CO 1Σ Research Articles

Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics on Ru(0001) Probed by Ultrafast X-Ray Spectroscopy and AbInitio Simulations.

We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from Ru(0001). In contrast to the precursor-mediated desorption mechanism on Ru(0001), the presence of surface oxygen modifies the electronic structure of Ru atoms such that CO desorption occurs predominantly via the direct pathway. This phenomenon is directly observed in an ultrafast pump-probe experiment using a soft x-ray free-electron laser to monitor the dynamic evolution of the valence electronic structure of the surface species. This is supported with the potential of mean force along the CO desorption path obtained from density-functional theory calculations. Charge density distribution and frozen-orbital analysis suggest that the oxygen-induced reduction of the Pauli repulsion, and consequent increase of the dative interaction between the CO 5σ and the charged Ru atom, is the electronic origin of the distinct desorption dynamics. Abinitio molecular dynamics simulations of CO desorption from Ru(0001) and oxygen-coadsorbed Ru(0001) provide further insights into the surface bond-breaking process.

Steric Effect in the Energy Transfer Reaction of Oriented CO (a 3Π, v′ = 0, Ω = 1 and 2) + NO (X 2Π) → NO (A 2Σ+, B 2Π) + CO (X 1Σ+)

The oriented CO (a (3)Π, v' = 0, Ω = 1 and 2) beam has been prepared by using an electric hexapole and applied to the energy transfer reaction of CO (a (3)Π, v' = 0, Ω = 1 and 2) + NO (X (2)Π) → NO (A (2)Σ(+), B (2)Π) + CO (X (1)Σ(+)). The emission spectra of NO (A (2)Σ(+), B(2)Π) have been measured at three orientation configurations (C-end, O-end, random). The shape of the emission spectra (and/or the internal excitation of products) turns out to be insensitive to the molecular orientation. The vibrational distributions of NO (A (2)Σ(+), v' = 0-2) and NO (B (2)Π, v' = 0-2) are determined to be N(v'=0):N(v'=1):N(v'=2) = 1:0.40 ± 0.05:0.10 ± 0.05 and N(v'=0):N(v'=1):N(v'= 2) = 1:0.6 ± 0.1:0.7 ± 0.1, respectively, and the branching ratio γ/β [=NO (A (2)Σ(+))/NO (B (2)Π)] is estimated to be γ/β ∼ 0.3 ± 0.1 by means of spectral simulation. These vibrational distributions of NO (A, B) can be essentially attributed to the product-pair correlations between CO (X, v″) and NO (A (2)Σ(+), v' = 0-2), NO (B (2)Π, v' = 0-2) due to energetic restriction under the vibrational distribution of CO (X, v″) produced from the vertical transition of CO (a (3)Π, v' = 0) → CO (X, v″) in the course of energy transfer. The steric opacity function has been determined at two wavelength regions: 220 < λ < 290 nm [NO (A → X) is dominant]; 320 < λ < 400 nm [NO (B → X) is dominant]. For both channels NO (A (2)Σ(+), B(2)Π), a significant CO (a (3)Π) alignment effect is recognized; the largest reactivity at the sideways direction with the small reactivity at the molecular axis direction is observed. These CO (a (3)Π) alignment effects can be essentially attributed to the steric asymmetry on two sets of molecular orbital overlap, [CO (2π) + NO (6σ (2π))] and [CO (5σ) + NO (1π (2π))]. All experimental observations support the electron exchange mechanism that is operative through the formation of a weakly bound complex OCNO.

Quantum mechanical embedding theory based on a unique embedding potential

We remove the nonuniqueness of the embedding potential that exists in most previous quantum mechanical embedding schemes by letting the environment and embedded region share a common embedding (interaction) potential. To efficiently solve for the embedding potential, an optimized effective potential method is derived. This embedding potential, which eschews use of approximate kinetic energy density functionals, is then used to describe the environment while a correlated wavefunction (CW) treatment of the embedded region is employed. We first demonstrate the accuracy of this new embedded CW (ECW) method by calculating the van der Waals binding energy curve between a hydrogen molecule and a hydrogen chain. We then examine the prototypical adsorption of CO on a metal surface, here the Cu(111) surface. In addition to obtaining proper site ordering (top site most stable) and binding energies within this theory, the ECW exhibits dramatic changes in the p-character of the CO 4σ and 5σ orbitals upon adsorption that agree very well with x-ray emission spectra, providing further validation of the theory. Finally, we generalize our embedding theory to spin-polarized quantum systems and discuss the connection between our theory and partition density functional theory.

