An ab initio molecular orbital study of the geometry of the dicationic Wallach rearrangement intermediate

2003 ◽  
Vol 81 (6) ◽  
pp. 535-541 ◽  
Author(s):  
Robin A Cox ◽  
David YK Fung ◽  
Imre G Csizmadia ◽  
Erwin Buncel
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Acid Catalysis ◽  
Parent Compound ◽  
Methyl Groups ◽  
Intermediate Structure ◽  
Reaction Intermediates ◽  
Aromatic Rings ◽  
Molecular Orbital Study ◽  
Good Agreement

Ab initio calculations have been performed on several different structures for the dicationic intermediate proposed for the Wallach rearrangement of aromatic azoxy compounds to hydroxy-substituted azo systems in strongly acidic media. For the unsubstituted parent compound azoxybenzene, these calculations reveal that the preferred structure for the intermediate is planar, as previously assumed, but bent rather than linear as we have formulated it. The presence of two methyl groups at the para positions of both aromatic rings does not change this situation, but six methyl groups at all para and ortho ring positions lead to a different preferred structure — still bent but with the two aromatic rings now at 90° to one another rather than being coplanar — undoubtedly due to steric interference between the ortho methyl groups. In all the cases examined the two positive charges reside primarily in the aromatic rings rather than on the nitrogens, which are sp2 hybridized and still have their lone pairs. The overall structures can best be regarded as two six-electron π systems joined together, with little communication between the two rings. For the most part the calculations are in good agreement with experimental observations. Recent calculations on other possible reaction intermediates by other groups are also discussed.Key words: Ab initio calculations, Wallach rearrangement, azoxyarenes, reaction intermediate, acid catalysis, intermediate structure, reaction mechanism.

ab initio MO Optimization of Molecular Structures of Fluoro- and Chloro-Substituted Dibenzo-p-dioxines and the Effect of Halogen Substitution at the 2,3,7,8-Positions on Metabolic Attack

1997 ◽  
Vol 52 (11) ◽  
pp. 1418-1431 ◽  
Author(s):  
Roland Weber ◽  
Till Kühn ◽  
Hanspaul Hagenmaier ◽  
Günter Häfelinger
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Energy Gap ◽  
Substituent Effects ◽  
Molecular Structures ◽  
X Ray ◽  
Degree Pattern ◽  
Chlorine Substituent ◽  
Homo Lumo ◽  
Good Agreement

Full ab initio optimizations were performed on the molecular structures of 24 fluorinated and chlorinated dibenzodioxines (PFDD/PCDD ) and dibenzofurans (PFDF/PCDF). Reasonable agreement was found by comparing the geometries of four calculated structures with known X-ray data from the literature. For the fluorine substituent, calculated electron densities (Mulliken total charges and π-electron charges) clearly demonstrate the opposite influence of the inductive (I) and mesomeric (M) effect. The changes in π-densities at carbons in ortho-, meta- and para-position are constant for each fluorine substituent (independent of degree, pattern, and position of substitution). It is thus possible to calculate the π-densities of the substituted dioxines by increments starting from dibenzodioxine. π-Charges from quantum mechanical calculations and the increment system show good agreement even for OctaFDD (O8FDD ), where eight substituent effects are acting additively. Compared with fluorine, the chlorine substituent exercises a smaller -I-effect and a clearly weaker +M-effect. The HOMO coefficients of the unsubstituted dibenzodioxine and dibenzofuran, extracted from ab initio calculations, yield a good explanation for the observed regioselective metabolic attack at the 2,3,7,8-positions. The squares of the HOMO-coefficients of the 2,3,7,8-positions in dibenzodioxine (DD ) are about ten times greater than those of the 1,4,6,9-positions. These HOMO coefficients are practically unaffected by halide substitution. But halogen substitution reduces strongly the electron density at the halogen-bound carbon, which, however, is a necessary prerequisite for the electrophilic oxygen transfer during metabolism. One would therefore expect halogen substitution of dibenzodioxine and dibenzofuran (DF) at the 2,3,7,8-position to hinder metabolism, as is indeed found. This provides a plausible explanation for the highly selective tissue retention of 2,3,7,8-substituted PCDDs and PCDFs. Our ab initio calculations of five tetra CDDs (T4CDDs) confirm the postulate of Kobayashi et al. [1 ] who, using semiempirical calculations, found a correlation between the toxicity of a dioxine congener and its absolute molecular hardness. The 2,3,7,8-T4CDD also exhibits the smallest absolute hardness (derived from the HOMO-LUMO energy gap) in our calculations.

