MNDO Calculations Research Articles

Исследование взаимодействия кислорода с поверхностью боросодержащих нанотрубок

Studies of semiconductor carbon nanotubes show that they are extremely sensitive to the chemical environment, and the effects of oxygen drastically change their properties. It has been found that narrow-zone semiconductor carbon tubes can be converted into an apparent metal by such action. Since other types of nanotubes, such as boron-carbon nanotubes of the BC3 type and boron-nitride BN nanotubes, are of great interest, it seems important to investigate whether they are capable of absorbing atomic and molecular oxygen. The study was conducted using the MNDO calculation method within molecular and ion-nested covalent-cyclic cluster models. The results of the study of the interaction of atomic and molecular oxygen with the external surfaces of boron-carbon (BC3), boron-nitride (BN) and boron nanotubes are considered: chair (n, n) and zigzag (n, 0) type. The study was performed by the MNDO method as part of a covalent-cyclic cluster model with embedded ions. Optimal geometry of adsorption complexes is determined and their main electronic and energy properties are described. Boron-carbon nanotubes have been shown to be better oxygen adsorbents than other types of nanotubes considered.

Computer simulation of the sensory interaction of carbon nanotubes with various modifications in relation to alkali metal atoms

The performed theoretical studies obtained by computer modeling and calculations performed by the semi-empirical method using the MNDO calculation scheme showed the possibility of creating sensitive chemically active probes, which are semiconductor sensor systems with boundary modification, which can act as the tip of the cantilever of the atomic force microscope. These studies allow us to predict that such a sensor based on a modified borocarbon nanotube will react to the presence of ultra-small amounts of substances, which opens up prospects for its use in the field of chemistry, biology, medicine, etc. The use of chemically modified nanotubes, including in atomic force microscopy, is the way to create probes with distinct chemical characteristics.

SENSOR PROPERTIES OF CARBON NANOTUBES

A review of publications over the last years dealing with the development of gas and electrochemical sensors, including biosensors, on the basis of carbon nanotubes is provided. Results of the experimental and theoretical study of their principles and operation mechanisms are provided. The structure of carbon nanotubes has been described. The main regularities of the structure, energetic characteristics and sensor properties of the modified semiconducting systems on the basis of carbon nanotubes have been studied. Analysis of the mechanisms of the interaction between nanotubes and the functional groups (including carboxyl and amino ones), metal nanoparticles and polymers for the formation of chemically active sensors has been reported. The possibility of application of the boundary modified monolayer carbon nanotubes for the identification of metals is discussed. Results of simulation of the interaction between nanotubes boundary modified by —COOH and —NH2 groups with atoms and ions of potassium, sodium and lithium obtained using the molecular cluster model and the MNDO and DFT calculation methods are provided. The sensors synthesized in this way will be used for establishment of the existence and identification of metal atoms and their ions included in salts and alkalis.

Carbon nanotubes: Sensor properties. A review

Recent publications dealing with dealing with the fabrication of gas and electrochemical biosensors based on carbon nanotubes have been reviewed. Experimental and theoretical data on the working principles of nanotubes have been presented. The main regularities of the structure, energy parameters and sensor properties of modified semiconducting systems on the basis of cabon nanotubes have been studied by analyzing the mechanisms of nanotubule interaction with functional groups (including carboxyl and amino groups), metallic nanoparticles and polymers leading to the formation of chemically active sensors. The possibility of using boundary modified nanotubes for the identification of metals has been discussed. Simulation results have been reported for the interaction of nanotubes boundary modified by –СООН and –NH2 groups with atoms and ions of potassium, sodium and lithium. The simulation has been carried out using the molecular cluster model and the MNDO and DFT calculation methods. Sensors fabricated using this technology will find wide application for the detection of metallic atoms and their ions included in salts and alkali.

Application of U(2) Algebraic Model in the Study of Stretching Vibrational Spectra of Large Fullerenes C180 and C240

In this study, the Hamiltonian describing the stretching vibrational spectra of fullerenes C180 and C240 are calculated within the framework of the vibron model by using the dynamical U(2) Lie algebra. Here, every C-C bond of the molecule is replaced by a corresponding Lie algebra and finally the Hamiltonian is constructed considering the interacting Casimir and Majorana operators. The fundamental stretching vibrational energy levels of the carbon cluster C180 and C240 are then calculated using this Hamiltonian to fit the semi-empirical MNDO calculation.

The chiral effect of adsorption of univalent atoms and diatomic molecules on the surface of carbon nanotubes

Quantum-chemical semiempirical calculations were performed of the adsorption of fluorine and hydrogen atoms and molecules on the surface of single-layered carbon nanotubes with various diameters. Semiempirical quantum-chemical MNDO calculations were based on the model of a molecular cluster with boundary pseudoatoms. The energy characteristics of adsorption were determined. Changes in physical properties caused by the adsorption of atoms and diatomic molecules were analyzed.

