Constrained Space Orbital Variation Analysis Research Articles

Estimating π binding energy of N-heterocyclic carbenes: the role of polarization.

In this work, the tuneability of the π acceptor or donor properties of a set of N-heterocyclic carbenes (NHCs) with a wide spectrum of electronic characteristics is established by means of density functional theory and energy decomposition analysis (EDA) tools. Even though the main orbital interaction contribution to the NHC coordination is the σ donation, a significant contribution of the π interactions to the bond is observed. By means of carefully selected coordination sites, different contributions to the π interactions could be identified and isolated. It includes not only the well known back donation and donation interactions, but also the intrafragment polarization, which has not been considered in previous studies. This can be obtained through the use of the extended transition state method for EDA combined with the natural orbitals for chemical valence and the constrained space orbital variation analysis. The contributions vary with the position of the heteroatoms and the presence of exocyclic substituents; the donation/backdonation π interactions between NHC and the coordination site can range between 2 and 61% of the total π orbital interactions, while the rest is owed to intrafragment polarization. Our results do not only contribute to the understanding of the electronic structure of NHC-based complexes, giving ways to improve their catalytic properties, but also provide comprehension on the modelization methods used to study their donor-acceptor interactions.

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.

On the bonding mechanism of CO to Pt(111) and its effect on the vibrational frequency of chemisorbed CO

The chemisorption of CO on the atop site of Pt(111) has been simulated by a Pt4 cluster model. Ab initio self consistent field (SCF) and complete active space self consistent field (CASSCF) cluster model wave functions have been obtained for the electronic ground state. Likewise, ab initio SCF wavefunctions have been obtained for two other electronic states. The optimum geometry and vibrational frequencies of chemisorbed CO are reported for the three states. The interaction energy and vibrational shift of chemisorbed CO, with respect to free gas phase CO, have been analyzed for the three electronic states. This analysis is carried out by means of the constrained space orbital variation (CSOV) method. In all cases the bond is found to be dominated by σ donation and π back-donation, known as Blyholder's mechanism. This mechanism is further supported by SCF calculations on a larger, Pt13, cluster model. For both clusters, the CSOV analysis of the vibrational frequency definitely shows that, contrary to previous recent studies, a major contribution to the experimentally observed vibrational shift comes from the π back-donation mechanism. However, we found that, contrary to common belief, σ donation also acts to lower the CO frequency and not to increase it. Physical reasons for such unexpected behaviour are given.

Ab initio cluster model study of geometry and bonding character of atomic nitrogen chemisorbed on the Cu(100) and Ag(100) surfaces

The chemisorption of atomic nitrogen on the four-fold site Cu(100) and Ag(100) surfaces has been modeled by Cu 5N, Ag 5N and Cu 41N cluster models. For these clusters, ab initio Hartree-Fock electronic wave functions have been obtained and the chemisorption bond analyzed using a variety of techniques. In particular, the interaction energy has been decomposed by means of the constrained space orbital variation (CSOV), method. To obtain a reliable description of the interaction energy, electronic correlation effects have been explicitly accounted for. It is concluded that, contrary to other adsorbates such as halides or oxygen, the bond is essentially covalent but with a large charge transfer from the substrate to the adsorbate. Use of the CSOV analysis permits us to conclude that the direct covalent participation of the metal d electrons to the bond is very important. Finally, a preliminary attempt to study the relaxation of the substrate has been performed for the larger cluster only.

Effects of relativity on the NiCO, PdCO, and PtCO bonding mechanism: a constrained space orbital variation analysis of density functional results

Using a density functional based constrained space orbital variation technique, we have analyzed the importance of relativistic effects on the various mechanisms which affect the metal-CO bond in Ni, Pd, and Pt monocarbonyls. The bonding is dominated by the π back-donation mechanism which, for Pd and Pt, is considerably reinforced by relativistic effects. The weaker PdCO bonding is rationalized by reduced back-donation. The combined effect of relativistic bond length contraction and of the atomic d orbital energies determines the trend in the metal-CO bond strengths. This trend is different from the one found for the coordinatively saturated M(CO) 4 tetracarbonyls.

Adsorption of CO molecules on a MgO(001) surface. Model cluster density functional study employing a gradient-corrected potential

A density functional model cluster study of CO adsorbed on a MgO(001) surface has been carried out in order to accurately estimate bonding and vibrational parameters of the adsorption complex and to make a thorough analysis of the observables. Computed data for extended stoichiometric cluster models of varying size were obtained by employing a gradient-corrected exchange-correlation energy functional and by correcting for the basis set superposition error. The surface Madelung field was represented by embedding the MgO cluster models in an array of point charges; their value, lower than the nominal ionic one, ±2 au, was derived from density functional model slab calculations of an MgO surface. The specified approach brings calculated observables, in particular the adsorption energy, in closer agreement with experiment as compared to previous studies at the local density functional level of theory. A constrained space orbital variation analysis reveals that σ donation from CO to MgO forms, along with the electrostatic attraction, a significant component of the adsorption interaction. A small stabilizing effect of the π back donation from the neighboring oxygen ions to CO has also been found.

