Structures and bonding nature of small monoligated copper clusters (HCN-Cun, n = 1-3) through high-resolution infrared spectroscopy and theory

The structures, C-H stretching frequencies, and dipole moments of HCN-Cun (n = 1-3) clusters are determined through high-resolution infrared spectroscopy. The complexes are formed and probed within superfluid helium droplets, whereby the helium droplet beam is passed over a resistively heated crucible containing copper shot and then through a gas HCN pickup cell. All complexes are found to be bound to the nitrogen end of the HCN molecule and on the "atop site" of the copper cluster. Through the experimental C-H vibrational shifts of HCN-Cun and ab initio calculations, it was found that the HCN-metal interaction changes from a strong van der Waals bond in n = 1 to a partially covalent bond in HCN-Cu3. Comparisons with existing infrared data on copper surfaces show that the HCN-Cun bond must begin to weaken at very large copper cluster sizes, eventually returning to a van der Waals bond in the bulk copper surface case.     

Unusual infrared spectrum of ethane adsorbed by gallium oxide

The present study reports an unusual diffuse reflection Fourier transform (DRIFT) spectrum of ethane adsorbed by gallium oxide. One of the stretching C-H bands in this spectrum with a maximum at 2753 cm(-1) is more than by 100 cm(-1) shifted toward lower frequencies in comparison with gaseous ethane. In addition, the relative intensity of this band is unusually high. This indicates a very strong polarizability of the corresponding chemical bonds resulting from perturbation of ethane by the low coordinated Ga(3+) cations. The assignment of this band to the very strongly perturbed initially fully symmetric nu(1) C-H stretching vibration is confirmed by a DFT modeling of ethane adsorption by the simplest Ga(2)O(3) cluster. The obtained results also indicate heterolytic dissociative adsorption and dehydrogenation of ethane by Ga(3+) Lewis sites at elevated temperature. This is evidenced by the appearance of new IR bands from zinc alkyl fragments and acidic protons followed by decomposition of resulting zinc ethyl species. In parallel, the most intense IR band at lower frequency from the most strongly polarized C-H chemical bond decreased in intensity. The obtained results indicate that these vibrations are involved in subsequent heterolytic dissociative adsorption. The obtained results demonstrate that, similar to the shifts of C-H stretching vibrations to the low-frequency, intensities of IR C-H stretching bands can be also used as an index of chemical activation of adsorbed paraffins via their polarization by the low-coordinated cations.     

Strongly blue-shifted C-H stretches: interaction of formaldehyde with hydrogen fluoride clusters

The equilibrium structures, binding energies, and vibrational spectra of the cyclic, hydrogen-bonded complexes formed between formaldehyde, H(2)CO, and hydrogen fluoride clusters, (HF)(1< or =n < or =4), are investigated by means of large-scale second-order M?ller-Plesset calculations with extended basis sets. All studied complexes exhibit marked blue shifts of the C-H stretching frequencies, exceeding 100 cm(-1) for n = 2-4. It is shown that these blue shifts are, however, only to a minor part caused by blue-shifting hydrogen bonding via C-H...F contacts. The major part arises due to the structural relaxation of the H(2)CO molecule under the formation of a strong C=O...H-F hydrogen bond which strengthens as n increases. The close correlation between the different structural parameters in the studied series of complexes is demonstrated, and the consequences for the frequency shifts in the complexes are pointed out, corroborating thus the suggestion of the primary role of the C=O...H-F hydrogen bonding for the C-H stretching frequency shifts. This particular behavior, that the appearance of an increasingly stronger blue shift of the C-H stretching frequencies is mainly induced by the formation of a progressively stronger C=O...H-F hydrogen bond in the series of H(2)CO...(HF)(1< or =n < or =4), complexes and only to a lesser degree by the formation of the so-called blue-shifting C-H...F hydrogen bond, is rationalized with the aid of selected sections of the intramolecular H(2)CO potential energy surface and by performing a variety of structural optimizations of the H(2)CO molecule embedded in external, differently oriented dipole electric fields, and also by invoking a simple analytical force-field model.     

