MNDO Calculations of silicon-containing molecules

A comparison is made of MNDO and MINDO /3 calculations for saturated silicon-containing molecules, and with experimental values, for heats of formation, molecular geometries, charge distributions, and ionization potentials. Except for bond angles, it is found that with the published parameter values the MINDO /3 program gives more reliable results than MNDO . For unsaturated molecules, a comparison of bond lengths and stabilities of Si multiple bonds as given by the two programs and ab initio methods is made, and large discrepancies between predicted structures are pointed out. Some reasons for the dicrepancies are discussed.     

Semiempirical and ab initio transition state calculations for the transformation of N-acetyltetrazoles into corresponding 1,3,4-oxadiazoles

The transformation of 2-acetyl-5-substituted-tetrazoles into the corresponding 1,3,4-oxadiazoles was studied with the semiempirical and ab initio methods. Two mechanisms, one with two transition states and the other with three, were elucidated by . The first mechanism supported by PM3 and MNDO has a two-step, almost concerted, mechanism for the elimination of a nitrogen molecule from the tetrazole ring and formation of the oxadiazole product from an open-chain intermediate through carbon C₅ and acetyl oxygen bond formation. The second mechanism supported by AM1 and MINDO/3 breaks the elimination of the nitrogen molecule into two steps: first breaking the N₄-C₅ and then the N₂-N₃ bonds. Even when the AM1 and MINDO/3 transition state structures were optimized by PM3 and MNDO, the obtained transition states present only one bond breaking. The HF/STO-3G and HF/3-21G ab initio methods agree with the first mechanism where two bonds are breaking almost simultaneously. Despite the disagreement in the mechanism of the nitrogen elimination, the transition state that presents the product formation from open-chain intermediates is quite similar for all methods studied. The semiempirical calculation of this transition state is possible only if it is assumed that it has biradical character. The activation energies calculated by PM3 seem to be insensitive to the nature of the substituents.     

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.     

Theoretical calculations of β-lactam antibiotics. III. AM1, MNDO,and MINDO/3 calculations of hydrolysis of β-lactam compound (azetidin-2-one ring)

Semiempirical AM1, MINDO/3, and MNDO methods have been used in the study of the alkaline hydrolysis of β-lactam antibiotics through a base-catalyzed, acyl-cleavage, bimolecular mechanism. In this work, the hydroxyl ion has been chosen as nucleophilic agent and the azetidin-2-one ring like a model of β-lactam antibiotic. The MINDO/3 method does not predict correctly the energies of small rings. This, together with the fact that, like MNDO, it cannot detect the occurrence of hydrogen bonds, gives rise to uncertain estimates of energy barriers. The AM1 method can be considered the most suitable for studying the hydrolysis of β-lactam compounds.     

Comparison of semiempirical calculations for silicon compounds

A comparison is made of the performance of the MINDO/3, MNDO, AM1, and PM3 methods in calculating the nature of the dimer reconstruction observed on the silicon (100) crystal surface. Based on this case study we conclude that MINDO/3 gives the most realistic results, with PM3 calculations being quite similar but both MNDO and AM1 missing some key features of this system and giving rather unrealistic charge distributions. Hence use of PM3 is recommended for Si containing molecules where a lack of parameters or other restrictions prevent the use of MINDO/3.     

A computational study of the reaction of methane with methyl radical

The activation energy and optimized transition-state geometry for the abstraction of a hydrogen atom from methane by methyl radical have been calculated by the semiempirical methods MINDO /3 and MNDO . These results are compared with other semiempirical and ab initio results. The MINDO /3 method, based upon accuracy of the computed energy of activation, appears to be the computational method of greatest reliability. A method of locating the transition state on semiempirical surfaces is demonstrated.     

硝酸酯化合物生成热的分子轨道研究

用MINDO/3, MNDO和AM1三种SCF-MO方法, 在全优化分子几何构型的基础上, 系统地计算研究了32个硝酸酯化合物的生成热, 与实验值相比, MNDO计算结果偏大很多; MINDO/3计算值与凝聚相实验值符合较好, 由于克服了MNDO法过高地估算原子之间Van der Waals核排斥能的缺点, AM1法给出了较满意的结果: 与七个气态实验值之间存在良好的线性关系(相关系数为0.992), 其间的平均绝对差值和平均差值分别只有10.28和-1.01kJ/mol。     

Development and status of MINDO/3 and MNDO

The historical development of MINDO/3 and MNDO is outlined in relation to the parallel developments of the ab initio SCF approach. It is pointed out that both treatments are purely empirical so far as chemistry is concerned and complement one another, MINDO/3 and MNDO allowing calculations to be carried out when ab initio methods of comparable performance are inapplicable because they need one-thousand times more computing time.     

Theoretical study of the proton affinities of 2-, 3-, and 4-monosubstituted pyridines in the gas phase by means of MINDO/3, MNDO,and AM1

Proton affinities (PAs) of 2-, 3-, and 4-monosubstituted pyridines in the gas phase are calculated using the MINDO/3, MNDO, and AM1 methods. The following substituents are considered: F, Cl, CN, CH₃, CF₃, CHO, NO₂, NH₂, N(CH₃)₂, OCH₃, and SCH₃. The results are compared with experimental values. It is found that all MINDO/3 PAs are ca. 6% too high (mean value) compared to the experimental results; on the other hand, the MNDO values are ca. 7% too low (mean value). However, a much better agreement has been observed for the AM1 method where the theoretical values are only ca. 2.4% too low (mean value). Correlations between the calculated proton affinities on one hand and the charges on the acid H atom and Hammett constants on the other hand are studied. Particularly good linear relationships are found for the 4-monosubstituted compounds within the AM1 formalism.     

