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

The
pure rotational spectrum of allyl isothiocyanate (CH2=CHCH2–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 34S, 13C, and 15N isotopologues and with the characteristic 14N 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)° 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 ∼sp1.5 in allyl–NCO as determined via
natural bond orbital analyses.