Nuclear Quadrupole Spectra of Boron Trichloride and Tribromide

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The pure quadrupole resonance spectra were observed on Cl³⁵ and Cl³⁷ in BCl₃ and Br⁷⁹ and B⁸¹ in BBr₃ at several temperatures. The spectrum of each nucleus consists of characteristic doublet (ν_I and ν_II) with the intensity ratio of ca. 3 to 5. The Zeeman study with a single crystal of BBr₃ was also carried out at liquid nitrogen temperature (77°K), to determine the asymmetry parameter η. The results are as follows:
(Remark: Graphics omitted.)
The ionic character and the double bond character were estimated to be i=0.42 and f=0.11, respectively for the B–Br bond from the above results. In BCl₃, f=0.06 was obtained by assuming i=0.50. The double bond characters of these compounds are fairly smaller than usually assumed.
The doublet separation cannot be explained by direct nuclear dipole-dipole interaction. A reasonable explanation was made by taking into account the effect of the intramolecular vibration: the isotopes of boron B¹⁰ and B¹¹ provide different mean square amplitudes to the bending vibrations of BX₃, which in turn produce a difference of q for B¹⁰X₃ and B¹¹X₃ molecules. The effect was calculated by use of the normal frequencies of BCl₃ and BBr₃. A fair agreement was obtained between the observed separation of the doublet and hte calculated one. The observed intensity is also compatible with the assumption.