CO Vibrational Frequency Research Articles

?Vibrational frequencies of CO adsorbed on silica-supported Mo atoms from density functional calculations?: a re-interpretation of results reported by N. L�pez, F. Illas, G. Pacchioni

?Vibrational frequencies of CO adsorbed on silica-supported Mo atoms from density functional calculations?: a re-interpretation of results reported by N. L�pez, F. Illas, G. Pacchioni

Local reactivity descriptors to predict the strength of Lewis acid sites in alkali cation-exchanged zeolites

Lewis acidity of alkali cation-exchanged zeolite is studied using local reactivity descriptors based on hard-soft acid–base (HSAB) concept. The local softness for nucleophilic attack ( s x +), local softness for electrophilic attack ( s x −) and their ratio, which is called `relative electrophilicity' ( s x +/ s x −), are calculated for the exchanged cations and Lewis acidity of the cations is found to decrease in the order: Li + > Na + > K + > Rb + > Cs +. Calculated blue shift of CO vibrational frequency (Δ ν) and interaction energy of CO molecule with alkali cation-exchanged zeolite clusters vary linearly with s x +/ s x − values.

Surface Reactivity of Pd Nanoparticles Supported on Polycrystalline Substrates As Compared to Thin Film Model Catalysts: Infrared Study of CO Adsorption

To identify the nature and the local structure of the surface of supported catalyst nanoparticles, we have performed a detailed comparative study of CO adsorption on two categories of oxide-supported Palladium catalysts: (1) polycrystalline MgO and γ-Al2O3 supported Pd metal catalysts prepared by impregnation techniques and characterized by different degrees of regularity and perfection and (2) single-crystal based Pd model catalysts prepared under ultrahigh vacuum (UHV) conditions. The assignment of the CO vibrational frequencies to different types of sites on these systems has allowed a detailed structural characterization. For the Pd model catalyst, at low CO coverage, the infrared (IR) reflection absorption spectra closely resemble the expected behavior for terminations by a majority of (111) facets and a minority of (100) facets. The spectral features are indicative of defect sites such as particle steps and edges. Occupation of the defect sites can be affected by surface contaminations such as atom...

The influence of metal cluster size on adsorption energies: CO adsorbed on Au clusters supported on TiO2.

Infrared reflection absorption spectroscopy (IRAS) has been used to study CO adsorption on Au clusters ranging in size from 1.8 to 3.1 nm, supported on TiO(2). The adsorbed CO vibrational frequency blue-shifts slightly (approximately 4 cm(-)(1)) compared to that adsorbed on bulk Au, whereas the heats of adsorption (-DeltaH(ads)) increase sharply with decreasing cluster size, from 12.5 to 18.3 kcal/mol.

Theoretical study of the chemisorption of CO on bimetallic RhCu surfaces and nanoparticles

The CO interaction with bimetallic RhCu surface models representing several compositions has been studied by first principles density functional theory calculations. The analysis of the bare bimetallic clusters Rh(4s) and Cu(3s) core-level binding energies indicates that is not possible to extract information about the oxidation state of the alloy components. The present calculations predict that CO does always sit on top sites, the influence of the alloy composition on the equilibrium geometry and vibrational frequency of CO chemisorbed at a given Rh or Cu site being very small. However, there is a large difference in the structural properties corresponding to CO chemisorption above either Rh or Cu. Therefore, the absolute value of the vibrational frequency of chemisorbed CO does not permit to extract any information about the alloy composition but afford to assign the chemisorption site. Finally, the CO adsorption energy does not follow a monotonic trend with composition. The use of the Constrained Space Orbital Variation analysis permits one to firmly establish that the difference in adsorption energy for different compositions cannot be explained through differences in the σ-donation and π-backdonation mechanisms.

