جستجوی مقالات مرتبط با کلیدواژه « hf » در نشریات گروه « شیمی »

The novelty of the work lies in the application of quantum computing analysis, specifically employing density functional theory (DFT) and Hartree-Fock (HF) techniques with various basis sets (aug-cc-pVQZ, 3-21G, 6-31G, 6-311G, and SDD), this work examined the structure and characteristics of naphthalene. The theoretical nature of naphthalene's structure and characteristics:  Highest Occupied Molecular Orbital (HOMO), Lowest Unoccupied Molecular Orbital (LUMO), band gap BG, density of state (DOS), Ultraviolet (UV), and Natural Bond Orbital (NBO) are explored. Several additional characteristics have been studied: thermochemical properties at standard temperature and pressure, and their optical properties (Optical BG with the indirect and direct transition).  The DFT/aug-cc-pVQZ basis was used with a fixed value of 4.75 eV to determine the HOMO-LUMO gap of naphthalene in this investigation. We find that the gaps of 4.71, 4.873, and 4.74 eV, respectively, in a recent density-functional theory (DFT) study that agrees with our results.

Conformationally-rigid para-sulfonato-calix[4]arene (C28H24O16S4) was isolated. The NMR parameters of the structure of calix[4]arenes have been compared. The study of organic structures to form nanoporous materials is a well-known chemical phenomena (supermolecular chemistry) that is necessary for finding the crystal forms of calix[4]arenes. We investigated and compared the hydrogen bonding, oxygen, and sulfur atom effects on calix[4]arene via Hartree-fock(HF) theory by the Gaussian 98 of program package.

Enaminones are those structures made up three various functional groups including carbonyl, alkeneand amine groups which arelocated along with each other in a conjugate fashion. These compoundsare of much attention due to special characteristics and numerous applications. In the paper, sixvarious enaminone structures were theoretically optimized and after concluding, were compared withequivalent experimental results. These enaminone structures have been studied for substituent effecton hydrogen bond, method and basic set effects on the geometrical parameters, vibrationalfrequencies, stability, complex forming, NBO computation and etc. Drawing molecular structures andcomputer calculations were performed with Gauss view (5.0) and Gaussian (09) softwarerespectively. The utilized methods in the paper were the HF and DFT and of DFT methods of basicfunctions BLYP, B3LYP, B3P86, B3PW91, SVWN, BVWN and BPV86. Also Basis sets of 6-311G,6-311G*, 6-311G**, 6-311+G*, 6-311+G**, 6-311++G* 6-311++G**, LANL2DZ and SDD havebeen utilized which have been applied as proportional to molecular structure and desirable purpose.Generally the results of computer calculations and comparing them with experimental equivalentsindicate that theoretical procedures could well be helpful and effective in enaminone studies and offerreasonable and well results.

The acidity constants (pKa) of thirty four (34) ;-substituted carboxylic acids in aqueous solution havebeen calculated using conductor-like polarizable continuum (C-PCM) solvation model. The gasphaseenergies at the Density Functional Theory (DFT-MPW1PW91) and solvation energies atHartree Fock (HF) are combined to estimate the pKa values which are very close to the experimentalvalues where, and the root-mean-square error (rmse) is less than 0.9. In addition, the pKa valuespredicted by Multilinear Regression (MLR), Principle Components Regression (PCR) and PartialLeast Square Regression (PLSR) methods showed an excellent correlation to the experimental valueswhere the root-mean-square error (rmse) decreases from 0.9 to 0.293, 0.326 and 0.319 respectively.Furthermore, the rmse of pKa values found less than 0.3 when the solvation models of some powerfulelectron withdrawing substituted acids are employed at the level of MPW1PW91 theory. Thecomputation by different regression methods showed reasonably a better calculation of pKa valuesthrough the lower root-mean-square errors (rmse).