ChemInform Abstract: Intermediates in the Reactions of Electron-Rich Germylenes with an Acyl Azide: An MNDO-SCF-MO Investigation.

The experimentally known reaction between the electron-rich germylene [(Me3Si)2N]2Ge andp-CH3C6H4SO2N3 has been modeled computationally by MNDO calculations on the reaction intermediates formed by [CH3Si)2E]2Ge (E=N or CH) and CH3CON3. Molecular and electronic structures have been established for the acyclic germaketimines [(CH3Si)2E]2Ge=N-N=N-COCH3 (the primary 1∶1 adducts formed by the reactants) and [CCH3Si)2E]2Ge=N-COCH3, and for the cyclic species [(H3Si)2E]2Ge-N=N-N=C(CH3)-O, [(H3Si)2E]2Ge-N=N-N(COCH3), and [(H3Si)2E]2-Ge-N=C(CH3-O. The intermediates [(H3Si)2E]2-GeN(N2)COCH3 were found, upon formation, to undergo smooth dissociation along the N(α)-N(β) bond, with loss of N2, to provide acyclic [H3Si)2E]2Ge=N-COCH3; the polymerizations of these latter species to form polygermazanes are extremely exothermic.

ChemInform Abstract: Cation, Carbene, and Radical Centers in a (4.4.4.5)Fenestrane.

Abstract Carbocation 2a, formed on photolysis of iodide 3a in methanol is captured by solvent to yield 4d exclusively. Carbene 2b, formed on photolysis of salt 1b in methanol, also yields only 4d. Radical 2c, formed on photochemical initiation of reaction between 3a and tributylstannane, abstracts hydrogen without rearrangement to give only 3c. These reactions and relevant MNDO calculations are discussed.

Photochemical naphthylation at α-carbon atom of carbonyl and its equivalences

The photostimulated reaction of halonaphthalene with a series of carbanions derived from propionic acid derivatives in liquid ammonia led to the naphthylation at α-carbon of the carbanions in an isolated yield ranging from 25% to 86%. In all cases, the dehalonaphthalenes were found to be by-products and the reaction was inhibited by p-dinitrobenzene. An electron transfer from the carbanion to the halonaphthalene followed by ejection of halogen led to naphthyl radical, as predicted by comparing the LUMOs of the carbanions and the halonaphthalenes, was involved in the process. Absence of the alkylnaphthalene and 1,2-dinaphthylalkane in the products indicates the smooth electron transfer between (Naph-Nu)− and Naph-X. C-vs. O-naphthylation is fully addressed in terms of MNDO calculations and acid-base principle.

ChemInform Abstract: IR Laser Induced Decomposition of Trifluoroacetic, Pentafluoropropionic and Heptafluorobutyric Anhydrides.

The continuous-wave CO2 laser induced gas-phase decomposition of trifluoroacetic, pentafluoropropionic and heptaftuorobutyric anhydride, sensitized by sulfur hexafluoride was studied in order to determine the course of the reactions in the absence of wall effects. The decompositions of all the compounds are assumed to involve cleavage of parent compound into acyl fluoride F(CF 2)n COF and' (CF 2)n CO2 ' biradical, the latter undergoing decay into CO and F(CF 2)n _ 1 COF compounds. The theoretical treatment of the biradical by MNDO calculations as well as trapping with ethene are presented.

ChemInform Abstract: An Analysis of the Electronic States of Aceanthrylene

The electronic spectrum of aceanthrylene (AA) is examined experimentally and theoretically by use of simple perturbational molecular orbital theory and by use of a semiempirical PPP SCF CI calculation. The long-wavelength transition near 540 nm is assigned to a new state characteristic of a 4N annulene perimeter called a S transition that is not present in the precursor analogue anthracene. Stretched polyethylene sheet spectra are used to verify the calculated polarization assignments. The excited states contain extensive configuration interaction. The lowest triplet state is characterized by MNDO calculation and its electron density is used partially to explain the triplet-state photodimerization of AA. The emission behavior of 2-methylaceanthrylene is characterized by a weak fluorescence near 460 nm. This is an upper excited state emission or anomalous fluorescence.

ChemInform Abstract: Strain Energy Effect on the Reactivity of Bridgehead Halides in Electron Transfer Reactions

Abstract We here report MNDO and AM1 calculations for halobridgehead compounds which indicate that an increase in their angular strain is aoocmpanied by more negative reduction potentials as measured through their LUMO energies. The theoretically predicted reactivity agrees with that determined experimentally. The dissociative or non-dissociative nature of the electron capture reaction for the compounds is estimated through the localization of the corresponding radical anion intermediates as minimum of the potential surfaces. Our calculations indicate that for bridgehead chlorides, which lead to pyramidal radicals, the electron transfer is a non-dissociative step, while for t-butyl chloride, the process is dissociative leading to a planar t-butyl radical. The presence of a substituent that lowers the LUMO of the halobridgehead compounds increases their reactivity in ET reactions. This effect is explained through an orbital mixing of the molecular orbitals involved in the ET process.

