Theoretical studies of the geometry and vibrations of molecular adsorbates: Acetylene

Abstract

Variations in the geometry and molecular vibrations of the acetylene molecule considered as an adsorbate species on a metal surface have been studied using ab-initio molecular orbital calculations. Introductory calculations at the 3-21G level of approximation involved Li 4 clusters representing (100) and (111) metal surfaces and a single C 2 H 2 molecule in different orientations parallel to the respective surfaces. The optimized molecular geometry representing a partial optimization of the total Li 4 C 2 H 2 system was calculated as a function of its distance from the metal surface. At the minimum energy representing the strongly chemisorbed species the C 2 H 2 molecule shows rehybridization to around sp 2.3 corresponding to a C−C bond length of 1.39±0.01 Å and CCH bond angles of 117°±2°. The theoretical C 2 H 2 vibration frequencies are consistent with the geometry changes associated with chemisorption and agree well with experimental data obtained from the high resolution electron energy loss spectroscopy of C 2 H 2 on Cu(100) and Cu(111). The mode of bonding to the metal surface is calculated as being mainly of di-σ type with some π-type contribution. The preferred orientation of the C 2 H 2 unit is a double-bridge site on (100) and a long-bridge site on (111). The calculations describe shifts in molecular orbital energy levels which agree with the data from photoionization studies of C 2 H 2 on Cu(100).