CO vibrational frequencies on methanol synthesis catalysts: a DFT study

Periodic, self-consistent density functional theory calculations are used to explain the observed decrease in the vibrational frequency of CO on methanol synthesis catalysts under severe reducing conditions (N.-Y. Topsøe, H. Topsøe, J. Mol. Catal. A Chem. 141 (1999) 95–105). Vibrational frequencies for CO on eight different models of the methanol synthesis catalyst surface have been determined. The calculated vibrational frequency of CO on Cu(111) (1/9 ML CO coverage) with 1/9 ML of Zn adatoms shows a decrease of between 15 and 38 cm −1 from the corresponding calculated CO frequency on clean Cu(111) (2073 cm −1). The calculated vibrational frequency of CO on Cu(111) with 1/9 ML of ZnO species shows a decrease of up to 72 cm −1 from the CO stretch frequency on clean Cu(111). These calculated CO vibrational frequency decreases agree with the experimentally measured decrease (ca. 50 cm −1), suggesting that Zn and/or ZnO species may be present in the vicinity of active Cu sites of methanol synthesis catalysts under highly reducing conditions. In addition, CO vibrational frequencies on partially oxidized Cu(111) surfaces are shown to increase from the corresponding frequencies on clean Cu(111), in agreement with experimental results.

A new interpretation of the IR bands of supported Rh(I) monocarbonyl complexes.

The characteristic CO vibrational frequency of supported monocarbonyl complexes Rh(I)CO, at 2014 and 1984 cm(-1) on dealuminated Y zeolite and alumina, respectively, is lower than the frequencies of both the symmetric and the antisymmetric CO normal modes of the corresponding stable supported Rh(I) dicarbonyls. The CO mode with a measured frequency between those of the symmetric and antisymmetric CO frequencies of the dicarbonyls, previously assigned to rhodium monocarbonyl, is reassigned to mixed carbonyl dihydrogen complexes Rh(H(2))(CO) or Rh(H)(2)(CO). This reassignment is based on a critical analysis of reported experimental data, supplemented by quantum chemical calculations.

In-situ vibrational spectroscopy on Pt electrocatalysts

CO adsorption on structurally very different electrocatalysts, namely carbon supported Pt catalysts and adsorbed colloids on surfaces (a model system), has been studied. From a comparison of the spectra of CO monolayers on these disparate electrocatalysts, conclusions can be derived regarding the structure of the particles. A correlation between CO vibrational frequency and structural properties, e.g. isolated particles versus agglomerated particles, is found. A dependence of vibrational frequency on the particle coverage is assigned to an increasing agglomeration with increasing Pt-coverage. The spectra of the technical electrocatalysts can be explained in terms of the simpler model systems.

Vibrational frequencies of CO adsorbed on silica supported Mo atoms from density functional calculations

The interaction of CO with silica supported molybdenum atoms has been studied by means of density functional calculations and cluster models. Experimentally two bands in the IR spectra of adsorbed CO have been observed at 2170 and 1990cm−1 with vibrational shifts of +27 and −153cm−1, respectively, with respect to the gas-phase molecule, the peak at +27cm−1 has been related to the presence of neutral Mo atoms anchored to two oxygen atoms of the SiO2 substrate. Possible reactive sites at the Mo/SiO2 interface have been explored as candidates for CO adsorption. Mo atoms in various formal oxidation states, from +II to +VI, have been considered. Both molecular and cluster models of the Mo/SiO2 interface have been employed. The analysis shows that a neutral Mo(II) atom, proposed to be responsible for the blue-shift of ν(CO), is not likely to be the origin of the IR band at 2170cm−1. Only Mo atoms in high oxidation states or Mo cations carrying a real positive charge can account for the positive shifts in the CO frequency.

Vibrational Dephasing of Carbonmonoxy Myoglobin

The autocorrelation function of vibrational frequency fluctuations of the CO ligand in carbonmonoxy myoglobin, Cδδ(t), is computed from molecular dynamics simulations. Electrostatic interactions are assumed to dominate the modulation of the CO vibrational frequency. The simulated Cδδ(t) is consistent with linear and nonlinear infrared spectroscopic measurements. The short-time decay of Cδδ is dominated by dynamics of the distal histidine, and of a water molecule that can occupy the heme pocket. Correlated protein and solvent dynamics induce spectral diffusion of the CO frequency on longer time scales.

Carbonmonoxy horseradish peroxidase as a function of pH and substrate: influence of local electric fields on the optical and infrared spectra.

