Electronic structures of 4d transition metal monoxides by density functional theory

Bond distances, dissociation energies, ionization potentials and electron affinities of 4d transition metal monoxides from YO to CdO and their positive and negative ions were studied by use of density functional methods B3LYP, BLYP, B3PW91, BPW91, B3P86, BP86, SVWN, MPW1PW91 and PBE1PBE. It was found that calculated properties are highly dependent on the functionals employed, especially for dissociation energy. For most neutral species, pure density functionals BLYP, BPW91 and BP86 have good performance in predicting dissociation energy than hybrid density functionals B3LYP, B3PW91 and B3P86. In addition, BLYP gives the largest bond distance compared with other density functional methods, while SVWN gives shortest bond distance, largest dissociation energy and electron affinity. For the ground state, the spin multiplicity of the charged species can be obtained by ± 1 of their corresponding neutral species.     

The confirmation of accurate combination of functional and basis set for transition‐metal dimers: Fe2, Co2, Ni2, Ru2, Rh2, Pd2, Os2, Ir2, and Pt2

Eight kinds of density functionals named B3LYP, PBE1PBE, B1B95, BLYP, BP86, G96PW91, mPWPW91, and SVWN along with two different valence basis sets (LANL2DZ and CEP‐121g) are employed to study the transition‐metal dimers for the elements of group VIII. By comparing the equilibrium bond distances, vibrational frequencies, and dissociation energies of the ground state of these dimers with the available experimental values and theoretical data, we show that the “pure” DFT methods (G96PW91, BLYP, and BP86) with great‐gradient approximation always give better results relative to the hybrid HF/DFT schemes (B3LYP, PBE1PBE, and B1B95). The striking case found by us is that the G96PW91 functional, which is not tested in previous systemic studies, always predicts the dissociation energy to be well. The Ru₂ and Os₂ dimers are sensitive to not only the functionals employed but also the valence basis sets adopted. The natural bond orbital population is analyzed, and the molecular orbitals of the unpaired electrons are determined. Furthermore, our results indicate that the s and d orbitals of these dimers always hybridize with each other except for Rh₂ and Pt₂ molecules. And by analyzing the electron configuration of the bonding atom, the dissociation limit of the ground state is obtained. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Electronic Structures of 5d Transition Metal Monoxides by Density Functional Theory

Bond distances, vibrational frequencies, electron affinities, ionization potentials, and dissociation energies of the diatomic 5d transition metal (except La) monoxides and their positively and negatively charged ions were studied by use of density functional methods B3LYP, BLYP, B3PW91, BPW91, B3P86, BP86, MPW1PW91, PBE1PBE, and SVWN. Our calculation shows that for each individual species, the calculated properties are quite sensitive to the method used. Compared with hybrid density functional method B3PW91 (B3P86), pure density functional method BPW91 (BP86) gives longer bond distance (lower vibrational frequency) from HfO to PtO for neutral species, HfO⁺ to IrO⁺ for cationic species, and HfO⁻ to AuO⁻ for anionic species. While for B3LYP and BLYP, the trend was observed for cationic species from HfO⁺ to IrO⁺ and anionic species from HfO⁻ to AuO⁻ (except TaO⁻), but not for neutrals. Pure density function methods BLYP, BPW91, and BP86 give larger dissociation energy compared with hybrid density functional methods B3LYP, B3PW91, and B3P86. SVWN in most cases gives the smallest bond distance, while BLYP gives the largest value. MPW1PW91 and PBE1PBE show the same performance in predicting the spectroscopic constants. In addition, useful empirical criteria that one has obtained the ground states of a species and its ions are the spin multiplicities of a neutral and its single charged ions which differs by ±1.     

