First high-resolution infrared observation of the symmetry-forbidden nu(5)band of (B2H6)-B-10

Spectra of B-10 monoisotopic diborane, B2H6, have been recorded at high res olution (2-3 X 10(-3) cm(-1)) by means of Fourier transform spectroscopy in the region 700-1300 cm(-1). A thorough analysis of the nu(18) a-type, nu(1 4) c-type, and nu(5) symmetry-forbidden band has been performed. Of particu lar interest are the results concerning the nu(5) symmetry-forbidden band, which is observed only because it borrows intensity through an a-type Corio lis interaction with the very strong nu(18) infrared band located approxima tely 350 cm(-1) higher in wavenumber. The nu(5) band has been observed arou nd 833 cm(-1) and consists of a well-resolved Q branch accompanied by weake r P- and R-branch lines. Very anomalous Line intensities are seen, with the low K-a transitions being vanishingly weak, and Raman-like selection rules observed. The determination of the upper state Hamiltonian constants prove d to be difficult since the corresponding energy levels of each of the band s are strongly perturbed by nearby dark states. To account for these strong localized resonances, it was necessary to introduce the relevant interacti ng terms in the Hamiltonian. As a result the upper state energy levels were calculated satisfactorily, and precise vibrational energies and rotational and coupling constants were determined. In particular the following band c enters were derived: nu(0) (nu(5)) = 832.8496(70) cm(-1), nu(0) (nu(14)) = 977.57843(70) cm(-1), and nu(0) (nu(18)) = 1178.6346(40) cm(-1). (Type A st andard uncertainties (1 sigma) are given in parentheses.) (C) 2000 Academic Press.