Infrared and optical spectra of carbonmonoxy horseradish peroxidase were monitored as a function of pH and substrate binding. The analyses of experimental results together with semiempirical calculations show that the CO-porphyrin complex is sensitive to environmental changes. The electronic Q(0,0) band of the porphyrin and the CO stretching mode respond to external perturbations with different symmetry dependencies. In this way, the complex is nonisotropic, and the combined spectral analyses constitute a valuable tool for the investigation of structure. In the absence of substrate and at pH 6.0, the low-spin heme optical Q(0,0) absorption band is a single peak that narrows as the temperature decreases. Under these conditions, the CO vibrational stretch frequency is at 1903 cm(-1). Addition of the substrates benzohydroxamic acid or naphthohydroxamic acid produces a split of approximately 320 cm(-1) in the Q(0,0) absorption band that is clearly evident at < 100 K and shifts the CO absorption to 1916 cm(-1). Increasing the pH to 9.3 also causes a split in the Q(0,0) optical band and elicits a shift in nu(CO) to a higher frequency (1936 cm(-1)). The splitting of the Q(0,0) band and the shifts in the IR spectra are both consistent with changes in the local electric field produced by the proximity of the electronegative carbonyl of the substrate near the heme or the protonation and/or deprotonation of the distal histidine, although other effects are also considered. The larger effect on the Q(0,0) band with substrate at low pH and the shift of nu(CO) at high pH can be rationalized by the directionality of the field and the orientation dependence of dipolar interactions.

Potential-dependent chemisorption of carbon monoxide on platinum electrodes: new insight from quantum-chemical calculations combined with vibrational spectroscopy

Abstract Density functional theory (DFT) using the finite cluster approach is utilized to compute binding energies, bond geometries, and vibrational properties of carbon monoxide adsorbed on Pt(111) as a function of the external interfacial field, focusing attention on the metal–CO bond itself. Comparison with electrode potential-dependent frequencies for the metal–CO ( ν M–CO ) as well as the much-studied intramolecular CO ( ν CO ) vibration, as measured by in-situ Raman and infrared spectroscopy, facilitate their interpretation in terms of metal-chemisorbate bonding for this archetypal electrochemical system. Decomposing the calculated metal–CO binding energy and vibrational frequencies into individual orbital and steric repulsion components enables the role of such quantum-chemical interactions to the field- (and hence potential-) dependent bonding to be assessed. No simple relationship between the field( F )-dependent binding energies and the ν M–CO frequencies is evident. While the DFT ν M–CO – F slopes are negative at positive and small–moderate negative fields, reflecting the prevailing influence of back-donation, a ν M–CO – F maximum is obtained at larger negative fields for atop CO, and a plateau for hollow-site CO. This Stark-tuning behavior reflects largely offsetting field-dependent contributions from π and σ surface bonding, and can also be rationalized on the basis of changes in the electrostatic component of ν M–CO from increasing M–CO charge polarization. A rough correlation between the field-dependent ν M–CO frequencies and the corresponding bond distances, r M–CO , is observed for hollow and atop CO in that r M–CO shortens towards less positive fields, but becomes near-constant at moderate–large negative fields. A more quantitative correlation between the field-dependent CO frequencies and bond lengths is also evident. In harmony with earlier findings (and unlike the ν M–CO – F behavior), the ν CO – F dependence is due chiefly to changes in the back-donation bonding component. The overall vibrational frequency-field behavior predicted by DFT is also in semi-quantitative concordance with experimental potential-dependent spectra.

Geometry and electronic structure of binuclear metal carbonyl cations, [M 2(CO) 2] 2+ and [M 2(CO) 6] 2+ (M=Ni, Pd, Pt)

The geometries and electronic structures of group 10 metal binuclear carbonyl cations of [M 2(CO) 2] 2+ and [M 2(CO) 6] 2+ (M=Ni, Pd, and Pt) were studied using the B3YLP density functional theory. [M 2(CO) 2] 2+ cations have two stable conformations; one is the CO-bridged structure in the D 2 h symmetry for M=Ni, and C 2 v symmetry for Pd and Pt, and the other is the linear structure in the D ∞ h symmetry. The former is more stable than the later for each of M=Ni, Pd, and Pt. The CO-bridged [M 2(CO) 2] 2+ are characterized by σ-donation and π back-donation which lead to longer C–O bond lengths of (R( CO)=1.130–1.137 A ̊ ) and lower CO vibrational frequencies of (ν( CO)=2055–2100 cm −1) than those of free CO [R( CO)=1.128 A ̊ , ν( CO)=2143 cm −1], while the ν(CO) is much higher than the values of the typical bridged metal carbonyls (1750–1850 cm −1). The linear [M 2(CO) 2] 2+ have shorter R(CO)s (1.112–1.115 Å) and higher ν(CO)s (2230–2260 cm −1) than those of free CO owing to the reduced π back-donation. [M 2(CO) 6] 2+ cations have only one stable conformation (minimum) in the D 2 d symmetry, which contains two essentially planar tricarbonyl metal units that are linked via a metal–metal bond about which they are twisted by 90.0° with respect to each other. The R(CO)s (1.115–1.119 Å) are shorter and ν(CO)s (2186–2240 cm −1) are higher than those of free CO. A transition state was found for [M 2(CO) 6] 2+ in which two carbonyls are symmetrically bridging onto two metals and the other four carbonyls are bound to each metal terminally with C 2 v symmetry. The R(CO)s for bridging CO are longer than those for terminal COs. The calculated ν(CO)s for the CO-bridged [Pd 2(CO) 2] 2+ and minimum [Pt 2(CO) 6] 2+ are in good agreement with available data given by IR and Raman spectroscopy for corresponding species. The bonding natures of M–C and C–O bonds are discussed in detail with reference to the molecular orbital energy diagrams, the results of population analysis, and the relativistic effects in metal atoms.

