Showing papers in "Journal of Molecular Structure-theochem in 2001"
TL;DR: In this paper, the two-component methods of relativistic quantum chemistry based on the Foldy-Wouthuysen (FW) transformations of the Dirac hamiltonian are reviewed.
Abstract: The two-component methods of relativistic quantum chemistry based on the Foldy–Wouthuysen (FW) transformations of the Dirac hamiltonian are reviewed. Following the strategy designed by Douglas and Kroll, the FW transformation is carried out in two steps. The first amounts to performing the exact free-particle FW transformation. At variance with other approaches, the second step is written in the form, which results in a nonlinear operator equation. This equation can be solved iteratively, leading to two-component hamiltonians of arbitrarily high accuracy in even powers of the fine structure constant. All these hamiltonians can be classified according to their completeness with respect to the leading order in the fine structure constant. On passing to the basis set representation one obtains the usual Douglas–Kroll hamiltonian and all possible higher-order approximations. By a simple modification of the operator equation which determines the block-diagonalizing transformation one can obtain numerical infinite-order solutions, i.e. one can obtain the exact numerical solution for the separation of the pure electronic part of the Dirac spectrum. This gives the exact two-component method for the use in relativistic quantum chemistry. The computational aspects of this approach are discussed as well. The transition from the Dirac formalism to any two-component approximation is accompanied by the change of all operators, including those which correspond to external perturbations and lead to properties of different orders. This so-called change of picture problem is given particular attention and its importance for certain operators is identified.
196 citations
TL;DR: In this paper, the authors present the results of an investigation of sulfur containing compounds using a tight-binding approach, which is a second-order approximation to density-functional theory (DFT).
Abstract: We present the results of an investigation of sulfur containing compounds using a tight-binding (TB) approach, which is a second-order approximation to density-functional theory (DFT). The performance of the so-called self-consistent charge density-functional based TB (SCC-DFTB) method is analyzed via comparison of the results for several molecular properties with other semi-empirical methods as well as with full DFT calculations. We also examine different conformers of the amino acid cysteine in order to estimate the reliability of our method for large-scale biomolecular systems. We find that SCC-DFTB provides, in general, good agreement with full DFT calculations and a similar deviation from experiment.
141 citations
TL;DR: In this paper, the most important findings are introduced in the present overview: nonplanarity of nucleobases, out-of-plane hydrogen bonds and amino acceptor interactions, structures and energies of hydrogen bonded base pairs, nature of base stacking, and interactions between metal cations and DNA bases, accuracy of density functional theory, semi-empirical methods, distributed multipole analysis and empirical potentials.
Abstract: Ab initio quantum chemical calculations with inclusion of electron correlation effects significantly contributed to our understanding of molecular interactions of DNA bases. Some of the most important findings are introduced in the present overview: nonplanarity of nucleobases, out-of-plane hydrogen bonds and amino acceptor interactions, structures and energies of hydrogen bonded base pairs, nature of base stacking, and interactions between metal cations and nucleobases. Also, accuracy of density functional theory, semi-empirical methods, distributed multipole analysis and empirical potentials is briefly commented on.
111 citations
TL;DR: In this paper, the relative stability of selected cis-isomers of Lycopene with respect to the all-trans isomer, was studied and the optimized molecular conformational study structures indicated that the central conjugated part of every lycopene isomer is planar and the two tail ends of the molecules, each containing three single C-C bonds, avoid coplanarity.
