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 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 Stability and Structural Determination of Vinyldichlorosilane by ab initio Calculations

1999 ◽  
Vol 54 (8-9) ◽  
pp. 503-506
Author(s):  
Mohammad A. Qtaitat
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Structural Parameters ◽  
Conformational Stability ◽  
Basis Sets ◽  
Structural Determination

The Conformational stability and barriers of interconversion between the eis and gauche conformers of vinyldichlorosilane, CH2CHSiHCl2 , have been studied using ab initio calculations employing the RHF/3-21G* and RHF/6-31G* basis sets. The eis conformer was found to be more stable than the gauche one by 45 cm-1 (539 J/mol) and 140 cm-1 (1.68 kJ/mol) from the RHF/3-21G* and RHF/6-31G* basis sets, respectively. Additionally, the structural parameters of both rotamers have been calculated. These results are compared with results of related molecules.

Saturated N,X-Heterocyclic Carbenes (X=N, O, S, P, Si, C, and B): Stability, Nucleophilicity, and Basicity

2015 ◽  
Vol 68 (9) ◽  
pp. 1438 ◽  
Author(s):  
Zahra Azizi ◽  
Mehdi Ghambarian ◽  
Mohammad A. Rezaei ◽  
Mohammad Ghashghaee
Keyword(s):  
Density Functional ◽  
Energy Gap ◽  
Energy Difference ◽  
Heterocyclic Carbenes ◽  
Structure Stability ◽  
Triplet Energy ◽  
Lumo Energy ◽  
Dft Methods ◽  
Functional Theory ◽  
Homo Lumo

Various saturated five-membered N,X-heterocyclic carbenes (X = N, O, S, P, Si, C, and B) have been studied by ab initio and density functional theory (DFT) methods. The substitutions alter the properties of the reference carbene from the viewpoint of electronic structure, stability, nucleophilicity, and basicity. Our study shows that the oxygen containing carbene (X = O) induces the highest HOMO–LUMO energy gap (ΔEHOMO–LUMO), while carbene with X = N has the widest singlet–triplet energy difference (ΔEs–t). The nucleophilicity of the carbene derivatives increased upon replacement of C, Si, and B, with the effect of the boron substituent being more pronounced. In addition, the basicity of the structure increased for the carbene derivatives with X = C and B with the latter substitution imposing a remarkably higher effect. Moreover, the substitution of boron at the α-position of the carbene increased the nucleophilicity and basicity, while inducing a reduction in the values of ΔEs–t and ΔEHOMO–LUMO.

Ab initio studies of isocyanatoborane and difluoroisocyanatoborane; bent or linear BNCO chains?

1991 ◽  
Vol 69 (6) ◽  
pp. 1000-1005 ◽  
Author(s):  
Susan Ellis ◽  
Edward G. Livingstone ◽  
Nicholas P. C. Westwood
Keyword(s):  
Carbon Atom ◽  
Ab Initio Calculations ◽  
Ab Initio ◽  
Potential Surface ◽  
Energy Difference ◽  
Basis Set ◽  
Basis Sets ◽  
Hartree Fock ◽  
Split Valence ◽  
Valence Basis Set

Ab initio calculating at the 3-21G, 6-31G*, 6-31G**, 6-311G*, and 6-311G** Hartree–Fock levels, have been performed on the unknown H2BNCO and F2BNCO molecules in order to establish the geometries of these isoelectronic propadienone analogues. For H2BNCO the double split valence basis sets lead to linear BNCO chains, whereas either a triple split valence basis set, or the inclusion of correlation to second order (MP2/6-31G*) gives trans-bent structures. These have angles at nitrogen of 153.6° (6-311G*) or 149.9° (MP2/6-31G*), with the potential surface for angle bending extremely flat, and 0.5 kJ mol−1 (6-311 G*) or 1.12 kJ mol−1 (MP2/6-31 G*) separating the C2v and Cs structures. For the bent structures there is a small trans-bend (4–6°) at the carbon atom. The F2BNCO molecule is also linear at the 3-21G level, but is, however, already trans-bent (145.6° at nitrogen, 175.9° at carbon) at the 6-31G* Hartree–Fock level; 1.47 kJ mol−1 separates the bent and linear structures. The triple split valence basis set 6-311G* leads to a further decrease in the angle at nitrogen (141.0°), and a similar NCO angle (175.9°), with the bent structure favoured by 3.85 kJ mol−1. MP2/6-31G* calculations give a minimum with an angle at nitrogen of 140.2°, and a bent-linear energy difference of 3.58 kJ mol−1. Key words: ab initio calculations, isocyanatoboranes, structures, quasilinearity, propadienone analogues.