ChemInform Abstract: The CO-CO Interactions in Transition-Metal Hexacarbonyls Studied by Penning Ionization Electron Spectroscopy: Cr(CO)6, Mo(CO)6 and W(CO)6.

Penning ionization electron spectra resulting from thermal collisions of He*(2 3S) metastable atoms with gaseous Cr(CO)6, Mo(CO)6, and W(CO)6 were measured to probe the CO–CO interactions. The energy splitting of the CO 5σ‐derived levels [I.P.(8a1g)−I.P.(8t1u)] was determined for the first time. Further, the spectra show that the through‐space interaction among the CO 5σ orbitals not only stabilizes the orbital energy of the CO 5σ‐derived 8a1g molecular orbital, but also gives rise to a diffuse electron distribution. The CO–CO interactions in the transition‐metal (TM) carbonyls were compared to those in the CO overlayer formed on TM surfaces.

Origin of the contrast inversion in the STM image of CO on Cu(1 1 1)

Using an accurate description of the Cu(1 1 1) surface state, its interaction with adsorbed CO, and the effect of the Scanning Tunneling Microscope (STM) tip, the decrease in electronic transmission spectra of the tunneling junction upon CO adsorption is studied. For the clean Cu(1 1 1) surface, the 4 p z components of the surface state dominate the local Density of States (DOS) near the Fermi energy. Their symmetry facilitates overlap with the tip, and they are the dominant channel for the tunneling current. Upon CO adsorption, the 4 p z surface state couples strongly with the CO 5σ highest occupied molecular orbital, creating bonding and antibonding states. This over-coupling locally depletes the DOS near the Fermi energy and attenuates the tunneling current. The enhanced overlap between the molecular orbitals and tip states does not compensate for the density depletion; hence leading to dark spots in the STM image where CO molecules are adsorbed. A 1-dimensional tight-binding model is constructed to illustrate the effect of the coupling between surface and tip, between surface and molecule, and between molecule and tip on the STM image contrast.

Plane-wave pseudopotential density functional theory periodic slab calculations of CO adsorption on Cu 2O(1 1 1) surface

Plane-wave pseudopotential Density Functional Theory (DFT) periodic slab calculations have been performed to investigate carbon monoxide adsorption on the (1 1 1) surface of Cu 2O. Negligible relaxation of this surface was found (surface energy = 0.71 J m −2), consistent with its non-polar nature. The on-top-Cu site of CO adsorption is favoured, with a binding energy of 1.50 eV. After adsorption, the Cu–C bond distance was found to be 1.82 Å and that of C–O was 1.15 Å. From the Density of States (DOS) plots, a distinct stabilization of both the Cu 3d and CO 5σ is observed with predominant back-bonding to CO 2π* from Cu. A red shift of −26 cm −1 in the C–O stretch was found in good agreement with experimental results.

A comparative study of the CO chemisorption on Ti 2O 3(1 0 [formula omitted] 2) and V 2O 3(1 0 [formula omitted] 2) non-polar surfaces

Density functional molecular cluster calculations have been used to investigate the coordination of CO to Lewis acid sites ( L s a) available on Ti 2O 3(1 0 1 ̄ 2) and V 2O 3(1 0 1 ̄ 2) non-polar surfaces. The electronic structure of the clean substrates, the adsorbate geometry and chemisorption enthalpies are computed and discussed. Properties of the clean surfaces are well described by the chosen cluster models. Moreover, the L s a–CO bonding is found to be very similar to that holding for transition metal carbonyls: i.e., a two-way electron flow implying a σ donation from the CO 5σ HOMO into empty L s a AOs, assisted by a π backdonation from L s a occupied orbitals into the CO 2π LUMO. Both the electronic and molecular structure of the adsorbate are significantly perturbed upon chemisorption and, consistently with experimental data, the C–O bond results strongly weakened. The chemisorption enthalpy of CO on V 2O 3(1 0 1 ̄ 2) (∼−30 kcal/mol) is about twice that computed for CO on Ti 2O 3(1 0 1 ̄ 2) (∼−16 kcal/mol).

