CO Vibrational Frequency Research Articles

Computational investigation of CO adsorbed on Aux, Agx and (AuAg)x nanoclusters (x = 1 \u2212 5, 147) and monometallic Au and Ag low-energy surfaces

Density functional theory calculations have been performed to investigate the use of CO as a probe molecule for the determination of the structure and composition of Au, Ag and AuAg nanoparticles. For very small nanoclusters (x = 1 − 5), the CO vibrational frequencies can be directly correlated to CO adsorption strength, whereas larger 147-atom nanoparticles show a strong energetic preference for CO adsorption at a vertex position but the highest wavenumbers are for the bridge positions. We also studied CO adsorption on Au and Ag (100) and (111) surfaces, for a 1 monolayer coverage, which proves to be energetically favourable on atop only and bridge positions for Au (100) and atop for Ag (100); vibrational frequencies of the CO molecules red-shift to lower wavenumbers as a result of increased metal coordination. We conclude that CO vibrational frequencies cannot be solely relied upon in order to obtain accurate compositional analysis, but we do propose that elemental rearrangement in the core@shell nanoclusters, from Ag@Au (or Au@Ag) to an alloy, would result in a shift in the CO vibrational frequencies that indicate changes in the surface composition.

Adsorption of O, O2 and CO on iridium clusters and the investigations of their stability

Density functional theory calculations were performed to study the stability of Irn clusters as well as the adsorption of O, O2 and CO adsorbates on selected structures. The clusters form three dimensional structures for n>4. Larger clusters of n>13 exhibit simple cubic structures up to n around 32, beyond which fcc structures become more favorable. The binding energy is found to increase as a function of cluster size to approach bulk cohesive energy asymptotically. The total magnetic moment is found to decrease as a function of the cluster size approaching the bulk nonmagnetic ground state. The top adsorption site is the most site of O, O2 and CO on small clusters, unlike Ir64 that exhibits hollow, bridge and top sites, respectively. The vibrational frequencies of CO (O2) on Ir2 and Ir4 are found to be less than those of free molecules of 2102.82 (1562.08)cm−1.

Scenarios of polaron-involved molecular adsorption on reduced TiO2(110) surfaces

The polaron introduced by the oxygen vacancy (Vo) dominates many surface adsorption processes and chemical reactions on reduced oxide surfaces. Based on IR spectra and DFT calculations of NO and CO adsorption, we gave two scenarios of polaron-involved molecular adsorption on reduced TiO2(110) surfaces. For NO adsorption, the subsurface polaron electron transfers to a Ti:3d-NO:2p hybrid orbital mainly on NO, leading to the large redshifts of vibration frequencies of NO. For CO adsorption, the polaron only transfers to a Ti:3d state of the surface Ti5c cation underneath CO, and thus only a weak shift of vibration frequency of CO was observed. These scenarios are determined by the energy-level matching between the polaron state and the LUMO of adsorbed molecules, which plays a crucial role in polaron-adsorbate interaction and related catalytic reactions on reduced oxide surfaces.

A reinvestigation of the CO vibration frequency as a probe to determine the species of Au in the Au/MOR catalyst

ONIOM calculations have been carried out to determine geometries, adsorption energies and vibrational frequencies of CO on Au2 and Au4-exchanged mordenite catalysts (Au2/MOR and Au4/MOR) using DFT with the B3LYP and ωB97X-D functionals. We considered structures with two Al atoms, MOR(TT), with at least one of them at the preferential site T4. To investigate the CO adsorption on Au2/MOR(TT) we examined three different situations: the adsorption of one single CO molecule bridged to two Au atoms (case I), one CO molecule adsorbed on a single Au atom (case II), and two molecules of CO absorbed on two Au atoms, one CO per Au atom (case III). From all the previous theoretically and experimentally proposed species of Au inside the MOR channels, including the ones considered in this work, only the μ2-CO-Au2/MOR configuration, with one CO adsorbed in a bridge form on two Au atoms, exhibits ν(CO) frequencies within the experimental range of 1949–2120cm−1 (low frequencies). Increasing the size of the Au particles shifts the ν(CO) to higher frequencies. The values of ΔHads decrease when the number of adsorbed CO molecules increases, similarly to what is experimentally observed when CO is adsorbed on Au/TiO2 and Au/Al2O3. Charge analysis shows that the Au2 in the Au2/MOR and the Au4 cluster in the Au4/MOR are cationic. Based on the present results we conclude that the use of ν(CO) as a probe to determine the Au species in the channels of MOR can only clearly identify Au2δ+ species inasmuch as the μ2-CO-Au2/MOR configuration is the only one among several others to exhibit ν(CO) frequencies within the experimental low frequency region.

