Present Density Functional Theory Research Articles

Impurities in Noncubic Crystals: Stabilization Mechanisms for Jahn-Teller Ions in Layered Perovskites

Mechanisms responsible for the local geometry around Jahn-Teller impurities in K2NiF4 type lattices are shown to be different from those generating the warping in cubic crystals. The present density functional theory calculations reveal that the elastic anisotropy of the host lattice (visible for closed shell impurities) and the electric field created by the rest of lattice ions upon active electrons make it possible to have d(9) ions in an elongated geometry but with a A(1g) ground state. The puzzling difference between equilibrium geometries for Cu2+ and Ni+ in layered perovskites can reasonably be understood.

Calculations of static and dynamic polarizabilities of excited states by means of density functional theory.

We present density functional theory and calculations for excited state second order, static or dynamic, properties. The excited state properties are identified from a double residue of a cubic response function. The performance of various functionals, including the generalized gradient approximation and fractional exact Hartree-Fock exchange, is compared to coupled cluster calculations. Applications on excited state polarizabilities of s-tetrazine and pyrimidine show a good agreement with ab initio correlated, coupled cluster, results.

Vibronic coupling and double excitations in linear response time-dependent density functional calculations: dipole-allowed states of N2.

The present study serves two purposes. First, we evaluate the ability of present time-dependent density functional response theory (TDDFRT) methods to deal with avoided crossings, i.e., vibronic coupling effects. In the second place, taking the vibronic coupling effects into account enables us, by comparison to the configuration analysis in a recent ab initio study [J. Chem. Phys. 115, 6438 (2001)], to identify the neglect of double excitations as the prime cause of limited accuracy of these linear response based TDDFRT calculations for specific states. The "statistical averaging of (model) orbital potentials (SAOP)" Kohn-Sham potential is used together with the standard adiabatic local-density approximation (ALDA) for the exchange-correlation kernel. We use the N2 molecule as prototype, since the TDDFRT/SAOP calculations have already been shown to be accurate for the vertical excitations, while this molecule has a well-studied, intricate vibronic structure as well as significant double excitation nature in the lowest 1Pi(u) state at elongated bond lengths. A simple diabatizing scheme is employed to obtain a diabatic potential energy matrix, from which we obtain the absorption spectrum of N2 including vibronic coupling effects. Considering the six lowest dipole allowed transitions of 1Sigma(u)+ and 1Pi(u) symmetry, we observe a good general agreement and conclude that avoided crossings and vibronic coupling can indeed be treated satisfactorily on the basis of TDDFRT excitation energies. However, there is one state for which the accuracy of TDDFRT/ALDA clearly breaks down. This is the state for which the ab initio calculations find significant double excitation character. To deal with double excitation character is an important challenge for time-dependent density functional theory.

A DFT study of asymmetric meso-substituted porphyrins and their zinc complexes

Abstract In the present work density functional theory (DFT) is applied to calculate geometries and electronic structures of 3,4-dimethyl- N -{2-[10,15,20-tris-(3,5-di- tert -butyl-phenyl)-porphyrin-5-yl]-phenyl}benzamide (H 2 P–O34) and its zinc complex (ZnP–O34). The influences of the substituents in the porphyrin ring on the energies of frontier molecular orbitals are evaluated. This is done by gradually increasing the amount of substituents in the investigated porphine and zinc porphine rings. The DFT framework used in this work is found to correctly describe the differences in electronic structures, which arise from differences in the location, number, and electronic effects of the substituents at the meso -positions of the porphyrin ring. In addition, the applicability of the ground state DFT method for evaluating the one-electron transition energies, which correspond to the near UV–Vis optical absorptions of porphyrin molecules, is investigated. The transition energies and the orbitals involved in electronic transitions are well reproduced by this method, as shown by comparing the results to our time dependent density functional calculations.

Adsorption and diffusion on a stepped surface: atomic hydrogen on Pt(211).

