The Microwave and Millimetre Wave Spectrum, Molecular Constants, Dipole Moment, and Structure of Isocyanic Acid, HNCO

Abstract

The microwave and millimetre wave spectra of six isotopic species of isocyanic acid, HNCO, have been measured. The molecule is a very slightly asymmetric prolate rotor. Besides the a type R branches, previously reported for two species, we have assigned for the first time a type Q branches and also some intense b type P and R branch transitions. Accurate values of the rotational constants have been determined. The rotational constant A0 is particularly large, with the result that the molecule undergoes very large centrifugal distortion, and terms up to the 12th power in the angular momentum are required to fit the spectrum. 14N nuclear quadrupole hyperfine splitting has been measured and accurate values for both χaa and (χbb − χcc) have been obtained, the latter for the first time. From the Stark effect of the b type transitions we have found the b component of the dipole moment to be 1.35 ± 0.1 D, an order of magnitude larger than previously supposed, and have used it to reinterpret earlier data indicating a variation of the a component with Ka. An improved molecular structure has been obtained. Astrophysical consequences of the large b component of the dipole moment are discussed. A table of transitions of potential astrophysical interest is presented.