Nuclear Quadrupole Spectra of Boron Trichloride and Tribromide

概要

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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 (νI and νII) 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.
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