High-Resolution Rotational Spectroscopy and Interstellar Search for Isopropyl Isothiocyanate

The pure rotational spectrum of isopropyl isothiocyanate was measured using a supersonic-jet Fourier transform microwave spectrometer in the frequency region of 2–20 GHz. The rotational transition assignments were extended to the monosubstituted ¹³C, one ³⁴S, and one ¹⁵N isotopic species in natural abundance. The experimental measurements yielded accurate spectroscopic parameters including rotational constants, quantic centrifugal distortion constants, and ¹⁴N nuclear quadrupole coupling constants. The determined rotational constants were used to derive the skeleton structure of isopropyl isothiocyanate. Rotational spectroscopy and structural analyses provide unambiguous evidence that the trans form of the molecule is the global minimum. Based on the spectroscopic results, an astronomical search for isopropyl isothiocyanate toward Sgr B2­(N) was carried out using a Shanghai Tianma radio telescope (TMRT); however, none of the predicted rotational transitions were detected. The upper limits for the abundance of trans (CH₃)₂CHNCS relative to H₂ in Sgr B2­(N) were estimated to be X((CH₃)₂CHNCS) < 8 × 10^–12.