MNDO Method Research Articles

Quantum-Chemical Study of the Relative Stability of N-Substituted Tetrazole Isomers

The enthalpies of formation and total energies of a series of 1- and 2-substituted tetrazoles in the gas phase and in solution were calculated by MNDO and AM1 semiempirical methods and by a set of nonempirical procedures. The effect of solvent on the relative stability of N-substituted tetrazoles was estimated in terms of the PCM, SM5, and SCRF models. The possibility for isomerization of N-substituted tetrazoles, depending on the substituent in position 1 or 2, was studied at the MP2/6-31G*//HF/6-31G* level. According to the results of nonempirical calculations, 2-substituted tetrazoles are more stable in the gas phase, in keeping with the experimental data for the corresponding isomers. The solvent nature is an important factor affecting isomerization of N-substituted tetrazoles: Rise in the solvent polarity leads to displacement of the equilibrium toward 1-substituted isomers.

Reactions of 4-Aryl-2-hydrazino-3-nitro-6-R-quinolines with HNO2

The reaction of 4-aryl-2-hydrazino-3-nitro-6-R-quinolines with NaNO2 in AcOH gives the corresponding tetrazolo[1,5-a]quinolines. In contrast to tetrazolo[1,5-c]pyrimidines they cannot be converted to 6-R-4-phenyl[1,2,5]oxadiazolo[3,4-b]quinoline-3-oxides by heating in THF, toluene, or AcOH. Total energy quantum-chemical calculations using the MINDO/3 and MNDO methods show that [1,2,5]oxadiazolo[3,4-b]quinoline-3-oxides are significantly higher in energy (230-280 kcal/mol) than the mentioned tetrazolo[1,5-a]quinolines and hence their formation is unlikely.

Systematic investigation of the molecular behaviors of heterofullerenes C48X2 (X=B, N)

A systematic investigation on all possible substituted fullerene isomers of C 48 B 2 and C 48 N 2 has been performed using the semiempirical methods AM1 and MNDO. The equilibrium geometrical structures, heats of formation, strain, aromaticity, HOMO–LUMO energy gaps, ionization potentials, electronic affinities, the absolute hardness and electronegativity have been studied. The results indicate that the isomer-78, which corresponds to 1,4-substitution in the six-membered ring located on the equator, is the most stable isomer for both C 48 B 2 and C 48 N 2 . The driving force governing the stabilities of the present studied C 48 X 2 (X=B, N) isomers is the strain being inherent in the C 50 cage. The contribution of the conjugation effect to the stabilization is not able to compete with that of the strain. From an application of the HSAB principle, the absolute hardness of the more stable isomers of both C 48 B 2 and C 48 N 2 are larger than that of C 50 , and the direction of electron flow for forming a complex among them may be C 48 N 2 →C 50 →C 48 B 2 according to the calculated absolute electronegativity. The more stable C 48 X 2 isomers have larger ionization potentials and smaller electronic affinities compared with C 50 , which suggests that it is more difficult to oxidize and reduce C 48 X 2 , i.e., the redox characteristics of C 50 can be weakened by doping. The vibrational spectra and electronic absorption spectra of these substituted fullerenes have been calculated, which could serve as a framework to interpret future experimental results. The computed nucleus independent chemical shifts (NICS) values also provide a basis for the possible characterization of these C 48 X 2 isomers.

Structures and stabilities of hydroazafullerenes C 59H 2 n+1 N ( n=1–5)

The geometries of the different isomers of C 59H 2 n+1 N ( n=1–5) have been optimized using the AM1 and MNDO methods and the program gaussian 94. The energies show that the most stable isomer of C 59H 3N is 6,18-(12,15-) isomer, and the most stable isomer of C 59H 5N is 6,18/12,15-isomer, they are 1,4-adduct and 1,4/1,4-adduct and far more stable than the other isomers, respectively. Successive addition of two hydrogen atoms results in a 1,2-adduct which is the most stable isomer, but the energy differences among the energy-favored isomers are very small. By replacing one C atom of the fullerene skeleton with a N atom, the regioselectivity of the carbon cage has been changed.