Theoretical Investigation of the Adsorption Properties of CO, NO, and OH on Monometallic and Bimetallic 13-Atom Clusters: The Example of Cu13, Pt7Cu6, and Pt13.

We report a density functional theory investigation of the adsorption properties of CO, NO, and OH on the Cu13, Pt7Cu6, and Pt13 clusters in the cationic, neutral, and anionic states with the aim to improve our atomistic understanding of the adsorption properties on bimetallic clusters compared with monometallic clusters. The adsorption energy of CO and NO are substantially stronger on Pt13 than on Cu13, and hence, CO and NO bind preferentially on Pt sites on Pt7Cu6. Thus, it can contribute to drive the migration of the Pt atoms from the core to the surface region in large PtCu nanoalloys. The CO and NO adsorption energies on the bimetallic cluster are enhanced by a few percent compared with the energies of the monometallic clusters, which shows that the Pt-Cu interaction can contribute to an increase in the adsorption energy. In contrast with CO and NO trends, the OH adsorption energies on Cu13, Pt7Cu6, and Pt13 deviates only up to 0.31 eV, and hence, there is no clear preference for Cu or Pt sites on Pt7Cu6 or an enhancement of the adsorption energy on the bimetallic systems. We found a reduction of the CO and NO vibrational frequencies upon adsorption, which indicates a weakening of the CO and NO binding energies, and it is supported by a slight increase in the bond lengths. However, the OH vibrational frequency increases upon adsorption, which indicates an enhancement of the OH binding energy, which is supported by a slight decrease in the bond length by about 0.01 Å. It can be explained by the large charge transfer from the clusters to the O atom, which enhances the electrostatic interaction in the O-H bonding.

Structures and vibrational frequencies of CO adsorbed on transition metals from calculations using the vdW-DF2 functional

We confirmed how well the exchange–correlation functional vdW-DF2, with its non-local correlation part, describes the adsorption of CO on close-packed surfaces and clusters of Co, Ni, Cu, Ru, Rh, Pd, and Pt. The calculated adsorption energies are fairly close to the experimental values. The carbonyl vibrational frequencies, calculated within the harmonic approximation, systematically underestimate the experimental results. To amend this deficiency, we explored multiplicative and additive correction schemes. Scaling the calculated harmonic CO frequencies by a uniform factor of 1.030 yields results with a mean absolute deviation of ∼13cm−1 from experiment. With this correction scheme we were able to clarify some controversial structures for CO adsorption on the surfaces Pd(111) and Co(0001).

Lateral interaction and spectroscopic constants of CO adsorbed on ZnO

The adsorption of carbon monoxide on a six-layer supercell ZnO(\(10\bar{1}0\)) was studied using DFT with periodic methodology, PW91 as functional and plane waves as basis set. Furthermore, vibrational energies and spectroscopic constants for the Zn–C interaction were determined. The geometric parameters of adsorbed CO are in accordance with available experimental and theoretical data. Carbon monoxide adsorption sites as a function of coverage were also studied, showing CO vibrational frequency decrease with coverage increases in agreement with experimental results. The adsorption energy shows a decrease with coverage, from 60.9 to 50.8 kJ mol−1, which is supported by experimental data. Structural analyses indicated that the change found in the adsorption energy and the associated frequency shift is explained by the surface relaxation upon adsorption. We have reported the calculated spectroscopic constants for CO adsorption, for the first time in the literature. Vibrational frequency found from potential energy curve has the same behavior of adsorption energies. Density of states analysis of CO adsorption also showed agreement with experimental results. A decrease of 4σ–5σ gap was obtained, resulting from the stabilization of both peaks in agreement with experimental results.

Conformation stability, halogen and solvent effects on C[dbnd]O stretching of 4-chloro-3-halogenobenzaldehydes

The effects of halogen and solvent on the conformation and carbonyl stretching of 4-chloro-3-halogenobenzaldehydes [C7H4ClXO; X=F (CFB), Cl (CCB) or Br (CBB)] were investigated using the density functional theory (DFT) method. The B3LYP functional was used by the 6-311+G(3df,p) basis set in combination with the polarizable continuum model (PCM). Computations were focused on the cis and trans isomers of the compounds in 18 different polar or non-polar organic solvents. The theoretical frequencies of the solvent-induced CO stretching vibrations were correlated with the empirical solvent parameters such as the Kirkwood–Bauer–Magat (KBM) equation, the solvent acceptor number (AN), Swain parameters and the linear solvation energy relationships (LSER). The present work explores the effect of both the halogen and medium on the conformational preference and CO vibrational frequency. The findings of this work can be useful to those systems involving changes in the conformations analogous to the compounds studied.

