Adsorption of CH-acids on magnesia -: An FTIR-spectroscopic study

C-H acids including monofluoro-, trifluoro- and trichloromethane, as well as acetylene and methylacetylene were tested as probe molecules for basic sites on MgO by FTIR-spectroscopy. Adsorption structures, frequency shifts, and the applicability of various probes are discussed. Acetylene and methylacetylene undergo H-bonding interactions with Lewis-basic sites on the MgO surface. The experimental frequency shift of the CH-stretching modes can be taken as a measure of the strength of the basic sites. Two adsorption complexes are observed for trichloromethane (CHCl3 and CDCl3) with the CH- (CD-) mode being shifted to lower wavenumbers. A red shift of 36 cm(-1) was obtained for the H-bonded O2- ... H-CCl3-complex. In contrast, the stronger CH-acid CF3H forms an adsorption complex in which hydrogen interacts with the basic oxygen sites and simultaneously one of the fluorine atoms is connected with a Mg2+-site. Adsorbed CH3F on MgO retains the C-3v-symmetry and therefore, the most likely adsorption complex is Mg2+ ... F-CH3. (C) 1999 Elsevier Science B.V. All rights reserved.