Molecular Orbital Approach Research Articles

B3LYP DFT molecular orbital approach, an efficient method to evaluate the thermochemical properties of MALDI matrices

The thermochemical properties of seven UV-MALDI matrices and two IR-MALDI matrices were evaluated by means of ab initio molecular orbital calculations using the B3LYP functional density approach with 6-31+G(d,p) and 6-311+G(2d,2p) basis sets. These thermochemical properties include ionization energy, proton and electron affinities of neutral molecules and proton affinity of deprotonated molecules. The choosen UV-MALDI matrices are: 2,5-dihydroxybenzoic acid (2,5DHB), nicotinic acid (NA) and 2-aminonicotinic acid (2ANA), picolinic acid (PA) and 3-hydroxypicolinic acid (3HPA), 2-aminobenzoic acid (2ABA) and 4-nitroaniline (4NiAn) while the studied IR-MALDI matrices are urea (U) and glycerol (G). The proton affinities of neutral and deprotonated molecules are in very good agreement with the available experimental values (the differences between experimental and calculated values are smaller than the precision on the experimental measurements); the calculated ionization energies are close to the experimental values of adiabatic electronic transitions; the calculated electron affinities are new and cannot be compared with corresponding experimental values since they are not available.

Electronic signature of the magicity and ionic bonding inAl13X(X=Li–K)clusters

Total energies and equilibrium geometries of neutral and anionic ${\mathrm{Al}}_{n}$ and ${\mathrm{Al}}_{n}X$ $(n=12--14,$ $X=\mathrm{Li},$ Na, K) clusters have been obtained following an optimization of their structure without any symmetry constraint. The calculations are based on molecular-orbital approach and the gradient-corrected density-functional theory. It is shown that although the binding energy of ${\mathrm{Al}}_{13}X$ clusters do not differ much from that of their neighbors, their ionization potentials, highest occupied molecular orbital and lowest unoccupied molecular orbital gaps, and electron affinities bear distinct signature of their magicity. That the binding of ${\mathrm{Al}}_{13}$ with alkali atoms is predominantly ionic is derived by comparing the electron affinity of ${\mathrm{Al}}_{13}X$ with other known salts.

Efficientab initiomolecular-orbital approach to quasi-one-dimensional molecular crystals based on neighboring-interaction-localized molecular orbitals

To obtain electronic states in molecular crystals efficiently, a quantum chemical method that utilizes the localization technique for providing the corresponding orbitals is presented. This localization technique enables us to diagonalize the large matrix for the entire system by means of eigenvalue problems for small dimensions of the number of molecules. To confirm the reliability of this treatment, the electronic states provided by this method were compared with those provided by the tight-binding method for periodic systems.

Valence Bond Configuration Interaction: A Practical ab Initio Valence Bond Method That Incorporates Dynamic Correlation

A post-VBSCF method, called valence bond configuration interaction (VBCI), is developed here. The method incorporates dynamic correlation, by use of configuration interaction (CI) to improve the energetics after a VBSCF calculation. The levels of CI are fashioned as in the corresponding molecular orbital approach. Thus, VBCIS involves only single excitations, while VBCISD involves also doubles, and so on. The VBCI method retains the simplicity of a VB presentation by expressing the wave function in terms of a minimal number of effective structures that dictate the chemistry of the problem. The method was tested by calculating the bond energies of H2, LiH, HF, HCl, F2, and Cl2 as well as the barriers of identity hydrogen abstraction reactions, X• + X‘H → XH + X‘• (X, X ‘ = CH3, SiH3, GeH3, SnH3, or PbH3). It is shown that VBCIS gives results that are at par with breathing orbital valence bond method. The VBCISD method is better and its results match those of the molecular orbital based coupled cluster CCSD method. Future potential directions of the development of the VBCI approach are outlined.

