TUNNELING IN A LINEAR B2H6-HCL DIMER

Abstract

Rotational spectra have been observed for eight isotopic species of the diborane-HCl complex with a Balle-Flygare, pulsed nozzle, Fourier transform microwave spectrometer. The dimer has a linear, or at most slightly bent B-B...H/DCl equilibrium structure with the H/D end of the HCl attracted symmetrically to a terminal BH2 group of the diborane. Three B2H6-HCl species homonuclear in the boron were observed to tunnel while those with (BBH6)-B-10-B-11 or DCl did not. The tunneling splits each rotational transition into two components of comparable intensity, separated by several MHz depending on J and K. The a-dipole transitions are characteristic of a prolate, very near symmetric top; only K = 0 and +/- 1 transitions were found. Rotational constants are reported for all species. The BBAR, D(J), H, (B-C), and D(JK) constants determined for (B2H6)-B-11-(HCl)-Cl-35 are for the A1 tunneling state 1273.364(1) MHz, 5.56(5) kHz, 1.0(8) Hz, 5.3(2) MHz, and -2.1(3) MHz; for the A2 tunneling state 1273.856(1) MHz, 11.64(7) kHz, 33.1(9) Hz, 5.70(4) MHz, and -3.21(5) MHz. The chlorine hyperfine structure gives the average torsional displacement of the H/DCl from the a axis to be 26.3-degrees for the HCl complexes and 22.5-degrees for the DCl. The torsional displacement of the B2H6 was found by an inertial analysis of the complexes with HCl. It is very anisotropic, being close to 0-degrees in the ethylene plane and about 9-degrees in the BH2B bridging plane. It is suggested that in the tunneling the B2H6 reorients by 180-degrees in the bridging plane, coupled with a gear-like counter rotation of the HCl by 360-degrees. The B...H distance is 2.693 angstrom in the dimers with HCl and the B...D distance is slightly longer, 2.702 angstrom.