CO Bond Strength Research Articles

Difluorodioxirane: An Unusual Cyclic Peroxide

The surprisingly high stability of the dioxirane CF2O2 (2), its unusual geometry, its infrared spectrum, and NMR chemical shifts are determined and analyzed on the basis of extended ab initio calculations including seven different methods and nine different basis sets. At the highest level of theory, the CCSD(T) approach has been used together with a cc-VTZ2P+f basis set, which leads to an accurate description of geometry and vibrational frequencies. Stabilizing CF,CF bond interactions add 19.5 kcal/mol and CF,CO bond interactions 12 kcal/mol to the stability of the dioxirane ring. The latter effect reduces ring strain from 32.8 (dioxirane) to 20.5 kcal/mol (2) where 17 kcal/mol are due to CO bond strengthening and 4.6 kcal/mol to OO bond weakening. Changes in CO and OO bond strength are caused by a transfer of negative charge from the CF2 group to the antisymmetric Walsh MO of the ring. The calculated ΔHf°(298) value of 2 is −102 ± 1.5 kcal/mol, which indicates that 2 is thermodynamically rather stable. Calculated 13C (133 ppm) and 17O NMR chemical shifts (403 ppm) are unusually positive for an organic cyclic peroxide, but should facilitate the identification of 2 in the presence of its isomers F2COO and FC(O)OF, which possess ΔHf°(298) values of −60 and −104 kcal/mol, respectively.

Theoretical study of the monocarbonyls of first-row transition metal atoms

The results of density functional calculations on the most stable high-spin and low-spin states of MCO are given, where M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. The ground states are found to be high spin for M=Sc, Ti, V, Cr, and Cu (2S+1=4, 5, 6, 7, and 2, respectively) and low spin for M=Mn, Fe, Co, and Ni (2S+1=4, 3, 2, and 1, respectively). From Sc to Cu, the M–CO binding energies with respect to ground state products are estimated to be 9, 16, 26, 13, −14, 14, 30, 54, and 19 kcal/mol. Where comparison with experiment is possible, the estimates are apparently too large by about 6 kcal/mol (FeCO), 13 kcal/mol (NiCO), and 12 kcal/mol (CuCO). The high-spin state MCO complexes with metal to the right of vanadium in the Periodic Table all have bent equilibrium geometries; all others are found to be linear. The calculated CO harmonic stretch frequencies generally overestimate the observed values, but follow a similar trend. The CO bond lengths, CO stretch frequencies, and metal–CO bond strengths all correlate well with the extent of π back donation. However, these correlations hold only within either the group of all high-spin states, or the group of all low-spin states. Thus, there are no simple trends in the calculated properties of ground state MCO complexes.

Fourier-transform infrared, attenuated total reflection spectroscopy: a complementary tool for the investigation of the adsorption of CO on thin metal layers

The application of the attenuated total reflection (ATR) technique to the study of the adsorption of CO on thin films of Pt or Pd from dry and wet gases and from water was investigated. Metal layers, thinner than the depth of penetration of IR light, were deposited on a germanium reflection element by sputtering or by evaporation. These films were characterized by different techniques such as STM, electron microprobe analysis and SIMS. Adsorption of CO was carried out in a flow-through cell. The effect of increasing coverage was studied.On Pt the adsorbed CO was mainly linearly bonded (> 2000 cm–1). The asymmetric shape of the bands indicated that different CO bond strengths coexist on the rough surface. Both linear and bridged-bonded CO adsorb at a lower frequency in the wet than in the dry state. The band position shifts to higher frequencies with increasing coverage. The platinum layer was not stable in contact with the liquid.On Pd the adsorbed CO was both linearly bonded (ca. 2060 cm–1) and bridged bonded (ca. 1880 cm–1). The relative intensity of the two bands varied considerably from one experiment to another. The asymmetry of the band for the bridged CO was greater than that for the linear CO. The relative intensityllinear/lbridged increased from dry to wet gas and decreased from wet gas to liquid. The frequency of the bridged CO was also increased from dry to wet gas and decreased from wet gas to liquid. Band positions within one experiment are constant, but vary with the Pd layer.