α-Amylase has been studied extensively from various sides. This enzyme is used in many industries. Many applications of this enzyme have encouraged us for greater attempts on the study of α-amylase and to search for more effective processes. In this investigation, the structure of nanotube - catalytic site of bacillus subtilis α- amylase was optimized by hyperchem 7.0 and then it was investigated with ab initio/hartree fock and density functional theory /B3LYP methods using the STO-3G, 3-21G and 6-31G basis sets for a physicochemical explanation of interactions within these nano biosystem. Then nuclear Magnetic Resonance (NMR) parameters and so charge, dipole moment, and stability energy were calculated on the optimized structure. We have found each of active atoms that indeed play an important role in imparting extra stability. In the current study, we have reported the NMR parameters of 8 atoms of catalytic site of bacillus subtilis alpha-amylasethe. Interesting finding of the present study is that in NMR shielding for each of active atoms, O8 and O14 had maximal shift in all of levels. In catalytic mechanism of this enzyme, O14 is adopting a chair structure leading to the easy cleavage of the glucoside bond (fixer for catalysis). This investigation suggests that nanotube interactions in this nano biosystems indeed play an important role in imparting extra stability of the catalytic site of the enzyme. Energy parameters in B3LYP level in different basis sets have more negative values than HF and have indicated the most stability in B3LYP6-31/G level and so dipole moment in this structure have observed that in HF3-21/G is maximum. The aim of this work was to discuss the aspects of the electronic structure of this nano biosystem to increase their advantages in practical applications.

Calixarenes are a readily available and important class of macrocycles in supramolecularchemistry. Calixarenes have generated considerable interest due to their basket shaped structureand as useful building blocks to synthesize selective receptors for the guest species, notably alkali,alkaline earth, lanthanide and transition metal cations. In this work studied on the complexingproperties of a para-sulfonato-calix [4] arene (C281424016S4) with alkali metal cations. Thecomplexation properties of para-sulfonato- calix [4] arene were studied by Hartree Fock method.The complexes showed different properties for the different cations, depending on the cations andthe position of the substituents grafted on the ligand.

An azobenzene bridge was introduced into the lower (or smaller) rim of p-tert-butylcalix[8]arene to form 1,5-singly bridged calix[8]arene derivatives, respectively. Bridged calix[8]arene of conformationally rigid wereisolated. The stability of the two structures of bridged calix[8]arenes have been compared.The study of organicstructure to form nanoporous structures is a well known in chemistry phenomena to finding crystal structure ofcalix[8]arene as supramolecular chemistry. The effect of hydrogen bonding on calix[8]arene was reported atDensity Functional Theory by Gaussian 98 of program package. The self assembled nanotubes in solid statethrough network hydrogen bonds between the chains of this structure were investigated.

In this paper,the molecular geometry for three tautomers of uracil and four tautomers of cytosine has been analyzed. vibrational IR spectra of the tautomers were   investigated at HF and B3LYP level using the AB initio 6-31G* and LANL2DZ basis sets from the program package Gaussian 98 (A.7 Public Domain version).
The physico-chemical and biochemical properties of uracil and cytosine are one of the principal criteria for the selection of these compounds. Calculated results were compared with the corresponding experimental data if available. The harmonic wave numbers for the main tautomers of uracil (U1) and cytosine (C2) will compare at the 25ºC, 37ºC, and 40ºC.

More recently medical chemistry research has been focused on proteins that drive and controlcell cycle progression. Among them, the cyclin dependent kinases (cdk’s) are a group ofserine/threonine kinases, which rule the transition between successive stages of the cell cycle. Theactivity of cdk’s is regulated by multiple mechanisms, including binding to cyclins, which is a broadclass of positive regulatory cdk-binding proteins. Among the chemical agents that act selectively ascdk inhibitors are flavonoids,flavopiridol is a semisynthetic flavonoid.Theoretical study is performedon flavopiridol using quantum chemical calculations. Interactions between flavopiridol withtransition metals were studied at HF/6-31G*, and HF/6-311G** levels of theory. Ab initio method at HF level of theory was used. Conformations, optimized parameters, bond length, were computed for metalated andisolated flavopiridol. Flavopiridol can be Metalated from its binding sites (oxo and hydroxyl groups) and theenergies of these compounds were computed.Abbreviations and notations: HF, Hartree-Fock; Cdk , Cyclin dependent kinases.