Tautomerism of oxopyridines and oxopyrimidines: Theoretical study with complete optimization of geometry

The semiempirical MINDO /3 method is employed to calculate the energies of various tautomers of model tautomeric compounds— 2-oxo- and 4-oxopyridines and pyrimidines. The results are compared with experimental data in the gas phase, where the solute–solvent interactions not included in theoretical calculations are absent. Although the relative energies obtained by the MINDO /3 method appear to be superior to other semiempirical and ab initio calculations, the accuracy of the method was determined to be as low as 3–4 kcal/mol. It indicates that it is exceedingly difficult to account theoretically for intrinsic stabilities of the tautomers. The importance of various factors influencing calculated free-energy differences is discussed. Particular attention is paid to the problem of geometry optimization.     

Comparison of the use of the MNDO and MINDO/3 methods with ab initio methods to study tautomerism in histamine, 2- and 4-methylhistamine

The semiempirical MNDO and MINDO/3 methods are used to study the various tautomeric forms of histamine, 2-methylhistamine, and 4-methylhistamine. Comparisons of the optimized structures and tautomerization energies are made with values obtained from ab initio Hartree-Fock calculations using the 3-21G and STO-3G basis sets. Based on these results and previous comparisons of STO-3G results with x-ray structures, the present results indicate that while there are some differences in the values of the structural parameters, the changes in structure upon tautomerization and/or protonation are very similar. Further analysis of the MNDO and MINDO/3 structures by means of their utilization in 3-21G and STO-3G calculations indicates that either of these semiempirical methods provides reliable values for the structural parameters. Both methods give good qualitative agreement with the ab initio calculations for the relative energies of the various tautomers in the three compounds. In these studies the MNDO method appears to give better quantitative agreement with the 3-21G and STO-3G results than the MINDO/3 method.     

Semiempirical quantum-chemical calculations on the conformations of methyl formate and methyl thiolformate

The cis, trans, and gauche conformations of the methyl esters of formic and thiolformic acids have been investigated by different semiempirical methods. Total geometry optimization (CNDO/2, MINDO/3) and bond angle optimization (PCILO, NDDO) have been performed for the O-alkyl ester. The S-alkyl ester has been studied by the MINDO/3 method at the total geometry optimization level and by CNDO/2 and PCILO methods at the bond angle optimization level. The influence of sulphur d orbitals on the optimized molecular geometry as well as on the magnitude and direction of the dipole moment vector has been investigated in the CNDO/2 framework. The total energy differences of the conformers are compared to the experimental and ab initio results. CNDO/2 and NDDO energy partitioning have been performed to obtain information on the origin of the cis—trans energy difference and of the rotation barrier. The extent of the lone-pair delocalization has been studied in the different conformations using localized molecular orbitals. Calculations have been performed on the staggered and eclipsed positions of the methyl group in the planar conformations of both esters.     

Quantenchemische modellierung der potentialhyperfläche des äthylkations im bereich der klassischen und nichtklassischen strukturen

Properties of the potential energy surface of protonated ethylene are calculated by semiempirical methods (CNDO/2-FK, MINDO/2, MINDO/3). These methods were chosen in respect of the agreement with new strong energy data concerning the relative stability of the nonclassical and a classical structure of the ethyl cations. They give no stationary point in that region of the hypersurfaces where until now was assumed to exist the most stable of the classical ethyl cations. The H-eclipsed classical cation is a saddle point on the CNDO/2-FK- and the MINDO/2-hypersurface, which represents the barrier of potential energy for the H-exchange in the ethyl cation; on the MINDO/3-surface a flat sea modified the character of the saddle point. Finally the results of the different quantum chemical methods (including NDDO and ab initio) are analysed concerning their agreement in calculating geometry and the distribution of electron density in the ethyl cations.     

