PM3, AM1, MINDO3 semi-empirical IR spectra simulations for some nitriles of interest for Titan's chemistry

A set of the semi-empirical methods (PM3, AM1, MNDO and MINDO3) has been tested to find the best auxiliary tool for the identification of nitriles by gas chromatography/Fourier transform IR spectroscopy/mass spectrometry, considering five nitriles of interest for Titan's chemistry as test compounds: acetonitrile, acrylonitrile, cyanoacetylene, 2-butynenitrile and dicyanoacetylene. Of the four semi-empirical methods, MNDO can be considered as the most advantageous auxiliary tool for the gas chromatography/Fourier transform IR spectroscopy/mass spectrometry (GC/FTIR/MS) identification of nitriles of interest for Titan's atmospheric chemistry, since (1) the simulated IR spectra best match the experimental (in some cases AM1 gives comparable results); (2) it provides the best linearity between the calculated and experimental frequencies (correlation coefficient of 0.990); a scaling factor of 0.90 can be applied to afford better correspondence between the calculated and experimental wavenumbers. At the same time, none of the methods is able to predict infrared intensities and a spectral intensity pattern.     

IR spectra simulation as auxiliary tool for gas chromatography/Fourier transform IR spectroscopy/mass spectrometry identification of unknown compounds. 2. PM3, AM1, MNDO and MINDO3 simulations for simple nitriles

A set of the semi-empirical methods (PM3, AM1, MNDO and MINDO3) supplied by the HyperChem™ package has been tested to find the best auxiliary tool for the gas chromatography/Fourier transform IR spectroscopy/mass spectrometry identification of nitriles, taking 23 relatively simple nitriles as test compounds. Of the four methods, MNDO can be considered as the most advantageous since: (1) for 17 compounds of 23 tested, the IR spectra simulated by this method best match the experimental spectra (and in additional 3 cases, the results are as good as those obtained by AM1 method); (2) within the range of experimental wavenumbers of 900–3100 cm⁻¹, MNDO provides the best linearity between the calculated and experimental values (with a correlation coefficient of 0.989). A scaling factor of 0.85 can be used to afford better correspondence between the calculated and experimental wavenumbers. A disadvantage of the MINDO simulations is underestimation of ν_CN (and sometimes ν_CH) band intensities.     

Theoretical prediction of gas-phase infrared spectra of imidazo[1,2-a]pyrazinediones and imidazo[1,2-a]imidazo[1,2-d]pyrazinediones derived from glycine

Imidazo[1,2-a]pyrazine-3,6-diones and imidazo[1,2-a]imidazo[1,2-d]pyrazine-3,8-diones can be produced by pyrolysis of simple amino acids. While such bicyclic and tricyclic amidines were detected and characterized by IR spectroscopy for some alpha-substituted amino acids, the parent systems composed of glycine fragments are unknown up to now. IR spectra for five amidines derived from glycine were calculated by using different semi-empirical (PM3, AM1, MNDO and MINDO/3), HF, and hybrid DFT (B3LYP, B3P86 and B3PW91) methods in conjunction with 6-31G(d) basis set (for HF and DFT). Vibration frequencies in the experimental IR spectra were predicted based upon the B3LYP data, by correcting the calculated wavenumbers by a scaling factor of 0.959. The behavior of most characteristic bands (nu(CX), nu(NH), etc.) and their shifts with respect to such bands in the spectra of alanine and alpha-aminoisobutyric acid derivatives studied before, are discussed. Performance of the semi-empirical methods was tested, bearing in mind possible future needs for IR spectra predictions for larger molecular systems of similar chemical nature; the use of MINDO/3 and MNDO is recommended. A basis set effect on the B3LYP fundamental vibration frequencies for hexahydroimidazo[1,2-a]pyrazine-3,6-dione was studied by varying Pople basis sets from minimal STO-3G to 6-311++G(d, p). No significant improvements were found beyond the 6-31G(d) basis set, which thus can be recommended to predict IR spectra for the amidines and similar molecules.     