A systematic investigation on the molecular behaviors of substituted fullerenes C 34X 2 (X=N, B)

A systematic investigation on the molecular behaviors of all the possible isomers of C 34B 2 and C 34N 2 formed from the initial C 36 fullerene with C 6 v and C 6 h symmetries have been performed using the semi-empirical AM1 and MNDO methods. The equilibrium geometrical structures, heats of formation, HOMO–LUMO gap energies, heats of atomization, ionization potentials (IP) and affinity potentials (AP), conjugate effect and deformation energies of C 34X 2 (X=N, B) have been studied. The calculation results show that the heterofullerenes are less stable than C 36, and the C 34N 2 should be more stable than the boron analog C 34B 2. Both empirical methods in this work indicate that the most stable isomer of C 34X 2 (X=N, B) corresponds to 1,4-substitution in the cyclohexatriene unit which locating at the equatorial belt of the C 36 cage. Generally speaking, the C 34X 2 (X=N, B) isomers with the doped atoms near the equatorial belt are more stable that the rest. The heterofullerenes C 34X 2 have bigger IP and smaller AP compared with C 36, thus the redox activity of C 36 can be reduced by doping. The results of π-orbital axis vector analysis show that replacements of carbon atoms with either nitrogen or boron can notably release the strains in local part of the cage. Both C 34N 2 and C 34B 2 are expected to have significantly different chemical and physical properties from those of the fullerenes.

Theoretical Study on the Rearrangement between the Isomers of C60X (X = O and S)

The rearrangements between the closed [6,6] and open [5,6] isomers of C 6 0 O, as well as the closed [6,6], closed [5,6], and open [5,6] isomers of C 6 0 S have been studied using semiempirical AM I and MNDO methods. The results show that the interconversion of the two isomers of C 6 0 O follows a two-step pathway involving an intermediate and two transition states. The calculated activation barriers for the migration of oxygen from [6,6]-bond to [5,6]-bond through an intermediate are 189.1 and 54.6 kJ mol - 1 , respectively. In the opposite way, the calculated activation barriers for the migration of oxygen from [5,6]-bond to [6,6]-bond through an intermediate are 293.2 and 2.7 kJ mol - 1 , respectively. The interconversion of the closed [6,6] and the open [5,6] isomers of C 6 0 S also follows a stepwise pathway via a local energy minimum corresponding to the closed [5,6] isomer. The calculated activation barriers for the migration of sulfur from [6,6]-bond to [5,6]-bond (in open [5,6] isomer) through the closed [5,6] isomer are 233.2 and 1.2 kJ mol - 1 , respectively. In the opposite way, the calculated activation barriers for the migration of sulfur from [5,6]-bond (in open [5,6] isomer) to [6,6]-bond via the closed [5,6] isomer are 82.0 and 150.5 kJ mol - 1 , respectively. The large barriers suggested that it should be possible to isolate the closed [6,6] and open [5,6] isomers of C 6 0 O at room temperature. This is consistent with experimental results. In addition, it can be inferred from our results that rearrangement between the two isomers of C 6 0 O can take place under heating or lighting conditions. Meanwhile, it can be deduced from our results that it should be possible to isolate both the closed [6,6] and the open [5,6] isomers of C 6 0 S at room temperature and to convert one into the other when certain energy is offered. It seems that the closed [5,6] isomer of C 6 0 S may not be observed experimentally at room temperature.

Quantum chemical studies on tautomerism of 2-, 3- or 4-hydroxyquinoline derivatives along with their thio and azo analogs

The geometries, relative stabilities and proton affinities for the different tautomers of 2-, 3- and 4-hydroxyquinoline derivatives and their thio and azo analogs along with their fixed forms (i.e. model molecules in which the proton migration is eliminated) were calculated with full geometry optimization using AM1, PM3 and MNDO methods. The predominance of oxo forms over hydroxy forms were confirmed with all three methods both in gas and liquid phases, as cited in the literature, with the exception of 3-hydroxyquinoline for which the AM1 and MNDO methods both in gas and liquid phases suggest the predominance of the hydroxy form. For the thio analogs the predominance of thione forms over mercapto forms and for the amino analogs the predominance of the amino forms over imino forms were confirmed both in gas and liquid phases with all three methods as indicated in the literature.

