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

In order to determine the molecular structure of alkali metal tetrahydroborates in a systematic way, we have extended the measurements of rotational transitions to LiBD4, NaBD4, and KBH4 in the ground vibrational states. The observed spectra, which all conformed well to the pattern expected for a symmetric top molecule, yielded rotational and centrifugal distortion constants for 7Li11BD4, 7Li10BD4, 6Li11BD4, 6Li10BD4, Na11BD4, Na10BD4, 39K11BH4, 39K10BH4, and 41K11BH4. The four observed rotational constants of LiBD4 gave rs(Li–B) to be 1.931 09(14) Å, which, when combined with an assumption that θ(Db–B–Dt)=113.0°, led to r(B–Db) and r(B–Dt) to be 1.250±0.025 Å and 1.212∓0.032 Å, respectively, where the uncertainties of the B–D distances are primarily due to that of θ(Db–B–Dt) estimated to be ∓1.0°. [The suffixes attached to the deuterium atom, b and t, denote bridge and terminal, respectively.] The rs(Li–B) distance in LiBD4 is significantly shorter than that in LiBH4, 1.939 38(10) Å. This large secondary isotope effect is ascribed partly to the large amplitude rocking (or internal rotation) motion of the BH4 group. By fixing the Li–B distance to the respective rs values, all of the eight rotational constants of LiBH4 and LiBD4 were simultaneously analyzed to determine the structure of the BH4 group, where the isotope effect was taken into account only for the B–H distances in a form δ=r(B–H)–r(B–D). Again θ(Hb–B–Ht)=θ(Db–B–Dt) had to be fixed to 113.0∓1.0°. The isotope shift δ was found to be not very dependent on the value of θ and was determined to be 0.006 26(6) Å. The two B–H distances were obtained to be r(B–Hb)=1.257±0.025 Å and r(B–Ht)=1.218∓0.032 Å. The data on NaBD4 were analyzed by assuming θ(Db–B–Dt)=111.0∓1.0° and r(B–Db)−r(B–Dt)=0.04(I) or 0.03(II) Å, yielding r(Na–B)=2.2978(I) or 2.2987(II)±0.0055 Å and r(B–Db)=1.269(I) or 1.258(II)∓0.040 Å. An alternative way of the analysis is to combine the data on the H and D isotopic species; the assumptions on θ(Hb–B–Ht)=θ(Db–B–Dt)=111.0∓1.0° and on the difference r(B–Hb or Db)–r(B–Ht or Dt)=0.04(I) or 0.03(II) Å lead to the following results: r(Na–B)=2.3075(I) or 2.3080(II)±0.0028 Å, r(Na–B) (in NaBH4)–r(Na–B) (in NaBD4)=0.0097(I) or 0.0092(II)∓0.0028 Å, r(B–Hb)=1.278(I) or 1.267(II)∓0.040 Å, and δ=0.0086∓0.0001 Å for both I and II. The rs(K–B) distance was obtained from the three observed rotational constants to be 2.656 41(20) Å, which, combined with the assumption of θ(Hb–B–Ht)=110.8∓1.0°, led to r(B–Hb)=1.272±0.030 Å and r(B–Ht)=1.233∓0.030 Å.