Structure of chemisorbed 1,3-butadiene on the Cu(111) surface

The adsorption of 1,3-butadiene (BD, C₄H₆) on Cu(111) was studied experimentally with reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) and computationally with density functional theory (DFT). The TPD results show that submonolayer BD desorbs at 217 K for the lowest BD coverages, while 2nd-layer and multilayer BD desorbs in the range of about 180 to 112 K. No carbon was observed with Auger electron spectroscopy (AES) after desorption of BD indicating that it does not dissociate on Cu(111). For multilayer coverages, the RAIRS peaks are close to those of solid BD. Comparing the multilayer RAIR spectra with calculated vibrational spectra of gas-phase BD indicates that the molecules are mostly oriented with their molecular planes parallel to the surface as the most intense peaks are due to out-of-plane bending modes. At low exposures at 85 K and after annealing the multilayer to 130 K, a set of peaks are observed that are distinct from those of gas-phase BD. This indicates that BD interacts with the surface in a way that significantly alters the molecule's internal bonding. Possible adsorption structures for BD on Cu(111) were explored through DFT calculations. A comparison of experimental and simulated RAIR spectra suggests that BD adsorbs as a mixture of s-trans and s-cis di-π, and possibly s-trans tetra-σ, structures. The s-trans and s-cis di-π structures were nearly iso-energetic, despite the s-cis isomer being less stable in the gas phase. An s-trans tetra-σ structure was found to be less stable by 0.2 eV than the di-π structures at 1/6 monolayer and become unstable at a lower coverage.