Zeeman Study on Nuclear Quadrupole Resonances in AsI₃ and AsI₃·3S₈

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Principal axis systems and asymmetry parameter of the electric field gradient tensors were determined by studying Zeeman effect of nuclear quadrupole resonances of As⁷⁵ and I¹²⁷. The iodine resonance in AsI₃ showed that there are three principal axis system swhose z axes form a trigonal pyramid having an apex angle of 91.7°±2.5°. The fact that this angle did not agree with the bond angle of 88°02′ supports the mechanism of resonance switching of the bond. The arsenic resonance in AsI₃ has only one principal axis system whose z axis is along the trigonal symmetry axis of arsenic tri-iodide molecule. The asymmetry parameter obtained from Zeeman study was nearly zero.
The iodine resonance in AsI₃·3S₈ also showed that there are three principal axis systems. This fact well agrees with the result from X-rays that the most possible crystal structure of AsI₃·3S₈ is C_3v¹. Three z axes are parallel to the edge of a trigonal pyramid with an apex angle of 101.7°±0.5° which is close to the bond angle 100°±2° (∠ I–As–I) of AsI₃ molecule in gaseous state. The electric field gradient tensor of arsenic resonance in AsI₃·3S₈ was nearly axial symmetric about the trigonal symmetry axis of the molecule.