Effect of adsorption site, size, and composition of Pt/Au bimetallic clusters on the CO frequency: a density functional theory study

Density functional theory (DFT) calculations were performed to investigate the C-O stretching frequency changes when a CO molecule was adsorbed to Pt/Au clusters of 2-4 atoms. Our calculations show that the adsorption site is the most sensitive quantity to the C-O stretching frequency shifts. All the bridge site adsorptions yield a CO frequency band of 1737-1927 cm-1 with the CO bond distance of 1.167-1.204 A regardless of cluster composition and size, and all the atop site adsorptions yield a CO frequency band of 2000-2091 cm-1 with the CO bond distance of 1.151-1.167 A. More detailed analysis of the two frequency bands shows that each band may consist of two emerging subbands with the lower frequencies corresponding to the CO adsorption to Pt atoms and the higher frequencies to the CO adsorption to Au atoms. The insensitivity of the CO frequency shift to the cluster size indicates that the trend discussed here for small clusters may be used to interpret the experimental observations for nanoparticles. Our results also illustrated that the Fourier transform infrared spectroscopy measurement may be used as a sensitive tool to identify adsorption sites of the Pt/Au nanoparticles using CO adsorption as the probe.     

High-resolution infrared spectroscopy of HCN-Agn (n = 1-4) complexes solvated in superfluid helium droplets

High-resolution infrared spectroscopy has been used to determine the structures, C-H stretching frequencies, and dipole moments of the HCN-Agn (n = 1-3) complexes formed in superfluid helium droplets. The HCN-Ag4 cluster was tentatively assigned based upon pick-up cell pressure dependencies and harmonic vibrational shift calculations. Ab initio and density functional theory calculations were used in conjunction with the high-resolution spectra to analyze the bonding nature of each cluster. All monoligated species reported here are bound through the nitrogen end of the HCN molecule. The HCN-Agn complexes are structurally similar to the previously reported HCN-Cun clusters, with the exception of the HCN-Ag binary complex. Although the interaction between the HCN and the Agn clusters follows the same trends as the HCN-Cun clusters, the more diffuse nature of the electrons surrounding the silver atoms results in a much weaker interaction.     

Intensities of C-H IR stretching bands of ethane and propane adsorbed by zeolites as a new spectral criterion of their chemical activation via polarization resulting from stretching of chemical bonds

Polarization of ethane and propane resulting from adsorption of these hydrocarbons by protons and different cations in mordenite, ZSM-5, and Y zeolites was studied by diffuse reflection Fourier transform IR spectroscopy (DRIFTS). Perturbation of adsorbed molecules by protons and sodium cations is weak, while positions of absorption bands for both these zeolites are very close to each other. In contrast, distributions of C-H IR stretching bands in intensities are somewhat different. This effect is pronounced much stronger for adsorption of light paraffins by bivalent alkaline earth and zinc cationic forms of these zeolites. Distribution of relative intensities of absorption bands strongly depends in this case both on the nature of cations and on the zeolites, while the most strongly perturbed vibrations are the initially fully symmetric C-H stretching vibrations. The corresponding low-frequency shifts and relative intensities of IR bands are increasing for different cations and zeolites in the following sequences: Na < Ca < Mg < Zn and Y < Mor approximately ZSM-5, while the difference in distribution of relative intensities of C-H stretching bands is pronounced much stronger than for the low-frequency shifts of these bands. Therefore, the relative intensities of IR C-H stretching bands are much better criterion of perturbation of light paraffins upon adsorption than the frequencies of these bands, which are traditionally used for this purpose. In addition, distribution of C-H IR stretching bands in intensity also provides unique information on anisotropy of polarizability of different C-H bonds created by their vibrations. For the acid and acid-base catalysis, where the main source of chemical activation arises from polarization of adsorbed molecules, such information is most important, while the anisotropy of polarizability provides a unique information on selective activation of different chemical bonds resulting from their stretching. The obtained results also demonstrate the possibility to use for testifying of the strength of Lewis acid sites instead of adsorption of the model molecular probes adsorption of the paraffins themselves.     

Periodic trends in electrode-chemisorbate bonding: benzonitrile on platinum-group and other noble metals as probed by surface-enhanced Raman spectroscopy combined with density functional theory.