Density functional theory study of trans-dioxo complexes of iron, ruthenium, and osmium with saturated amine ligands, trans-[M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), and detection of [Fe(qpy)(O)2]n+ (n=1, 2) by high-resolution ESI mass spectrometry

Density functional theory (DFT) calculations on trans-dioxo metal complexes containing saturated amine ligands, trans-[M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), were performed with different types of density functionals (DFs): 1) pure generalized gradient approximations (pure GGAs): PW91, BP86, and OLYP; 2) meta-GGAs: VSXC and HCTH407; and 3) hybrid DFs: B3LYP and PBE1PBE. With pure GGAs and meta-GGAs, a singlet d2 ground state for trans-[Fe(O)2(NH3)2(NMeH2)2]2+ was obtained, but a quintet ground state was predicted by the hybrid DFs B3LYP and PBE1PBE. The lowest transition energies in water were calculated to be at lambda approximately 509 and 515 nm in the respective ground-state geometries from PW91 and B3LYP calculations. The nature of this transition is dependent on the DFs used: a ligand-to-metal charge-transfer (LMCT) transition with PW91, but a pi(Fe-O)-->pi*(Fe-O) transition with B3LYP, in which pi and pi* are the bonding and antibonding combinations between the dpi(Fe) and ppi(O(2-)) orbitals. The FeVI/V reduction potential of trans-[Fe(O)2(NH3)2NMeH2)2]2+ was estimated to be +1.30 V versus NHE based on PW91 results. The [Fe(qpy)(O)2](n+) (qpy=2,2':6',2':6',2':6',2'-quinquepyridine; n=1 and 2) ions, tentatively assigned to dioxo iron(V) and dioxo iron(VI), respectively, were detected in the gas phase by high-resolution ESI-MS spectroscopy.     

DFT study of the structural characteristics of the yttrium(3+) aqua ion

Density functional theory (DFT) using SVWN5, B3LYP, B3P86, O3LYP, B3PW91, B1LYP, B971, MPW1PW91, PBE1PBE, BHandH, and BHandHLYP density functionals was employed to study the structural characteristics of the Y(H₂O) ₈ ³⁺ yttrium aqua ion. The nonlocal hybrid GGA functionals show worse predictive ability in structural calculations of the Y(H₂O) ₈ ³⁺ aqua ion compared to the relatively simple combined functional BHandH and to the simplest SVWN5 functional in LSDA theory.     

Reaction mechanism of HSH and CH3SH with NH2CH2COCH2X (X = F and Cl) molecules

The reaction mechanism of model compounds H₂S and CH₃SH for cysteine proteases with NH₂CH₂COCH₂X (X = F and Cl) molecules has been investigated using DFT methods with B3LYP and B3PW91 hybrid density functionals at 6‐31+G* basis sets. The single point energy has been calculated for the above reactions with B3LYP and B3PW91 functionals using aug‐cc‐PVDZ infinite basis set in both gas and solution phases. The intrinsic reaction coordinates calculations have been performed to confirm that each transition state is linked by the desired reactants and products. The geometries and relative energies for various stationary points have been determined and discussed. The zero point vibrational energy corrections have been made to predict the reliable energy. The negative value of reaction energy indicates that the overall reaction profile is found to be exothermic. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Theoretical study of the vibrational frequencies of carbon disulfide

A benchmark comparison for different computational methods and basis sets has been presented. In this study, five computational methods (Hartree–Fock (HF), MP2, B3LYP, MPW1MP91, and PBE1PBE) along with 18 basis sets have been applied to optimize the geometry of carbon disulfide (CS₂), and further calculate the vibrational frequencies of the optimized geometries. The differences between the calculated frequencies and corresponding experimental data are used to evaluate the efficiency of each combination of computational method and basis set. The comparison of frequency difference indicates that B3LYP generally gives the best prediction of frequencies for CS₂, whereas the other two density functional theory (DFT) methods, i.e., MPW1PW91 and PBE1PBE, often give parallel results. Although MP2 predicts the frequencies with accuracy almost as good as those from DFT methods, in a particular case, HF calculation outperforms MP2 as well as MPW1PW91 and PBE1PBE for prediction of the frequency of asymmetrical stretching for CS₂. © 2013 Wiley Periodicals, Inc.     

Why are hexavalent uranium cyanides rare while U–F and U–O bonds are common and short?