The six-membered-ring azomethineN-((E)-{5-[(E)-(pyridin-3-ylimino)methyl]thiophen-2-yl}methylidene)pyridin-3-amine

2013 ◽  
Vol 69 (10) ◽  
pp. 1196-1199 ◽  
Author(s):  
Andréanne Bolduc ◽  
Stéphane Dufresne ◽  
W. G. Skene
Keyword(s):  
Energy Gap ◽  
Three Dimensional ◽  
Energy Difference ◽  
Membered Ring ◽  
Intermolecular Hydrogen Bonding ◽  
Lumo Energy ◽  
Azomethine Group ◽  
Compound C ◽  
The Mean ◽  
Homo Lumo

The title compound, C16H12N4S, forms a three-dimensional layered network structureviaintermolecular hydrogen bonding and π-stacking. The azomethine molecule adopts the thermodynamically stableEregioisomer and the pyridine substituents are antiperiplanar. The mean planes of the pyridine rings and the azomethine group to which they are connected are twisted by 27.27 (5) and 33.60 (5)°. The electrochemical energy gap of 2.3 eV based on the HOMO–LUMO energy difference is in agreement with the spectroscopically derived value.

scholarly journals Vibrational Spectroscopic and Computational Analysis of 5-chloro-2-hydroxy Acetophenone

2020 ◽  
Vol 8 (4S4) ◽  
pp. 254-259
Keyword(s):  
Density Functional ◽  
Computational Analysis ◽  
Energy Gap ◽  
Molecular System ◽  
Basis Sets ◽  
Geometrical Parameters ◽  
Lumo Energy ◽  
Functional Theory ◽  
Infrared Intensities ◽  
Homo Lumo

Fourier Transfer infrared and Raman spectra in the range of 4000-400 cm-1 and 3500-50 cm-1 were recorded to study the vibrational spectra of 5-chloro-2-hydroxyacetophenone (CHAP). Using density functional theory (DFT/B3LYP) with 6-31+G(d,p) and 6-311++G(d,p) basis sets the various geometrical parameters such as Raman activities, infrared intensities and optimum frequencies were calculated. The HOMO-LUMO energy gap has been computed which confirms the charge transfer of the molecular system. Mulliken’s atomic charges associated with each atom and thermodynamic parameters have also been reported with the same level of DFT.

scholarly journals Effects of Zinc Binding on the Structure and Stability of Glycylglycine Dipeptide: A Computational Study

2012 ◽  
Vol 9 (3) ◽  
pp. 1244-1250 ◽  
Author(s):  
Mohammad T. Baei ◽  
S. Zahra Sayyad-Alangi
Keyword(s):  
Electronic Properties ◽  
Energy Gap ◽  
Computational Study ◽  
Bidentate Ligand ◽  
Zinc Binding ◽  
Basis Sets ◽  
Stable Complex ◽  
Stable Isomer ◽  
Zinc Cation ◽  
Homo Lumo

Energies and structures of six different isomers resulted of complex between glycylglycine dipeptide and zinc cation were calculated at the BLYP and B3LYP levels of theory with 6-311G** and 6-311++G** basis sets. The energies of the glycylglycine-Zn2+different complexes at the BLYP/6-311++G** obtained more stable than another methods. In this method, the most stable complex was zinc cation bound to the two oxygen sites of the glycylglycine zwitterion and acted as a bidentate ligand. The vibrational frequencies, thermodynamic and electronic properties, HOMO, LUMO, and energy gap for the most stable isomer in different solvents were calculated at the BLYP/6-311++G** level of theory.

Ab Initio Studies on the Interaction Between Copper(I) and 5-Nitrotetrazolate Anion

2017 ◽  
Vol 68 (1) ◽  
pp. 12
Author(s):  
V. Venkatesan
Keyword(s):  
Binding Energy ◽  
Ab Initio ◽  
Molecular Orbital ◽  
Natural Bond Orbital ◽  
Energy Difference ◽  
Atoms In Molecules ◽  
Basis Sets ◽  
Molecular Orbital Calculations

<p class="p1">Ab initio molecular orbital calculations on the interaction between Copper(I) and 5-nitrotetrazolate anion were done using different basis sets, at the HF, B3LYP and MP2 levels of theory. Three minima were found to 1A, 1B and IC structures of Cu(I)-5-NTz complex. At the B3LYP/LanL2DZ level, the energy difference between 1A and 1B was computed to be 8.18 kcal/mol, while that between 1A and 1C is 22.76 kcal/mol. The presence of both Cu-N and Cu-O interactions is revealed in 1A structure using both natural bond orbital and atoms-in-molecules analyses, which makes more stable than those of 1B and 1C complexes. The binding energy corrected for both ZPE and BSSE for 1A is found to be -150.59 kcal/mol at the B3LYP/LanL2DZ level. The barrier for the 1B <span class="s1">→</span> 1A and 1C <span class="s1">→</span> 1B conversion is calculated to be 7.80 kcal/mol and 9.40 kcal/mol, respectively.</p>

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