Detailed intramolecular vibrational spectra obtained by means of surface-enhanced Raman scattering (SERS) for benzonitrile adsorbed on seven electrode surfaces-four Pt-group metals (platinum, palladium, rhodium, and iridium) and the Group IB metals (copper, silver, and gold)-are reported with the aim of exploring the metal-dependent nature of surface-chemisorbate interactions. The Pt-group surfaces were prepared as ultrathin electrodeposited films on gold, enabling the SERS activity inherent to the substrate to be imparted to the overlayer material. Benzonitrile was selected as a "model" organic adsorbate since it displays a rich array of coupled aromatic ring as well as substituent modes which collectively can provide insight into the various molecular perturbations induced by surface coordination via the nitrile substituent. The experimental spectra are compared with ab initio calculations of vibrational frequencies, bond geometries, and charge distributions obtained by means of Density Functional Theory (DFT), which yields valuable insight into the underlying structural reasons for the sensitivity of the experimental coordination-induced frequency shifts to the nature of the intramolecular mode and the metal surface. The DFT results also form an invaluable aid in making SER spectral assignments, along with providing detailed information on the coupled atomic displacements involved in each vibrational mode. Benzonitrile surface coordination was modeled in the DFT calculations by binding the nitrile group to metal atoms and small metal clusters. While the majority of the aromatic-ring SER frequencies are altered only slightly (approximately < 5 cm(-1)) upon surface coordination, several modes (especially nu(1), nu(6a)) are blue-shifted substantially (by up to 50 cm(-1)). These shifts were identified by DFT as arising from mode coupling to the nitrile substituent, especially involving the C-CN bond that is compressed upon nitrile coordination, associated with metal-adsorbate back-donation. The small (<5 cm(-1)) red-shifts seen for ring vibrations not involving coupled substituent motion apparently arise from increased antibonding aromatic electron density. The metal-dependent frequency shifts seen for these coupled aromatic vibrations as well as for the more localized C-N nitrile stretching mode are consistent with increased back-donation anticipated in the sequence d(10) < d(9) < d(8) within a given Periodic row. Overall, the findings provide a benchmark illustration of the virtues of DFT in interpreting complex vibrational spectra for larger polyatomic adsorbates.     

Infrared and ab initio studies on 1,2,4,5-tetrafluorobenzene clusters with methanol and 2,2,2-trifluoroethanol: presence and absence of an aromatic C-H...O hydrogen bond

The (1:1) clusters of 1,2,4,5-tetrafluorobenzene (TFB) with methanol and with 2,2,2-trifluoroethanol (TFE) were studied both experimentally and computationally. Through use of fluorescence-detected infrared spectroscopy, the (1:1) clusters were identified in supersonic jets. Intermolecular interactions in the clusters were characterized by the spectral shifts of the aromatic C-H stretching modes in the TFB moiety owing to the cluster formation. The molecular structures, stabilization energies, and vibrational frequencies of the clusters were computed at the MP2/6-31+G level. Both computational and experimental data indicate that an aromatic C-H...O hydrogen bond is present in the TFB-methanol cluster, while it is absent in the TFB-TFE cluster.     

Adsorption of molecular hydrogen and hydrogen sulfide on Au clusters

The authors present theoretical results describing the adsorption of H2 and H2S molecules on small neutral and cationic gold clusters (Au(n)((0/+1)), n=1-8) using density functional theory with the generalized gradient approximation. Lowest energy structures of the gold clusters along with their isomers are considered in the optimization process for molecular adsorption. The adsorption energies of H2S molecule on the cationic clusters are generally greater than those on the corresponding neutral clusters. These are also greater than the H2 adsorption energies on the corresponding cationic and neutral clusters. The adsorption energies for cationic clusters decrease with increasing cluster size. This fact is reflected in the elongations of the Au-S and Au-H bonds indicating weak adsorption as the cluster grows. In most cases, the geometry of the lowest energy gold cluster remains planar even after the adsorption. In addition, the adsorbed molecule gets adjusted such that its center of mass lies on the plane of the gold cluster. Study of the orbital charge density of the gold adsorbed H2S molecule reveals that conduction is possible through molecular orbitals other than the lowest unoccupied molecular orbital level. The dissociation of the cationic Au(n)SH2+ cluster into Au(n)S+ and H2 is preferred over the dissociation into Au(m)SH2+ and Au(n-m), where n=2-8 and m=1-(n-1). H2S adsorbed clusters with odd number of gold atoms are more stable than neighboring even n clusters.     

