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.

本研究采用傅里叶变换微波（Fourier transform microwave, FTMW）光谱法，在4~26 GHz频段采集了异硫氰酸烯丙酯（allyl isothiocyanate, CH₂=CHCH₂–NCS）的纯转动光谱。对光谱的分析表明，由于CCCN与CCNC二面角的变化，该分子存在两种构象体。观测得到的光谱与基于B3LYP-D3(BJ)和MP2理论水平的量子化学计算所得到的势能面结果一致。本实验与理论研究共同明确鉴定出一种新构象体（I）为全局最低能量几何构型。通过观测与其34S、13C及15N同位素取代体匹配的跃迁信号，以及特征性的14N四极超精细结构，验证了该新构象体的光谱指认结果。针对构象体I，我们推导得到了取代结构（rₛ）与有效基态（r₀）结构，结果显示CCNC角存在大振幅运动的贡献。其余几何参数与通过B3LYP-D3(BJ)/cc-pVQZ计算得到的平衡结构（rₑ）吻合良好。本研究得到异硫氰酸烯丙酯构象体I的CNC键角为152.6(3)°，较异氰酸烯丙酯（allyl–NCO）的137.5(4)°大约15°，这与自然键轨道分析结果相符：即氮原子的杂化类型从异硫氰酸烯丙酯中近似sp杂化转变为异氰酸烯丙酯中的近似sp1.5杂化。