Dissociation Energy of Boron Monofluoride from Mass-Spectrometric Studies

Abstract

Gaseous equilibria involving BF, BF3, Ca, CaF, and CaF2 were studied by mass spectrometry in the range 1583° to 1734°K. The vapor species were produced in a graphite Knudsen cell containing a mixture of elemental boron and calcium fluoride, and they were identified from their characteristic mass spectra and ionization-efficiency curves. From ion-intensity measurements made at ionizing energies a few volts above threshold, the equilibrium constants and heats of several reactions involving BF (g) were evaluated. These results yield the standard heat of formation −27.5±3 kcal/mole for BF (g) and the dissociation energy, D0°, of 7.85±0.15 eV. The thermochemical dissociation energy of BF is significantly larger than a spectroscopic value obtained from linear extrapolation of the ground-state vibrational levels, but it agrees well with a value taken from a short extrapolation of the vibrational levels in the excited 1π state.