Spatial electron distribution of CO adsorbed on Ni(100) and Ni(111) surfaces probed by metastable impact electron spectroscopy

Electron emission spectra obtained by thermal collisions of He*(2 3S) metastable atoms with CO on Ni(100) in the c(2×2) structure and on Ni(111) in the c(4×2) structure were measured to probe directly the spatial electron distribution. For a systematic comparison, the metastable spectra of free CO, condensed CO on Ni(111), and gaseous Cr(CO)6 were also measured under the same beam conditions. Our data showed that the relative ionization cross sections for the CO 4σ-, 1π-, and 5σ-derived states depend drastically on the molecular orientation of CO with respect to the metastable beam, reflecting the local electron density of CO in the impact region. Moreover, it was found that the 4σ- and 5σ- derived states of CO at hollow sites on Ni(111) are strongly modified in space by mixing with each other, where considerable charge transfer occurs from the C site to the O site in the 5σ-derived state and in the opposite way in the 4σ-derived state. In contrast, such a strong charge redistribution was not seen in the cases of terminal CO on Ni(100) and Cr(CO)6. These findings were in good accordance with the crystal orbital overlap population obtained by density functional theory through a generalized gradient approximation.

CASSCF study of bonding in NiCO and FeCO

A series of CASSCF calculations were performed on the ground states of NiCO and FeCO. The contributions of the σ/π interactions are checked by examining the validity of the CASSC calculation to describe the molecule with a particular choice of the active space. The calculation results substantiate that the stability of MCO is determined by a balance between π donation from the metal 3dπ to the CO 2π and repulsion between the metal σ electrons and the CO 5σ lone pair and, at the same time, emphasizes the importance of the synergistic σ/π interactions between the metal and the CO group. The relative importance of σ/π interactions depends on the nature of the metal. In the case of NiCO, it is the π donation from Ni 3dπ to CO 2π that makes the largest contribution to the formation of the NiCO bond, while in the case of FeCO, it is the correlation of σ electrons that holds the metal and CO together. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 221–231, 1999

Carbonyl bonding on ZnO(0001), CuCl(111) and Cu 2O(111) 3d 10 metal ion surfaces: photoelectron spectroscopy of CO/Cu 2O(111) and electronic structure calculations

The adsorption of CO on Cu 2O(111) has been investigated using He II UPS. With increasing doses of CO two peaks are observed at 17.8 and 14.3 eV ionization potential which are assigned to the CO 4σ and 1π/5σ levels. The interaction of CO with ZnO(0001), CuCl(111) and Cu 2O(111) surfaces has also been investigated using SCF-Xα Scattered Wave (SW) electronic structure calculations. The SCF-Xα-SW results demonstrate that the bonding of CO to Zn ++ and Cu + sites is dominated by the sigma donor interaction of the CO 5σ level with the empty 4s and 4p, metal ion levels. The sigma donation on Zn ++ was calculated to be one third as large as on the Cu + sites. π-Backbonding is calculated to be present on both Cu + sites but not on ZnO which is consistent with previous experimental data. The net charge on the carbon atom in the bound CO is calculated to be positive and larger for Zn ++ than for CO bound to Cu + sites. The electronic structure origins of the differences in CO bonding among the Zn ++ and Cu + sites are addressed and insight is gained into the contribution of π-backbonding to metal carbonyl reactivity.