Abstract: Lycopene is an effective antioxidant in vivo. Although lycopene is present in its all-trans isomeric form in fruits and vegetables, serum and tissue samples show a predominance of various cis-isomers of lycopene. The present study was undertaken to investigate the molecular structure of several cis-isomers of lycopene using an ab initio molecular modeling procedure. The relative stability of selected cis-isomers of lycopene with respect to the all-trans isomer, was studied. The following sequence of stability was observed. 5-cis> all -trans>9-cis>13-cis>15-cis>7-cis>11-cis The first four of these isomers had relative energies within +1 kcal mol−1, but the fifth isomer (i.e. 15-cis) was within 3 kcal mol−1. However, the last two isomers were less stable than the all-trans-isomer with more than 5 kcal mol−1, difference in energy. The optimized molecular conformational study structures indicated that the central conjugated part of every lycopene isomer is planar. However, the two tail ends of the molecules, each containing three single C–C bonds, avoid coplanarity.
101 citations
TL;DR: In this paper, the theoretical similarities and differences between Si and C structures were established through the use of two alternative theoretical approaches, the first principles calculations and empirical potential, the electronic and structural properties of this hypothetical material are examined.
Abstract: Even though silicon nanotubes have never been observed, this paper attempts to establish the theoretical similarities and differences between Si and C structures. Through the use of two alternative theoretical approaches, the first principles calculations and empirical potential, the electronic and structural properties of this hypothetical material are examined. The first principles calculations are based on the density-functional theory and it is shown that depending on their chiralities and diameters, the silicon nanotubes may present metallic (armchair) or semiconductor (zigzag and mixed) behaviors, similar to carbon structures. It is shown that the gap decreases in inverse proportion to the diameter, thus approaching zero for planar graphite, as was expected. In the second alternative approach, the Monte Carlo simulations are used with the Tersoff's empirical potential to present a systematic study on the thermal behavior of these new structures. It is shown that similarities like band structures and density of states are observed between the C and Si nanotubes. Nevertheless, there are relevant discrepancies in the thermal stabilities and energy differences between the cohesive energies per atom for the two tubes, compared with the corresponding bulks, implying the very improbable structure of the silicon nanotubes.
97 citations
TL;DR: A review of recent advances in the theory of nanoparticle optical spectra using classical electrodynamics can be found in this article, where the authors describe several applications of DDA to the determination of extinction spectra, including extensions to describe solvent and substrate effects.
Abstract: This article reviews recent advances in the theory of nanoparticle optical spectra using classical electrodynamics. Although Mie theory has dominated work in this field for nearly a century, this theory is limited to spherical particles and spherical shells, and thus it cannot describe many experiments of recent interest to the chemical and biological sensor community in which nonspherical particles, including particles with only C1 symmetry, and particles in asymmetric environments (i.e. on surfaces and in nonspherical aggregates) are studied. There are now several approaches available for describing nonspherical particles, but the majority of recent applications to isolated particles have been done using the finite element discrete dipole approximation (DDA) theory. The review describes several applications of DDA to the determination of extinction spectra, including extensions to describe solvent and substrate effects. We also describe theories of nanoparticle aggregates based on approaches that explicitly describe electromagnetic coupling between the particles, and effective medium approaches for nonspherical aggregates.
96 citations
TL;DR: In this paper, the effect of basis sets on the structures and energetics of the rotation for the peptide bond of N -methylacetamide (NMA) has been investigated at high levels of theory.
Abstract: The effect of basis sets on the structures and energetics of the rotation for the peptide bond of N -methylacetamide (NMA) has been investigated at high levels of theory. The structures, dipole moments and thermodynamic properties for trans - and cis -NMA have been determined at the HF, MP2 and B3LYP levels with 6-31G(d), 6-31G(d,p), 6-31+G(d), 6-31+G(d,p), 6-31++G(d,p), 6-311+G(d,p) and 6-311++G(d,p) basis sets. By comparing the calculated structure and dipole moment of trans -NMA and the relative enthalpy (Δ H ) of cis -NMA in the gas phase with corresponding experimental data, the B3LYP level with 6-31G(d) or 6-31G(d,p) basis sets appears to be the most appropriate among several levels of theory and basis sets considered here. Trans -NMA is computed to be favored by a total free energy of 2.7 kcal/mol than cis -NMA in water at the B3LYP/6-31G(d,p) level with the SCI-PCM method, which corresponds to 1.1% population of cis -NMA. This is consistent with the results of NMR experiments in aqueous solution that estimate the cis population to be 1.5±0.1% and the free energy difference of 2.5±0.3 kcal/mol. In addition, the change in relative solvation free energies is computed to be −0.1 kcal/mol at the HF and B3LYP levels with the 6-31G(d,p) basis set as transferred from water to chloroform, which corresponds to the increase of the cis population by about 0.3%, consistent with the value from NMR measurements. Therefore, the SCRF method with proper levels of theory and basis sets seems to be reasonable to describe the solvation of NMA. Our calculated results confirmed that the relative distribution of trans and cis conformers of NMA is insensitive to the solvent polarity.
