Knowledge Bank

Rotational spectra of $B2H6-H/DX$ complexes are being studied using a Flygare Fourier transform microwave spectrometer with a pulsed supersonic nozzle as the molecular source. The most extensive results are for several isotopic species of a $B2H6-H/DF$ dimer, observed at 4-18 GHz. The dimer is a near symmetric prolate top $(k=-0.9995)$: it has a novel, linear BB-H/DF equilibrium structure with the H/D end of the H/DF attracted axially (a-axis) to a terminal $BH2$ group. The B, C, $DJ$, and $DJK$ rotational constants for $\mathrm{<mrow\; data-mjx-texclass="ORD">11</mrow>}B2H6-HF$ are 2 111.601(1) and 2 091.308(1) MHz, and 5.83(3) and 46.1(5) kHz. The spectra are insensitive to the value of A. The hyperfine structure of the $J=0-1$ transitions for $\mathrm{<mrow\; data-mjx-texclass="ORD">11</mrow>}B2H6-HF$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">10</mrow>}B11BH6-HF$ shows that the outer boron nucleus in the dimer has a very small quadrupole coupling ($|xaa|\le 15$ KHz for $\mathrm{<mrow\; data-mjx-texclass="ORD">11</mrow>}B$) while that for the inner boron is appreciable ($xaa$ of -220 KHz for $\mathrm{<mrow\; data-mjx-texclass="ORD">11</mrow>}B$). The implications of these results are considered. The hyperfine structure also gives an average torsional amplitude for the HF and DF of $21\circ$ and $25\circ$ with respect to the a axis. In-plane torsional amplitudes of $13.5\circ$ were determined for the $\mathrm{<mrow\; data-mjx-texclass="ORD">11</mrow>}B2H6$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">10</mrow>}B11BH6$ from the rotational constants for the HF dimers. With allowance for torsional oscillations, the $B\dots H$ distance is 2.5032 and 2.5038 A in the two species. A somewhat smaller $B\dots D$ distance of 2.4955 A is estimated for $\mathrm{<mrow\; data-mjx-texclass="ORD">11</mrow>}B2H6-DF$. The attractive potential between $B2H6$ and HF is estimated from $DJ$ to have a well depth 0 of $530cm-1$, comparable with that reported for $N2-HF$. Rotational transitions have also been identified for the HCl analogue of the ``linear'' complex. A study of them is being made to determine from their hyperfine structure whether the torsional oscillations of the HX are affected by the anisotropy of the terminal $BH2$ group. A search is under way for a non-linear $B2H6-HF$ dimer, the possible existence of which is indicated by observation of three weak unassigned lines requiring both $B2H6$ and HF.