ChemInform Abstract: A 400- and 600-MHz 1H NMR Conformational Study on Nucleoside Cyclic 3′, 5′ PV-TBP Systems. Conformational Transmission Induces Diequatorial Orientation of the 3′,5′-Dioxaphosphorinane Ring in a Nonchair Conformation.

The novel nucleoside cyclic 3’3’ PV-TBP compounds 4 and 5 were studied as models for the proposed activated state of cyclic adenosine 3’,5’-monophosphate (CAMP). Compound 4 features equatorial-axial (e,a) orientation of the 3’,5’-dioxaphosphorinane ring. The design of compound 5, which incorporates OCH2CH20Me as a conformational probe, was essentially based on our previous work on conformational transmission in Pv-TBP compounds. ‘H NMR analysis of compound 5 showed conformational transmission in the probe fragment, which indicates that the molecular structure with diequatorial (e,e) orientation of the 3‘,5’-ring, and axial location of OCH2CH20Me contribute significantly to the pseudorotational equilibrium. Conformational transmission in 5 was clearly established via comparison with the PV-TBP compounds 13, in which 0 n B u replaces the OCH2CH20Me group, 14, in which a trans-fused cyclopentane ring replaces the furanose ring of thymidine, and 15, in which the OCH2CH20Me group is locked in an equatorial position of the PV-TBP. The results reveal that conformational transmission can help to stabilize diequatorial orientation of the 3’,5’-dioxaphosphorinane ring, which may be of relevance for the trigger function of cAMP with respect to protein kinases. The detailed conformational properties of 4 and 5 were investigated further on the basis of MNDO calculations on the models 22-24. Structural data as obtained from ‘H NMR were used in the start of the optimizations. The calculations showed that axial-quatorial orientation of the 3’,5’-dioxaphosphorinane ring is favored by 3-4 kcal/mol over diequatorial orientation. The optimized structures show a twist conformation of the e,a-oriented 3’,5’-dioxaphosphorinane ring, whereas the e,e orientation corresponds with a half-chair geometry.

ChemInform Abstract: Spatial Structure of 4nπ Helicene Dianions.

Helicene dianions, e.g. phananthrene derivatives, once believed to be non-helical, maintain their chirality as 4nπ dianions. Phenanthrene 5, as well as substituted phenanthrene derivatives, undergo a two-electron reduction to form the respective 4nπ dianion e.g., 52–/2Li+. Phenanthrene derivatives substituted at the 4- and 5-positions (bay substituents)e.g.6–11, which are helical, afford stable dianions. These dianions are also prepared by a two-electron reduction of the (4n+ 2)π electron hydrocarbons and show, in their 1H NMR spectra, a quench of the paratropicity compared to 52– as well as a line shape dependence on their twist angle. The quench of paratropicity was also observed in the closely related charged helicenes derived from the benzo[g]chrysene system, i.e. anions 122– and 132–. The twist angles were calculated by MMX and MNDO calculations for the neutral systems, i.e., 5–11, and by MNDO for the dianions. MNDO calculations also included the preferred location of the counter cation. A dynamic NMR spectroscopic study proves experimentally the helicity of anion 112– thus shedding light on the behaviour of this novel class of dianions.

ChemInform Abstract: A New Synthesis of 6-Oxopyrimidinium-4-olates. Theoretical Study of the Regioselective Cycloaddition of Arylisocyanates with a 1,3- Thiazolium-4-olate System.

Abstract A new and regioselective synthesis of 6-oxopyrimidinium-4-olate systems (13–21) is described, involving the 1,3-dipolar cycloaddition of the 1,3-thiazolium-4-olate 12 with arylisocyanates. MNDO and AM1 calculations rationalise the reactivity and regioselectivity experimentally observed.

ChemInform Abstract: Structure and Reactivity of Lithium Diisopropylamide in the Presence of N,N,N′,N′-Tetramethylethylenediamine.

