ROTATIONAL SPECTRA AND STRUCTURE OF DIBCRANE-HYDROGEN HALIDE DIMERS
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Date
1986
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Ohio State University
Abstract
Rotational spectra of $B_{2}H_{6}-H/DX$ 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 $B_{2}H_{6}-H/DF$ 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 $BH_{2}$ group. The B, C, $D_{J}$, and $D_{JK}$ rotational constants for $^{11}B_{2}H_{6}-HF$ 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 $^{11}B_{2}H_{6}-HF$ and $^{10}B^{11}BH_{6}-HF$ shows that the outer boron nucleus in the dimer has a very small quadrupole coupling ($|x_{aa}|\le 15$ KHz for $^{11}B$) while that for the inner boron is appreciable ($x_{aa}$ of -220 KHz for $^{11}B$). 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 $^{11}B_{2}H_{6}$ and $^{10}B^{11}BH_{6}$ from the rotational constants for the HF dimers. With allowance for torsional oscillations, the $B\ldots H$ distance is 2.5032 and 2.5038 A in the two species. A somewhat smaller $B\ldots D$ distance of 2.4955 A is estimated for $^{11}B_{2}H_{6}-DF$. The attractive potential between $B_{2}H_{6}$ and HF is estimated from $D_{J}$ to have a well depth 0 of $530 cm^{-1}$, comparable with that reported for $N_{2}-HF$. 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 $BH_{2}$ group. A search is under way for a non-linear $B_{2}H_{6}-HF$ dimer, the possible existence of which is indicated by observation of three weak unassigned lines requiring both $B_{2}H_{6}$ and HF.
Description
Work supported by NSF and PRF. Address: Noyes Chemical Laboratory, University of Illinois, Urbana, IL 61801.
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