Isomerism and aromaticity of heterofullerene C70-nPn (n=2-10).

A systematic investigation on possible structures of heterofullerenes C70-nPn (n=2-10) has been performed employing semiempirical MNDO and ab initio methods. The stabilities decrease with increasing number of heteroatoms. The structures whose carbon atoms are substituted in the para pattern across the equatorial hexagons correspond to the most stable isomers. The isomeric pattern of P-doped C70 systems follows our previously proposed correlation between the isomerism of the fullerene adducts C60Xn/C70Xn and those of the heterofullerenes C60-nNn or C60-nBn /C70-nNn or C70-nBn. The aromaticity of the most stable structures of heterofullerenes is studied to investigate the heteroatom doping effect on the electron delocalization of the fullerene cage.

Investigation of the structure and properties of some indazole derivatives using the AM1, PM3 and MNDO semiempirical methods. 2. An aqueous phase study

In this work the aqueous phase physicochemical properties of some 3-substituted indazole derivatives were computed by using semiempirical methods and the obtained results were evaluated by searching a possible correlation with the previously obtained experimental properties. The aqueous phase geometries, relative stabilities, acidity constants, tautomerism, proton affinities and dipole moments for the tautomeric forms of some 3-substituted indazoles and their fixed forms (model compounds in which proton migration is eliminated by replacing the mobile hydrogen atom with a methyl group) were calculated with full geometry optimization using AM1, PM3, and MNDO methods. The results of aqueous phase semiempirical calculations indicate that 1H form of the studied molecules are more stable than that of 2H form as stated in the literature.

A systematic investigation on the molecular behaviors of boron- or nitrogen-doped C40 clusterElectronic supplementary information (ESI) available: Heats of formation for all possible isomers of C38B2, C38N2 and C38BN. See http://www.rsc.org/suppdata/cp/b1/b111443c/

A systematic investigation on all possible substituted fullerene isomers of C38B2, C38N2 and C38BN has been performed using the semiempirical methods AM1 and MNDO. The heat of formation, stabilization energy, HOMO–LUMO gap, the strain, the absolute hardness and electronegativity have been studied. The calculations show that the order of stability is C40 > C38B2 > C38BN > C38N2, which is similar to heterofullerenes C36 though somewhat different from the hetero-C60 cluster. The strain in C38XY rises relative to the C40 cluster according to POAV analysis, and the absolute hardness and the electronegativity of C40 is larger from an application of the HSAB principle. The selectivity of the substitution positions is commonly determined by the conjugative effect and tension. The vibration spectra for the doped fullerenes have been calculated, which could serve as a framework to interpret future experimental results.

Experimental and Theoretical Study of a New Class of Acceptor Group in Chromophores for Nonlinear Optics: 2-Substituted 4-Methylene-4H-oxazol-5-ones

Push−pull organic molecules that combine a polyfunctionalized electron-donating carbazole moiety with a new acceptor group, 4-methylene-4H-oxazol-5-ones, were synthesized and characterized as nonlinear optical chromophores with promising μβ values. First-order hyperpolarizability and dipole moments were calculated by MNDO, AM1 and PM3 methods. Theoretical μβ values were compared to experimental data measured in solution by electric-field-induced second-harmonic generation (EFISH). The different factors that could influence the NLO response, especially the gain or loss of aromatization stabilization during the charge-transfer process, were studied. The thermal stability of these new chromophores is also reported.

A study of the oxidation and fluorination of single-walled carbon nanotubes by the MNDO method

This paper reports on the results of MNDO calculations of the atomic adsorption of oxygen and fluorine on the surface of armchair and zigzag single-walled carbon nanotubes with a cylindrical symmetry. The calculations are carried out within the molecular cluster and ion-incorporated covalent-periodic cluster models at the modern quantum-mechanical semiempirical MNDO level. The electronic and energy characteristics of the oxidation and fluorination processes are analyzed, and the most energetically favorable oxide structure of the (6, 6) nanotube is determined. It is found that narrow-gap tubulenes show a tendency to metallic behavior as their surface is saturated with oxygen atoms.