系列化合物C-H伸缩频率的自然杂化轨道研究

在计算C-H核自旋-自旋偶合常数的新公式及其与C-H伸缩频率之间的关系的基础上, 得出了计算C-H伸缩频率的新的一般关系式。并利用CNDO/2分子轨道和自然杂化轨道方法, 具体计算了三种不同系列化合物的原子净电荷和自然杂化轨道。给出了计算不同系列化合物C-H伸缩频率的良好线性关系式。结果表明, 碳氢化合物中的C-H伸缩频率主要由原子的轨道杂化作用所决定, 而对于含杂原子的取代碳氢化合物, C-H键的极性成为影响伸缩频率的重要因素。     

HCN Adsorption on the Fe(100), Fe(111) and Fe(110) Surfaces: a Density Functional Theory Study

Twenty kinds of adsorptions of HCN on the Fe(100), Fe(111) and Fe(110) surfaces at the 1/4 monolayer coverage are found using the density functional theory. For Fe(100), the adsorption energy of the most stable configuration where the HCN locates at the fourfold site with the C-N bonded to four Fe atoms is 1.928 eV. The most favored adsorption structure for HCN on Fe(111) is f-η3(N)-h-η3(C), in which the C-N bond is almost parallel to the surface, and the adsorption energy is 1.347 eV. On Fe(110), the adsorption energy in the most stable configuration in which HCN locates at the two long-bridge sites is 1.777 eV. The adsorption energy of the parallel orientation for HCN is larger than that of the perpendicular configuration. The binding mechanism of HCN on the Fe(100), Fe(111) and Fe(110) surfaces is also analyzed by Mulliken charge population and the density of states in HCN. The result indicates that the configurations in which the adsorbed HCN becomes the non-linear are beneficial to the formation of the addition reaction for hydrogen. The nature that the introduction of Fe into the catalyst could increase the catalytic activity of the bimetallic catalyst in the addition reaction of hydrogen for nitriles is revealed.     

Theoretical study of oxygen adsorption on pure Au(n+1)+ and doped MAu(n)+ cationic gold clusters for M = Ti, Fe and n = 3-7

A comparative study of the adsorption of an O2 molecule on pure Au(n+1)+ and doped MAu(n)+ cationic gold clusters for n = 3-7 and M = Ti, Fe is presented. The simultaneous adsorption of two oxygen atoms also was studied. This work was performed by means of first principles calculations based on norm-conserving pseudo-potentials and numerical basis sets. For pure Au4 +, Au6+, and Au7+ clusters, the O2 molecule is adsorbed preferably on top of low coordinated Au atoms, with an adsorption energy smaller than 0.5 eV. Instead, for Au5+ and Au8+, bridge adsorption sites are preferred with adsorption energies of 0.56 and 0.69 eV, respectively. The ground-state geometry of Au(n)+ is almost unperturbed after O2 adsorption. The electronic charge flows towards O2 when the molecule is adsorbed in bridge positions and towards the gold cluster when O2 is adsorbed on top of Au atoms, and both the adsorption energy and the O-O bond length of adsorbed oxygen increase when the amount of electronic charge on O2 increases. On the other hand, we studied the adsorption of an O2 molecule on doped MAu(n)+ clusters, leading to the formation of (MAu(n)O2+) ad complexes with different equilibrium configurations. The highest adsorption energy was obtained when both atoms of O2 bind on top of the M impurity, and it is larger for Ti doped clusters than for Fe doped clusters, showing an odd-even effect trend with size n, which is opposite for Ti as compared to Fe complexes. For those adsorption configurations of (MAu(n)O2+) ad involving only Au sites, the adsorption energy is similar to or smaller than that for similar configurations of Au(n)+1O2 + complexes. However, the highest adsorption energy of (MAu(n)O2+) ad is higher than that for (Au(n)+1O2+) ad by a factor of approximately 4.0 (1.2) for M = Ti (M = Fe). The trends with size n are rationalized in terms of O-O and O-M bond distances, as well as charge transfer between oxygen and cluster substrates. The spin multiplicity of those (MAu(n)O2+) ad complexes with the highest O2 adsorption energy is a maximum (minimum) for M = Fe (Ti), corresponding to parallel (anti-parallel) spin coupling of MAu(n)+ clusters and O2 molecules. Finally, we obtained the minimum energy equilibrium structure of complexes (Au(n)O2+) dis and (MAu(n)O2+) dis containing two separated O atoms bonded at different sites of Au(n)+ and MAu(n)+ clusters, respectively. For (MAu(n)O2 (+)) dis, the equilibrium configuration with the highest adsorption energy is stable against separation in MAu(n)+ and O2 fragments, respectively. Instead, for (Au(n)O2+) dis, only the complex n = 6 is stable against separation in Au(n)+ and O2 fragments. The maximum separation energy of (MAu(n)O2+) dis is higher than the O2 adsorption energy of (MAu(n)O2+) ad complexes by factors of approximately 1.6 (2.5), 1.6 (1.7), 1.5 (2.4), 1.5 (1.3), and 1.6 (1.8) for M = Ti (Fe) complexes in the range n = 3-7, respectively.     

