinate expressing the redundancy condition in first-order approximations). It is shown that the quadratic force constants for such null coordinates are indeterminate. It is also shown that, when redundant coordinates are used, the linear force constants and intramolecular tensions must in general be considered; exceptions to this occur in the cases of non-totally-symmetric null coordinates, where the linear constants and tensions vanish, and of null coordinates which express a redundancy condition exactly, where the same constants are indeterminate. The physical meaning of the intramolecular-tension force constants is also discussed.