A combined theoretical and experimental study of supported vanadium oxide catalysts

In order to investigate the nature of VO x species supported on SiO 2, γ-Al 2O 3 and TiO 2, a combination of a theoretical study and an experimental characterization is carried out. A molecular orbital approach of the extended Hückel type is performed to measure the plausibility of different postulated VO x species (monovanadates, divanadates, polyvanadates, etc.). The experimental characterization is performed by TPR, oxygen uptake measurements and FTIR of adsorbed pyridine. Both theoretical and experimental results indicate that the structure of VO x and its interaction with the support are strongly influenced by the nature of the oxide carrier. On TiO 2 and γ-Al 2O 3 dimers are the most abundant species, while on SiO 2 only monomers are formed.

Aqueous Solution Inclusion of the Nonionic Surfactant C 12 E 4 in β -Cyclodextrin: Implications of Micellization in Stoichiometry Determination and Model Calculations

The inclusion of α-n-dodecyl-ω-hydroxytetra(oxyethylene), C12E4, in β-cyclodextrin (βCD) has been studied in aqueous solution. Guest encapsulation is recognized by the upfield variations in the chemical shifts of the H3 and H5 inner protons of βCD, and the chemical shift differences of the H5 protons are used for determining the complex stoichiometry (2βCD : 1C12E4) by the continuous variation method. Self-association (micellization) of the surfactant molecules is considered, and the relative amount of surfactant involved in micellar systems at the stoichiometric point estimated. A two-layered integrated molecular orbital and molecular mechanics approach with the PM3 and UFF model chemistries for the guest and host, respectively, was used to perform full geometry optimizations and frequency calculations on the host-guest systems. Energies for the optimized structures were subsequently obtained by single point calculations at the Hartree–Fock level using the STO-3G basis set. These calculations showed that one C12E4 molecule encapsulated by a head-to-head βCD dimer is a stable model system in consonance with the experimentally determined stoichiometry, and that the 1 : 1 complex is not stable with respect to dissociation. In the stable 2 : 1 model system, the guest molecule is appreciably tilted with respect to the βCD dimer axis and presents a gradually bent alkylic chain in clear manifestation of conformational flexibility. Model calculations for βCD inclusion complexes of other oligo(oxyethylene) molecules further indicate that the number and strength of H⋯H intermolecular close contacts reflect the position and conformational flexibility of the guest hydrophobic chain inside βCD.

An analytical molecular orbital approach for modeling of low-dimensional conductors in molecular crystals

In low-dimensional molecular crystals that exhibit Peierls instability, the intercolumnar interaction plays an important role in the determination of the conducting property. To predict the crystal structure that exhibits an electronic conduction, we proposed a simple formula based on the molecular orbital theory. In this approach, analytical molecular orbitals of polyene were extended to be applicable for molecular crystals of charge–transfer complexes, and the Peierls distortion was represented by using the first- and second-order perturbation theory. It was concluded that the energetic behaviors and structures calculated by the analytical method developed in the present work are identical to those obtained by the tight-binding ab initio molecular orbital calculation. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Theoretical characterization of alumina and sulfated-alumina catalysts for n-butene isomerization

A theoretical study was carried out in order to characterize the changes produced by the presence of sulfate on the alumina surface. A molecular orbital approach of the extended Hückel (EH) type together with dynamic molecular calculation were performed to evaluate adsorption and desorption energies of the intermediate species for skeletal and double-bond isomerization reactions of n-butene The bimolecular and monomolecular mechanisms for skeletal isomerization were evaluated.

Molecular and electronic structures of six-coordinate W complexes and polyanions containing tri-oxo groups

The molecular and electronic structures of [WO 3(OL) 3] (L: CN, H 2, H) complexes and of the [W 2O 9] 6− and [W 4O 16] 8− polyanions have been investigated using density-functional methods. Structural and bonding analyses have been performed employing various molecular-orbital and population approaches. Correlations of the trans-influence type between bonds to unshared and to high-coordinate oxygen atoms have been observed for geometrical and electronic parameters, such as WO distances and orders. The ( trans) bridging bonds have been found to be (separately) rather weak, as suggested by their (long) lengths, but the valency and orbital-interaction results have indicated that the overall bonding capacity and strength are similar for all oxygen atoms in each individual polyanion.