We present density functional theory calculations for atomic hydrogen interacting with a stepped surface, the Pt(211) surface. The calculations have been performed at the generalized gradient approximation level, using a slab representation of the surface. This is the state-of-the-art method for calculating the interaction of atoms or molecules with metal surfaces, nevertheless only few studies have used it to study atoms or molecules interacting with stepped surfaces, and none, to the best of our knowledge, have considered hydrogen interacting with stepped platinum surfaces. Our goal has been to initiate a systematic study of this topic. We have calculated the full three-dimensional potential energy surface (PES) for the H/Pt(211) system together with the vibrational band structure and vibrational eigenfunctions of H. A deep global minimum of the PES is found for bridge-bonded hydrogen on the step edge, in agreement with experimental results for the similar H/Pt(533) system. All the local vibrational excitations at the global minimum have been identified, and this will serve as a helpful guide to the interpretation of future experiments on this (or similar) system(s). Furthermore, from the calculated PES and vibrational band structure, we identify a number of consequences for the interpretation or modelling of diffusion experiments studying the coverage and directional dependence of atomic hydrogen diffusion on stepped platinum surfaces.

Quantum-Chemical Study of Structure and Activity of Chain Ends in Metal-Free Anionic Polymerization of Methacrylates

Kinetic and NMR studies of the anionic polymerization of methyl methacrylate using tetraphenylphosphonium and other phosphorus-containing, metal-free counterions indicated an equilibrium between an active ion pair and a corresponding ylide as a dormant species. We present density functional theory (DFT) calculations of the energy difference between the ion pair and the ylide for the ester enolate of methyl isobutyrate with various counterions which support this view. It is possible to establish a correlation based on the calculated energy difference between the ylide and the ion pair and the apparent activity of the chain end. The ylide formed after the first polymerization step is extremely stable compared to the ion pair, whereas after addition of one molecule of methyl methacrylate, the ylide of the dimer is destabilized through steric interactions and the polymerization can proceed much faster. This explains the observation of a pronounced induction period when using the ester enolate of methyl isobut...

Density functional theory investigation of the reaction of isodiiodomethane with acetylene: potential utility of isodiiodomethane for cyclopropenation reactions.

We present density functional theory calculations for the reactions of CH2I-I and CH2I with acetylene (HC triple bond CH) to form a cyclopropene product. CH2I-I readily reacts with HC triple bond CH to form a cyclopropene product and an I2 leaving group via a rate-determining step barrier of approximately 3.9 kcal/mol (B3LYP/Sadlej-pVTZ). Calculations indicated that the CH2I radical reacts to form an iodopropenyl radical, which can close to a cyclopropene only with difficulty. Our results indicate that CH2I-I may act as an effective carbenoid to produce cyclopropenated products from alkynes.

Transient Resonance Raman Spectroscopy and Density Functional Theory Investigation of Iso-Dibromoacetic Acid Photoproduct Produced From Ultraviolet Photolysis of Dibromoacetic Acid in the Solution Phase

Abstract We present transient resonance Raman spectra and density functional theory calculations that indicate the iso-dibromoacetic acid (Br–BrCHCOOH) species can be produced in noticeable amounts following ultraviolet photolysis of dibromoacetic acid in cyclohexane solution. We compare the properties of the iso-dibromoacetic acid species to those previously found for the related iso-dibromomethane species. We briefly discuss the potential utility of using iso-dibromoacetic acid in cyclopropanation reactions to make substituted cyclopropane products.

Hydrogen-bond-mediated tuning of the redox potential of the non-heme Fe site of superoxide dismutase.

The highly homologous proteins of Fe-containing superoxide dismutase (FeSOD) and MnSOD from Escherichia coli nonetheless exert very different redox tuning on the active site metal ion [Vance; Miller J. Am. Chem. Soc. 1998, 120, 461-467; Biochemistry 2001, 40, 13079-13087]. This was proposed to stem from different hydrogen bonding between the protein and the metal ion's coordinated solvent molecule, and the tight coupling between the protonation state of coordinated solvent and the oxidation state of the metal ion. We now present density functional theory (DFT) calculations on Fe2+ and Fe3+ bound to models of both FeSOD and MnSOD. The calculations support a very important role for the conserved second sphere Gln in MnSOD in specifically destabilizing coordinated H2O relative to coordinated OH-, and thus disfavouring the oxidized state of the metal ion. To test these results we have mutated this Gln to Glu, which is isosteric and isoelectronic to Gln but functions as an H-bond acceptor instead of an H-bond donor and thus should increase the stability of Fe2+-bound H2O. In accordance with the calculations, Q69E-FeSOD displays a significantly higher reduction potential than wild-type FeSOD. Thus we have demonstrated that hydrogen bonds to coordinated solvent can exert strong redox tuning on a metal ion.