Electronic structures in the valence region of chemisorbed and physisorbed species on Pd(110)

Electronic structures of CO, CH 4 and Xe on Pd(110) were studied by means of photoelectron spectroscopy (PES) using synchrotron radiation. In order to enhance the sensitivity for the adsorbate derived states in valence PE spectra, photon energy at the Cooper minimum ( hv = 120–130 eV) was utilized. In the case of the CO saturated Pd(110) surface at 90 K, peaks are observed at ∼ 3.4 eV, 8.0 eV (with a shoulder at ∼ 7.2eV), 11.0 eV and 14.4 eV below the Fermi level ( E F). In addition to the previously reported peaks (1 π and 5σ at ∼ 8 eV and 4σ at ∼ 11 eV), two peaks were newly observed. The 3.4 eV peak could be assigned to the bonding state hybridized between CO 2π and Pd 4d orbitals. The 14.4 eV peak is tentatively assigned to the CO 4σ shake-up satellite. In the ‘physisorption’ systems, i.e. CH 4 and Xe on Pd(110), only the smooth attenuation of photoemission from the Pd 4d-band was observed in the PE spectra.

State-to-state differential cross sections for rotationally inelastic collisions of NO(2Π1/2,j⩽2.5) with CO(1Σ+) and O2(3Σg−) at a kinetic energy of 442 cm−1

Crossed molecular beam measurements of state-resolved differential cross sections for NO+O2 and NO+CO inelastic collisions at a relative kinetic energy of 442 cm−1 are reported. The initial states (NO 2Π1/2, ν=0, j⩽2.5, CO 1Σ+, ν=0, O2 3Σg−, ν=0) were prepared by pulsed supersonic expansions of pure NO, O2, and CO gas. Scattered NO products were detected by resonance enhanced two-photon ionization. Product distributions were measured in both center-of-mass scattering angle and final rotational state (j′). Intensity maxima were found in both types of scans and comparable populations were found in both of the spin–orbit manifolds (2Π1/2 and 2Π3/2). The results are compared to previous inelastic scattering experiments of NO collisions with NO, Ar, CO, and O2.

Orbital mixing in CO chemisorption on transition metal surfaces

The chemisorption of CO on metal surfaces is widely accepted as a model for understanding chemical bonding between molecules and solid surfaces, but is nevertheless still a controversial subject. Ab initio total energy calculations using density functional theory with gradient corrections for CO chemisorption on an extended Pd{110} slab yield good agreement with experimental adsorption energies. Examination of the spatial distribution of individual Bloch states demonstrates that the conventional model for CO chemisorption involving charge donation from CO 5σ states to metal states and back-donation from metal states into CO 2π states is too simplistic, but the computational results provide direct insight into the chemical bonding within the framework of orbital mixing (or hybridisation). The results provide a sound basis for understanding the bonding between molecules and metal surfaces.

A molecular cluster approach to the study of the bonding of CO and NH 3 to a d 10 ion on ZnO(0001) and CuCl(111)

The local-density-functional approximation coupled to the molecular cluster approach is used here to compare the electronic structure of CO and NH 3 molecules chemisorbed on the ZnO(0001) and CuCl(111) polar surfaces. For both substrates the interaction with the adsorbate is strongly dependent on the charge carried by the atom representative of the Lewis acid site. In particular, a realistic description of the surface-adsorbate bonding scheme is only obtained by forcing the partially occupied dangling bonds on ZnO(0001)/CuCl(111) to be empty. The bonding of CO to CuCl(111) looks similar to that present in metal-carbonyl complexes, with a donation from the CO 5σ HOMO into the empty levels of the coordinatively unsaturated Cu surface ions assisted by a significant backdonation from the fully occupied Cu 3d orbitals into the CO 2π * LUMO. At variance to that, the bonding of CO to ZnO(0001) is limited to a donation from the CO HOMO into the empty levels of the surface Zn 2+ ions. The surface-molecule electrostatic interaction, negligible for CuCl(111), plays for ZnO(0001) an important role in determining the relative energy position of CO based MOs. As far as the bonding of NH 3 to CuCl(111) and ZnO(0001) is concerned, it has been found to be characterized in both cases by a donation from the NH 3 3a 1 HOMO into the empty levels of the unsaturated metal sites. Any backbonding from the 3d orbitals of the Lewis acid sites is prevented by the high energy of the NH 3 2e LUMO. Finally, for NH 3 on ZnO(0001), the electrostatic interaction between the permanent NH 3 dipole moment and the high value of the Lewis acid site effective charge plays a leading role in determining the binding energy.