The quantum chemical basis of the Fischer-Tropsch reaction

The electronic features determining the reactivity of CO and CH X on transition metal surfaces are reviewed. Focus is on the relevant features that control the Fischer-Tropsch synthesis. The CO dissociation reaction path is controlled by the interaction with the CO bond strength weakening 2π* orbitals. CH3 fragment adsorption is controlled by σ type molecule fragment orbitals. This directs the CH3 fragment to the atop adsorption site on those late transition metals that have strongly interacting d-valence electrons. Adsorbed C and O have a stronger bond strength than CH3 because they have also unoccupied atomic p orbitals available to bonding. Because the bond strength of adsorbed C and O increases more rapidly with depletion of d-valence electron occupation than that of CO, the activation energy for CO dissociation decreases for the corresponding transition metals towards the left of the periodic system. The rate of methanation versus chain growth is controlled by the strength of the M-CH3 bond versus the activation energy of carbon-carbon bond formation. The first appears to be more sensitive to variations in metal carbon bond strength than the latter.

CO chemisorption on free gas phase metal clusters

Pulsed fast flow reactor techniques have been used to study the reactivity of CO toward clusters of many different transition metals; V, Fe, Co, Ni, Cu, Nb, Mo, Ru, Pd, W, Ir, and Pt as well as Al, for clusters containing up to 14 atoms. Clusters are produced by pulsed laser vaporization of metal substrates, injected into the reactor, formed into a molecular beam, and detected by photoionization time of flight mass spectrometry. Our results show that CO is readily chemisorbed on most transition metal clusters containing five or more metal atoms, and that the reactivity for larger clusters varies by a factor of 2 or 3, depending on both cluster size and metal type. Depending on the metal, certain atoms, dimers, trimers, and tetramers exhibit little evidence of reactivity toward CO. This observation is explained in terms of a competition between unimolecular decomposition and collisional stabilization, and leads to a prediction of the ordering of the metal–CO bond strengths.

Estimation of the number of co molecules adsorbed in various modes on different crystal planes of alumina supported polycrystalline palladium

The chemisorbed CO species formed on different crystal faces of polycrystalline palladium is not uniformly held on the metal surface. Parallel infrared and chemisorption measurements have demonstrated that the two-fold coordinated CO molecules on the Pd(111) plane are the most strongly adsorbed species. Under room-temperature evacuation of a sample with monolayer coverage about 20% of the double-bonded CO molecules on Pd(100) are desorbed. As to be expected, the weakest adsorbed CO species is the linear one on both the Pd(111) and Pd(100) faces. Using the variation in the CO bond strength some specially conducted experiments have made possible to find out a correlation between the integrated band intensities and the number of CO molecules adsorbed in various modes on the exposed crystal planes of the metal. Ratios between the integrated absorption coefficients of the IR bands from CO adsorbed in various modes were estimated. Empirical equations expressing a correlation between the number of CO molecules adsorbed on a definite face in a particular mode and the integrated intensities of the respective IR bands are proposed. Speculations concerning the crystal structure of the palladium particles are also presented.

Size dependence of surface cluster models: CO adsorbed on Cu(100).

We have studied the strong variation of cluster-model binding energies of the CO/Cu(100) adsorbate system for clusters with between 1 and 34 atoms to represent the Cu surface. Based on the constrained space orbital variation approach the metal-CO interaction is decomposed into three terms: (1) charge superposition of the free CO and metal subunits, (2) charge polarization within the subunits, and (3) charge transfer between the subunits. The results show that one can identify bonding contributions which vary slowly with cluster size and shape. Further, relations between the cluster representation of the Cu(100) conduction band and the metal--CO bond strength can be established. These relations make it possible to determine the importance of cluster artifacts for the metal--CO bonding from electronic properties of the bare metal cluster. Our results provide further support for the validity of using clusters with small numbers of substrate atoms to describe chemisorption on metal surfaces.

