Studies of unoccupied molecular orbitals of the B-O bond by molecular orbital calculations, X-ray absorption near edge, electron transmission, and NMR spectroscopy

Journal Article · · Am. Mineral.; (United States)

OSTI ID:6913807

Tossell, J A

The nature of the unoccupied molecular orbitals (MOs) associated with the B-O bond in gas-phase molecules and solids is determined by performing molecular orbital calculations using the Multiple Scattering X..cap alpha.. molecular orbital method and by comparison of the calculated properties with those obtained from X-ray absorption near-edge spectroscopy (XANES) and electron-transmission spectroscopy (ETS). For three-coordinate B, the lowest-energy unoccupied MO is essentially a nonbonding or weakly antibonding orbital of a/sub 2/ symmetry. In B(OH)/sub 3/, this orbital is less stable than its analogue in the isoelectronic gas-phase molecule BF/sub 3/ by about 3 eV, and its energy changes only slowly with B-O distance. It generates an absorption about 4 eV below the B1s IP in XANES and a scattering resonance about 4 eV above threshold in ETS. It is the final state for the lowest-energy UV absorptions of the BO3-/3 group, and excitations to it from the e' B-O bonding MO dominate the paramagnetic contribution to the /sup 11/B nuclear magnetic resonance (NMR) chemical shift. The only other unoccupied orbital observed for three-coordinate B is the e' B-O antibonding orbital that appears above threshold in both XANES and ETS. The energy of this e' antibonding orbital is highly distance dependent and thus may provide useful information on B-O distance in amorphous materials. For four-coordinate B, the lowest-energy unoccupied orbitals lie just above threshold in XANES, giving a qualitatively different spectrum than that of the three-coordinate borates. Parallels are drawn between the properties of the solid borates and those of isoelectronic gas-phase analogues like BF/sub 3/ and CF/sub 4/. The experimental ETS is reported for B(OCH/sub 3/)/sub 3/, a gas-phase analogue of the BO3-/3 group.

• Research Organization: Univ. of Maryland, College Park
• OSTI ID: 6913807
• Journal Information: Am. Mineral.; (United States), Vol. 71:9-10
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
BORON OXIDES
STRUCTURAL CHEMICAL ANALYSIS
BORATES
CHEMICAL BONDS
ELECTRON SPECTROSCOPY
NUCLEAR MAGNETIC RESONANCE
X-RAY SPECTROSCOPY
BORON COMPOUNDS
CHALCOGENIDES
MAGNETIC RESONANCE
OXIDES
OXYGEN COMPOUNDS
RESONANCE
SPECTROSCOPY
580300* - Mineralogy
Petrology
& Rock Mechanics- (-1989)