Simultaneous optimization of Gaussian type function exponents for electron and positron with full-CI wavefunction – application to ground and excited states of positronic compounds with multi-component molecular orbital approach

In order to obtain the best wavefunction of positronic compound with molecular orbital (MO) treatment, the full-configuration interaction (full-CI) fully variational MO (FVMO) method is proposed for multi-component systems, in which all the variational parameters in electronic and positronic wavefunctions are optimized under the full-CI scheme. We have applied the full-CI multi-component FVMO method to the ground and positronic-excited states of [H −;e +] system. Our treatment gives good improvement in the basis functions for positronic compounds owing to the extension of flexibility in the variational space, though the convergence of electron–positron correlation term is slower than that of conventional electron correlation.

A scattered wave model of electronic structure of Eu 2+ in BaMgAl 10O 17 and associated excitation processes

The excitation spectrum of a commercial blue phosphor, BaMgAl 10O 17:Eu 2+ (BAM) has been investigated assuming a distribution of the activator ions at three different sites: Beevers–Ross, anti-Beevers–Ross and mid-oxygen sites in the intermediate plane. The energy levels and wavefunctions of the Eu 2+ ions located at these sites have been calculated using the self-consistent field Xα scattered wave cluster molecular orbital approach (SCF-Xα-SW-MO). The molecular orbital eigenvalues are then utilized to develop an energy level diagram and the associated excitation spectrum. A comparison of the calculated excitation peaks with those from experiment provides support for distribution of Eu 2+ ions at multiple sites in BAM.

Comment on the Electronic Reorganization in 1,3-Dipolar Cycloaddition of Fulminic Acid to Acetylene

Department of Chemistry, UniVersity of LeuVen,Celestijnenlaan 200F, B-3001 LeuVen, Belgium, NationalInstitute of AdVanced Industrial Science and Technology,AIST Tsukuba Central 5, Tsukuba 305-8565, Japan, andDepartment of Information Systems Engineering, Faculty ofEngineering, Osaka Sangyo UniVersity, Daito 574, JapanReceiVed: January 23, 2001; In Final Form: July 5, 2001In this note, we comment on the electronic mechanism ofthe concerted 1,3-dipolar cycloaddition reaction of fulminic acidto acetylene (HCNO + HCCH), which has recently been studiedby valence bond and molecular orbital approaches by fourdifferent groups including ourselves: (i) Sakata, K. J. Phys.Chem. A 2000, 104, 10001, (ii) Harcourt, R. D.; Schulz, A. J.Phys. Chem. A 2000, 104, 6510, (iii) Nguyen, M. T.; Chandra,A. K.; Sakai, S.; Morokuma, K. J. Org. Chem. 1999, 64, 65,and (iv) Karadakov, P. B.; Cooper, D. L.; Gerratt, J. Theor.Chem. Acc. 1998, 100, 222. The main discrepancy arising fromthese theoretical studies concerns the electronic reorganization,and in particular, the direction of the electron flow accompany-ing the nuclear deformation at the transition structure. Wepresent additional results supporting our earlier findings that inthe 1,3-dipolar cycloaddition of fulminic acid (HCNO) andprobably substituted nitrile-oxides (RCNO), to alkynes andphosphaalkynes, the oxygen (O) atom behaves as a bondacceptor center and the carbon (C) as a bond donor.The 1,3-dipolar cycloaddition is a general and powerfulmethod for the synthesis of five-membered heterocyclic com-pounds owing to the great variety and availability of dipolesand dipolarophiles.