Interaction of carbon monoxide with Ru/γ-Al2O3. An electron spin resonance investigation

Interaction of carbon monoxide with Ru/γ-Al2O3 leads to the formation of RuI and RuIII carbonyl derivatives. The relative amounts of the two species are a function of the temperature used in decarbonylating the Ru3(CO)12/γ-Al2O3 precursor. The thermal and vacuum stabilities of the RuI derivative [g⊥= 2.050, g‖= 1.992, A‖(99Ru, 101Ru)= 28 G] and the RuIII derivative (g‖= 2.046, g⊥= 2.005) are different, the RuIII species being the more stable. The Ru—CO bond strength of these carbonyls is high in comparison with that of the analogous Rh/γ-Al2O3 carbonyl derivatives. The enhanced bond strength may account for the difference in the catalytic behaviour between the Ru and Rh systems.

Molecular orbital studies on the CO/Ni chemisorption system in presence of alkali additives and steps

The atom superposition and electron delocalization molecular orbital theory has been used to study the effect of alkali additives and steps on the dissociative chemisorption of CO molecule on Ni(001) surface. We have found that with increasing concentration of alkali atoms the CO bond length increases, the CO bond strength weakens and the CO dissociation barrier decreases. The results are in qualitative agreement with experimental results. The steps have also been found to reduce the dissociation barrier quite significantly compared with the reduction by alkali additives.

Surface Chemistry of Thin Palladium Films

ABSTRACTThe properties of Pd monolayers on several transition metal substrates are dramatically different from the surface properties of bulk Pd single crystals. The changes include a large rehybridization of the Pd atoms to a noble metal electronic configuration and a concomitant reduction in CO and H2 chemisorption bond strengths. We present here temperature programmed desorption (TPD) studies of CO, NO, H2, and C2H4 chemisorption on thin epitaxial Pd films on Nb(110). Our resuAs confirm for all of these gases a large reduction in the chemisorption bond energy for adsorption on Pd thin (<2 ML) films. Our TPD results for CO, which give a CO binding energy of about 10 kcal/mole, confirm previous UPS studies which show that a Pd monolayer forms only a weak chemisorption bond to CO. Thicker Pd films ( > 5 ML ) have CO TPD spectra that agree qualitatively with previous studies on Pd(111) single crystals, where the CO binding energy is about 30 kcal/mole. These results for Pd/Nb(110) will be discussed with respect to recent work on related systems involving thin Pd films on other transition metal substrates.

Substrate effect on the growth of iron clusters in Y zeolite

Investigation of the decomposition process and of the thermolytic products obtained from Fe(CO) 5/faujasite adducts by thermogravimetric, IR-spectroscopic, Mössbauer spectroscopic and X-ray absorption measurements (EXAFS) provides evidence for a substrate effect on the growth process of iron clusters. CsY substrate increases the FeCO bond strength. The stabilized intermediates generated by this effect upon thermolysis at 500 K are easily oxidized to small iron(III) oxide clusters, whereas with NaY substrate to a large extent iron(O) particles are generated. The latter show Mössbauer effect and EXAFS spectra comparable to those obtained from bulk iron. An inner oxidation process is assumed to be involved in the generation of the zeolite-supported iron oxide.

Paramagnetic metal and oxygen species observed with [CO4(CO)12] and [Rh4(CO)12] carbonyl clusters pyrolysed on γ-alumina and zirconia supports

The pyrolysis of γ-Al2O3- and ZrO2-supported [Co4(CO)12] and [Rh4(CO)12] gives highly dispersed metal systems whose electronic properties have been studied by e.s.r. spectroscopy. Paramagnetic formal RhII species (g∥= 2.21, g⊥= 2.11) were observed on Rh/γ-Al2O3 samples. The pattern of the hyperfine structure suggests that metal–metal interactions are present and that paramagnetic transition-metal carbonyl clusters can be assumed as model compounds. No paramagnetic species involving the metal were observed for either γ-Al2O3- or ZrO2-supported cobalt samples. This behaviour is consistent with a higher acidic character of γ-Al2O3 with respect to ZrO2. On contact with O2 and CO the following paramagnetic species were observed: O–2—Al3+(g∥= 2.034, g⊥= 2.00) over Co/γ-Al2O3 samples, O–2—RhIII(g1= 2.09, g2= 2.03, g3= 2.00) over Rh/γ-Al2O3 samples, O–2—RhIII and O–2—Zr4+(g∥= 2.03, g⊥= 2.00) over Rh/ZrO2 samples and CO—RhII(text-decoration:overlineg= 2.034) over Rh/γ-Al2O3 samples. The O2 and CO bond strengths are higher for the γ-Al2O3-supported samples than for the ZrO2-supported samples and are tentatively related to the catalytic activity of these metal systems.

