Knowledge Bank

The microwave and millimeter wave spectra of sixteen isotopic species of isothiocyanic acid (HNCS) have been measured. The molecule is a very slightly asymmetric rotor. Besides the a-type R branches, previously assigned for four species, we have assigned for the first time some very weak a-type Q branches. For the deuterated species we have assigned, also for the first time, some intense high J b-type P and R branch transitions. Accurate values for the rotational constants have been obtained. The rotational constant $Ao$ is extremely large, with the result that the molecule undergoes very large centrifugal distortion, requiring very high order centrifugal distortion constants to fit the spectrum. We have obtained improved values for the $\mathrm{<mrow\; data-mjx-texclass="ORD">14</mrow>}N$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">33</mrow>}S$ nuclear quadrupole coupling constants. From the Stark effect of the b-type transitions the b-component has been measured, and found to be much larger Chan previously supposed. The spectra have enabled an improved structure to be obtained, and should also facilitate searches for HNCS in interstellar space.