CO adsorption on PdGa(1 0 0), (1 1 1) and(1¯ 1¯ 1¯)surfaces: A DFT study

Fil: Bechthold, Pablo Ignacio. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Bahia Blanca. Instituto de Fisica del Sur. Universidad Nacional del Sur. Departamento de Fisica. Instituto de Fisica del Sur; Argentina

Theoretical study for the adsorption of CO on neutral and charged Pd13 clusters

Adsorption of carbon monoxide, CO, on the surface of magnetic Pd13, Pd13–, and Pd13+ clusters, showing magnetic moments of 8, 7, and 7 Bohr magnetons (μB), respectively, was studied by means of density functional methods, allowing partial inclusion of relativistic effects. The favorable adsorption modes are on top, bridge, and threefold, with the binding energy of CO with Pd13– increasing in this same order as 39.4, 48.0 and 50.2 kcal/mol. In addition, the experimental results for the [Pdn–CO]–, n = 4–12, anions show a decrease of the vibrational frequency of CO along this triad of modes, 1940, 1800, and 1680 cm−1, with respect to the free CO value, 2143 cm−1, which conforms to our estimated frequencies, 1956, 1784, and 1679 cm−1, for CO in the [Pd13–CO]– complex. Also, the threefold mode shows a significantly longer bond length for CO, 1.210 Å, with respect to the free case, 1.139 Å. Then the bond of CO is considerably weakened in the negatively charged [Pd13CO]– cluster when the adsorption occurs in a threefold site. These results are mainly accounted for by charge transfer effects from the Pd13 cluster to the CO molecule. Smaller CO activation was found in neutral Pd13–CO and in [Pd13–CO]+, where hollow adsorption yields bigger structural and electronic changes on CO than the respective bridge and on-top modes. Overall, CO adsorption notably quenches the magnetization of neutral and charged Pd13 particles.

Analytic Morse/long-range potential energy surfaces and predicted infrared spectra for CO–H2 dimer and frequency shifts of CO in (para-H2)N N = 1–20 clusters

A five-dimensional ab initio potential energy surface (PES) for CO-H2 that explicitly incorporates dependence on the stretch coordinate of the CO monomer has been calculated. Analytic four-dimensional PESs are obtained by least-squares fitting vibrationally averaged interaction energies for vCO = 0 and 1 to the Morse/long-range potential function form. These fits to 30,206 points have root-mean-square (RMS) deviations of 0.087 and 0.082 cm(-1), and require only 196 parameters. The resulting vibrationally averaged PESs provide good representations of the experimental infrared data: for infrared transitions of para H2-CO and ortho H2-CO, the RMS discrepancies are only 0.007 and 0.023 cm(-1), which are almost in the same accuracy as those values of 0.010 and 0.018 cm(-1) obtained from full six-dimensional ab initio PESs of V12 [P. Jankowski, A. R. W. McKellar, and K. Szalewicz, Science 336, 1147 (2012)]. The calculated infrared band origin shift associated with the fundamental of CO is -0.179 cm(-1) for para H2-CO, which is the same value as that extrapolated experimental value, and slightly better than the value of -0.176 cm(-1) obtained from V12 PESs. With these potentials, the path integral Monte Carlo algorithm and a first order perturbation theory estimate are used to simulate the CO vibrational band origin frequency shifts of CO in (para H2)N-CO clusters for N = 1-20. The predicted vibrational frequency shifts are in excellent agreement with available experimental observations. Comparisons are also made between these model potentials.

Trends with coverage and pH in Stark tuning rates for CO on Pt(1 1 1) electrodes

The general understanding of so-called electrochemical Stark tuning rates, that is, the potential dependence of vibrational frequency of CO adsorbed on Pt(111), has developed over the past thirty years in terms of two semiempirical models. The first is the Fermi level shift model used in non-self-consistent-field one-electron molecular orbital theory. This approach has provided qualitative understanding in terms of Fermi level-dependent variations in σ and π orbital bonding between CO and the electrode surface atoms. The second is the use of self-consistent-field theory with surface charging to create adjustable electric fields. Adsorbed CO then reacts to the field in a classical Stark effect with some small uncharacterized Fermi level shift superimposed. It is now possible, using two-dimensional density functional theory, including electrolyte polarization from surface charging, and the dielectric continuum to approximate solvation energy, to calculate the tuning rate in response to shifts in the Fermi level and electrode potential caused by changing the surface charge density. Here we apply this first principles method to calculate trends in the tuning rate for CO adsorbed on 1-fold Pt(111) sites with changes in CO(ads) coverage and with changes in electrolyte pH. The tuning rate is calculated to decrease as the coverage is increased and, for high coverage, to increase as the pH is increased. These trends are shown to be in qualitative agreement with the very little existing experimental data for these trends.