Relativistic small-core pseudopotential B3LYP and CCSD(T) calculations and frozen-core PW91–PW91 studies are reported for the series UF ₄X ₂ ( X=H, F, Cl, CN, NC, NCO, OCN, NCS and SCN). The bonding in UF ₆ is analyzed and found to have some multiple-bond character, approaching at a theoretical limit a bond order of 1.5. In addition to these s and p orbital interactions, the electrostatic attraction is important. Evidence for p bonding in the other systems studied was also found. The triatomic pseudohalides as well as fluorine and chlorine are in this sense better ligands than cyanide. The –CN group is a s donor and p acceptor, as uranium itself, and hence is unfit to bond to U(VI). The s-bonded UH ₆ is octahedral.     

Interaction of Dihydrogen with Transition Metal (Pd,Ni, Ag,Cu) Clusters

Quantum calculations of interaction of the molecular hydrogen with transition-metal clusters have been performed. The aim of the project is to compare the results for different metals and different methods of calculations. The calculations have been mostly based on the gradient-corrected methods of the density functional theory. The list of the exchange-correlation functionals includes: the gradient corrected functional BP86, the hybrid functionals B3P86, B3LYP, B3PW9, and the local SVWN functional. The calculations of the potential energy surface (PES) for the hydrogen molecule positioned over the planar Pd₅ clusters have been performed. It was found that the H–H bond activation is without barrier for most of the functionals used. However, the results obtained for the B3LYP functional suggest very small potential barrier, of the order of 0.003 eV. The calculations of the PES for dihydrogen in contact with metal clusters have been performed for Ni₅, Ag₅, Cu₅ clusters and for mixed clusters Ag₄Pd, AgPd₄, NiCu₄, and NiPd₄. The dissociation paths for all the cases with the exception of Ag₅ and Cu₅ have been found and the dissociation energies and activation barriers have been estimated.     

Mechanistic insights into triterpene synthesis from quantum mechanical calculations. Detection of systematic errors in B3LYP cyclization energies

Most quantum mechanical studies of triterpene synthesis have been done on small models. We calculated mPW1PW91/6-311+G(2d,p)//B3LYP/6-31G* energies for many C30H51O+ intermediates to establish the first comprehensive energy profiles for the cationic cyclization of oxidosqualene to lanosterol, lupeol, and hopen-3beta-ol. Differences among these 3 profiles were attributed to ring strain, steric effects, and proton affinity. Modest activation energy barriers and the ample exothermicity of most annulations indicated that the cationic intermediates rarely need enzymatic stabilization. The course of reaction is guided by hyperconjugation of the carbocationic 2p orbital with parallel C-C and C-H bonds. Hyperconjugation for cations with a horizontal 2p orbital (in the plane of the ABCD ring system) leads to annulation and ring expansion. If the 2p orbital becomes vertical, hyperconjugation fosters 1,2-methyl and hydride shifts. Transition states leading to rings D and E were bridged cyclopropane/carbonium ions, which allow ring expansion/annulation to bypass formation of undesirable anti-Markovnikov cations. Similar bridged species are also involved in many cation rearrangements. Our calculations revealed systematic errors in DFT cyclization energies. A spectacular example was the B3LYP/6-311+G(2d,p)//B3LYP/6-31G* prediction of endothermicity for the strongly exothermic cyclization of squalene to hopene. DFT cyclization energies for the 6-311+G(2d,p) basis set ranged from reasonable accuracy (mPW1PW91, TPSSh with 25% HF exchange) to underestimation (B3LYP, HCTH, TPSS, O3LYP) or overestimation (MP2, MPW1K, PBE1PBE). Despite minor inaccuracies, B3LYP/6-31G* geometries usually gave credible mPW1PW91 single-point energies. Nevertheless, DFT energies should be used cautiously until broadly reliable methods are established.     

