Infrared investigation of hydrogen bonding on the surface of beryllium fluoride glass

Abstract

Infrared absorbance spectra were obtained from a dry BeF2 glass surface, and from the same surface after exposure to moist air. No absorbance resulted from the dry surface, but an intense, broad, structured contour having maximum absorbance near 3170±20 cm−1 was produced by the wetted surface. Continuous evaporation for 905 days at 15 mTorr and room temperature was then carried out, as the integrated absorbance of the 3170 cm−1 contour was measured relative to the intense, sharp 3690±5 cm−1 absorbance from internal Be–OH groups. The 3170 cm−1 contour is thought to arise from the combined stretching vibrations of surface Be–OH⋅⋅⋅F−⋅⋅⋅H–OH+2(H2O)2(H2O)n units. In these, the primary hydration sphere of H3O+ contains two H2O molecules and one F− ion, all strongly bound, and the second hydration sphere contains up to six H2O molecules, n≤6. Upon evacuation, the second-sphere H2O molecules are removed consecutively, but the two primary-sphere H2O molecules, and the very strong F−⋅⋅⋅H hydrogen bond are unaffected. The relative integrated 3170 cm−1 absorbance AT was observed to decrease at a decreasing rate by a factor of about 2.5 after 50 days of pumping, and then to decline to ∼20% of the t=0 absorbance, upon pumping for 905 days. The temporal behavior of A was found to fit first-order kinetics of the form −(dA/dt)=[0.244 045/(1+t)]A. Here, [0.244 045/(1+t)]=B exp(−∼(ΔH)/RT), where B=9.65×105 d−1 at 298.15 K. ∼(ΔH) is the species-dependent, and therefore time-dependent, enthalpy in cal mol−1, and t is the pumping time in days.