Coordination chemistry of CO and NH 3 on CuCl(111): an experimental and theoretical study of the CO and NH 3 bonding to a d 10 ion

A detailed investigation of the electronic structure of CO chemisorbed on CuCl has been carried out by coupling diffuse reflectance infrared (IR) spectroscopy to the local-density-functional molecular-cluster approach. IR measurements indicate a red-shift of the CO stretching mode on passing from the free molecule to the chemisorbed one, proving a slight weakening of the CO bond upon chemisorption. Theoretical results, obtained for CO on CuCl(111) by using the Cu 22Cl 24CO model cluster, reproduce such a slight bond weakening giving for the CO stretching a red-shift of 38 cm −1 with respect to the free molecule. The bonding of CO to CuCl(111) implies a donation from the CO 5σ HOMO into the empty levels of the coordinatively unsaturated Cu surface ions assisted by a significant backdonation from the fully occupied 3d orbitals of the Lewis acid site into the CO 2π ∗ LUMO. The interaction of NH 3 to CuCl(111) has been also investigated by employing the Cu 22Cl 24NH 3 model cluster. The bonding interaction is here limited to a donation from the NH 3 3a 1 HOMO into the empty levels of the unsaturated Cu sites. Any backbonding from the 3d orbitals of the Lewis acid site is prevented by the high energy of the NH 3 2e LUMO. At variance to NH 3 on ZnO(0001), the low value of the Lewis acid site effective charge makes negligible the electrostatic interaction with the permanent dipole moment of NH 3.

CO on Fe(110): an angle-resolved photoemission study

CO chemisorption on Fe(100) at 80 K was reinvestigated by angle-resolved ultraviolet photoemission spectroscopy with synchrotron radiation. At 0.25 L, the CO 5σ peak and the CO 4σ peak are observed at 7.5 and 10.5 eV below the Fermi level, and they shift to 8.3 and 10.7 eV respectively, as the coverage increases up to 3.0 L. In contrast to previous observations for CO on Fe(100) [Phys. Rev. B 27 (1983) 3338] where the 1π − 5σ separation was considerably larger (~1.5 eV), the 1π and 5σ peaks are not observed separately as usually. Their separation is found to be ~0.5 eV. The lower binding-energy peak previously assigned to CO 1π emission is due to oxygen contamination. The present data also indicate that the orientation of the CO axis undergoes a two-stage transition depending on the amount of CO exposure: inclined (⩽ 0.5 L) → upright (~2 L) → inclined (4 L).

Metal-metal bonding on surfaces: molecular orbital study of Pd/Ti(001) and Pd/Ru(001)

The interaction of a Pd atom with Ti(001), Ru(001) and Pd(111) surfaces has been studied using semiempirical MO-SCF calculations (INDO/1) and cluster models. In addition, the electronic properties of the diatomic PdTi, PdRu and Pd 2 molecules have been examined using ab initio SCF calculations. The results of the calculations indicate that the charge transfer in the Pd-Ti and Pd-Ru bonds is small. For supported Pd monolayers, the Pd-substrate bonds can be described as mainly metallic, with a small degree of ionic character. Adsorption of Pd on an early transition metal induces a reduction in the electron population of the Pd(4d) orbitals by: (1) charge transfer from Pd to the metal substrate, and (2) rehybridization of the Pd(4d,5s,5p) levels. The magnitude of both phenomena increases when the fraction of empty orbitals in the valence band of the metal substrate rises. The Pd(4d) → Pd(5s,5p) electron transfer plays an important role in the strength of the bimetallic bonds: the larger this rehybridization, the stronger the Pd-substrate bond. Electronic perturbations induced by Ti or Ru on Pd reduce the CO-chemisorption ability of Pd by weakening simultaneously the Pd(4d)-CO(2π ∗) and Pd(5s,5p)-CO(5σ) bonding interactions. For adsorption of CO on supported Pd, the π back-donation and strength of the Pd-CO bond decrease when the fraction of empty states in the valence band of the support increases.