95 citations
TL;DR: In this article, the authors present an overview of the past ten years of work relating to the design, characterization and evaluation of new and proposed energetic compounds, at both ab initio and density functional levels.
Abstract: We present an overview, focusing primarily upon the past ten years, of our work relating to the design, characterization and evaluation of new and proposed energetic compounds. Our approach has been entirely computational, at ab initio and density functional levels. Several areas are discussed in some detail: (a) assessment of thermodynamic and kinetic stabilities; (b) investigation of factors affecting impact/shock sensitivities; (c) calculation of gas, liquid and solid phase heats of formation; (d) determination of thermodynamic/kinetic data and reaction mechanisms for decomposition and combustion processes; and (e) evaluation of possible synthetic pathways.
92 citations
TL;DR: In this paper, gas phase quantum chemical calculations for the efficiency of some imidazole derivatives as corrosion inhibitors, along with the relative stability and proton affinity determination were performed by means of the AM1, PM3, MINDO/3 and MNDO semi-empirical SCF molecular orbital methods.
Abstract: Gas phase quantum chemical calculations for the efficiency of some imidazole derivatives as corrosion inhibitors, along with the relative stability and proton affinity determination were performed by means of the AM1, PM3, MINDO/3 and MNDO semi-empirical SCF molecular orbital methods. The agreement with the experimental data was found to be satisfactory.
87 citations
TL;DR: In this paper, the B3LYP density functional approach was used to compute the gas-phase affinities of neutral and protonated glycine, serine, and cysteine.
Abstract: Proton affinities and gas-phase basicities of glycine, serine and cysteine have been computed using the three-parameter B3LYP density functional approach. For that, the geometry and vibrational frequencies of several conformations of neutral and protonated glycine, serine and cysteine have been explored. The preferred site for protonation in all aminoacids is the amino group. The lowest conformation always shows an intramolecular hydrogen bond between NH3+ and the carbonylic oxygen. For serine and cysteine, additional hydrogen bonds may be formed, the favored interaction being that in which the oxygen or sulfur atoms of the side chain interact, as proton acceptor, with NH3+. The computed B3LYP proton affinities and gas-phase basicities are in very good agreement with the known experimental data.
82 citations
TL;DR: In this paper, a summary of the studies of hydrogen-bonded complexes during the decade of the 90's is provided. But, significant discrepancies were observed for others, including complexes of HCl and HBr with ammonia, trimethylamine, and 4-substituted pyridines.