Temperature induced effects in the coadsorption system: CO/D 2/Ni(100)

The temperature dependent changes in the CO/D/Ni(100) system have been examined using thermal desorption spectroscopy, work function change measurements, ultra-violet photoelectron spectroscopy and X-ray photoelectron spectroscopy. Three forms of CO(a) have been found, one of which exists only in the intermediate temperature range of 130–215 K. This form is believed to be a CO(a) tilted with respect to the surface normal, with significant ONi interaction. In addition, a new interpretation of several UPS parameters measured for CO(a) is presented. It is proposed that for CO(a), the 5σ−1π separation is a probe of bonding geometry, the 4σ−1π separation reflects COCO repulsion, and the 4σ 5σ intensity ratio reflects the NiCO bond strength.

Temperature induced effects in the coadsorption system: CO/D 2/Ni(100)

Abstract The temperature dependent changes in the CO/D/Ni(100) system have been examined using thermal desorption spectroscopy, work function change measurements, ultra-violet photoelectron spectroscopy and X-ray photoelectron spectroscopy. Three forms of CO(a) have been found, one of which exists only in the intermediate temperature range of 130–215 K. This form is believed to be a CO(a) tilted with respect to the surface normal, with significant ONi interaction. In addition, a new interpretation of several UPS parameters measured for CO(a) is presented. It is proposed that for CO(a), the 5σ−1π separation is a probe of bonding geometry, the 4σ−1π separation reflects COCO repulsion, and the 4σ 5σ intensity ratio reflects the NiCO bond strength.

Theoretical studies of CO/Ni(100): Geometry, vibrational frequencies and ionization potentials for the on-top site

The chemisorption of CO on the (100) surface of Ni has been studied using an Ni 14 cluster and generalized valence bond (GVB) methods. CO is found to bond perpendicular to the Ni surface with optimized NiC and CO bond distances of 1.94 and 1.15 Å, respectively. The calculated NiCO bond strength is 29.7 kcal (experimental values 30–32 kcal). Vibrational frequencies are calculated to be 401 cm −1 for NiC stretch, 327 cm −1 for NiCO bend, and 2129 cm −1 for CO stretch. This decrease of the CO frequency by 71 cm −1 from the free molecule value is consistent with experiment based on self-consistent calculations of the positive ion states. We propose a new explanation for the loss of one PES peak upon chemisorption.

Interactions of chlorine and bromine with chemisorbed carbon monoxide on evaporated platinum, rhodium, and iridium films

The action of bromine and chlorine on surface carbonyl complexes of platinum, rhodium, and irdium has been studied by ir spectroscopy. At room temperature chlorine and bromine cause partial removal of chemisorbed CO molecules, and produce two other main effects. The stretching frequency of the remaining CO molecules progressively shifts to higher values, corresponding to an increase of the CO bond strength, due to a lowering of the electron back donation dπ → 2π ∗ from the metal to the 2π ∗ antibonding orbitals of the chemisorbed CO molecules. This electron-withdrawing ligand effect of halogen is a long-range effect which uses the collective properties of the metal surface. In the case of Rh and Pt, new carbonyl species which exhibit ir bands at higher frequencies (range 2100–2135 cm −1) have also been detected; the corresponding metal atoms are oxidized. The observation that these metal atoms are insensitive to the ligand electron-donor effect of P(CH 3) 3 is justified in terms of “demetallization”.

Vibrational EELS studies of CO chemisorption on clean and carbided (111), (100) and (110) nickel surfaces

Room temperature adsorption of CO on bare and carbided (111), (100) and (110) nickel surfaces has been studied by vibrational electron energy loss spectroscopy (EELS) and thermal desorption. On the clean (100) and (110) surfaces two configurations of CO adsorbed species, namely “terminal” and bridge bonded CO, are observed simultaneously. On Ni(111), only two-fold sites are involved. The presence of superficial carbon lowers markedly the bond strength of CO on Ni(111)C and Ni(110)C surfaces, while no adsorption has been detected on the Ni(100)C surface. Moreover, on the carbided Ni(110)C surface, the adsorption mode for adsorbed CO is changed with respect to the clean surface; only “terminal” CO is then observed.

