P—R Separations and Coriolis Constants for Symmetric-Top Molecules: Force Constants for NF3, PF3, and AsF3

Abstract

Based on the method of Edgell and Moynihan for computing the Coriolis constants of the degenerate vibrations of symmetric‐top molecules from gas‐phase infrared band envelopes, an equation was developed which relates the P—R branch separation to the zeta for the case of a narrow Q branch. A computer program was written for the solution of this equation and was used to determine the Coriolis constants for the v₄ vibrations of NF₃, PF₃, AsF₃, and NH₃ using the previously determined P—R separations. Significant deviation between the calculated and observed value of ζ₄ for NH₃ was found, indicating a breakdown in the method of Edgell and Moynihan for light molecules whose rotational structure has been smoothed by an infrared‐inert broadening gas. The zetas for the Group V trihalides, together with the zeta sum rules and the vibrational frequencies, were used to determine general harmonic force constants for the E‐species vibrations. Equations were also developed which permit approximate zetas to be calculated from the P—R separations of the perpendicular bands.