A study of FeCO− and the 3Σ− and 5Σ− states of FeCO by negative ion photoelectron spectroscopy

The 488 and 514 nm negative ion photoelectron spectra of FeCO−, obtained at an instrumental resolution of 5 meV (40 cm−1), show vibrationally resolved transitions from the anion ground state to the ground state and a low-lying excited state of the neutral molecule. The ground state of FeCO is assigned as the 3Σ− state and the excited state, lying 1135±25 cm−1 higher in energy, as the 5Σ− state. The fundamental vibrational frequencies are νCO=1950±10, νFeC=530±10, and νbend=330±50 cm−1 in the 3Σ− state, and νCO=1990±15, νFeC=460±15, and νbend=180±60 cm−1 in the 5Σ− state. Principal force constants are estimated from these results. Based on a Franck–Condon analysis of the spectrum and other considerations, the Fe–C bond is determined to be 0.15±0.04 Å shorter, and the C–O bond 0.05±0.02 Å longer, in the 3Σ− state than in the 5Σ− state. These results demonstrate the importance of sdσ hybridization in reducing the σ repulsion between the metal 4s electron and the CO 5σ lone pair, a mechanism that is available only when the electrons in the singly occupied 3dσ and 4s orbitals are singlet coupled as in the 3Σ− state. The FeCO− anion displays a high Fe–C stretching frequency (465±10 cm−1), as well as an asymptotic Fe–CO bond energy, a bending frequency (230±40 cm−1) and equilibrium bond lengths intermediate between those in the 3Σ− and 5Σ− states. Since the FeCO− ground state is assigned as a 4Σ− state in which the extra electron occupies a σ orbital, these results indicate that the increased σ repulsion is partially offset by stronger metal–CO π bonding in the anion. The electron affinity of FeCO is measured to be 1.157±0.005 eV.

CO bonding and vibrational modes on a perfect MgO(001) surface: LCGTO-LDF model cluster investigation

The adsorption of isolated CO molecules on a perfect MgO(001) surface is investigated theoretically by the LCGTO-LDF cluster method. Internuclear distances, adsorption energies as well as frequencies and absolute intensities of the MgOCO and CO vibrational modes are calculated for a number of MgO/CO and MgO/OC model clusters containing 2–18 substrate atoms and a large array of surrounding point charges. The convergence of the computed observables with cluster size is discussed. The LDF results are compared to those of previous HF studies. MgOCO adsorption is found to be mainly due to the electrostatic interaction of the CO molecule with the Madelung field on the surface. A small charge transfer of about 0.1 au from the CO 5σ orbital to the substrate also takes place and, besides Pauli repulsion, contributes to the overall blue shift of the CO vibrational frequency. The cluster models predict an approximate doubling of the CO vibrational intensity upon adsorption. This intensity enhancement derives to a large extent from a change of the π component of the CO dynamical dipole moment due to Pauli repulsion between the adsorbate at the cation atop position and the nearest neighbour surface anions. The calculated change of the CO vibrational intensity is at variance with that obtained in a previous analysis of the observed coverage dependence of the CO frequency. The MgOCO vibrational mode is calculated to have a frequency lower than 200 cm −1 and an absolute intensity of about 0.5 km/mol.

CO interaction with ultrathin MgO films on a Mo(100) surface studied by infrared reflection–absorption spectroscopy, temperature programmed desorption, and x-ray photoelectron spectroscopy

The interaction of CO with MgO ultrathin films grown on a Mo(100) surface is studied using infrared reflection–absorption spectroscopy, temperature programmed thermal desorption (TPD), and x-ray photoelectron spectroscopy. CO adsorbed on 7 ML of MgO shows an infrared band at 2178 cm−1. This blue-shift of the CO stretching frequency relative to that of CO in the gas phase (2143 cm−1) is attributed to electron charge transfer from the CO 5σ orbital to the MgO surface. It is further shown that CO adsorption induces a 0.4 eV shift in the Mg(2p) and O(1s) core levels of the MgO thin films to lower binding energy, consistent with the charge transfer from CO molecules to the MgO thin films. The TPD spectra indicate that CO molecules on the MgO thin films desorb in the 100–180 K sample temperature range. The CO adsorption heat on 7 ML of MgO is deduced to be 9.9 kcal/mol using an isothermal adsorption method.