Theoretical characterization of the potential energy surface of the ground state of the HCO system

Large scale, ab initio configuration interaction calculations are reported on the ground state potential energy surface of the HCO system. The calculated equilibrium geometry of HCO of RCH=1.12 Å, RCO=1.19 Å, and ϑHCO=126° is in good agreement with that determined experimentally, although the calculated H–CO bond strength De=16 kcal/mole is too small by ∼3 kcal/mole. A barrier of 7.5 kcal/mole is found for recombination of H+CO; the height of the barrier is estimated to be too large by 5–6 kcal/mole. The COH isomer, with a calculated equilibrium geometry of ROH=0.98 Å, RCO=1.29 Å, and ϑCOH=114°, is predicted to lie 40 kcal/mole above the HCO isomer, or 24 kcal/mole above the H+CO limit. For COH the barrier for dissociation of 16 kcal/mole is substantially less than that for isomerization (29 kcal/mole) in spite of the fact that isomerization is energetically more favorable. The COH isomer has not yet been identified experimentally. For the HCO system there seems to be no significant difference between the variational and perturbation calculations which have been reported, aside from a constant shift in the energy.

CNDO/2 study of the isoelectronic series of complexes dicarbonyl(dinitrogen)- and dicarbonyl-η-benzenechromium, -η-cyclopentadienyl-manganese, -η-cyclobutadieneiron, and -trismethylenemethane-iron obtained by the matrix–isolation technique

The electronic structure of the isoelectronic series of [ML(CO)2(N2)] and [ML(CO)2]complexes [ML = Cr–(η-C6H6), Mn(η-C5H5), Fe(η-C4H4), or Fe{C(CH2)3}] has been investigated within a CNDO/2 formalism, using both experimental and standard geometries. The computed trends for CO and N2 bond strengths in the complexes, as measured by Wiberg indices, charge distributions, and orbital populations, are well correlated with the experimental CO and N2 stretching frequencies and energy-factored stretching force constants, i.e. Cr < Mn < Fe and [ML(CO)2] < [ML(CO)2(N2)] < [ML(CO)3]. The computed total energies of the [ML(CO)2] complexes suggest that in all cases a tilted geometry is preferred, i.e. the plane of the L ligand is not perpendicular to the M(CO)2 plane. New CNDO/2 calculations have been carried out for [Fe{C(CH2)3}(CO)3] which is shown to fit in well with other [ML(CO)3] complexes.

ChemInform Abstract: NEGATIVE‐ION MASS SPECTRA OF TRINUCLEAR CARBONYL CLUSTERS

Negative-ion mass spectra at 70 eV of the clusters [Fe3(CO)12], [FeRu2(CO)12], [Ru3(CO)12], [Ru2Os(CO)12], [RuOs2(CO)12], and [Os3(CO)12] are reported. Negative molecular ions are absent, and the trinuclear gragments formed are due to the loss of CO groups mainly by dissociative electron-capture processes. A decreasing intensity of [M3(CO)11]–, the base peak in the spectrum of [Os3(CO)12], is observed on passing from Os to Ru and to Fe. The ion [Ru3(CO)10]– is the base peak in the spectrum of [Ru3(CO)12] and [Fe3(CO)9]– in that of [Fe3(CO)12]. The same behaviour is observed in the mixed-metal carbonyls. This trend is explained by the different stability of the ionic species, which it is suggested is mainly related to the strength of the metal–metal bonds. The negative-ion mass spectra of [Ru3(CO)11(PEt2Ph)] and [Os3(CO)11(PEt2Ph)] indicate that both the M–CO bond strengths and the ionisation potentials of the metals do not play a major role. The negative-ion mass spectra of the compounds [Fe3(CO)9X2](X = S, Se, or Te) follow a similar behaviour and exhibit [Fe3(CO)7X2]– as the base peak with a weak [Fe3(CO)8X2]– when X = Te.