Conformational Preferences of Anticodon 3′-Adjacent Hypermodified Nucleic Acid Base cis-or trans-Zeatin and its 2-methylthio Derivative, cis-or trans-ms2Zeatin

Conformational preferences of the hypermodified nucleic acid bases N6-(Δ2 -cis-hydroxyisopentenyl)adenine, cis-io6Ade also known as cis-zeatin, and N6-(Δ2 -trans-hydroxyisopentenyl)adenine, trans-io6ade or trans-zeatin, and 2-methylthio derivatives of these cis-ms2io6Ade or cis-ms2zeatin, and trans-ms2io6Ade or trans-ms2zeatin have been investigated theoretically by the quantum chemical Perturbative Configuration Interaction with Localized Orbitals (PCILO) method. Automated geometry optimization using quantum chemical MNDO, AMI and PM3 methods has also been made to compare the salient features. The predicted most stable conformation of cis-io6Ade, trans-io6Ade, cis-ms2io6Ade and trans-ms2io6Ade are such that in each of these molecules the isopentenyl substituent spreads away (has “dista” conformation) from the five membered ring imidazole moiety of the adenine. The atoms N(6), C(10) and C(11) remain coplanar with the adenine ring in the predicted preferred conformation for each of these molecules. In cis-io6Ade as well as cis- ms2io6Ade the hydroxyl oxygen may participate in intramolecular hydrogen bonding with the H-C(10)-H group. In trans-io6Ade the hydroxyl group is oriented towards the H-C(2) instead. This orientation is retained in trans-ms2io6Ade, possible O-H…S hydrogen bonding may be a stabilizing factor. In all these four modified adenines C(11)-H is favourably placed to participate in intramolecular hydrogen bonding with N(1). In cis-ms2io6Ade as well as trans-ms2io6Ade the 2-methylthio group preferentially orients on the same side as C(2)-N(3) bond, due to this nonobstrusive placing, orientation of the hydroxyisopentenyl substituent remains unaffected by 2-methylthiolation. Thus the N(1) site remains shielded irrespective of the 2-methylthiolation status in these various cis-and trans-zeatin analogs alike. Firmly held orientation of hydroxyisopentenyl substituent in zeatin isomers and derivatives, in contrast to adaptable orientation of isopentenyl substituent in i6Ade and ms2i6Ade, may account for the increased efficiency of suppressor tRNA and reduced codon context sensitivity accompanied with the occurrence of ms2-zeatin (ms2io6Ade) modification.

MNDO, AM1, and PM3 study of the protonation tautomerization and valence tautomerization equilibria of some pyrrole macrocycles

The geometries, relative stabilities, proton affinities for tautomers of 1,3,6,10,12,15-hexamethyl-6,7,15,16-tetrahydropyrrolo[3,4- a]pyrrolo[3,4- i][16]annulene ( 1) and its possible fixed models (in which proton migration is eliminated) of 1,2,3,6,10,11,12,15-octamethyl-6,7,15,16-tetrahydropyrrolo[3,4- a]pyrrolo[3,4- i][16]annulene, of 1,2,3,6,8,10,12,15-octamethyl-6,7,8,15,16-pentahydro-2H-pyrrolo[3,4- a]pyrrolo[3,4- i][16]annulene, 1,3,5,7,10,12,14,16-octamethyl-5,6,7,14,15,16-hexahydro-2H-pyrrolo[3,4- a]2H-pyrrolo[3,4- i][16]annulene, molecules ( 2) were calculated with fully geometry optimization using MNDO, AM1 and PM3 methods in the gas and aqueous phases. A parallelism between the experimental acidity constant values p K a and of the computed acidity constants, p K a was search to clearing the tautomeric forms.