Conformational Structure of Chloromethyldichlorophosphines

2002 ◽  
Vol 57 (6-7) ◽  
pp. 333-336
Author(s):  
Evgenii A. Romanenko ◽  
Alexander M. Nesterenko
Keyword(s):  
Experimental Data ◽  
Ab Initio Calculations ◽  
Ab Initio ◽  
Electron Correlation ◽  
Basis Set ◽  
Conformational Structure ◽  
Gauche Conformation ◽  
Nuclear Quadrupole ◽  
Set Up ◽  
Good Agreement

IThe 35Cl nuclear quadrupole resonances (77 K) and ab initio calculations of trichloromethyldichlorophosphine () show that it exists in the chess conformation form. The barrier to internal rotation about the P-C bond in I at the RHF/6-31++ G(d,p) level equals to 38.1 kJ mol-1. In chloromethyldichlorophosphine (II) the extension of the basis set up to the RHF/6-311++G(df, pd) level does not improve the description of the most preferable gauche-conformation; only if electron correlation (at the MP2 level) is taken into account the results are in a good agreement with experimental data.

Ab initio studies on amides: conformational preferences of diacetamide and barriers to interconversion of the conformers

1980 ◽  
Vol 33 (11) ◽  
pp. 2337 ◽  
Author(s):  
L Radom ◽  
NV Riggs
Keyword(s):  
Hydrogen Atom ◽  
Ab Initio ◽  
Internal Rotation ◽  
Model Systems ◽  
Methyl Groups ◽  
Electrostatic Repulsion ◽  
Conformational Preferences ◽  
Out Of Plane ◽  
Model Transition ◽  
Good Agreement

Diacetamide, like other diacylamines, is capable of existing in three basic conformations about the N-C bonds. Optimization (STO-3G) of model systems in which all first-row atoms and the amido hydrogen atom are held coplanar predicts that the E,Z conformer (3) is of lowest energy, the Z,Z conformer (2) of somewhat higher energy (4.2 kJ mol-1), and the E,E conformer (1) of highest energy (23.6 kJ mol-1); 4-31G evaluation of the energies suggests that (1) and (2) are each of higher energy than (3) by 27-28 kJ mol-1. It is suggested that (2) is destabilized with respect to (3) by electrostatic repulsion of the two negatively charged oxygen atoms whereas destabilization of (1) is due to substantial methyl-methyl steric interactions as reflected in the very wide <CNC (136°); the energy of (1) is, however, raised by out-of-plane or rotational movements of the methyl groups, i.e., the preferred structure (excluding methyl hydrogens) is planar. The calculated height of the barrier to internal rotation of (3) by either of two model transition states is 41-45 kJ mol-1, in good agreement with an experimental value of 45.2 kJ mol-1 in solution at -60°.

AB-INITIO CALCULATIONS OF SUPERCONDUCTING PROPERTIES OF YBa2Cu3O7

1999 ◽  
Vol 13 (29n31) ◽  
pp. 3579-3581 ◽  
Author(s):  
G. L. ZHAO ◽  
D. BAGAYOKO
Keyword(s):  
Electronic Structure ◽  
Ab Initio Calculations ◽  
Ab Initio ◽  
High Energy ◽  
Optical Phonons ◽  
Superconducting Properties ◽  
S Wave ◽  
Gap Equation ◽  
Self Consistent ◽  
Good Agreement

We present ab-initio calculations for the electronic structure and superconducting properties of YBa 2 Cu 3 O 7 (YBCO). The electronic structure was calculated using a self-consistent ab-initio LCAO method. We solved the anisotropic Eliashberg gap equation numerically. The strong coupling of the high energy optical phonons around 60-73 meV, with the electrons at the Fermi surface, leads to a high Tc in YBCO. The calculated Tc is about 89 K for μ*=0.1. The good agreement of the calculated results with experimental measurements and the ab-initio nature of the calculations support the scenario of an anisotropic s-wave superconductor for YBCO.