Correlation of singlet-triplet gaps for aryl carbenes calculated by MINDO/3, MNDO,AM1, and PM3 with Hammett-type substituent constants

Heats of formation, atomic charges, and geometries of some 110 structures involving substituted singlet and triplet phenyl and 4,4-dimethyl-1,4-dihydronaphthalene carbenes and the corresponding diazomethanes were calculated by MINDO/3, MNDO, AM1, and PM3 semiempirical molecular orbital methods. The singlet-triplet gaps for AM1 and PM3 calculations for the para derivatives in both systems have been successfully correlated with Brown σ⁺ constants. Good correlations with σ⁺ were found for the charges on the carbenic centers of the singlets as well as with the energy barrier for rotation of the aryl group about the C-C single bond in substituted singlet phenylcarbenes. Comparisons of these results with experimental data indicate that AM1 and PM3 are much better than MNDO and MINDO/3 in predicting the intrinsic substituent effects in singlet carbenes.     

Quantenchemische untersuchungen zum mechanismus der elektrophilen substitution. I. Zur potentialhyperfläche des systems benzol/h+

Results of semiempirical calculations (CNDO/2-FK and MINDO/2 methods) for the σ-π complex problem on protonated benzene are given and compared with previous ones. The semiempirical methods were chosen according to the agreement of their results with new theoretical energy data (E_HF + E_korrel) concerning the classical–nonclassical problem of protonated ethylene. By these methods the corresponding part of the energy surface of the benzene/H⁺ system is simulated. The stationary points of this surface are found by a gradient method with complete optimization of the geometry. On the basis of this method we determined the energy profile of a reaction coordinate between the classical (σ-complex) and nonclassical (π-complex) cation. The so called strong π-complex is a saddle point between two σ-complex minima and can be interpreted as transition state of 1,2-proton shifts. Hypotheses for possible minimum energy paths of electrophilic attacks in the given region of the surface are discussed.     

Theoretical studies of [n]paracyclophanes and their valence isomers. I. Geometries,strain energies,and enthalpies of the inter-conversions of [n]paracyclophanes and their Dewar benzee isomers

Four semiempirical methods (AM1, MNDO, PM3, and MINDO/3) are used to calculate the deformation angles of [n]paracyclophanes and their Dewar benzene isomers for n = 3… 10. The results obtained by all these methods are in good agreement with data from X-ray studies. We have determined the strain energies that, in both series of compounds, are due to two components: (1) the strain energy of deformation of the cycle (aromatic or Dewar Benzene skeletons) and (2) the strain energy of the oligomethylene chain. In [6]paracyclophane, the strain energy [SE_ring(MNDO) ≈? 32.9 kcal/mol] almost compensates the resonance energy (E_resonance ≈ 36 kcal/mol) so that its chemical properties are closer to alkenes than to benzenic compounds. To better reproduce the enthalpy of the valence isomerization [n]Dewar bezene → [n]paracyclophane, which is poorly calculated with these methods, a correction is proposed and the reaction enthalpy of [6]paracyclophane is estimated to be about ΔH_r ≈ 15 ± 15 kcal/mol. It is found that MNDO and MINDO/3 need the smallest corrections, but MNDO leads to better geometries than MINDO/3. In conclusion, MNDO seems to be the best technique for further studies of these compounds. © 1992 by John Wiley & Sons, Inc.     

Semiempirical implementation of strictly localized geminals for analysis of molecular electronic structure

Approximate electronic trial wave function taken as the antisymmetrized product of strictly localized geminals (APSLG) is implemented for semiempirical analysis of molecular electronic structure of “organic” compounds and for calculations of their heats of formation. This resulted in an O(N)‐scaling method. Using the MINDO/3 form of the semiempirical Hamiltonian with reparameterized β_AB values in combination with the APSLG form of the wave function yields the computational procedure BF'98. Calculations on the heats formation and the equilibrium geometries for a wide range of molecules show that the APSLG‐MINDO/3 approach is more favorable than its self‐consistent field‐based counterpart. Also, the APSLG formalism allows to interpret molecular electronic wave function in chemically sensible terms. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 752–764, 2001     

Numerical sensitivity of trajectories across conformational energy hypersurfaces from geometry optimized molecular orbital calculations: AM1, MNDO,and MINDO/3