Quantum chemical calculations on the annular tautomerism of some indazole derivatives. 1. A gas phase study

In this work, physicochemical properties of some indazole derivatives were computed by using semi-empirical methods in the gas phase and the obtained results were evaluated by searching a possible correlation with the previously obtained experimental properties. The relative stabilities, proton affinities and dipole moments for the tautomeric forms of some indazoles and their fixed forms (model compounds in which proton migration is eliminated by replacing the mobile hydrogen atom with a methyl group) were calculated with full geometry optimization using AM1, PM3 and MNDO methods. When the annular tautomerism is taken into account the results of semi-empirical AM1, PM3 and MNDO calculations indicate that 1H-form of the studied molecules are more stable than that of 2H-form as stated in the literature.

Quantum-Chemical Study of the Equilibrium Geometry and Electronic Structure of the Defensin Molecule

The molecular geometry of rabbit NP-1 defensin is fully optimized by means of molecular mechanics and MNDO methods. Atomic charges are calculated, and an estimate for the dipole moment of the molecule is obtained (μ 4.0 D). The possible mechanism of defensin-receptor interactions is discussed.

Quantitative structure–property relationships in the pyridine series

AbstractBy means of the MNDO, AM1, and PM3 methods, standard heats of formation, entropies, ionization potentials, and molecular dipole moments of the pyridine series compounds have been computed. The information on the experimental values of the above quantities has been systematized. Linear dependencies allowing a priori evaluation of thermodynamic and molecular characteristics of pyridines have been stated. Correlations of the 2,2′‐bipyridines pKa values for aqueous solutions with the gaseous‐phase proton affinities have been found. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:229–241, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10013

On the Structure of Lead(II) Complexes in Aqueous Solutions. III. Hexanuclear Clusters

Optimum geometries based on experimental structures and corresponding electronic structures of [Pb6(μ3-OH)8]4+ and [Pb6O(μ3-OH)6]4+ clusters as well as of their hydrated [Pb6(μ3-OH)8(H2O)6]4+ and [Pb6O(μ3-OH)6(H2O)6]4+ analogues are investigated using the semiempirical MNDO method of quantum chemistry. Direct Pb-Pb and O-O bonds are vanishing in the systems under study. Small differences in interatomic Pb-Pb and Pb-O distances between these clusters indicate that they cannot be distinguished by recent experimental techniques in solution. Both the mentioned structures are stable and might coexist in solution.

Relative Stability of Linear Zn Clusters

The MNDO method was used to study the potential energy surface for Zn$n and ZnnXm (n = 2 – 4) linear zinc clusters with terminal and lateral coordinations of the X acceptor groups. The results agree with the formerly proposed hypothesis on the modes of Hg cluster stabilization. Possible effects of the individual properties of the element and the value of the acceptor strength of the ligands on the relative stability of the clusters are discussed.

Dinaphtho[2′,3′:5,6][1,4]dithiino[2,3-b:2,3-e]pyridine, its 16-butyl derivative and dinaphtho[2′,3′:5,6][1,4]dioxo[2,3-b:2,3-e]pyridine: novel heterocycles as electron donor compounds

Dinaphtho[2′,3′:5,6][1,4]dithiino[2,3-b:2,3-e]pyridine (4a), its 16-butyl derivative (4b), and dinaphtho[2′,3′:5,6][1,4]dioxo[2,3-b:2,3-e]pyridine (5) have been prepared and fully characterized. The electrochemical properties of 4a, 4b and 5 have been studied by cyclic voltammetry in CH2Cl2∶trifluoroacetic acid (1∶1); in agreement with their donor character, they exhibit a first reversible oxidation wave to their radical cations with very similar potential peak values and a second irreversible wave to their dications, the lowest potential peak value of this second oxidation corresponding to 5. Their radical cations, generated by oxidation of the parent compounds, are relatively stable and have been analyzed in liquid solution by electron paramagnetic resonance (EPR). Spin density distributions in the SOMO have been calculated by the semiempirical MNDO method. Electronic spectra of 4a and 4b in trifluoroacetic acid show peaks at 417 and 401 nm, respectively, and in the presence of thallium(III) trifluoroacetate two characteristic peaks at λmax 411 and 868 nm for 4a˙+, and at 411 and 872 nm for 4b˙+. X-Ray analysis of 4b shows a molecular structure with a stable chair-shaped conformation with interplanar angles between naphthalenes and the pyridine ring of 139.0(1) and 146.4(1)°.