Abstract: This article provides a summary of our studies of hydrogen-bonded complexes during the decade of the 90's. These studies began with systematic investigations of the methodological dependence of the computed structures and binding energies of these complexes. The MP2/6-31+G(d,p) level of theory was identified as the minimum level required to obtain reliable structures, while reliable energetics required larger polarized split-valence basis sets that include diffuse functions. While the experimental frequency shift of the A–H stretching band upon formation of an A–H–B hydrogen bond could also be reproduced at MP2/6-31+G(d,p) for a variety of hydrogen-bonded complexes, significant discrepancies were observed for others, including complexes of HCl and HBr with ammonia, trimethylamine, and 4-substituted pyridines. Resolving these discrepancies became the primary focus of our work, and redefined our research efforts. We solved a model two-dimensional nuclear Schrodinger equation to obtain anharmonic dimer- and proton-stretching frequencies, modeled matrix effects with external electric fields, and characterized hydrogen bond types as traditional, proton-shared, and ion-pair. We were able to resolve the observed discrepancies between theory and experiment, and explain the rather disparate effects of matrices on the IR spectra of closely related complexes. We also initiated studies of the NMR properties of the chemical shift of the hydrogen-bonded proton, and the A–B spin–spin coupling constant across the A–H–B hydrogen bond. We demonstrated the dominance of the Fermi-contact term for determining coupling constants in complexes with N–H–N, N–H–O, O–H–O, and Cl–H–N hydrogen bonds, and the distance dependence of this term. We also showed that the IR anharmonic proton-stretching frequency and the NMR spin–spin coupling constant are spectroscopic fingerprints of hydrogen bond type, which provide information about intermolecular distances in hydrogen-bonded complexes.
TL;DR: In this paper, a multireference wave operator formalism was proposed for coupled-cluster equations (MMCC) with noniterative energy corrections that, when added to the ground-and excited-state energies obtained in approximate SUCC calculations, recover the exact energies.
Abstract: The recently proposed method of moments of coupled-cluster equations (MMCC) is extended to a multireference wave operator formalism. After reviewing the single-reference MMCC theory and its performance in calculations of potential energy surfaces involving bond breaking, we introduce the method of moments of the generalized Bloch equation and the method of moments of the state-universal coupled-cluster equations (MM-SUCC). The main idea of the MM-SUCC theory is that of the noniterative energy corrections that, when added to the ground- and excited-state energies obtained in approximate SUCC calculations, recover the exact energies. Approximate variants of the MM-SUCC formalism that may lead to significant improvements in the results of the standard SUCC calculations are discussed.
TL;DR: In this article, the ground-state and excited-state dipole moments of fluorescein were studied at room temperature in n -alcohols (methanol n-hexanol) and acetonitrile and acetitrile-benzene solvent mixtures.
Abstract: The ground-state and excited-state dipole moments of fluorescein were studied at room temperature in n -alcohols (methanol– n -hexanol) and acetonitrile and acetonitrile–benzene solvent mixtures. The excited-state dipole moments were estimated from Lippert's, Bakhshiev's and Chamma–Viallet's equations by using the variation of the Stokes' shift with the solvent dielectric constant and refractive index. Experimental ground-state dipole moments for fluorescein in n -alcohols, acetonitrile and acetonitrile–benzene solvent mixtures were estimated by the Guggenheim–Smith method (GSM). It was determined that dipole moments of the excited-state were higher than those of the ground-state in n -alcohols while they were lower than those of the ground-state in acetonitrile and acetonitrile–benzene solvent mixtures. The solute–solvent interactions on the ground-state and the excited-state dipole moments of fluorescein are discussed.
TL;DR: In this paper, the application of Boys-Bernardi's function counterpoise procedure to correct for the basis set superposition error when computing barrier heights associated with transition structures using well-balanced basis sets is analyzed in detail.
Abstract: The application of Boys–Bernardi's function counterpoise procedure to correct for the basis set superposition error when computing barrier heights associated with transition structures using well-balanced basis sets is analyzed in detail. It is shown that Emsley et al.’s formulation of the counterpoise method including the fragment relaxation terms may involve considerable errors when the relative orientation of the reference fragments is very different in the two chemical species defining the energy barrier.
TL;DR: In this paper, the authors introduce the concept of the inward product of two matrices, which is defined as an operation of internal composition in matrix spaces and present a wide prospect of applications, as well as evident connections with other mathematical structures like tagged sets and vector semispaces, useful in theoretical chemistry applications.
