Isothiocyanato-Containing Carbon Chains: The Laboratory Detection of HCCCCNCS and NCCCNCS via Rotational Spectroscopy

Two new NCS-containing carbon chains of potential astronomical interest, HC₄NCS and NC₃NCS, were produced through dc electric discharge in a molecular beam and detected by Fourier transform microwave spectroscopy. Rotational transitions from the two species were recorded between 4 and 26 GHz, and assignments were confirmed by the distinct hyperfine structures due to the ¹⁴N nuclear quadrupole moments of these two linear, isoelectronic species. The rotational constants of HC₄NCS and NC₃NCS were determined experimentally to be 595.09300(3) and 596.26344(2) MHz, respectively, and are in good agreement with theoretical predictions at the MP2 and CCSD(T) levels of theory. The use of diacetylene in the precursor mixture to form HC₄NCS improved the intensity of the spectrum by a factor of 10 and allowed the observation of the rotational transitions of the ³⁴S singly substituted form in natural abundance.