Theoretical analysis of the bonding of oxygen to Cu(100).

The chemisorption of O/Cu(100) has been modeled by a ${\mathrm{Cu}}_{5}$O cluster; ab initio self-consistent-field electronic wave functions have been obtained for this cluster. The bonding has been analyzed using several new theoretical methods: (1) the variation of the ${\mathrm{Cu}}_{5}$O dipole moment with the distance of O from the surface; (2) the projection of the O orbitals from ${\mathrm{Cu}}_{5}$O; and (3) the constrained space orbital variation (CSOV) method for the development of the bond. It is concluded that the bond is dominantly ionic but with a significant covalent contribution. Our results indicate that the excess charge on O is \ensuremath{\sim}1.5 electrons. We have computed the CSOV analysis for ${\mathrm{Cu}}_{5}$O and for ${\mathrm{Cu}}_{5}$ with point charges. The comparison of these two clusters has allowed us to have a definitive measure of the contribution of the Cu d electrons to the covalent bond. The total contribution of the d electrons to the bond is rather large (1.2 eV). Once the different contributions are separated, it is shown that purely electrical polarization effects account for \ensuremath{\sim}0.5 eV, while the direct participation of the d electrons in the covalent bond is \ensuremath{\sim}0.7 eV.

On the uniqueness of the constrained space orbital variation (CSOV) technique

Several CSOV analyses are performed for the 1∑ + state of NiCO, and it is shown that the importance of the CO σ donation, Ni π back donation and inter-unit polarizations are virtually independent of the order of the CSOV steps, provided that the open-shell 3d σ and 4s Ni orbitals are orthogonalized to the CO. This order of orthogonalization is consistent with the polarization of the Ni observed in the unconstrianed SCF wavefunction. If instead the CO is orthogonalized to the open-shell Ni orbitals, the frozen orbital repulsion and entire CSOV analysis becomes unphysical. A comparison of the SCF and CAS SCF descriptions for the NICO 1∑ + state shows the importance of the s to d promotion and sd hybridization in reducing the repulsion and increasing the Ni to CO π bonding. For LIF, CSOV analyses starting from both the neutral and ionic asymptotes show the bonding to be predominantly Li + − F −. These examples show the uniqueness of the CSOV decomposition.

A b i n i t i o calculations on the positive ions of the alkaline-earth oxides, fluorides, and hydroxides

Theoretical spectroscopic parameters are presented at the self-consistent-field and singles-plus-doubles configuration-interaction (SDCI) levels for the ground states of the positive ions of the alkaline-earth fluorides, oxides, and hydroxides. The alkaline-earth fluoride and hydroxide ions are found to have X 1Σ+ ground states corresponding to a M+2X−1 structure. The alkaline-earth oxide positive ions, on the other hand, undergo a change in ground state symmetry between CaO+(X 2Π) and SrO+(X 2Σ+), whereas the analogous alkali oxides undergo the change one row higher between NaO and KO. The greater stability of the 2Π states for the alkaline-earth oxide positive ions is due to the increased electrostatic field created by the larger metal ion charge and reduced bond lengths. Calculations on the 3Π states of BeF+, MgF+, BeOH+, and MgOH+ indicate that these states, corresponding to the M+1X structure, lie much higher in energy. The bond distances of the positive ions are uniformly 0.07±0.02 Å shorter than the analogous electronic states of the neutral. By correcting for the relatively small errors in the theoretical bond distances for the neutrals, bond distances accurate to about 0.005 Å are obtained for the ions where experimental values are available for the neutral species. Since extensive one-particle basis sets are employed and differential correlation effects are minimized by dissociating directly to ions, theoretical dissociation energies and ionization potentials (I.P.) accurate to about 0.15 eV can be obtained. The dissociation energies (D0) in eV for the ground states at the SDCI level are found to be: BeO+(4.04), MgO+(2.31), CaO+(3.29), SrO+(3.28), BeF+(6.25), MgF+(4.46), CaF+(5.67), SrF+(5.71), BeOH+(6.00), MgOH+(3.62), CaOH+(4.73), and SrOH+(4.67). For all of the systems studied, the bonding is dominated by electrostatic interactions. However, the molecules containing Be show some covalent character owing to the large I.P. of Be+. We have found that the constrained space orbital variation analysis, which decomposes the bonding into intrafragment polarization and interfragment donations, is an excellent method for delineating the degree of ionic bonding in a molecular system.

Origin of lone pair binding energy shifts in photoemission from adsorbed molecules: CSOV analysis for the Cu 5CO cluster

The chemisorptive interaction of the CO/Cu(100) system is studied with a Cu 5(1,4)CO cluster model using the constrained space orbital variation (CSOV) analysis. This analysis has been used previously to examine the origin of the CO-metal bond and to show that the energetic importance of the metal to CO π donation is larger than that of the CO to metal σ donation. In the present work, we use it to determine the origin of the differential CO 5σ ionization potential shift observed in the photoemission spectra of CO/Cu(100). This shift is shown to be largely due to electrostatic, environmental potential effects. It does not indicate the nature of the CO-metal bond but does indicate the CO adsorption geometry.