Rotational spectrum and structure of a linear B2H6–H/DF dimer

Abstract

Rotational spectra have been observed at 4–18 GHz for several isotopic species of a diborane–hydrogen fluoride complex, using a Flygare, Fourier transform microwave spectrometer with a pulsed supersonic nozzle as the molecular source. The dimer is a near symmetric prolate top (κ=−0.9995); it has a linear BB–H/DF equilibrium structure with the H/D end of the H/DF attracted axially to one of the diborane’s terminal BH2 groups. The B, C, DJ, and DJK rotational constants are: for 11B2H6–HF, 2111.601(1) and 2091.308(1) MHz, and 5.83(3) and 46.1(5) kHz; for 11B2H6–DF, 2092.991(2) and 2072.772(2) MHz, and 5.34(13) and 56.6(11) kHz; and for 10B11BH6–HF, 2176.086(1) and 2154.566(1) MHz, and 6.28(3) and 56.1(5) kHz, respectively. The spectra are insensitive to the value of A, which is assumed to be that of diborane itself (79.6 GHz). The hyperfine structure of the J=0→1 transitions for 11B2H6–HF and 10B11BH6–HF shows that the outer boron nucleus in the dimer has a very small quadrupole coupling constant (‖χaa‖ ≲15 kHz for 11B) while that for the inner boron is appreciable (χaa ∼−220 kHz for 11B). The implications of these results are considered. The hyperfine structure also gives an average torsional amplitude for the HF of 27° with respect to the a axis. In-plane torsional amplitudes of 13.5° were determined for the 11B2H6 and 10B11BH6 from the rotational constants for the HF dimers. With allowance for the torsional oscillations, the B⋅⋅⋅H distance is 2.5032 and 2.5038 Å in the dimers. A somewhat smaller B⋅⋅⋅D distance of 2.4955 Å is estimated for 11B2H6–DF. The attractive potential between B2H6 and HF is estimated from DJ to have a well depth ε of ∼530 cm−1, which is comparable with that reported for the N2–HF dimer. The novel structure of the B2H6–H/DF complex is discussed.