CO Adsorption on Molybdenum Nitride's γ-Mo2N(100) Surface: Formation of NCO Species? A Density Functional Study

To gain insight into the electronic surface properties of γ-Mo2N(100) and the CO chemisorption, density functional calculations have been undertaken on periodic surface models using the generalized gradient approximation for the exchange-correlation functional. Structure relaxation of γ-Mo2N(100) has been performed for different models depending on the surface stoichiometry (Mo/N ratio, mimicking the nitridation effect) or composition (O adatom, mimicking the passivation effect). The reaction γ-Mo2N(100) + X2(g) → γ-Mo2N(100)−X, with X = N and O, is calculated to be exothermic. This suggests that stoichiometric (100) surfaces containing 4-fold vacancy sites are unstable with respect to the dissociative adsorption of N2 and O2 molecules. The chemisorption sites of CO and the associated CO vibrational frequencies have been determined for the Mo2N(100)−X/CO surfaces. On a stoichiometric (100) surface, a μ4-bonding mode of CO is localized on the potential energy surface with the same binding energy as the one...

Photooxidation of Toluene and p-Xylene in Cation-Exchanged Zeolites X, Y, ZSM-5, and Beta: The Role of Zeolite Physicochemical Properties in Product Yield and Selectivity

The photooxidation of toluene and p-xylene with molecular oxygen and visible light has been investigated in several cation-exchanged zeolites. In general, the yield of the photooxidation products, for fixed irradiation time and intensity, was found to correlate with the electric field intensity at the cation sites within the zeolites. On the basis of measurements of CO vibrational frequencies, electric fields of approximately 3−7 V nm-1 are indicated for the cation-exchanged zeolites X, Y, ZSM-5, and Beta used in these studies. These large electric fields are thought to promote photooxidation by stabilizing an intermolecular charge transfer state (R+·O2-) that is formed upon excitation with visible light. The measured electric field was found to correlate with the product yield and was highest in divalent cation-exchanged zeolites with high Si/Al ratios, such as BaZSM-5 and BaBeta. For zeolites containing the same cation, the selectivity of toluene to form benzaldehyde and p-xylene to form p-tolualdehyde ...

Role of the axial ligand in hemeCO backbonding; DFT analysis of vibrational data

The FeCO backbonding correlation, a plot of CO versus FeCO vibrational frequencies, is a useful tool for investigating proximal ligand and distal environmental effects in CO adducts of heme proteins and models. The data fall on parallel lines of negative slope, shifted along the FeCO axis for proximal ligands of different donor strength. However, new resonance Raman spectra of five-coordinate Fe(II)(porphyrins)(CO) with electron-withdrawing or -donating substituents reveal that the backbonding slope is reduced in the absence of a proximal ligand. This phenomenon has been explored with non-local density functional theory on Fe(II)(porphines)(CO) with fluorine, chlorine and amino substituents. The calculations likewise give lines of negative slope, displaced according to trans ligand donor strength. Strikingly, the slopes are nearly the same for pyridine and methylthiolate trans ligands, but halved for five-coordinate complexes, as in the experimental plots. However, the magnitude of the slopes, as well as the displacements of the correlations, are about twice the experimental values. The origin of the backbonding behavior can be seen in the bond-length relationships for the calculated structures. For a given CO length, the FeCO bond is lengthened systematically by increasing the donor strength of the axial ligand, whose sigma donor orbital competes with the CO donor sigma orbital for the vacant Fe d z 2 orbital. For a given proximal ligand, the FeCO and CO bond lengths are negatively correlated, as expected for backbonding. In addition, the slopes increase systematically with increasing ligand donor strength, because the longer FeCO distance decreases the π overlap, and increases the FeCO sensitivity to backbonding changes.