The monocyclic β-lactam [[4(S)-methyl-2-oxo-1-azetidinyl]thia]acetic acid was studied by the semiempirical molecular orbital methods AM1, MNDO, and MINDO/3. Using the reaction coordinate option in the program MOPAC on VAX and Cray X-MP computers, the potential energy curve was calculated for rotation of the C2-N1-S-C torsional angle in the conformationally flexible side chain while optimizing all other geometrical variables in the molecule. The trajectory taken during geometry optimization was found to be sensitive to the computer, the program version, the convergence criteria, and the degree of code optimization used in the calculation. In order to reduce the likelihood of spurious results, conformational or reaction energy hypersurfaces need to be calculated with the more precise SCF convergence and minimization criteria available in programs for MINDO/3, MNDO, and AM1 calculations. The nitrogen in the model β-lactam antibiotic is predicted to invert periodically as the dihedral angle to the exocyclic N-substituent sweeps through 360°.     

Partitioning scheme for theab initio SCF energy

As a tool for the interpretation ofab initio SCF calculations, an energy partitioning scheme is presented. When performed within an orthogonalized basis, the scheme allows the deduction of well transferable, almost basis independent two-center terms which characterize bond strengths and non-bonded interactions. The results for a large number of molecules are given. The construction of an orthogonal minimal basis (OMBA) from arbitrary basis sets as a generalization of the symmetrical orthogonalization is described. The transferability of Fock matrix elements is discussed. The energy partitioning quantities are related to the corresponding terms obtained with the semiempirical schemes CNDO and MINDO/3.     

SCRCR'CO重排反应的量子化学研究

用ＭＩＮＤＯ／３方法研究ＳＣＲＣＲ’ＣＯ的热重排反应的机理，给出了活化能和ＩＲＣ途径，讨论了活化能与取代基Ｒ’和迁移基Ｒ的性质之间的内在联系。     

Photo- and thermochromic spirans. 18. Theoretical study of the mechanisms of the photocolorization and photodecolorization of 2H-pyrans and their structural analogs. Triplet state

The critical sections of the potential-energy surface of the excited first triplet state (T₁) of the electrocyclic cyclization of compounds that contain a 2H-pyran ring were studied by means of the semiempirical MINDO/3 method. A theoretical explanation is given for the experimentally observed difference in the mechanisms of the photocolorization and photodecolorization of spiropyrans. The activation barriers of the reaction in the first triplet state range from 20 to 110 kJ/mole.See [1] for communication 17.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 310–315, March, 1991.     

Solvatons. II. Aqueous dissociation of hydrides in the MINDOS approximation

A simple electrostatic model of solvation is presented which allows the interaction with solvent to be included systematically within semiempirical SCF calculations. Solvent effects are incorporated into the Hamiltonian for a solute molecule through a series of imaginary particles, solvatons, which represent the oriented solvent distribution around the solute.The proposed model is based on an algorithm for approximating the enthalpy of solvation of each atomic center from its charge in the molecular system and the experimental hydration enthalpies of its various ions. The calculated atomic solvation energy of one center is then modified to include the interaction with other charged atomic centers in the molecule. The method, developed here for the MINDO/3 approximation, has been applied to the calculation of the aqueous dissociation of a series of hydrides. In general, it leads to fairly accurate solvation enthalpies andpK _a values when applied to systems with fixed molecular geometries. A general discussion of the problems associated with the development of a solvation model within a semiempirical framework is also presented.     

Tautomerism of pyrimidine bases—uracil,cytosine, isocytosine: Theoretical study with complete optimization of geometry

The semiempirical MINDO /3 method with complete optimization of geometry is employed to calculate the electronic ground-state properties (dipole moments and ionization potentials) and the energies of various tautomeric forms of uracil, uracil monoanion, cytosine, and isocytosine. The results are shown to be consistent with most of the experimental data. Accuracy of various quantum–mechanical methods is discussed. Particular attention is paid to the influence of the geometry optimization on energy differences between various tautomers. Some qualitative conclusions of biological importance are drawn from these calculations.     