A study of hydrogen abstraction reactions by halogen atoms with HFCO and HClCO: determination of transition state structures, barrier heights, and vibrational frequencies

1993 ◽  
Vol 71 (1) ◽  
pp. 135-140 ◽  
Author(s):  
J. S. Francisco ◽  
N. Mina-Camilde
Keyword(s):  
Transition State ◽  
Ab Initio Calculations ◽  
Ab Initio ◽  
Chlorine Atom ◽  
Fluorine Atom ◽  
Hydrogen Abstraction ◽  
Barrier Heights ◽  
Optimized Geometries ◽  
Good Agreement

Theoretical barriers and reaction energetics were determined using ab initio calculations at the PMP4/6-311 + +G** level with UMP2/6-311G** optimized geometries for hydrogen abstraction reactions of HFCO and HClCO by fluorine and chlorine atoms. The largest barriers are predicted for fluorine and chlorine atom abstractions of hydrogen from HFCO. Fluorine and chlorine atom abstractions of hydrogen from HClCO are predicted to be barrierless. The predicted barrier for fluorine atom abstraction of hydrogen from HFCO is calculated to be 1.3 kcal mol−1 and is in good agreement with the experimental value of 1.8 ± 0.4 kcal mol−1.

Multireference Ab Initio Calculations on Reaction Intermediates of the Multicopper Oxidases

2006 ◽  
Vol 45 (26) ◽  
pp. 11051-11059 ◽  
Author(s):  
Jakub Chalupský ◽  
Frank Neese ◽  
Edward I. Solomon ◽  
Ulf Ryde ◽  
Lubomír Rulíšek
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Reaction Intermediates ◽  
Multicopper Oxidases

scholarly journals Dynamics Studies of Nitrogen Interstitial in GaN from Ab Initio Calculations

Materials ◽  
2020 ◽  
Vol 13 (16) ◽  
pp. 3627
Author(s):  
Huan He ◽  
Wenbo Liu ◽  
Pengbo Zhang ◽  
Wenlong Liao ◽  
Dayin Tong ◽  
...  
Keyword(s):  
Optical Properties ◽  
Ab Initio Calculations ◽  
Ab Initio ◽  
Separation Distance ◽  
Semiconductor Materials ◽  
Stable Configuration ◽  
Anisotropic Behavior ◽  
Out Of Plane ◽  
First Time ◽  
Good Agreement

Understanding the properties of defects is crucial to design higher performance semiconductor materials because they influence the electronic and optical properties significantly. Using ab initio calculations, the dynamics properties of nitrogen interstitial in GaN material, including the configuration, migration, and interaction with vacancy were systematically investigated in the present work. By introducing different sites of foreign nitrogen atom, the most stable configuration of nitrogen interstitial was calculated to show a threefold symmetry in each layer and different charge states were characterized, respectively. In the researches of migration, two migration paths, in-plane and out-of-plane, were considered. With regards to the in-plane migration, an intermediated rotation process was observed first time. Due to this rotation behavior, two different barriers were demonstrated to reveal that the migration is an anisotropic behavior. Additionally, charged nitrogen Frenkel pair was found to be a relatively stable defect complex and its well separation distance was about 3.9 Å. Part of our results are in good agreement with the experimental results, and our work provides underlying insights of the identification and dynamics of nitrogen interstitial in GaN material. This study of defects in GaN material is useful to establish a more complete theory and improve the performance of GaN-based devices.

scholarly journals Nitrogen Dissolution in Liquid Ga and Fe: Comprehensive Ab Initio Analysis, Relevance for Crystallization of GaN

Materials ◽  
2021 ◽  
Vol 14 (5) ◽  
pp. 1306
Author(s):  
Jacek Piechota ◽  
Stanislaw Krukowski ◽  
Petro Sadovyi ◽  
Bohdan Sadovyi ◽  
Sylwester Porowski ◽  
...  
Keyword(s):  
Experimental Data ◽  
Molecular Dynamics ◽  
Ab Initio Calculations ◽  
Ab Initio ◽  
Chemical Potential ◽  
Nitrogen Solubility ◽  
Molecular Nitrogen ◽  
Ideal Gas ◽  
The Difference ◽  
Good Agreement

