ALBERT

Notes: The rotational spectrum of NaBH4 was observed in the millimeter-wave region using a high temperature absorption cell. The observed spectrum of NaBH4 showed the pattern of a symmetric top molecule: Strong and weak for K=3n and 3n±1, respectively, because of the nuclear spin statistical weight for C3v symmetry. The rotational and centrifugal distortion constants for the 11B and 10B species were determined. The observed rotational constants of Na11BH4 and Na10BH4, combined with the assumption that r(B–Hb)−r(B−Ht)=0.04 A(ring) and θ (Hb–B–Ht)=111°, gave estimates for r(Na–B) and r(B–Hb) to be 2.308±0.006 A(ring) and 1.28±0.10 A(ring), respectively, where the uncertainties are mainly due to those of the assumed values. This bond length obtained for Na–B is much shorter than the reported value in crystal: 3.08 A(ring). The bond lengths derived indicate that NaBH4 has a tridentate molecular structure with three bridging hydrogens. This result agrees with those of ab initio calculations.

Notes: The rotational spectrum of LiBH4 was observed in the millimeter-wave region using a high- temperature absorption cell. The observed spectrum of LiBH4 showed the pattern of a symmetric top molecule: strong and weak for K=3n and 3n±1, respectively, because of the nuclear spin statistical weight for C3v symmetry. The rotational constants and centrifugal distortion constants for 7Li11BH4, 7Li10BH4, 6Li11BH4, and 6Li10BH4 species were determined. Four observed rotational constants gave rs (Li–B) to be 1.939 38±0.000 10 A(ring), which, combined with the assumption that θ(Hb–B–Ht)=113 °, led to r(B–Hb) and r(B–Ht) to be 1.256±0.015 A(ring) and 1.216(minus-plus)0.015 A(ring), respectively, where the signs should be taken in the stated order. The uncertainties of r(B–Hb) and r(B–Ht) are mainly due to that of θ (Hb–B–Ht), which is estimated to be (minus-plus)1.5 °. This bond length obtained for Li–B is much shorter than the reported value in crystal; 2.47 A(ring). The bond lengths derived indicate that LiBH4 has a tridentate molecular structure with three bridging hydrogen atoms as in the case of NaBH4, in agreement with predictions by ab initio calculations. Two sets of vibrational satellites were observed and analyzed for both 7Li11BH4 and 7Li10BH4. These satellites were assigned to the nondegenerate Li–B stretching and the doubly-degenerate Li–B–H bending mode.

Notes: The microwave spectrum of BH2Cl existing as an intermediate in the reaction between diborane and boron trichloride was observed. The lifetime of this molecule in a brass waveguide cell was found to be less than 1 s. The observed spectrum indicated the molecule to be planar with C2v symmetry. The rotational constants were determined for 11BH2 35Cl, 11BH2 37Cl, 10BH2 35Cl, and 11BD2 35Cl, and were employed to calculate the structure parameters (rs):r(B–H)=1.1916(2) A(ring), r(B–Cl)=1.7353(5) A(ring), and (angle)HBH=124.22(3)°. The quadrupole coupling constants of Cl, an extraordinarily small χzz and a large η compared to related molecules, were obtained. The dipole moment was determined to be μa=0.75 (5) D for BH2Cl.

Notes: 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) A(ring), 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 A(ring) and 1.212(minus-plus)0.032 A(ring), respectively, where the uncertainties of the B–D distances are primarily due to that of θ(Db–B–Dt) estimated to be (minus-plus)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) A(ring).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(minus-plus)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) A(ring). The two B–H distances were obtained to be r(B–Hb)=1.257±0.025 A(ring) and r(B–Ht)=1.218(minus-plus)0.032 A(ring). The data on NaBD4 were analyzed by assuming θ(Db–B–Dt)=111.0(minus-plus)1.0° and r(B–Db)−r(B–Dt)=0.04(I) or 0.03(II) A(ring), yielding r(Na–B)=2.2978(I) or 2.2987(II)±0.0055 A(ring) and r(B–Db)=1.269(I) or 1.258(II)(minus-plus)0.040 A(ring). 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(minus-plus)1.0° and on the difference r(B–Hb or Db)–r(B–Ht or Dt)=0.04(I) or 0.03(II) A(ring) lead to the following results: r(Na–B)=2.3075(I) or 2.3080(II)±0.0028 A(ring), r(Na–B) (in NaBH4)–r(Na–B) (in NaBD4)=0.0097(I) or 0.0092(II)(minus-plus)0.0028 A(ring), r(B–Hb)=1.278(I) or 1.267(II)(minus-plus)0.040 A(ring), and δ=0.0086(minus-plus)0.0001 A(ring) for both I and II. The rs(K–B) distance was obtained from the three observed rotational constants to be 2.656 41(20) A(ring), which, combined with the assumption of θ(Hb–B–Ht)=110.8(minus-plus)1.0°, led to r(B–Hb)=1.272±0.030 A(ring) and r(B–Ht)=1.233(minus-plus)0.030 A(ring). © 1995 American Institute of Physics.

Notes: The infrared spectrum of a transient species HBNH was observed in the gas phase using a diode laser spectrometer. The HBNH molecule is formed in an ac discharge plasma of a diborane/ammonia or diborane/nitric oxide mixture. The ν3 band origin and the rotational constants of H11BNH were determined to be ν0=1786.193 08(72), B0=1.099 335(81), and B3=1.093 481(72) cm−1, with three standard deviations in parentheses. The ν3 band of H10BNH was also observed and analyzed.

Notes: The FBO molecule was generated by a dc discharge in a mixture of BF3, O2, and He and its vibration–rotation and rotation spectra were observed by using infrared diode laser spectroscopy and microwave spectroscopy, respectively. The ν1 B–O stretching mode of F11BO and F10BO and the ν1+ν2−ν2 hot band of F11BO were observed in the region 2050 to 2140 cm−1. Subsequently, the pure rotational spectrum was observed for the following isotopic species: F11BO, F10BO, F11B18O, and F10B18O. The rotational, centrifugal distortion, and l-type doubling constants were determined from the observed spectra, and the partial rs structure of FBO was calculated from rs coordinates and the first moment equation to be r(F–B)=1.2833(7) A(ring) and r(B–O)=1.2072(7) A(ring).