Extension of the density functional derived scaled quantum mechanical force field procedure

The density functional derived scaled quantum mechanical (SQM) force field method of Rauhut and Pulay [J. Phys. Chem. 99 (1995) 3093] has been extended. The original procedure (employing B3-LYP/6-31G* computations and 11 transferable scale factors for the different kinds of internal coordinates) was capable to reproduce the vibrational fundamentals of 31 simple organic (H, C, N, O) molecules with a total mean deviation of about 13 cm −1. The present Density Functional Theory based SQM force field method is an extension of the original one: with the help of 20 transferable scale factors can reproduce the fundamentals of 20 inorganic, organic and organosilicon molecules containing nonmetallic first and second-row atoms with a total mean deviation of 10.8 cm −1. The transferability and reliability of the new set of scale factors are demonstrated on the examples of the a priori SQM vibrational spectra of cis and gauche-cyclopropylchlorosilane.

Density Functional Calculations on Alanine-Derived Radicals: Influence of Molecular Environment on EPR Hyperfine Coupling Constants

The amino acid L-R-alanine and its associated radiation-induced radicals display particular characteristics in solid-state that make it very appropriate for use in Electron Paramagnetic Resonance (EPR) dosimetry. In contrast to the number of experimental studies, relatively few theoretical studies have been published concerning the EPR parameters of these radicals. However, these studies inadequately account for the molecular environment of the alanine radicals in the crystalline lattice. Here, we present Density Functional Theory (DFT) calculations on one of the stable radiation-induced radicals of L-R-alanine both in molecular cluster models and in periodic models. An extensive investigation is presented on the various geometrical ingredients which have a substantial impact on the hyperfine coupling constants as the planarity of the radical backbone and the internal rotations of the final methyl and amino group vary. It is found that the accurate modeling of the hydrogen bonds with neighboring molecules is of utmost importance for an adequate reproduction of the experimental data.

Theoretical Study of the Adsorption of Acetylene on the Si(001) Surface

Recent STM measurements and electronic structure calculations have revealed that the Si(001)/acetylene surface is far more complex than previously assumed, with there being at least three different binding arrangements of an acetylene molecule on the surface. In the present study density functional theory with slab models and periodic boundary conditions are used to study the adsorption of acetylene on the Si(001) surface. Nine different local potential energy minima, including three not previously described, are characterized. The relative stability of the two lowest energy structures is found to reverse with increasing surface coverage. The minimum energy pathways between selected pairs of local minima are calculated.

Do Nonplanar Porphyrins Have Red-Shifted Electronic Spectra? A DFT/SCI Study and Reinvestigation of a Recent Proposal

It is commonly believed that nonplanar distortions of the porphyrin skeleton bring about significant red shifts in the electronic absorption spectra. However, based on absorption spectroscopy and semiempirical AM1 studies of meso-tetrakis(perfluoroalkyl)porphyrins, DiMagno and co-workers have challenged this notion. Here we present density functional theory based configuration interaction singles calculations which uphold the traditional view. Both ruffling and saddling deformations do bring about significant red shifts in the electronic spectra. Nonplanarity-induced destabilization of the porphyrin HOMOs appears to be the principal cause of these red shifts.

A density functional theory based on the universality of the free energy density functional

A methodology for classical density functional theory is proposed which expresses the first order direct correlation function for nonuniform systems in terms of the second order direct correlation function and the bridge function of the corresponding uniform systems. Its applicability to nonuniform systems is verified by applying the approach to three concrete cases, namely, to a hard sphere fluid in a spherical cavity, a hard sphere fluid near a hard wall, and a hard sphere fluid near a 9–3 soft wall. The present density functional theory (DFT) provides a recipe by which the integral equation theories for uniform systems can be transformed into the corresponding DFTs for nonuniform systems.