On the applicability of the MINDO/3 method in studies of molecular structures in the vapor phase. Geometries,conformational energies,and vibrational amplitudes of alkenes and chloroalkanes

The geometries and force fields of the different conformers of five alkenes and five chloroalkanes (17 conformers in total) were calculated in the MINDO /3 approximation and compared with the corresponding experimental values. The direct use of geometrical or vibrational parameters derived from MINDO /3 either as starting values or as a source of constraints in the analysis of gas-phase electron diffraction data presents great risks. After a scaling procedure in which the calculated values are fitted to the experimental ones for a small number of compounds, scale factors can be obtained which, with due caution, are transferable to other molecules in the same class. Scaled MINDO /3 produces geometries and force fields that fit to experiment considerably better than their unscaled counterparts. However, scaled MINDO /3 does not produce better results than molecular mechanics. Calculated energy differences between rotamers are generally too high.     

Molecular structure and vibrational investigation of benzenesulfonic acid methyl ester using DFT (LSDA, B3LYP, B3PW91 and MPW1PW91) theory calculations

The FT-Raman and FT-IR spectra for benzenesulfonic acid methyl ester (BSAME) have been recorded in the region 4000-100 cm(-1) and compared with the harmonic vibrational frequencies calculated using DFT (LSDA, B3LYP, B3PW91 and MPW1PW91) method by employing 6-311G (d, p) basis set with appropriate scale factors. IR intensities and Raman activities are also calculated by DFT (LSDA, B3LYP, B3PW91 and MPW1PW91) methods. Optimized geometries of the molecule have been interpreted and compared with the reported experimental values for sulfonic acid and some substituted sulfonic acids. The experimental geometrical parameters show satisfactory agreement with the theoretical prediction from DFT. The scaled vibrational frequencies at LSDA/B3LYP/6-311G (d, p) seem to coincide with the experimentally observed values with acceptable deviations. The theoretical spectrograms (IR and Raman) have been constructed and compared with the experimental FT-IR and FT-Raman spectra. Some of the vibrational frequencies of the sulfonic acid are effected upon profusely with the methyl substitution in comparison to benzene sulfonamide and these differences are interpreted.     

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.     

Vibrational spectra and force field of terephtalonitrile

The IR and Raman spectra of the two molecules terephtalonitrile and terephtalonitrile-¹⁵N were recorded to permit the general assignment of the vibrational bands observed, in agreement with a D_2h symmetry for these molecules. The general quadratic force field was calculated by the semi-empirical MINDO/3 method from an optimized geometry obtained by the same method. The resulting force field was refined by employing the experimental vibrational frequency data of the two molecules and those of terephtalonitrile-d₄. The final differences between the calculated end experimentally observed frequencies for B_2g and B_3u terephtalonitrile species were within the range ± 0.1 cm⁻¹.     

The vibrational and NMR spectra,conformations and ab initio calculations of aminomethylene,propanedinitrile and itsN-methyl derivatives

The IR and Raman spectra of aminomethylene propanedinitrile (AM) [H₂N-CH=C(CN)₂], (methylamino)methylene propanedinitrile (MAM) [CH₃NH-CH=C(CN)₂] and (dimethylamino)methylene propanedinitrile (DMAM) [(CH₃)₂N-CH=C(CN)₂] as solids and solutes in various solvents have been recorded in the region 4000-50 cm^–1. AM and DMAM can exist only as one conformer. From the vibrational and NMR spectra of MAM in solutions, the existence of two conformers with the methyl group orientedanti andsyn toward the double C=C bond were confirmed. The enthalpy difference H ⁰ between the conformers was measured to be 3.7±1.4 kJ mol^–1 from the IR spectra in acetonitrile solution and 3.4±1.1 kJ mol^–1 from the NMR spectra in DMSO solution. Semiempirical (AM1, PM3, MNDO, MINDO3) and ab initio SCF calculations using a DZP basis set were carried out for all three compounds. The calculations support the existence of two conformersanti andsyn for MAM, withanti being 7.8 kJ mol^–1 more stable thansyn from ab initio and 8.6, 13.4, 11.6, and 10.8 kJ mor^–1 from AM1, PM3, MNDO, and MINDO3 calculations, respectively. Finally, complete assignments of the vibrational spectra for all three compounds were made with the aid of normal coordinate calculations employing scaled ab initio force constants. The same scale factors were optimized on the experimental frequencies of all three compounds, and a very good agreement between calculated and experimental frequencies was achieved.     

