X-ray emission spectroscopy and density functional study of CO/Fe(100)

Molecular Bonding of Predissociative CO on Fe(100): Molecular Orbital Perspective

On Fe(100), CO molecules can get activated efficiently so that CO bond breaking occurs with its transition state in close connection to the adsorbate. The CO bonding thus serves as a prototype model, nicely representing a balance of two simultaneous processes, namely, bond making with the surface and bond breaking within the adsorbate. Such unique configuration highlighting the interplay of two fundamental processes in one adsorption geometry, about which chemists have often fantasized, provides a viable solution to understand the very fundamental aspects of chemistry exemplified in a broad range of disciplines. Using density functional theory calculations, in this paper, we get a glimpse into how the CO bond activation is gestated and initiated in the adsorbate, wherein orbital cooperation in CO activation is evidenced by external CO bond making with the metal and internal CO bond breaking. We find that the symmetry breaking of occupied molecular orbitals in both 5σ and 1π symmetries marks efficient CO bond activation, which is reinforced by 1π → 2π excitations and 2π backdonation that are coupled with the symmetry transition of partially occupied 2π orbitals to a rotational symmetry. Our findings promote our knowledge of CO bond activation beyond the established picture of 5σ donation and 2π backdonation without symmetry breaking and may have insightful implications on orbital control of molecular activation, with further possible impact on elucidating the physical basis of heterogeneous catalysis.

Micro-Kinetic Modelling of CO-TPD from Fe(100)—Incorporating Lateral Interactions

The experimentally determined temperature programmed desorption profile of CO from Fe(100) is characterized by four maxima, i.e., α1-CO, α2-CO, α3-CO, and β-CO (see e.g., Moon et al., Surf. Sci. 1985, 163, 215). The CO-TPD profile is modeled using mean-field techniques and kinetic Monte Carlo to show the importance of lateral interactions in the appearance of the CO-TPD-profile. The inclusion of lateral interactions results in the appearance of a new maximum in the simulated CO-TPD profile if modeled using the mean-field, quasi-chemical approach or kinetic Monte Carlo. It is argued that α2-CO may thus originate from lateral interactions rather than a differently bound CO on Fe(100). A detailed sensitivity analysis of the effect of the strength of the lateral interactions between the species involved (CO, C, and O), and the choice of the transition state, which affects the activation energy for CO dissociation, and the energy barrier for diffusion on the CO-TPD profile is presented.

Quantum Chemical Calculations of X-ray Emission Spectroscopy

The calculation of X-ray emission spectroscopy with equation of motion coupled cluster theory (EOM-CCSD), time-dependent density functional theory (TDDFT), and resolution of the identity single excitation configuration interaction with second-order perturbation theory (RI-CIS(D)) is studied. These methods can be applied to calculate X-ray emission transitions by using a reference determinant with a core-hole, and they provide a convenient approach to compute the X-ray emission spectroscopy of large systems since all of the required states can be obtained within a single calculation, removing the need to perform a separate calculation for each state. For all of the methods, basis sets with the inclusion of additional basis functions to describe core orbitals are necessary, particularly when studying transitions involving the 1s orbitals of heavier nuclei. EOM-CCSD predicts accurate transition energies when compared with experiment; however, its application to larger systems is restricted by its computational cost and difficulty in converging the CCSD equations for a core-hole reference determinant, which become increasing problematic as the size of the system studied increases. While RI-CIS(D) gives accurate transition energies for small molecules containing first row nuclei, its application to larger systems is limited by the CIS states providing a poor zeroth-order reference for perturbation theory which leads to very large errors in the computed transition energies for some states. TDDFT with standard exchange-correlation functionals predicts transition energies that are much larger than experiment. Optimization of a hybrid and short-range corrected functional to predict the X-ray emission transitions results in much closer agreement with EOM-CCSD. The most accurate exchange-correlation functional identified is a modified B3LYP hybrid functional with 66% Hartree-Fock exchange, denoted B(66)LYP, which predicts X-ray emission spectra for a range of molecules including fluorobenzene, nitrobenzene, acetone, dimethyl sulfoxide, and CF3Cl in good agreement with experiment.