Lithium diisopropylamide (LDA) crystallizes from N,N,N',-tetramethylethylenediamine (TMEDA)/hexane mixtures as an infinite array of dimers linked by bridging (nonchelating) TMEDA ligands. 6Li and I5N NMR spectroscopic studies reveal that LDA in neat TMEDA exists as a cyclic dimer bearing a single $-coordinated TMEDA ligand on each lithium. The equilibrium of solvent-free LDA and the TMEDA-solvated dimer shows a very strong temperature dependence. TMEDA coordinates readily only at low temperature. High TMEDA concentrations are required to saturate the lithium coordination spheres at ambient temperatures. One equivalent of THF readily displaces TMEDA from the coordination sphere to produce the dimeric LDA/THF solvate characterized previously. Kinetics of metalation of 2-methylcyclohexanone N,N-dimethylhydrazone monitored by Fourier-transform infrared (FT-IR) spectroscopy are consistent with a rapid, spectroscopically undetectable dimer-monomer preequilibrium followed by rate-limiting proton transfer. The reaction rate is independent of the TMEDA concentration. Related spectroscopic and rate studies using dimethylethylamine lacking the capacity to chelate show strong parallels with the TMEDA data. We conclude that TMEDA chelates of LDA are of no measurable consequence throughout the reaction coordinate. MNDO calculations of THF and TMEDA solvates of Me,NLi and LDA monomers and dimers qualitatively support the conclusions derived from the experimental data.

ChemInform Abstract: Azomethine Ylide Generation via the Dipole Cascade.

Abstract A series of N-acyl 2-diazo-3-oxobutanoates, when treated with a catalytic quantity of a rhodium(II) carboxylate, were found to afford substituted pyrroles derived from an azomethine ylide intermediate. The initial reaction involves generation of the expected carbonyl ylide dipole by intramolecular cyclization of the keto carbenoid onto the oxygen atom of the neighboring amide group. The primary cycloadduct undergoes a subsequent rearrangement-fragmentation reaction to give the pyrrole derivative. When the α-position of the α-diazoketone was blocked with two methyl groups, the rhodium(II)-catalyzed cycloaddition with dimethyl acetylenedicarboxylate led to the carbonyl vlide cycloadduct in high yield. MNDO calculations show that the cyclic carbonyl ylide derived from ethyl 2-diazo-4-dibenzoylamino-3-oxobutyrate is 3.3 kcal lower in its heat of formation than the corresponding azomethine ylide. This thermodynamic difference in stability of dipoles nicely accounts for why the carbonyl ylide cycloadduct is the major product from the rhodium(II) catalyzed reaction of ethyl 2-diazo-4-dibenzoylamino-3-oxobutyrate.

ChemInform Abstract: Structural Studies of Some 5-(Perfluoroalkenyl) Uracils.

Abstract Structures of the most stable conformers of derivatives of 5-vinyluracil and fluorinated analogues have been determined using MM and semiempirical MNDO calculations. It has been found that the most stable conformers of 5-trifluorovinyl and 5- E / Z -pentafluoropropenyl groups are almost perpendicular to the plane of the uracil ring. These findings are in good agreement with X-ray data obtained for 1,3,6-trimethyl-5-trifluorovinyluracil. The calculated energy barrier for rotation of exocylic 5-substituents depends on the structure of the fluorinated fragments as well as on the nature of the substituent at the 6-position of uracil. A methyl group at the 6-position causes steric hindrance and significantly disturbs the rotation of fluorinated groups.

Comparison between Ab Initio and semiempirical net atomic charges of some nicotinic acetylcholine receptor agonists

We have calculated the net atomic charges and molecular electrostatic potentials of two potent nicotinic acetylcholine receptor agonists, isoarecolone and acetylpiperazine, by three different methods to see how well they correlate and if the simplest method gives the same predictive results. The calculational methods involved calculating net atomic charges by semiempirical (MNDO from MOPAC) and ab initio (Mulliken) and ab initio (potential derived) at STO-3G basis set level. Some deviations were observed when comparisons were made atom by atom, but when group comparisons were made, good correlations were observed. When these partial charges were used to calculate the respective molecular electrostatic potentials on the van der Waals surface, very good correlations were obtained. This study shows that for routine electrostatic calculations, semiempirical MNDO Calculations give similar results and thus lead to similar predictions.

Thermodynamics of binding of angiotensin-converting enzyme inhibitors to enzyme active site model

The density functional theory (DFT) using Becke3LYP functional and the two-layered ONIOM Becke3LYP:MNDO calculations have been carried out to investigate the structural and thermodynamic properties of 29 neutral and deprotonated angiotensin-converting enzyme inhibitors (H 2O, enalaprilat, cilazaprilat, imidaprilat, perindoprilat, quinaprilat, ramiprilat, spiraprilat, trandolaprilat, fosinoprilat, omapatrilat, captopril, zofenoprilat, silanediol and keto-ACE) in complex with zinc cation and three first-shell ligands as models of active site of angiotensin-converting enzyme. The influence of deprotonation on the structure and relative energetics of model complexes was examined. Interaction enthalpies and Gibbs energies between inhibitors and angiotensin-converting enzyme active site model were calculated. The structure and thermodynamics of optimized complexes are discussed from the point of view of their biological importance.