The dissolution of molecular nitrogen in Ga and Fe was investigated by ab initio calculations and some complementary experiments. It was found that the N bonding inside these solvents is fundamentally different. For Ga, it is between Ga4s and Ga4p and N2p states whereas for Fe this is by N2p to Fe4s, Fe4p and Fe3d states. Accordingly, the energy of dissolution of N2 for arbitrarily chosen starting atomic configurations was 0.535 eV/mol and −0.299 eV/mol for Ga and Fe, respectively. For configurations optimized with molecular dynamics, the difference between the corresponding energy values, 1.107 eV/mol and 0.003 eV/mol, was similarly large. Full thermodynamic analysis of chemical potential was made employing entropy-derived terms in a Debye picture. The entropy-dependent terms were obtained via a normal conditions path to avoid singularity of ideal gas entropy at zero K. Nitrogen solubility as a function of temperature and N2 pressure was evaluated, being much higher for Fe than for Ga. For T=1800 K and p=104 bar, the N concentration in Ga was 3×10−3 at. fr. whereas for Fe, it was 9×10−2 at. fr. in very good agreement with experimental data. It indicates that liquid Fe could be a prospective solvent for GaN crystallization from metallic solutions.

Analysis of magnesium–carbon bonding in magnesium anthracene systems

1996 ◽  
Vol 74 (6) ◽  
pp. 801-809 ◽  
Author(s):  
Ralf Stegmann ◽  
Gernot Frenking
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Electron Density ◽  
Chemical Shifts ◽  
Topological Analysis ◽  
Nbo Analysis ◽  
Experimental Values ◽  
A Charge ◽  
Good Agreement ◽  
Carbon Bonding

Ab initio calculations at the MP2/3-21G(*) level of theory have been carried out for the magnesium–anthracene complexes 9,10-magnesiumanthracene•3H2O (1) and the 9-methyl (2), dimethyl (3), and 9,10-bis(methylsilyl) (4) substituted derivatives. The theoretically predicted geometries of the anthracene ligands are also reported. The calculated geometries of 1–4 are in very good agreement with experimental values for the corresponding THF complexes. The Mg—C9,10 bonds of the bridged structures are rather long and the anthracene ligands are folded by ~40° along the C9–C10 line in the complexes. Analysis of the electronic structure shows clearly that the Mg—C9,10 bonds should be considered as purely ionic. This is revealed by topological analysis of the electron density distribution and its associated Laplacian. The electron density at the Mg—C9,10 bond critical points ρ(rb) is very low and the Laplacian [Formula: see text] and the energy density Hb have positive values. The ionic nature of the Mg—C9,10 bond is also indicated by the natural bond order (NBO) analysis, which gives a Lewis structure with two lone pairs at C9 and C10 but no Mg—C9,10 bonds. The NBO method gives a charge donation from Mg to the anthracene ligand of nearly two. The theoretically predicted NMR chemical shifts using the GIAO method give 13C resonances for the complex 1 and for anthracene and anthracene dianion that are in good agreement with experimental values. Key words: magnesium–anthracene complexes, ab initio calculations, analysis of magnesium–carbon bonding.

Structure of 6H–SiC(0001) Surfaces from AB-Initio Calculations

1998 ◽  
Vol 05 (01) ◽  
pp. 199-205 ◽  
Author(s):  
M. Sabisch ◽  
P. Krüger ◽  
A. Mazur ◽  
J. Pollmann
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Local Density ◽  
Substrate Surface ◽  
Basis Sets ◽  
Smooth Norm ◽  
Optimal Surface ◽  
Substrate Surfaces ◽  
Gaussian Orbital ◽  
Good Agreement

We report the results of ab-initio calculations of structural properties of hexagonal 6H–SiC(0001) surfaces. The calculations have been carried out self-consistently within local density approximation employing supercell geometries, smooth norm-conserving pseudopotentials in separable form and Gaussian orbital basis sets. We have investigated several structural models for adatom-induced [Formula: see text] reconstructions with adsorbed Si or C adatoms or trimers residing in threefold-symmetric T 4 or H 3 positions above Si- or C-terminated substrate surfaces, respectively. In the case of the Si-terminated substrate surface our results favor Si adatoms in T 4 sites as optimal configuration in very good agreement with experimental data. For the C-terminated substrate surface our results indicate that none of the investigated [Formula: see text] adatom or trimer configurations is the optimal surface structure.

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