Nitrogen-15 chemical shift in the pyridine–methanol complex

This Letter presents density functional theory calculations of the 15 N chemical shifts of pyridine in the presence of a methanol molecule. These calculations were done to study the effects of hydrogen bonding (HB) on the principal components of the aromatic nitrogen chemical shift tensors. Calculations have been done in three dimensions, i.e. varying the distance and the bond and dihedral angles between the pyridine and the methanol molecule. The results show that HB effects on the chemical shifts are significantly more sensitive to the distance between molecules than to the angles.

Adiabatic integration formula for the correlation energy functional of the Hartree-Fock density

An adiabatic integration formula for the quantum chemistry correlation energy functional of the Hartree–Fock density, EcQC[n], is presented. The functional EcQC[n] is meant to be added to the completed Hartree–Fock energy to produce the exact ground-state energy of the system under consideration. The initial slope of the integrand in this connection formula is identified as a second-order energy and an explicit expression for the initial slope of the integrand is presented. Our expression should be useful for arriving at new improved approximations to EcQC[n]. Previous numerical results by Huang and Umrigar (1997) Phys Rev A 56:290, for two-electron densities are proved, and a generalization to more than two electrons is presented. Results obtained by means of the present density functional theory correlation energy functionals, when used to approximate the initial slope in our adiabatic integration formula for EcQC[n], are compared against exact numbers.

Energetics of the naphthalene/azulene monocation isomerization: density functional and coupled cluster calculations †‡

The energetics of isomerization between the azulene and naphthalene radical cations have been investigated using the hybrid density functional method B3LYP with the cc-pVDZ basis sets. CCSD/cc-pVDZ energy calculations were also carried out for selected points along the reaction coordinate. The transition state barrier energies for isomerization are lower than the dissociation limit of C8H6˙+ (benzocyclobutadiene˙+) + C2H2 deduced earlier. A key intermediate is a hydrogen shifted naphthalene isomer analogous to the intermediate suggested in the Dewar–Becker isomerization mechanism for neutral azulene. The norcaradiene isomer of the Dewar–Becker mechanism was found to be a transition structure in the ionic system. Results of the present density functional theory (DFT) and coupled cluster calculations are discussed in the light of recent experimental evidence.

Extended Defects in GaN: a Theoretical Study

We present density–functional theory studies for a variety of surfaces and extended defects in GaN. According to previous theoretical studies1{100} type surfaces are electrically inactive. They play an important role in GaN since similar configurations occur at open–core screw dislocations and nanopipes as well as at the core of threading edge dislocations. Domain boundaries are found to consist of four–fold coordinated atoms and are also found to be electrically inactive. Thus, except for full–core screw dislocations which possess heavily strained bonds all investigated extended defects do not induce deep states into the band–gap. However, electrically active impurities in particular gallium vacancies and oxygen related defect complexes are found to be trapped at the stress field of the extended defects.

Femtosecond β-cleavage dynamics: Observation of the diradical intermediate in the nonconcerted reactions of cyclic ethers

Femtosecond (fs) dynamics of reactions of cyclic ethers, symmetric and asymmetric structures, are reported. The diradical intermediates and their β-cleavages, which involve simultaneous C-C, C-H σ-bond breakage and C-O, C-C π-bond formation, are observed and studied by fs-resolved mass spectrometry. To compare with experiments, we present density functional theory calculations of the potential energy surface and microcanonical rates and product distributions.

Surface Relaxation and Ferromagnetism of Rh(001)

The significant discrepancy between first-principles calculations and experimental analyses for the relaxation of the (001) surface of rhodium has been a puzzle for some years. In this paper we present density functional theory calculations using the local-density approximation and the generalized gradient approximation of the exchange-correlation functional. We investigate the thermal expansion of the surface and the possibility of surface magnetism. The results throw light on several, hitherto overlooked, aspects of metal surfaces. We find, that, when the free energy is considered, density-functional theory provides results in good agreement with experiments.