The harmonic force field and absolute infrared intensities of diacetylene

Abstract

The frequencies, harmonic force field and absolute IR intensities for C4H2 and C4D2 are reported. The experimental harmonized frequencies obey the Teller—Redlich product rule very well.

An approximate harmonic force field was obtained from a refinement procedure in which the starting values are adjusted in order to fit the experimental harmonized C4H2 and C4D2 frequencies. The starting force constant values were taken from the harmonic force field of propyne. The integrated IR intensities were determined according to the Wilson—Wells—Penner—Weber method, using nitrogen as a broadening gas at a pressure of 60 atm. The results of the F-sum rule clearly reveal the internal consistency of the measurements. Two sets of ∂μ→/∂S values can be derived from the experimental intensities by use of an iterative least-squares fitting procedure. One final set can be selected by applying the isotopic invariance criterion and by comparing with the corresponding derivatives of propyne. The final ∂μ→/∂S values are reduced to bond-charge parameters and atomic charges. The bond-charge parameter values are compared with the corresponding values of some common set of intensity parameters recently reported for the series C2H2, CH3CCH and CH3CCCH3.