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.     

Rearrangements of bicyclo[n.1.1]alkylium cations

In accord with experimental observations, AM1, MNDO and MINDO/3 calculations suggest facile rearrangement pathways available to bicyclo[n.1.1]alkylium cations (n=1,2,3).     

SINDO1 II. Application to ground states of molecules containing carbon,nitrogen and oxygen atoms

Molecular geometries, binding energies, ionization potentials and dipole moments are calculated by the SINDO1 method for a large number of molecules containing C, N and O atoms. Comparison is made with MINDO/3, MNDO and where possible with STO-3G results. The explicit data and an error statistics show the relative merits of SINDO1.     

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°.     

A semiempirical and ab initio MO study of the tautomers ofN-unsubstituted pyrazolones [hydroxy pyrazoles]

Semiempirical (MNDOC, MINDO/3, AM1, and MNDO) and ab initio (STO-3G and 4-31G basis sets) calculations on the relative stabilities, structures, and dipole moments of the 8 theoretically possible tautomeric forms of pyrazolone are reported. It is shown that MNDO + CI and MINDO/3 predict that 5-hydroxy pyrazole, 3-hydroxy pyrazole, and 2-pyrazolin-5-on are the most stable. These results correspond to the known experimental data. Of all used quantum chemical methods, the MINDO/3 results for the dipole moments of the investigated tautomers are in best agreement with the known experimental data. The electronic excitation energies were calculated using the CNDO/S-CI method. The results are in good agreement with the experimental UV spectra.     

Hydrocarbon acidities calculated with MINDO/3, MNDO,and AM1

Acidities of 32 hydrocarbons have been calculated using MINDO/3, MNDO, and AM1. All three semiempirical procedures have systematic errors and reproduce experimental acidities poorly. A linear correlation, however, does exist between the calculated and experimental results. Correction of the AM1 or MNDO acidities leads to good agreement with literature values even for acids, such as methane and ethylene, whose conjugate bases are small localized anions. Predictions for several hydrocarbons are given.     

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.     

哌嗪二酮的气相Hel光电子能谱及其量化计算

给出了哌嗪二酮的气相HeI紫外光电子能谱(UPS), 并进行了化合物分子的HAM/3, MNDO, MINDO/3, INDO, CNDO/2和EHMO等量子化学计算研究. UPS谱低电离能(<11.00 eV)区的四重峰被指认为分子体系中氧-氧, 氮-氮原子孤对轨道间的通过键相互作用导致的分裂峰. 表明HAM/3和MNDO计算法是预指该化合物实验电离能正确次序、轨道对称性类型以及通过键相互作用导致分裂大小的较好方法.     

在不加对称性限制的条件下采用不同的半经验方法研究酞菁的电荷分布和几何构型（英文）

在不加对称性限制的条件下采用不同的半经验方法CNDO,INDO,MINDO/3,MNDO和Pm3对自由酞菁(H2Pc)进行几何构型最优化和电荷密度布居分析,并将所得结果与Am1方法以及X射线衍射和中子衍射数据的数据进行比较.结果表明只有从INDO方法得到的是桥式构型,而从CNDO, MINDO/3, MNDO,Am1, Pm3方法得到的都是键式构型.除INDO之外所有的这几种方法都对先前报道的几何构型扭曲给出进一步的支持证据.新近建立的半经验方法如MNDO,An1,Pm3等看起来更适合于用来研究大分子如酞菁及其类似物的几何构型扭曲和电荷分布极化等问题.     

Evaluation of MINDO/3 calculated structures. II. Branching errors in alkanes and cycloalkanes

MINDO/3 calculations have been carried out for a series of branched chain alkanes in order to assess effects of branching on calculated geometries and heats of formation (ΔH_f). With vicinal branching, MINDO/3 calculates the central C? C bond to be too long. Bond angles are also found to be distorted. Errors in calculated heats of formation are large when geminal branching is present and significant with vicinal branching. Branching error corrections for ΔH_f have been derived and applied to a separate series of branched acyclic and cyclic compounds. For the test sample, application of the branching error corrections gave calculated structures of acyclic branched hydrocarbons with heats of formation having an average absolute error of 1.3 kcal/mole rather than 17.3 kcal/mole before correction. Cyclic branched hydrocarbons are shown to be less well corrected. Calculations of heats of reaction have also been carried out for some isomerization and cyclization reactions using the MINDO/3 and MNDO methods. It is clear from the comparisons that MNDO calculations give less severe errors for highly branched compounds but the errors are still substantial. For prediction of heats of reaction, the error-corrected calculations are shown to be superior to the “raw” calculations obtained by MINDO/3 or MNDO.     

A density functional treatment of organolithium compounds: Comparison to ab initio,semiempirical, and experimental results

Density functional calculations on several classes of organolithium compounds are described. The compounds studied include lithium bonds to carbon, oxygen, and nitrogen and are representative of most types of organolithium compounds that have appeared in the recent literature. The computational results are compared to those using MNDO, which has been shown to have some serious deficiencies in compounds involving carbon–lithium bonds, and to PM3 results, which offer some improvement over MNDO for many organolithium compounds. Most of the density functional calculations with a large basis set are in good agreement with available ab initio and experimental data. Calculated carbon–lithium bond lengths were slightly shorter than those calculated by other ab initio methods and were substantially longer than those calculated by MNDO, which is known to underestimate carbon–lithium bond lengths severely. Dimerization energies of methyllithium, calculated by DMol, were also in good agreement with those of other ab initio calculations. Lithium–nitrogen bonds in lithium amides were calculated to be slightly shorter by DMol than by MNDO, although the two methods were in qualitative agreement for this type of compound. © 1995 by John Wiley & Sons, Inc.