Effects of the HCN adsorption on the structural and electronic parameters of the beryllium oxide nanotube

The adsorption behavior of the HCN on the surface of beryllium oxide nanotube (BeONT) is studied by the density functional theory. Geometrical parameters, electronic properties and adsorption energies have been calculated for the BeONT and fourteen different HCN configurations on the nanotube. According to the obtained results, the process of the HCN molecule adsorption on different sites on the external surface of the nanotube is exothermic and all of the configurations are stable, while the process of HCN molecule adsorption on the internal surface of the BeONT is endothermic. The adsorption energy values indicate that the HCN molecule can be physically adsorbed on the surface of the BeONT. Furthermore, the HOMO–LUMO gap (Eg) of the BeONT decreases upon the HCN adsorption, resulting in the enhancement of the electrical conductivity. The AIM theory has been also utilized to analyze the properties of the bond critical points: their electron densities and their Laplacians. NBO analysis indicates that the HCN molecule can be adsorbed on the surface of the nanotube with a charge transfer from nanotube to HCN molecule. Due to the physisorption, NQR parameters of nanotube are also altered. In order to examine the deformation degree of the nanotube after HCN molecule adsorption, deformation energy is calculated, which indicates that no significant curvature in the geometry of the nanotubes is occurred when HCN adsorbs onto the surface of BeONT.     

Local analysis and comparative study of the hydrogen bonds in the linear (HCN)n and (HNC)n clusters

B3LYP/6-311+G(2d,p), the density functional theory method of 98 package, is applied to study the hydrogen bonding of a series of linear (HCN)_n and (HNC)_n molecular clusters (for n=1–10). By the localization analysis methods we developed, pair-wised σ type H-bond orders and bond energies are calculated for each pair of the two near-by molecules in both (HCN)_n and (HNC)_n clusters. The calculated results are checked well with the shortening of N–H or C–H distance, the elongation of CH or NH bond distance, and the red shift of stretching frequencies of CH or NH. All pieces of evidence show that the central pair of the two molecules forms the strongest H bond when n of (HCN)_n or (HNC)_n is even, and the two middle pairs form the two strongest H bonds when n is odd. Two terminal pairs of HCN or HNC molecules always form the two weakest H-bonds in each molecular cluster. When comparing molecular cluster energies between (HCN)_n and (HNC)_n for various values of n, the well-known (HCN)_n is found more stable than the related (HNC)_n from energy calculation. However, if outcomes of H-bond local analysis are contrasted, our analysis significantly shows that inter-molecular H-bonds inside of (HNC)_n clusters are much stronger than the corresponding H-bonds in (HCN)_n with the same n. In comparing energy differences between these related clusters per monomer, [E_(HNC)n−E_(HCN)n]/n is found decreasing monotonically as n increases. All pieces of evidence from this theoretical prediction indicate that (HNC)_n with large n is probably constructed by its relative strong H-bonds.     

A germanium isocyanide complex featuring (n → π*) back-bonding and its conversion to a hydride/cyanide product via C-H bond activation under mild conditions

Reaction of the diarylgermylene Ge(Ar(Me(6)))(2) [Ar(Me(6)) = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-(CH(3))(3))(2)] with tert-butyl isocyanide gave the Lewis adduct species (Ar(Me(6)))(2)GeCNBu(t), in which the isocyanide ligand displays a decreased C-N stretching frequency consistent with an n → π* back-bonding interaction. Density functional theory confirmed that the HOMO is a Ge-C bonding combination between the lone pair of electrons on the germanium atom and the C-N π* orbital of the isocyanide ligand. The complex undergoes facile C-H bond activation to produce a new diarylgermanium hydride/cyanide species and isobutene via heterolytic cleavage of the N-Bu(t) bond.     