On the accuracy of density-functional theory exchange-correlation functionals for H bonds in small water clusters: benchmarks approaching the complete basis set limit

The ability of several density-functional theory (DFT) exchange-correlation functionals to describe hydrogen bonds in small water clusters (dimer to pentamer) in their global minimum energy structures is evaluated with reference to second order Moller-Plesset perturbation theory (MP2). Errors from basis set incompleteness have been minimized in both the MP2 reference data and the DFT calculations, thus enabling a consistent systematic evaluation of the true performance of the tested functionals. Among all the functionals considered, the hybrid X3LYP and PBE0 functionals offer the best performance and among the nonhybrid generalized gradient approximation functionals, mPWLYP and PBE1W perform best. The popular BLYP and B3LYP functionals consistently underbind and PBE and PW91 display rather variable performance with cluster size.     

Hydrogenation of conjugated CC and CO bonds: Quantum chemical preview before metal catalysis

Hydrogenation of acrolein, ethylene and formaldehyde by molecular hydrogen in gas phases, as a preview before metal catalysis, is investigated using density functional theory (PW91 and B3LYP), ab initio (MP2), and composite theoretical methods (G2, CBS-QB3, and CBS-APNO). Compared to the most accurate CBS-APNO method used in this paper, PW91 functional underestimates the barrier while B3LYP reproduces close results. All the reaction barriers are predicted to be no less than 70 kcal/mol and the CC hydrogenation is thermodynamically favored. Kinetically, however, the hydrogenation of CO bond is more favorable than that of the CC bond, especially for the isolated CO. The conjugation effect in acrolein greatly reduces the kinetic preference for the isolated CO hydrogenation. It is revealed that there is a good relationship between the energy barrier and the increase of the molecular H₂ bond length from the reactant to transition state.     

Ab initio calculation of bowl, cage, and ring isomers of C20 and C20-

High-level ab initio calculations have been carried out to reexamine relative stability of bowl, cage, and ring isomers of C(20) and C(20)(-). The total electronic energies of the three isomers show different energy orderings, strongly depending on the hybrid functionals selected. It is found that among three popular hybrid density-functional (DF) methods B3LYP, B3PW91, PBE1PBE, and a new hybrid-meta-DF method TPSSKCIS, only the PBE1PBE method (with cc-pVTZ basis set) gives qualitatively correct energy ordering as that predicted from ab initio CCSD(T)/cc-pVDZ [CCSD(T)-coupled-cluster method including singles, doubles, and noniterative perturbative triples; cc-pVDZ-correlation consistent polarized valence double zeta] as well as from MP4(SDQ)/cc-pVTZ [MP4-fourth-order Moller-Plesset; cc-pVTZ-correlation consistent polarized valence triple zeta] calculations. Both CCSD(T) and MP4 calculations indicate that the bowl is most likely the global minimum of neutral C(20) isomers, followed by the fullerene cage and ring. For the anionic counterparts, the PBE1PBE calculation also agrees with MP4/cc-pVTZ calculation, both predicting that the bowl is still the lowest-energy structure of C(20)(-) at T=0 K, followed by the ring and the cage. In contrast, both B3LYP/cc-pVTZ and B3PW91/cc-pVTZ calculations predict that the ring is the lowest-energy structure of C(20)(-). Apparently, this good reliability in predicting the energy ordering renders the hybrid PBE method a leading choice for predicting relative stability among large-sized carbon clusters and other carbon nanostructures (e.g., finite-size carbon nanotubes, nano-onions, or nanohorns). The relative stabilities derived from total energy with Gibbs free-energy corrections demonstrate a changing ordering in which ring becomes more favorable for both C(20) and C(20)(-) at high temperatures. Finally, photoelectron spectra (PES) for the anionic C(20)(-) isomers have been computed. With binding energies up to 7 eV, the simulated PES show ample spectral features to distinguish the three competitive C(20)(-) isomers.     