Abstract: A special matrix product, the inward product, of two matrices is defined as an operation of internal composition in matrix spaces. A generalisation and an extension of the inward product make it a very flexible algorithmic tool. This product, known from a long time, also called Hadamard or Schur product, presents characteristic properties, which made such an operation interesting enough due to its potential use in many fields, such as quantum chemistry, where matrix manipulation is a common trait. Here, besides the introduction to the main features of inward product, it is shown that there is a wide prospect of applications, as well as evident connections with other mathematical structures like tagged sets and vector semispaces, useful in theoretical chemistry applications. Approximate least-squares solutions forced to belong to a vector semispace are also discussed. This constrained least-squares procedure furnishes a new theoretical basis to the foundation of QSAR or QSPR in the framework of quantum similarity and other approaches.
TL;DR: In this paper, a series of computational investigations on MoSin (n=1-6) clusters is reported, where geometry optimizations are performed at the B3LYP/LanL2DZ level, subsequently, electronic and bonding features of the resulting equilibrium structure are discussed.
Abstract: A series of computational investigations on MoSin (n=1–6) clusters is reported. For each unit, geometry optimizations are performed at the B3LYP/LanL2DZ level, subsequently, electronic and bonding features of the resulting equilibrium structure are discussed. The emphasis of this study is placed on the magnetic properties of the MoSiN (N=1–6) systems and, more specifically, the impact of their spin on the internal electron transfer between the Sin subsystem and the Mo atom. As a general trend for MoSiN clusters with N>1, we record a change of the natural charge on the Mo atom from negative to positive as the spin constraint on the system increases from S=0 to S=3. This feature is shown to be associated with a pronounced drop in the stability of the cluster as one goes from the singlet to the septet spin configuration.
TL;DR: In this paper, the authors used a graph of atomic orbitals (GAO) to represent molecular structures for mutagenicity modeling, based on the optimization of correlation weights of local invariants (OCWLI) of the labeled hydrogen-filled graphs and the GAO.
Abstract: Graphs of atomic orbitals (GAOs) have been used to represent molecular structures. Rules by which the labeled hydrogen-filled graphs (LHFGs) were converted into the GAOs are described. The GAO is an attempt at taking into account the structures of atoms (i.e. atomic orbitals, such as 1s 1 , 2p 2 , 3d 10 ) for QSPR/QSAR analyses. As a method of mutagenicity modeling, optimization of correlation weights of local invariants (OCWLI) of the LHFGs and the GAOs has been used. Statistical characteristics of such models based on the OCWLI of GAO are better than those based on the OCWLI of the LHFGs.
TL;DR: In this paper, the conformational equilibrium of thiacalix[4]arene, a heterocalixarene in which the phenol groups are bridged by sulphur atoms, is compared to that of calix [4] are compared.
Abstract: Density functional theory calculations for the structure and conformational equilibrium of thiacalix[4]arene are reported. The conformational equilibrium of thiacalix[4]arene, a heterocalixarene in which the phenol groups are bridged by sulphur atoms is compared to the conformational equilibrium of calix[4]arene. Thiacalix[4]arene conformational energies relative to the cone conformer (ΔE's) are reduced in comparison with calix[4]arene. This conformational change is in qualitative agreement with recent NMR spectroscopy measurements of the conformational equilibrium for a tetraethylether of thiacalix[4]arene in a CDCl3 solution which indicates an enhanced chemical exchange of thiacalixarene conformers in comparison with similar methylene bridged structures. Density functional theory results for the structure of thiacalix[4]arene are in good agreement with recent X-ray diffraction measurements. The electrostatic potentials in the cone conformers of thiacalix[4]arene and calix[4]arene suggest that their complexation or recognition abilities can be significantly different. Dipole moments of the four thiacalix[4]arene conformers are in the order: cone>1,2-alternate>partial-cone>1,3-alternate.
TL;DR: In this paper, a spin-adapted open-shell CCSD method was proposed for the doublet state, which was implemented in an automated fashion and applied to a number of variations on the general theme.
