Alternation of the spin–orbit coupling in the 2Π ground state of HC n S (n = 1–12) radicals

First-order calculations of spin–orbit constants, dipole moments and carbon–sulphur distances have been performed for HC n S (n = 1–12) radicals in the 2Π electronic ground state. It is found that these molecular properties alternate with even or odd numbers of carbon atoms in the chains and the spin–orbit constant A SO is around −300 cm−1 for n even and about +120 cm−1 for n odd throughout the series. This agrees with the experimentally determined value of about −270 cm−1 for HC2S, but the theoretically predicted A SO values are much larger than the values given for HC3S and HC4S from a fit of their millimeter-wave spectra. Values that were too low were also assumed in the analysis of the rotational spectra of n = 4–8.