The IR spectra and hydrogen bonding of toluene-2,6-bis(methyl) and 4,4′-diphenylmethane-bis(methyl) carbamates

The IR spectroscopy has been used to study models of polyurethanes containing different hard segments. The spectra of toluene-2,6-bis(methyl) and 4,4′-diphenylmethane-bis(methyl) carbamates at different temperatures were studied. The absorption curves of the free and associated carbamate molecules were compared with experimental IR spectra. The characteristic features of toluene-2,6-bis(methyl) carbamate and methyl-N-methyl carbamate clusters were revealed. The IR spectra for the two most stable toluene-2,6-bis(methyl) carbamate conformations were compared. The origin of the multiplet structure of bands in the experimental IR spectra of polyurethanes was discussed. The results obtained can be used for the analysis of the chemical and physical transformations in urethanes and polyurethanes.     

Barriers to rotation in methyl formate by dynamic NMR spectroscopy and barriers to 1,3 oxygen-to-oxygen migration in methyl formate and trifluoromethyl formate by ab initio calculations

Free-energy barriers of 9.85 and 11.91 +/- 0.15 kcal/mol at -70.8 degrees C were found by dynamic NMR spectroscopy for the E-to-Z and Z-to-E conversions, respectively, of methyl formate (1) enriched in 13C to 99% for the carbonyl carbon [methyl formate 13C (2)]. These barriers are higher than the literature values reported for -53 degrees C. The free-energy barrier to 1,3 oxygen-to-oxygen migration of the methyl group in methyl formate was determined by ab initio calculations at several levels. The value of 58.7 kcal/mol obtained at the MP2/6-311+G (df,pd) level was compared to a literature barrier for this process (MINDO/3) and to barriers for related compounds. A free-energy barrier of 63.0 kcal/mol for the oxygen - to - oxygen migration of the CF3 group in trifluoromethyl formate (3) was calculated at the MP2/6-31+G level.     

多硝基金刚烷生成热和稳定性的理论研究

在密度泛函理论（DFT）B3LYP/6-31G水平下，通过不破裂金刚烷分子骨架（即 选择金刚作为参考物）的等键反应设计，比较精确地计算了系列多硝基金金刚烷的 生成热。经验性基团加和法和半经验MO法（AM1，PM3，MNDO，MNDO/3）均不适用于 标题生成熟的估算。4种半经验MO方法中，以MP3计算结果略好些。探讨了生成热和 分子结构的关系，发现桥头C上硝基使生成热减小，而偕二硝基使生成热增大。运 用生成热、前沿轨道能级差和C-NO_2键级等计算结果，阐明了标题的相对稳定性， 为新一代高能量密度材料（HEDM）的分子设计提供了基础数据和规律性。     

Study of the electronic energy band structure of polyethylene using MINDO/3

The electronic structure of polyethylene has been calculated using crystal orbital theory and the latest version (MINDO/3) of the MINDO semi-empirical SCF MO method. The results are in better agreement with the observed photoelectron spectrum than those given by an analogous calculation using MINDO/2, which in turn has been shown superior to treatments based on EHT, CNDO, INDO, or ab initio SCF procedures.     