Abstract: We present the outline for a general internally contracted multireference coupled cluster method, and indicate some open ended issues that are not yet resolved. The detailed equations for this general scheme are rather complicated and it is necessary to automate the implementation of the method. In this paper, we investigate the simplest variant of this scheme, a spin-adapted open-shell CCSD method applicable to open-shell doublet states. The method was implemented in an automated fashion, and we consider a number of slight variations on the general theme. Applications are presented for the cationic states of N 2 , C 2 H 4 , C 2 and O 3 , and anionic states of C 2 , O 3 and F 2 , which include various highly correlated states that illustrate the potential of the general methodology.
TL;DR: In this article, a critical review of the computational study of the Cope rearrangement of 1,5-hexadiene and its derivatives is presented and it is demonstrated that a realistic potential energy surface in the transition region can be obtained only at very high levels of approximation.
Abstract: This critical review highlights intricacies of the computational study of the Cope rearrangement of 1,5-hexadiene and its derivatives It is demonstrated that a realistic potential energy surface in the transition region can be obtained only at very high levels of approximation The emphasis is on the reasons why some generally reliable methods fail to make even qualitatively correct predictions about this reaction
TL;DR: In this paper, the role of peptide folding and its role in the reductionistic approach towards the understanding of protein folding is discussed from the points of view of past, present and possible future developments.
Abstract: Ab initio peptide folding, and its role in the reductionistic approach towards the understanding of protein folding are discussed from the points of view of past, present and possible future developments. It is believed that after the initial holistic approach, we are now at a new epoch, which will be dominated by reductionism. New quantitative mathematical models will be the result of the reductionistic approach that will lead toward a new, more sophisticated holistic era.
TL;DR: In this article, the electronic states of anatase and rutile TiO 2 crystals and of TiO2 with Ti 4+ ions with various metal ions were calculated using the DV-Xα method, which is a density functional method.
Abstract: The electronic states of anatase and rutile TiO 2 crystals and of TiO 2 crystals with Ti 4+ ions isomorphically replaced by various metal ions (V 3+ , V 4+ , V 5+ , Cr 3+ , Mn 3+ , Fe 3+ , Co 3+ , Ni 2+ , Ni 3+ , and Rh 3+ ) were calculated using the DV-Xα method, which is a density functional method. It was found that the band gap changed in parallel to the radius of the replacing ion, except for a few cases. This tendency agrees with an experimental result in the literature that the light absorption band of anatase shifts to a longer wavelength region, when metal ions are injected. Calculations showed that the replacement of Ti 4+ by Ni 3+ and V 5+ gave the largest decreases in the band gap, suggesting that these metal ions are the most efficient replacing ions for TiO 2 . The isomorphic replacement resulted in the formation of new energy levels of the metal 3d orbitals in the forbidden band of TiO 2 . These levels seemed to interact with the 2p orbitals of oxide ions in TiO 2 , resulting in a decreased band gap. An increase in the degree of covalent bonds between the replacing ions and the oxide ions probably contributes to the decrease in the band gap.
TL;DR: In this article, a posteriori correction suggested previously for the state-specific multireference Brillouin-Wigner coupled-cluster singles and doubles (MR BW CCSD) theory is applied to eliminate its size-extensivity error.
Abstract: We tested a posteriori correction suggested previously for the state-specific multireference Brillouin–Wigner coupled-cluster singles and doubles (MR BW CCSD) theory, to eliminate its size-extensivity error. The correction was applied to a two-reference BW CCSD model by using the cc-pVXZ basis sets ( X =2,3,4) and it was tested by calculating the spectroscopic constants of the F 2 molecule.
TL;DR: In this paper, the structural and electronic properties of armchair and zigzag models of single-wall BN nanotubes have been investigated by performing semi-empirical molecular orbital self-consistent field calculations at the level of the Austin Model 1 (AM1) method within the RHF formulation.
