Conformers of Allyl Isothiocyanate: A Combined Microwave Spectroscopy and Computational Study.

The pure rotational spectrum of allyl isothiocyanate (CH ₂ =CHCH ₂ -NCS) was collected from 4 to 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 ( r _s ) and effective ground state ( r ₀ ) 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 ( r _e ) from B3LYP-D3(BJ)/cc-pVQZ calculations. The derived CNC bond angle of 152.6(3)° for conformer I of allyl-NCS is found to be ∼15° larger than that of allyl-NCO (137.5(4)°), 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.