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Anomalous hyperfine splittings; Fourier transform microwave spectroscopy; Internal rotation barrier; Permutation–inversion group theory; Stark effect; Equivalent methyl tops
Abstract:
The rotational spectrum of the symmetric top trimethyl tin chloride (CH₃)₃SnCl has been studied using a pulsed molecular beam Fourier transform microwave spectrometer in the frequency range from 3 to 24 GHz. The spectrum is exceedingly complicated by the internal rotation motions of the three equivalent methyl tops, the high number of Sn- and Cl-isotopes and the quadrupole hyperfine structure of the chlorine nucleus. In this paper, we present the microwave spectrum, ab initio calculations, permutation inversion (PI) group-theoretical considerations, Starkeffect measurements and finally the assignments and fits of the different torsionrotation species. Based on the Stark-effect measurements, the dipole moment is μ =3.4980(30) D. Due to K = ±1-mixing effects we observe linear Stark-effect behavior and additional quadrupole splitting for some K = 0 torsion rotation transitions in (CH₃)₃SnCl, which can be group-theoretically explained. The symmetric rotor fit of A1 states leads to an effective B-constant of 1680.040124(50)MHz for the main isotopologue (CH₃)₃120Sn 35Cl. A global fit of 182 K = 0 torsion-rotation transitions yields a V3 torsional barrier of 148.299(54) cm-1.
