Fourier Transform Microwave Measurements of the Quadrupole Hyperfine Structure of NaBH4and KBH4

The lowest rotational transitions J = 1 ← 0 of NaBH 4 and KBH 4 were observed, using a Fourier transform microwave spectrometer combined with a laser ablation apparatus. Hyperfine structure has been observed at good resolution yielding precise values of the nuclear quadrupole coupling constants eQq for both the alkali metal and boron atoms. The eQq values of the alkali metals, −3.385(31) and −4.256(29) MHz for M = 23 Na and 39 K in M 11 BH 4 , respectively, are close to those in Na 35 Cl (−5.6698 MHz) and K 35 Cl (−5.656 MHz), indicating that NaBH 4 and KBH 4 are also ionic molecules. The boron eQq ’s are small, −0.670(53) and −0.722(59) MHz for Na 11 BH 4 and K 11 BH 4 , respectively, much smaller in magnitude than those reported for known boron-containing compounds, indicating that the BH 4 group retains a structure of approximate T d symmetry.