Dipole Moment and Microwave Spectrum of Bromo-acetylene

Abstract

THE electric dipole moments of the mono-halogen acetylenes are known to decrease to very small values as the electronegativity of the halogen decreases. The moment of bromo-acetylene, in particular, was reported to be indistinguishable from zero by Brockway and Coop¹. Although no estimates of possible errors were stated for this molecule, the accuracy of the methods was in general such that the error was not expected to exceed 0.01 D. Tyler and Sheridan² prepared and characterized iodo-acetylene, and were unable to detect its microwave spectrum in sensitive instruments, although the B₀ value assumed was subsequently proved to be correct by rotational Raman spectroscopy³. Hence it was concluded that the dipole moment of iodo-acetylene is not more than 0.05 D, perhaps much less. In view of the improbability that bromo- and iodo-acetylene should both possess such small dipole moments, we have examined the microwave rotation spectrum of bromo-acetylene. This spectrum proved easy to observe and indicated that the dipole moment of the molecule is more than 0.01 D.