Monoxide carbon frequency shift as a tool for the characterization of TiO2 surfaces: Insights from first principles spectroscopy

The adsorption and vibrational frequency of CO on defective and undefective titanium dioxide surfaces is examined applying first-principles molecular dynamics simulations. In particular, the vibrational frequencies are obtained beyond the harmonic approximation, through the time correlation functions of the atomic trajectories. In agreement with experiments, at low CO coverages we find an upshift in the vibration frequency with respect to the free CO molecule, of 45 and 35 cm(-1) on the stoichiometric rutile (110) and anatase (101) faces, respectively. A band falling 8 cm(-1) below the frequency corresponding to the perfect face is observed for the reduced rutile (110) surface in the low vacancy concentration limit, where the adsorption is favored on Ti(4+) sites. At a higher density of defects, adsorption on Ti(3+) sites becomes more stable, accompanied by a downshift in the stretching band. In the case of anatase (101), we analyze the effect of subsurface oxygen vacancies, which have been shown to be predominant in this material. Interestingly, we find that the adsorption of CO on five coordinate Ti atoms placed over subsurface vacancies is favored with respect to other Ti(4+) sites (7.25 against 6.95 kcal/mol), exhibiting a vibrational redshift of 20 cm(-1). These results provide the basis to quantitatively assess the degree of reduction of rutile and anatase surfaces via IR spectroscopy, and at the same time allow for the assignment of characteristic bands in the CO spectra on TiO2 whose origin has remained ambiguous.

Ab initio study of CO adsorption on PdGa(1 1 0)

CO adsorption is analyzed using Density Functional Theory (DFT) calculations. Changes in the electronic structure of PdGa(110) surface and CO bond after adsorption are addressed. CO is located only on Pd atop geometry with a tilted configuration (9.13° from the perpendicular to the surface) and no interaction with Ga is detected. The Pd–Pd bond strength decreases 54.2% as the new Pd–CO bond is formed. The C–O bond length change less than 1%, compared to the gas phase value, while its bond overlap population decrease 46.2%. The effect of CO is limited to its first Pd neighbor. Analysis of orbital interaction reveals the Pd–CO bond mainly involves s–s and s–p orbitals with less participation of Pd 4d orbitals. The computed CO vibration frequencies after adsorption shows a red shift from vacuum to ward 2013.85cm−1, which agrees with previous experimental data on PdGa intermetallic.

Precise Identification of the Infrared Bands of the Polycarbonyl Complexes on Ni–MOR Zeolite by12C16O–13C18O Coadsorption and Computational Modeling

We report a combined computational and experimental IR study of polycarbonyl species on Ni–MOR zeolite formed after adsorption of 12C16O/13C18O isotopic mixtures, which allowed us to suggest a justified assignment of the vibrational bands in the complex IR spectra. For identification of all individual IR bands of the polycarbonyl complexes containing either the same or mixed isotopic probe molecules, we modeled the corresponding species with density functional method. In order to provide reliable computed values for comparison with experimentally measured vibrational frequencies of CO in the complexes, we combined periodic density functional calculations (with gradient corrected exchange-correlation functional) with isolated cluster calculations using hybrid functional. In this way, we confirm that the Ni+ cations in this zeolite material are able to form di- and tricarbonyl complexes and thus feature high coordinative unsaturation as suggested earlier.

Finite temperature path integral Monte Carlo simulations of structural and dynamical properties of ArN−CO2 clusters

We report finite temperature quantum mechanical simulations of structural and dynamical properties of Ar(N)-CO(2) clusters using a path integral Monte Carlo algorithm. The simulations are based on a newly developed analytical Ar-CO(2) interaction potential obtained by fitting ab initio results to an anisotropic two-dimensional Morse/Long-range function. The calculated distributions of argon atoms around the CO(2) molecule in Ar(N)-CO(2) clusters with different sizes are consistent to the previous studies of the configurations of the clusters. A first-order perturbation theory is used to quantitatively predict the CO(2) vibrational frequency shift in different clusters. The first-solvation shell is completed at N = 17. Interestingly, our simulations for larger Ar(N)-CO(2) clusters showed several different structures of the argon shell around the doped CO(2) molecule. The observed two distinct peaks (2338.8 and 2344.5 cm(-1)) in the υ(3) band of CO(2) may be due to the different arrangements of argon atoms around the dopant molecule.