Abstract: The structural and electronic properties of armchair and zigzag models of single-wall BN nanotubes have been investigated by performing semi-empirical molecular orbital self-consistent field calculations at the level of the Austin Model 1 (AM1) method within the RHF formulation. It has been found that these structures are stable and exothermic. The armchair model has a zero net dipole moment, whereas the zigzag model has a nonzero net dipole moment. The interfrontier molecular energy gap of these systems are close to each other, thus the gap is independent of the chirality of the tube.
TL;DR: In this paper, the density functional theory and the cluster model approach have been used to study the adsorption of the ethylene molecule on the (100) surfaces of platinum, palladium and nickel.
Abstract: The density functional theory and the cluster model approach have been used to study the adsorption of the ethylene molecule on the (100) surfaces of platinum, palladium and nickel. For each metal surface, two adsorption sites have been considered: the atop site (π) and the bridge site (di-σ). For each case, optimized geometries, adsorption energies and vibrational wavenumbers have been calculated. The results show that ethylene adsorbs strongly on the three metal surfaces. Upon adsorption, the degree of distortion of the ethylene molecule (relative to its gas phase geometry), as well as the binding strength to the metal surface increases in the order Pd
TL;DR: In this article, the first two catalytic reactions, the oxidative addition and migratory 1,1-insertion of the Monsanto and Cativa processes, were investigated using density functional theory with hybrid B3LYP exchange and correlation functional.
Abstract: Density functional theory with hybrid B3LYP exchange and correlation functional has been used to investigate the first two catalytic reactions, the oxidative addition and migratory 1,1-insertion of the Monsanto and Cativa processes. One of the main interests has been to study if the previously unidentified trans forms of the active catalytic species [Rh(CO)2I2]− (1) or [Ir(CO)2I2]− (2) have any significance in these processes. The oxidative addition has been studied using both cis and trans forms of 1 and 2. We have also studied the oxidative addition of methyl iodide to [Rh(CO)2I3]2− (3). In addition, different isomers of dicarbonyls [CH3Rh(CO)2I3]− (4) and [CH3Ir(CO)2I3]− (5) and tricarbonyl [CH3Ir(CO)3I2] (8) has been used in the 1,1-insertion study to see if these could provide new, alternative reaction pathways. The calculated free energies of activation for the oxidative addition of methyl iodide to cis-1 and cis-2 are 20.8 and 16.9 kcal/mol, respectively. The corresponding free energy barriers for trans-1 and trans-2 are 15.0 and 13.2 kcal/mol, respectively. The oxidative addition is the rate-determining step in the Monsanto process and the reaction with trans-1 is predicted to accelerate that step. The presence of 3 could enhance the addition even more; the free energy of activation is only 5.6 kcal/mol. For the 1,1-insertions we have found similar activation energies in fac,cis- and mer,trans-structures. In the rhodium system, the free energies of activation are in the order of 20 kcal/mol and in the iridium system 30 kcal/mol. Interestingly, the insertions in mer,cis-dicarbonyls have considerably lower activation energies, half of those calculated for the insertions in mer,trans- and fac,cis-structures. The iodide dissociation from fac,cis-5 could provide the path to mer,cis-5 and so significantly enhance the rate of the insertion in the iridium system. The rate of the insertion should also increase if experimentally proposed tricarbonyl 8 is used instead of 5. According to our calculations of different isomers of 8, this seem to be true although the insertion barrier in mer,cis-5 is calculated to be even lower. Our results are consistent with the experiments and other computational results. These results show that the geometrical arrangement of the ligands has a very large effect on the catalytic activity of the complexes and this suggests possible improvements to these industrially important processes.
TL;DR: In this article, a qualitative valence bond (VB) mechanism has been suggested for the interconversion of (i) N 12 →N 10 +N 2 ; and (ii) N 10 →N 8 +N2 (TS2, C 1 ).