On the luminescence efficiency of polymer light-emitting diodes: a quantum-chemical investigation

Correlated quantum-chemical techniques are applied to the description of electronic excitations in luminescent conjugated polymers. We first address the role of intermolecular interactions on the emission properties of organic conjugated materials. The nature of the lowest excited states in molecular aggregates is discussed and a special emphasis is devoted to the chain-length dependence of the exciton coupling. By applying a molecular orbital perturbation approach, we then calculate the formation rates for singlet and triplet molecular excitons associated with intermolecular charge-transfer processes. Application of our approach to a model system for poly(paraphenylenevinylene) shows that the ratio between the electroluminescence and photoluminescence quantum yields generally exceeds the 25% spin-degeneracy statistical limit.

Adsorption and Absorption of H on Ni(111) A Theoretical Study based on Molecular Orbitals

The adsorption of monoatomic H on Ni(111) was studied theoretically, taking into account the possibility of H diffusion and eventual incorporation of H as an interstitial. The desorption of H2 as a diffusion-controlled process from the bulk and the influence on adsorption properties due to a H sub-surface phase very near to the adsorbed phase were considered. Our semiempirical molecular orbital approach allows us to analyze the results from an electronic structure perspective, showing that in general the H–Ni bond is weakened by the presence of the H sublayer.

Structure and magnetic properties of Fe-Ni clusters

Using a first-principles molecular-orbital approach, we study the equilibrium geometries and binding energies of small Fe-Ni clusters containing up to four Fe and four Ni atoms. We find that the clusters are ferromagnetic with magnetic moments enhanced relative to the bulk values. For some specific cluster sizes and compositions, different isomers (corresponding to different geometries and clearly different energies) are found to have the same magnetic moment. This behavior is in contrast to the common understanding that magnetism usually depends on the cluster geometry. Calculations have also been carried out by modeling Fe{sub n-1}Ni (n=2,9,15,27) clusters as fragments of a bcc lattice to determine the preferred site of Ni and its effect on the magnetic properties. The results are used to analyze the recent experimental study of the magnetic properties of FeNi nanoparticles.

ADSORPTION AND ABSORPTION OF H ON Ni(111): A THEORETICAL STUDY BASED ON MOLECULAR ORBITALS

The adsorption of monoatomic H on Ni(111) was studied theoretically, taking into account the possibility of H diffusion and eventual incorporation of H as an interstitial. The desorption of H 2 as a diffusion-controlled process from the bulk and the influence on adsorption properties due to a H sub-surface phase very near to the adsorbed phase were considered. Our semiempirical molecular orbital approach allows us to analyze the results from an electronic structure perspective, showing that in general the H–Ni bond is weakened by the presence of the H sublayer.

A combined theoretical and experimental study of the effects of residual chlorine on the behavior of Pd/γ-Al 2O 3 catalysts for methane oxidation

Studies of the methane oxidation on Pd/γ-Al 2O 3 catalysts with and without chloride were made. The reaction was investigated at temperatures in the range 20–500°C using stoichiometric reactant mixture. Dissociation of methane and oxygen and desorption of carbon dioxide and water on Pd catalysts have been investigated using a Molecular Orbital approach of the Extended Hückel type, including repulsion terms.

ChemInform Abstract: Quantitative Harmonization of the Three Molecular Orbital, Valence Bond, and Broken Symmetry Approaches to the Exchange Coupling Constant: Corrections and Discussion

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

A Molecular Orbital Approach to Conformational Study of the Polyisocyanates

A Molecular Orbital Approach to Conformational Study of the Polyisocyanates

A Molecular Orbital Approach to Conformational Study of the Polyisocyanates

A conformational study of poly(alkyl isocyanates) with both non-chiral and chiral side groups (R) is presented. For this purpose the conformational preferences of model compounds CH 3 -(CONR) m -CH 3 , where m is the number of monomers, were investigated by means of quantum mechanical methods. The influence of the number of monomers and the side chain conformation on the relative stability of the different helical minima has been systematically studied. Finally, the influence of the solvent chloroform has been examined by using a self-consistent reaction-field. The results provide a detailed picinte of the modulation exerted by these factors on the helical preferences of these compounds.