IR Spectra of Ethane Adsorbed on the Hydrogen,Sodium, and Zinc Forms of a Y-Type Zeolite: Interpretation Using <Emphasis Type="Italic">ab initio</Emphasis> Quantum Chemical Calculations

Ethane adsorption on the hydrogen, sodium, and zinc forms of faujasite brings about polarization anisotropy of the C-H stretching vibrations. This anisotropy shows itself most clearly as perturbation of the vibrational mode analogous to the breathing C-H mode ν₁ of free ethane. The relative intensity and the low-frequency shift of the absorption band due to the distorted ν₁ mode increase with increasing perturbation caused by the C₂H₆-adsorption site interaction. The polarizing power of adsorption sites increases in the order H⁺ < Na⁺ < Zn²⁺. The C-H stretching vibrations in ethane adsorbed on the cationic forms of the Y zeolite are not symmetry-forbidden; accordingly, adsorbed ethane gives more absorption bands than gaseous ethane. The interaction between ethane and zinc cations in the Y zeolite structure eliminates not only the symmetry forbiddenness but also the twofold degeneracy of the C-H stretches.__________Translated from Kinetika i Kataliz, Vol. 46, No. 3, 2005, pp. 434–440.Original Russian Text Copyright © 2005 by Pidko, Kazanskii.     

Overtone spectroscopy of chlorine substituted benzene derivatives

The absorption spectra of di-, tri- and tetra-derivatives of chlorobenzene have been studied in their pure form in the spectral range 400-20,000 cm(-1). A large number of bands associated with the fundamental, the overtones and the combination frequencies of C-H stretching mode have been observed. Vibrational frequencies, anharmonicity constants and dissociation energies, for the C-H stretch vibrations for the six molecules have been determined using local mode model. The C-H stretch frequencies obtained from experiments are compared with the corresponding frequency determined theoretically using RHF and DFT methods with same 6-31+G* basis set. This information has been used for the assignment of several combination bands as well as some weak overtone bands. Effect of hydrogen atom substitution by chlorine atom has been studied by measuring changes in the vibrational frequency of the C-H stretching mode and the C-H bond length. Frequency changes have been well correlated with the change in charge density on carbon as well as chlorine atoms.     

Infrared and DFT investigations of the XC[triple bond]ReX3 and HC[triple bond]ReX3 complexes: Jahn-Teller distortion and the methylidyne C-X(H) stretching absorptions

The XC[triple bond]ReX3 complexes (X = F, Cl) are produced by CX(4) reaction with laser-ablated Re atoms, following oxidative C-X insertion and alpha-halogen migration in favor of the carbon-metal triple bond and are identified through the observation of characteristic absorptions in the argon matrix infrared spectra and comparison with vibrational frequencies calculated by density functional theory. The methylidyne C-F and C-Cl stretching absorptions are observed near 1584 and 1328 cm-1, and the C-H stretching modes for HC[triple bond]ReX3 at 3104 and 3097 cm(-1), respectively, which are substantially higher than the precursor stretching modes and in agreement with the general trend that higher s-orbital character in carbon hybridization leads to a higher stretching frequency. The Jahn-Teller effect in the doublet-state XC[triple bond]ReX3 and HC[triple bond]ReX3 complexes gives rise to distorted structures with Cs symmetry and two equivalent longer Re-X bonds and one slightly shorter Re-X bond.     

Molecular water on exposed Al3+ cations is a source of acidity in silicoaluminas

Evidence about the remarkable acidic nature of molecular water adsorbed at the surface of mesoporous MCM-41 silicoaluminas (MSA) with a ratio of Si/Al = 25 is reported, coming from both infrared spectroscopy and ab initio calculations. By using CO as a probe, OH stretching and HOH bending modes of water adsorbed on coordinatively unsaturated Al ions (Lewis sites) at the surface of MSA have been detected for the first time. CO adsorption causes OH stretching frequencies to suffer a bathochromic shift of about -200/-250 cm(-1), whereas the HOH bending frequency undergoes a hypsochromic shift by about +10/+20 cm(-1). B3LYP/6-31+G(d,p) calculations on model clusters designed to mimic both Br?nsted and Lewis sites validate the assignments, showing that water adsorbed on the surface Lewis site, when interacting with CO, shows an acidity comparable to that of a classical zeolitic Br?nsted site.