用HAM/3法研究胞嘧啶及其甲基衍生物的光电子能谱与紫外光谱

本文用半经验SCF-MO-HAM/3方法计算了胞嘧啶和它的某些甲基衍生物的电离能、激发能和振子强度.指认了这些分子的紫外光电子能谱和紫外电子光谱.讨论了在紫外光电子能谱指认上与CNDO/S的不同之处.分析了胞嘧啶在磷酸三甲酯中可能存在的主要异构体形式.     

(艹力仒)酸及其类似物紫外光电子能谱的量子化学研究

用一种新的半经验SCF MO方法HAM/3计算菸酸,菸酸甲酯,吡啶,甲酸甲酯和甲酸的分子轨道电离能。提出菸酸与菸酸甲酯紫外光电子能谱的轨道指认,并且研究了这些分子基态的电子结构。在取代效应基础上用经验方法分析了HAM/3指认的可靠性。文中还给出和讨论CNDO/2的计算结果。     

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.     

Semi-empirical molecular orbital calculations of the properties of 2:1 hydrogen bonded complexes of water with some ketones and ethers

The properties (molecular geometries and hydrogen bond energies) of a number of 2:1 complexes formed between water and some ketones and ethers have been calculated using both the CNDO/2 and the MINDO/3 semi-empirical molecular orbital methods. The CNDO/2 results have been compared with those obtained using the MINDO/3 method, and have been correlated with the i.r. spectroscopic properties of the complexes. The results for the 2:1 complexes have also been compared with those of 1:1 complexes formed with the same solvents.     

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.     

A quantum chemical study on structure of 1,2-bis(diphenylphosphinoyl)ethane and phenol cocrystal

The cocrystal of 1,2-bis(diphenylphosphinoyl)ethane (DPPEO) with phenol (1:1) were studied theoretically with AM1, PM3, MNDO and MINDO/3 semi-empirical methods to elucidate its structure. The bond lengths and angles from theoretical studies of molecule DPPEO/phenol (1:1) were found to be as expected. Theoretical results, concerning with intermolecular van der Waals forces in cocrystal, were compared with the previously obtained experimental data and AM1 results were found to be the best fit for bond lengths and angles of DPPEO/phenol.     

Molecular orbital calculations on solvents and other small molecules: Correlation between electronic and molecular properties ν, αMOL, π*, and β.

Molecular orbital calculation by the MINDO/3 method are reported for 50 compounds used as solvents in chemical reactions. Relationships between various parameters of electronic structure and molecular properties such as dipole moment, polarizability and Taft solvent parameters are presented. Comparison between experimental values of π* and calculated values is given.     

Synthesis,characterization and antibacterial activity of 1-([6-bromo-2-hydroxy-naphthalen-1-yl]arylphenyl)methyl)-3-chloro-4-(arylphenyl)-azetidin-2-one

One-pot synthesis of 1-([6-bromo-2-hydroxy-naphthalen-1-yl]-aryl-phenyl)methyl)-3-chloro-4-(aryl-phenyl)azetidin-2-ones has been reported in the present research work via Staudinger [2 + 2] ketene-imine cycloaddition reaction pathway. The reaction of 1-((Benzylideneamino)(aryl)methyl)-6-bromo-naphthalen-2-ols with chloroacetic acid and triethylamine afforded 1-([6-bromo-2-hydroxynaphthalen-1-yl]aryl-phenyl)methyl)-3-chloro-4-(aryl-phenyl)azetidin-2-ones. For the structural elucidation of series of compounds, different analytical and spectroscopic techniques such as elemental analysis, IR spectra, ¹H-NMR spectra and mass spectra were used. All the newly synthesized compounds were tested for their anti-bacterial activity studies. It revealed that some of the compounds possesses moderate to good activities as compared to standard drugs. The widest spectrum of anti-bacterial activities against both gram-positive and gram-negative bacterial strains among the examined compounds possessed having more hydroxyl group along with β-lactam ring compared to other substituted azetidinones.