A hydrogen-bonding network plays a catalytic role in photosynthetic oxygen evolution

In photosystem II, oxygen evolution occurs by the accumulation of photo-induced oxidizing equivalents at the oxygen-evolving complex (OEC). The sequentially oxidized states are called the S(0)-S(4) states, and the dark stable state is S(1). Hydrogen bonds to water form a network around the OEC; this network is predicted to involve multiple peptide carbonyl groups. In this work, we tested the idea that a network of hydrogen bonded water molecules plays a catalytic role in water oxidation. As probes, we used OEC peptide carbonyl frequencies, the substrate-based inhibitor, ammonia, and the sugar, trehalose. Reaction-induced FT-IR spectroscopy was used to describe the protein dynamics associated with the S(1) to S(2) transition. A shift in an amide CO vibrational frequency (1664 (S(1)) to 1653 (S(2)) cm(-1)) was observed, consistent with an increase in hydrogen bond strength when the OEC is oxidized. Treatment with ammonia/ammonium altered these CO vibrational frequencies. The ammonia-induced spectral changes are attributed to alterations in hydrogen bonding, when ammonia/ammonium is incorporated into the OEC hydrogen bond network. The ammonia-induced changes in CO frequency were reversed or blocked when trehalose was substituted for sucrose. This trehalose effect is attributed to a displacement of ammonia molecules from the hydrogen bond network. These results imply that ammonia, and by extension water, participate in a catalytically essential hydrogen bond network, which involves OEC peptide CO groups. Comparison to the ammonia transporter, AmtB, reveals structural similarities with the bound water network in the OEC.

A density functional study of carbon monoxide adsorption on small cationic, neutral, and anionic aluminum phosphide clusters

The chemisorption of CO on aluminum phosphide (AlP)n (n=1–12) clusters are studied with density functional theory. Among various possible CO adsorption sites, the on-top site is identified to be the most favorable chemisorption sit. The Al-top sites are the preferred one in the most cases for one CO adsorption in (AlP)n (n=3–12) clusters, irrespective of the charge state of the clusters. The Al–P bond lengths decrease generally as the size of the cluster increases. There is a slight increase in the mean Al–P bond lengths after CO adsorption on the lowest-energy sites of the most AlP clusters. In general, adsorption energies of CO are found to decrease with an increase in the cluster size for both neutral and ionic clusters. The adsorption energies of CO on the cationic (AlP)nCO+ (n=2–12) clusters are greater than those on the neutral and anionic complexes. The result shows that large adsorption energies of CO on neutral and ionic AlP clusters and large highest occupied and lowest unoccupied molecular-orbital gaps for (AlP)CO, (AlP)CO+ and (AlP)CO− make these species behaving like magic clusters. The CO vibrational frequencies νCO in all the cationic complexes are always larger than the corresponding quantities in the neutral and anionic clusters. All of them are smaller than those in the free CO molecule.

Influence of histidine tag attachment on picosecond protein dynamics.

Polyhistidine affinity tags are routinely employed as a convenient means of purifying recombinantly expressed proteins. A tacit assumption is commonly made that His tags have little influence on protein structure and function. Attachment of a His tag to the N-terminus of the robust globular protein myoglobin leads to only minor changes to the electrostatic environment of the heme pocket, as evinced by the nearly unchanged Fourier transform infrared spectrum of CO bound to the heme of His-tagged myoglobin. Experiments employing two-dimensional infrared vibrational echo spectroscopy of the heme-bound CO, however, find that significant changes occur to the short time scale (picoseconds) dynamics of myoglobin as a result of His tag incorporation. The His tag mainly reduces the dynamics on the 1.4 ps time scale and also alters protein motions of myoglobin on the slower, >100 ps time scale, as demonstrated by the His tag's influence on the fluctuations of the CO vibrational frequency, which reports on protein structural dynamics. The results suggest that affinity tags may have effects on protein function and indicate that investigators of affinity-tagged proteins should take this into consideration when investigating the dynamics and other properties of such proteins.

A Stable Acyclic Ligand Equivalent of an Unstable 1,3‐Dithiol‐5‐ylidene

A Stable Acyclic Ligand Equivalent of an Unstable 1,3‐Dithiol‐5‐ylidene