Mindo/3 and mndo calculations of closed- and open-shell cations containing C,H, N,and O

Heats of formation of 119 closed- and open-shell carbocations calculated by the semiempirical quantum chemical methods MINDO/3 and MNDO are reported and compared with experimental data. With proper consideration of failures in specific areas, both methods can be used for the thermodynamics of carbocations containing C, H, N, and O. MINDO/3 predicts unrealistic values for nitrogen containing cations with nitrogen multiple bonds and is not suited for closed-shell cations containing oxygen. Saturated acyclic hydrocarbon radical cations often are computed with abnormally long CC bonds by MNDO. Otherwise, the standard deviation of the two methods is not very different, being in the range of ±13 kcal/mol. MINDO/3 tends to overestimate the cation stabilities, whereas MNDO calculates cations usually too high in energy. Some of the errors which were found in the calculations of the ions are related to the computed values for the parent neutral structures, but others are not.     

Hybridization in fulvene and some related cycloalkenes by the iterative maximum overlap approximation

The hybrid orbitals in cyclopentadiene, fulvene and 6,6-dimethylfulvene were calculated by the iterative maximum overlap method. The hybridization parameters obtained were then used for the calculation of the proton chemical shifts and spin-spin coupling constants J(¹³C-¹H) of the directly bonded carbon and hydrogen nuclei. They can be favourably compared with available experimental data. The predicted bond lengths and bond angles are compared with the results of the more sophisticated semiempirical MINDO/2 and MINDO/3 methods. It is shown that the iterative maximum overlap method gives equally good, or sometimes even better, agreement with experiment. The calculated bond lengths and angles have been used for prediction of molecular diamagnetic susceptibilities by using the additivity formulae of Maksi? and Bloor. The results are within experimental error.     

Modified MINDO/3 13C chemical shift calculations for simple hydrocarbons

Calculations of ¹³C chemical shifts in some simple hydrocarbons have been carried out using the GIAO approach in the MINDO/3 semiempirical formalism. In order to achieve reasonable agreement with experiment it is necessary to modify (increase) the vacant orbital energies in the MINDO/3 calculation in order to reduce the magnitude of the paramagnetic contribution, and to also modify this dominant term by generally reducing it as a function of the number of hydrogen and carbon atoms bonded to the resonant nucleus in question. For a set of 34 resonant nuclei of the simpler hydrocarbons, agreement with experiment of the order of 7.8 ppm is attained; however, pathological cases such as cyclopropane and some simple allenes continue to cause problems, increasing the standard deviation of the full set to 12.5 ppm. Our results indicate that the MINDO/3 approach is as viable for ¹³C chemical shift calculations as other semiempirical approaches, all of which seem currently to be limited to a standard deviation of the order of 10 ppm.     

N-4-戊烯基硝酮的分子内环加成反应的区域选择性的理论研究

用MINDO/3方法求出了N-4-戊烯基硝酮分子内环加成反应的过渡态和反应途径.两个环加成区域异构体是由N-4-戊烯基硝酮的两个不同的构象经过各自的过渡态得到的. 理论分析满意地解释了实验结果.     

The Electronic Structure of Phenylene and Naphthylene Bicyclobutanes. Photoelectron spectroscopy and model calculations

The photoelectron (PE.) spectra of 1,2,3-methenoindane ( 2 ), 1,2,3-metheno-2,3-dihydro-1H-cyclopenta [b]naphthalene ( 3 ) and 1,2,3-metheno-2,3-dihydro-1H-phenalene ( 4 ) are investigated. The PE. spectrum of 1,3-methano-2,3-dihydrophenalene ( 7 ) is reported and compared with that of 4 . The experimental results are analyzed in terms of empirical correlation diagrams and the results of semiempirical MINDO/3 and CNDO/S calculations. The analysis indicates that the strong impact of the bicyclobutylene group on the spectroscopic properties of the aromatic π-systems in 2 , 3 and 4 is due to hyperconjugative interactions involving bicyclobutane Walsh orbitals.