A mixed experimental and theoretical study on chelidamate copper(II) complex with 4-methylpyrimidine

A new chelidamate complex, [Cu(chel)(H₂O)₂(mpd)] (chel = chelidamate; mpd = 4-methylpyrimidine), has been synthesized and characterized through a combination of single crystal X-ray analysis, electron paramagnetic resonance (EPR), ultraviolet-visible (UV-vis), and fourier transform infrared spectroscopy (FT-IR). The complex has six-coordinate distorted octahedral geometry around Cu(II). The theoretical vibrational frequencies and optimized geometric parameters (bond lengths and angles) have been calculated using Density Functional Theory (DFT)/B3LYP and Hartree Fock quantum chemical methods with 6-31G(d, p) basis set by Gaussian 09W software. The EPR spectrum of the compound showed that the paramagnetic center has rhombic symmetry. The EPR studies were carried out using the following unrestricted hybrid density functionals: B3LYP, CAM-B3LYP, HSEH1PBE, WB97XD, MPW1PW91, and BPV86. The UV–vis absorption spectra have been examined in different media and compared with the calculated one using TD-DFT method by applying the polarizable continuum model. Natural bond orbital property of complex has been performed by DFT/B3LYP with 6-31G (d, p) basis set.     

Theoretical calculation of bond dissociation energies and heats of formation for nitromethane and polynitromethanes with density functional theory

The C? NO₂ bond dissociation energies (BDEs) and the heats of formation (HOFs) of nitromethane and polynitromethanes (dinitromethane, trinitromethane, and tetranitromethane) system in gas phase at 298.15 K were calculated theoretically. Density functional theory (DFT) B3LYP, B3P86, B3PW91, and PBE0 methods in combination with different basis sets were employed. It was found that the C? NO₂ bond BDEs can be improved from B3LYP to B3PW91 to B3P86 or PBE0 functional. Levels of theory employing B3P86 and PBE0 functionals were found to be sufficiently reliable without the presence of diffusion functions. As the number of NO₂ groups on the same C atom increases, the PBE0 functional performs better than the B3P86 functional. Regarding the calculated HOFs, all four functionals can yield satisfactory results with deviations of <2 kcal mol^?1 from experimental ones for CH₂(NO₂)₂ and CH(NO₂)₃, when the diffusion functions are not augmented. For the C(NO₂)₄ molecule, the large basis sets augmented with polarization functions and diffusion functions are required to yield a good result. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Theoretical determination of lowest structures of all‐metal aromatic clusters M4L2 (M = Al,Ga, In,Tl; L = Li,Na, K,Rb, Cs)

The researches of all‐metal aromatic clusters have been a thermic theme in inorganic aromaticity domain both experimentally and theoretically since the Al₄L^? (L = Li, Na, Cu) clusters were created by laser vaporization. In systemic determination of the lowest structures of 20 gaseous all‐metal aromatic clusters M₄L₂ (M = Al, Ga, In, Tl; L = Li, Na, K, Rb, Cs), the isomer energy differences of four low‐lying structures of each cluster were evaluated at high‐quality quantum chemistry levels. Single point calculations at the coupled cluster level were performed at geometries optimized at the MP2, B3LYP, and B3PW91 levels, and harmonic frequency calculations and zero point energy corrections were implemented following optimizations at the B3LYP and B3PW91 levels. In addition to Li‐ and Na‐containing species, theoretical investigations came down to those new clusters including K, Rb, and Cs. For many clusters, the most convincing theoretical evidences indicate that the lowest structures are a square bipyramidal isomer rather than an edge‐caped square pyramidal species. A few discrepancies were addressed at the MP2, B3LYP, and B3PW91 levels in comparison with the coupled cluster results. These findings are significant because some clusters were generated by laser vaporization and served as theoretical prototypes to test the new means for assessing aromaticity. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

ChemInform Abstract: An 18‐Electron System Containing a Superheavy Element: Theoretical Studies of Sg@Au12.

With reference to the high‐symmetric structure and significant stability of M@Au₁₂ cage molecules (M = group 6 transition element), the geometric and electronic structures as well as bonding of various Sg@Au₁₂ isomers are investigated by DFT (PW91, PBE, B3LYP) and wave function theory (MP2, CCSD(T)) approaches.