Abstract: Ab initio MO computations have been performed at HF/6-31G(d) and electron correlated MP2/6-311G(d) levels of theory for N 12 (diazobispentazole, N 5 –N N–N 5 , C 2 h ), N 10 (pentaazopentazole, N 5 –NNNNN, C 1 ), N 8 (azidopentazole, N 5 –N 3 , C s ) and the two transition states connecting (i) N 12 and N 10 +N 2 (TS1, C 1 ); and (ii) N 10 and N 8 +N 2 (TS2, C 1 ). In addition, a qualitative valence bond (VB) mechanism has been suggested for the interconversion of (i) N 12 →N 10 +N 2 ; and (ii) N10→N 8 +N 2 .
TL;DR: In this article, the Fukui function, condensed Fukui functions and relative nucleophilicy indices were calculated for 2-methyl-2-nitrosopropane nitroso group in the gas phase and in the water phase using SCRF SCI-PCM.
Abstract: The Fukui function, condensed Fukui function and relative nucleophilicy indices DFT calculations for nitrosobenzene and 2-methyl-2-nitrosopropane nitroso group in the gas-phase and in the water solution using SCRF SCI-PCM have been presented and discussed. Considering the calculated Fukui functions, both the N and the O atomic centres are found to be suitable nucleophilic reactive sites in the gas-phase and in the water solution. Condensed Fukui function and relative nucleophilicity indices support this view. Also, condensed Fukui function and relative nucleophilicity indices calculated from charges obtained with orbital charge calculation methods (MPA and NPA) show that the O atom of the nitroso group is a more reactive nucleophilic centre, while indices derived from the electrostatic potential charges (MKS, CHelp and CHelpG) emphasize the N atom as a more reactive nucleophilic site.
TL;DR: In this article, a method is devised to construct spherical large fullerenes (C 240, C 540, C 960, C 2160, C 2940, C 3840, C 4860 ) by using the triangular motif.
Abstract: According to the experimental investigation, the carbon nano-particles have spherical multi-layer structure (also called onion-like carbon structure). Theoretically, the optimum structures of these large fullerenes contain highly faceted shapes with icosahedral symmetry. This discrepancy in structure may be attributed to the formation mechanism. Thus, a method is devised to construct spherical large fullerenes (C 240 , C 540 , C 960 , C 2160 , C 2940 , C 3840 , C 4860 ) by using the triangular motif. The 5–7–5–7 shape defect is applied in this method for assembling the large spherical fullerenes which could transform the graphene sheet to a spherical motif via SW rearrangement. The geometry-optimized structures of large spherical fullerenes have been generated by molecular mechanics calculation. Then, the average radius and standard deviation of these large fullerenes were obtained to verify the spherical shape. The multi-layer fullerene with spherical shape was confirmed by the TEM observation. According to the structure analysis, the distance between two neighboring encapsulating carbons is about 3.5 A, which approximately coincides with the distance between two layers of graphite. The van der Waals force per carbon atom and of multi-layer fullerene with the spherical shape generated by force field calculation, predict their relative stability.
TL;DR: In this paper, six different exchange-correlation functionals were assessed in a quantum chemical study of the geometry and energetics of silanol, its conjugated base and both conformers of disiloxane, using Dunning's augmented correlation consistent basis set.
Abstract: Six different exchange-correlation functionals were assessed in a quantum chemical study of the geometry and energetics of silanol, its conjugated base and both conformers of disiloxane, using Dunning's augmented correlation consistent basis set. It was found that the B3LYP and the B3PW91 hybrid functionals perform the best, their performance was comparable with MP2/VQZ calculations. A deprotonation energy of −365.4 kcal/mol for silanol was obtained at the B3PW91/AVQZ level. For disiloxane, a very small difference between both disiloxane conformers, 0.037 kcal/mol (extra stabilization for the doubly staggered conformation) and 149.3° for the very sensible Si–O–Si angle was found, or 154.7° temperature corrected (300 K).