Microwave spectrum of alkali metal tetrahydroborate. III. Rotational transitions of LiBD4, NaBD4, and KBH4 in the ground vibrational states and molecular structures of MBH4 (M=Li, Na, and K)

Abstract

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

“…Recently we have investigated LiBH 4 (1,2,4), NaBH 4 (2-4), and KBH 4 (2,4) in the gaseous phase by using a millimeter-wave spectrometer and have found that all of these molecules take tridentate structure with the alkali metal to boron distance being much shorter than reported for the crystal phase (5)(6)(7). In addition, we noticed (4) that the Li-B distance (r s ) was shortened by 0.0083 Å when the BH 4 group was replaced by BD 4 ; the bond shortening is normally referred to as the secondary isotope effect.…”

“…In addition, we noticed (4) that the Li-B distance (r s ) was shortened by 0.0083 Å when the BH 4 group was replaced by BD 4 ; the bond shortening is normally referred to as the secondary isotope effect.…”

“…If the bond between the alkali metal and BH 4 is really ionic, we expect eQq of the alkali metal atom to be as small as that in alkali halide, which is a most typical ionic molecule. The eQq value of boron would supply information on how much the structure of the BH 4 group deviates from T d symmetry.…”

“…The eQq value of boron would supply information on how much the structure of the BH 4 group deviates from T d symmetry. If the spectral resolution is high, we might be able to observe splittings due to the internal motion of BH 4 , provided that the barrier height between two tridentate configurations, i.e., the potential barrier at the bidentate configuration, is low enough. This tunneling splitting was calculated by an ab initio method (8) and the tunneling effect on the rotational spectrum was discussed in some detail (9).…”

“…It may be interesting to compare the structures of two ionic molecules, NaBH 4 stants, that both NH 4 Cl (10) and NH 4 Br (11) in the gaseous phase have hydrogenbonded structures NH 3 --HX rather than NH / 4 --X 0 . The observation of hyperfine structure and Stark effect may be most conveniently achieved with Fourier transform microwave spectroscopy, because its high sensitivity allows us to observe low J transitions for which hyperfine splittings are large and its high resolution allows us to resolve small splittings.…”