Analysis of electronic contributions to the vibrational frequency of CO/Cu 2O(111)

The change in the CO vibrational frequency after adsorption on low-coordinated copper atoms at the Cu 2O(111) surface has been studied theoretically with gradient-corrected density-functional theory (DFT) methods. The surface was modeled with a Cu 28O 14 cluster embedded in point charges and total-ion model potentials. An analysis of the contributions to changes in the CO stretching mode based on the electron density was performed. We show that the proper description of the Cu 3d→CO 2π ∗ back donation, which is responsible for the experimentally observed small red shift, requires the inclusion of electron correlation in a self-consistent way. In addition to this main physical effect, the absolute value of the frequency shift depends on the form of the exchange functional and on the description of electrostatic interactions in the cluster model.

Ab Initio Cluster Model Study of the Chemisorption of CO on Low-Index Platinum Surfaces

A systematic theoretical study of the adsorption of CO on the Pt{100}, Pt{110}, and Pt{111} surfaces is presented. The calculated equilibrium geometries and vibrational frequencies have been found to be rather independent of the cluster model chosen to represent the surface. However, calculated interaction energies are found to be very sensitive to the surface cluster model. The analysis of the chemisorption bond has been carried out by means of the constrained space orbital variation, CSOV, and of projection operator techniques. These analysis reveal that the bonding interactions are dominated by the π-back-donation although σ-donation plays a significant role. It is also clearly shown that all bonding mechanisms, other than Pauli repulsion, but specially π-back-donation, contribute to the observed red shift. However, the π-back-donation contribution to the red shift is very similar for CO on different sites. Hence, π-back-donation cannot be the mechanism responsible for the observed difference for the CO vibrational frequency on on-top and bridge sites. The CSOV decomposition reveals that the leading term contributing to this difference in vibrational frequency of chemisorbed CO is the initial Pauli repulsion or “wall effect”; this is a new, important and unexpected conclusion.

CO Adsorption as a Probe of Acid Sites and the Electric Field in Alkaline Earth Exchanged Zeolite Beta Using FT-IR and ab Initio Quantum Calculations

Fourier transform infrared (FT-IR) spectra were obtained for carbon monoxide adsorbed on a series of alkaline earth exchanged beta zeolites. A cation-dependent vibrational band was observed at 2184, 2190, 2195, and 2210 cm-1 for CO adsorbed on Ba-beta, Sr-beta, Ca-beta, and Mg-beta, respectively. Using the experimental CO vibrational frequencies, the calculated electric fields ranged from 7.3 V/nm (Ba-beta) to 12.9 V/nm (Mg-beta). A vibrational band at 2228 cm-1 was observed in all of the IR spectra and was assigned to CO adsorbed on Lewis acid sites. A shoulder observed at 2188 cm-1 in some of the infrared spectra was assigned to CO interacting with Bronsted acid sites. The FT-IR experimental studies were combined with ab initio calculations to understand the effect of the different alkaline earth cations on the observed vibrational frequencies. Model compounds M(OH)2−CO (where M = an alkaline earth cation) and Al(OH)3−CO were used in the calculations to approximate the zeolitic environment. The computat...

Electric field-induced shifts of vibrational frequencies of CO adsorbed on Ni, Pd, Pt, Cu, Ag and Au metal (100) surfaces A theoretical comparative study

A theoretical comparative study of electric field(F)-induced vibrational frequency shifts of CO adsorbed on Ni, Pd, Pt, Cu, Ag and Au metal (100) surfaces is presented. The calculated vibrational tuning rates dωC–O/dF for the various metals are similar, thereby corroborating the experimental results of Ikezawa etal. The calculated absolute values are close to Lambert's observed values for CO on Ni in ultrahigh vacuum (UHV). The resulting estimates of the vibrational tuning rate with potential V are in qualitative agreement with experimental data in aqueous electrolytes. The small experimental value of tuning rate for CO on Pt in UHV is not supported by the calculation. The M–CO bonding properties and their trends among the metals are also investigated.