Variations in carbonyl mode intensities in inelastic electron tunneling and multiple reflection absorption infrared spectroscopy Part II: effect of internal carbonyl group location for selected aliphatic di -carboxylic acids on alumina

Abstract In this, the second in a series of three articles, inelastic electron tunneling (IET), and Fourier transform MRAIR spectra are obtained for a series of aliphatic di-carboxylic acids and oxo-substituted aliphatic di-carboxylic acids. The acids are chosen in order to investigate the effect of the internal carbonyl group location within the molecule itself on spectral intensities. The oxo-substituted di-carboxylic acids produce a carbonyl absorption in the region 1700–1760 cm−1 for IET spectra. These are attributed to the internal carbonyl group present in these molecules, since this vibration is absent in the corresponding spectra for the non-oxo-substituted di-carboxylic acids. A comparison of MRAIR spectra for samples prepared with and without a lead cover film shows that both carbonyl, and C–H mode intensities for the MRAIR samples are attenuated to various degrees by the presence of a lead cover film depending on their location within the molecules. This indicates steric hindrance of these groups by the lead cover film, which causes the molecular backbone to be canted at the alumina surface. Evidence for this is obtained from the unusually high intensity of the asymmetric carboxylate ion stretch in the MRAIR spectra for samples with a lead cover film.