The pure rotational spectrum of allyl isothiocyanate (CH2=CHCH2-NCS) was collected from 4-26 GHz using Fourier transform microwave (FTMW) spectroscopy. Its analysis revealed the presence of two conformers that arise due to variation in the CCCN and CCNC dihedral angles.The observed spectrum is consistent with the accompanying potential energy surfaces derived using quantum chemical calculations at the B3LYP-D3(BJ) and MP2 levels of theory. Together, this experimental and theoretical study unequivocally identifies a new conformer (I) as the global minimum geometry. The spectral assignment of this new conformer is verified by the observation of transitions consistent with its 34 S, 13 C and 15 N isotopologues and with the characteristic 14 N quadrupole hyperfine patterns. For conformer I, the substitution (rs) and effective ground state (r0) structures were derived and reveal contributions from a large amplitude motion in the CCNC angle.The remaining geometric parameters compare well with the equilibrium structure (re) from B3LYP-D3(BJ)/cc-pVQZ calculations. The derived CNC bond angle of 152.6(3) o for conformer I of allyl-NCS is found to be ~15 o larger than that of allyl-NCO (137.5(4) o ), which is in line with a change in the hybridization at nitrogen from an orbital with more ~sp character in allyl-NCS to ~sp 1.5 in allyl-NCO as determined via natural bond orbital analyses.

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A spectroscopic analysis of allyl isothiocyanate (CH 2 = CHCH 2 NCS) was conducted via chirped-pulse and Balle-Flygare Fourier Transform microwave (FTMW) spectroscopy in the 7-25 GHz range. Rotational transitions associated with the different conformers were assigned based on information gathered from quantum-chemical calculations computed at B3LYP-D3(BJ) and MP2 levels of theory using the Dunning cc-pVTZ basis set. The results showed the existence of two conformers, the gauche conformer, which was reported in previous work a as well as the first reported existence of a second conformer which corresponds to the global minimum. Spectroscopic assignments included transitions due to both parent conformers, 34 S, 13 C and 15 N singly substituted isotopic species as well as hyperfine splitting due to the presence of 14 N.