scholarly journals Engineering energy gap of the carbon saturated nanowire and investigation of ammonia molecule doping effects by using initial calculations (Ab initio).

2018 ◽  
Vol 18 (2) ◽  
pp. 313-320
Author(s):  
F Marsusi ◽  
S M Monavari ◽  
◽  
Keyword(s):  
Ab Initio ◽  
Energy Gap ◽  
Ammonia Molecule ◽  
Doping Effects

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.

scholarly journals Diradicalar Character and Ring Stability of Mesoionic Heterocyclic Oxazoles and Thiazoles by Ab Initio Mono and Multi-Reference Methods

Molecules ◽  
2020 ◽  
Vol 25 (19) ◽  
pp. 4524
Author(s):  
Antonio João da Silva Filho ◽  
Lucinêz da Cruz Dantas ◽  
Otávio Luís de Santana
Keyword(s):  
Ab Initio ◽  
Energy Gap ◽  
Experimental Studies ◽  
Energy Difference ◽  
Basis Sets ◽  
Neutral Compounds ◽  
Diradical Character ◽  
Electronic Configurations ◽  
Homo Lumo

Mesoionics are neutral compounds that cannot be represented by a fully covalent or purely ionic structure. Among the possible mesomeric structures of these compounds are the diradical electronic configurations. Theoretical and experimental studies indicate that some mesoionic rings are unstable, which may be related to a significant diradical character, that until then is not quantified. In this work, we investigated the diradical character of four heterocycles: 1,3-oxazol-5-one, 1,3-oxazol-5-thione, 1,3-thiazole-5-one, and 1,3-thiazole-5-thione. The oxazoles are known to be significatively less stable than thiazoles. DFT and ab initio single (B3LYP, MP2, CCSD, and QCISD) and ab initio multi-reference (MR-CISD) methods with three basis sets (6-311+G(d), aug-cc-pVDZ, and aug-cc-pVTZ) were employed to assess the diradical character of the investigated systems, in gas phase and DMSO solvent, from three criteria: (i) HOMO-LUMO energy gap, (ii) determination of energy difference between singlet and triplet wave functions, and (iii) quantification of the most significant diradical character (y0, determined in the unrestricted formalism). All of the results showed that the diradical character of the investigated systems is very small. However, the calculated electronic structures made it possible to identify the possible origin of the oxazoles instability, which can help the design of mesoionic systems with the desired properties.

AB INITIO PATH INTEGRAL STUDY ON ISOTOPE EFFECT OF AMMONIA MOLECULE

2005 ◽  
Vol 04 (01) ◽  
pp. 175-181 ◽  
Author(s):  
MASANORI TACHIKAWA ◽  
MOTOYUKI SHIGA
Keyword(s):  
Molecular Dynamics ◽  
Ab Initio ◽  
Isotope Effect ◽  
Path Integral ◽  
Quantum Chemical ◽  
Degrees Of Freedom ◽  
Dynamics Simulation ◽  
Ammonia Molecule ◽  
Quantum Chemical Approach ◽  
Quantum Feature

We have applied ab initio path integral molecular dynamics simulation to study the quantum feature and proton/deuteron isotope effect of ammonia molecule. This method treats all the rotational and vibrational degrees of freedom fully quantum mechanically, while the potential energies of the respective molecular configurations are calculated "on the fly" using ab initio quantum chemical approach. The differences on the geometry and the electronic structure between NH 3 and ND 3 molecules are investigated in detail.

scholarly journals Why base-catalyzed isomerization of N-propargyl amides yields mostly allenamides rather than ynamides

2015 ◽  
Vol 11 ◽  
pp. 1441-1446 ◽  
Author(s):  
Armando Navarro-Vázquez
Keyword(s):  
Ab Initio ◽  
Equilibrium Position ◽  
Structural Changes ◽  
Energy Gap ◽  
Equilibrium Mixture ◽  
Experimental Results ◽  
Exchange Equilibrium ◽  
Small Energy ◽  
Dft Computations ◽  
Ab Initio And Dft

The base-catalyzed isomerization of N-propargylamides or carbamates may furnish N-allenyl compounds (allenamides/allencarbamates) or further evolve to N-alkynyl compounds (ynamides or yncarbamates). The particular fate of this reaction varies from experiment to experiment and there is no clear rule for predicting the reaction outcome for a particular structure. With the support of ab initio and DFT computations, this work shows that observed results can be explained by assuming an exchange equilibrium between energetically close N-propargyl, allenyl and N-alkynyl forms and that the reaction outcome correlates to a particular equilibrium mixture. Due to the very small energy gap between the N-allenyl and N-alkynyl forms, small structural changes may easily alter the equilibrium position, explaining the variety of observed experimental results. Based on CBS-QB3 computations, the ωB97 functional provided reasonably accurate isomerization energies and could successfully predict the experimentally observed behavior for several examples from the literature.

scholarly journals Electronic Structure of Hydrogenated and Surface-Modified GaAs Nanocrystals: Ab Initio Calculations

2012 ◽  
Vol 2012 ◽  
pp. 1-5 ◽  
Author(s):  
Hamsa Naji Nasir ◽  
Mudar A. Abdulsattar ◽  
Hayder M. Abduljalil
Keyword(s):  
Electronic Structure ◽  
Ab Initio ◽  
Valence Band ◽  
Density Functional ◽  
Energy Gap ◽  
Unit Cell ◽  
Large Unit ◽  
Geometrical Optimization ◽  
Large Unit Cell ◽  
Core Part

Two methods are used to simulate electronic structure of gallium arsenide nanocrystals. The cluster full geometrical optimization procedure which is suitable for small nanocrystals and large unit cell that simulates specific parts of larger nanocrystals preferably core part as in the present work. Because of symmetry consideration, large unit cells can reach sizes that are beyond the capabilities of first method. The two methods use ab initio Hartree-Fock and density functional theory, respectively. The results show that both energy gap and lattice constant decrease in their value as the nanocrystals grow in size. The inclusion of surface part in the first method makes valence band width wider than in large unit cell method that simulates the core part only. This is attributed to the broken symmetry and surface passivating atoms that split surface degenerate states and adds new levels inside and around the valence band. Bond length and tetrahedral angle result from full geometrical optimization indicate good convergence to the ideal zincblende structure at the centre of hydrogenated nanocrystal. This convergence supports large unit cell methodology. Existence of oxygen atoms at nanocrystal surface melts down density of states and reduces energy gap.

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