The covalent binding of acetyl cyanide on Si(100)-2 x 1 has been investigated using high-resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. The absence of the C=O and C equivalent to N stretching modes in the EELS spectra for chemisorbed molecules indicates the direct interactions between carbonyl, cyano groups and the dangling bonds on Si(100)-2 x 1. The characteristic C=C=N asymmetric stretching mode at 2031 cm(-1) and C=N stretching mode at 1667 cm(-1) suggest the existence of both ketenimine species and tetra-sigma adduct at the interface, further supported by XPS results and DFT calculations. (c) 2005 Elsevier B.V. All rights reserved.