可乐定分子构象异构和互变异构的理论研究

采用HF/6-31G*方法, 对氨基型和亚胺型可乐定中性分子以及质子化的可乐定分子进行了构象分析. 之后采用B3LYP/6-31G**方法对处于势能面上的能量极小构象异构体进行全自由度几何优化和频率分析. Onsager反应场溶剂模型用于水相的计算. 结果表明, 在气相和水相中可乐定中性分子主要以亚胺型存在, 这同已有实验结果一致. 进一步, 寻找构象异构化过渡态和氨基型亚胺型互变异构化过渡态, 探讨质子化和水溶剂化效应对异构化过程的几何结构和能量的影响. 为了考察氯苯的共轭效应对可乐定互变异构体稳定性的影响, 在B3LYP/6-31G**水平上, 研究了2-氨基-2-咪唑啉的互变异构化反应机理.     

Conformations of dimethoxydimethylsilane: matrix isolation infrared and ab initio studies

Conformations of dimethoxydimethylsilane (DMDMS) were studied using matrix isolation infrared spectroscopy, by trapping the silane in argon and nitrogen matrixes. The matrix was deposited using both an effusive and a supersonic jet source. The effusive source was maintained at two different temperatures, viz. 298 and 433 K, during deposition to alter the conformational population of the silane. The experimental results were supported by computations performed at both the HF and B3LYP levels, using 6-31++G** basis set. Vibrational frequency calculations were carried out to assign the experimental features and also to ensure that the computed structures did indeed correspond to minima. A conformer with a G+/-G-/+ structure was found to be the ground state, while G+/-T and G+/-G+/- structures were the next higher energy conformers with energies of 1.32 and 1.48 kcal/mol, respectively. Natural bond orbital analysis was carried out at both HF/6-31++G** and B3LYP/6-31++G** level which indicated that the charge-transfer hyperconjugative interactions largely determine the conformational preferences in this molecule. This interaction appears to be smaller in DMDMS than in the corresponding carbon analogue, dimethoxypropane (DMP).     

The structure, Raman spectra, and force fields of methylbromosilanes CH3SiClnBr3−n (n = 0, 1, 2)

The laser-excited Raman spectra of liquid CH₃SiCl_nBr_3?n (n = 0, 1, 2) were studied. Quantumchemical calculations of these substances with geometry optimization were performed to determine their harmonic force fields and vibrational frequencies. The calculations were made using the HF/6-31G* and HF/6-311++G** approximations and density functional theory at the B3LYP/6-31G* and B3LYP/6-311++G** levels. An interpretation of the spectra was suggested and the calculated force fields were discussed in comparison with the data on related compounds.     

Tert-butyl N-(2-bromocyclohex-2-enyl)-N-(2-furylmethyl)carbamate的结构和振动光谱

The molecular structure, conformafional stability, and vibrational frequencies of ten-butyl N-（2- bromocyclohex-2-enyl）-N-（2-furylmethyl）carbamate （TBBFC） were investigated by utilizing the Hartree-Fock （HF） and density functional theory （DFT） ab initio calculations with 6-31G ^＊ and 6-31G^＊ ＊ basis sets. The optimized bond length and angle values obtained by HF method showed the best agreement with the experimental values. Comparison of the observed and calculated fundamental vibrational frequencies indicated that B3LYP was superior to the scaled HF approach for molecular problems. Optimal uniform scaling factors calculated for the title compound are 0.899/0.904, 0.958/0.961, and 0.988/0.989 for HF, B3LYP, and BLYP （6-31G ^＊/6-31G ^＊ ＊）, respectively.     

FT-Raman and FT-IR spectra, ab initio and density functional studies of 3,4-dichlorobenzyl alcohol

The Fourier transform Raman and Fourier transform infrared spectra of 3,4-dichlorobenzyl alcohol were recorded in the solid phase. The equilibrium geometry, harmonic vibrational frequencies, infrared intensities and Raman scattering activities, depolarization ratios, reduced masses were calculated by HF and density functional B3LYP method with the 6-311 G** basis set. The scaled theoretical wavenumbers showed very good agreement with the experimental values. The thermodynamic functions of the title compound were also performed at HF/6-311G** and B3LYP/6-311G** levels of theory. A detailed interpretations of the infrared and Raman spectra of 3,4-dichlorobenzyl alcohol is reported. The theoretical spectrograms for FT-IR spectra of the title molecule have been constructed.     

Conformational study of the structure of free 18-crown-6

A conformational search was performed for 18-crown-6 using the CONLEX method at the MM3 level. To have a more accurate energy order of the predicted conformations, the predicted conformations were geometry optimized at the HF/STO-3G level and the 198 lowest energy conformations, according to the HF/STO-3G energy order, were geometry optimized at the HF/6-31+G level. In addition, the 47 nonredundant lowest energy conformations, according to the MP2/6-31+G energy order at the HF/6-31+G optimized geometry, hereafter the MP2/6-31+G//HF/6-31+G energy order, were geometry optimized at the B3LYP/6-31+G level. According to the MP2/6-31+G//B3LYP/6-31+G energy order, three conformations had energies lower than the experimentally known Ci conformation of 18c6. At the MP2/6-31+G//B3LYP/6-31+G level, the S6 lowest energy conformation is more stable by 1.96 kcal/mol than this Ci conformation. This was confirmed by results at the MP2/6-31+G level with an energy difference of 1.84 kcal/mol. Comparison between the structure of the S6 conformation of 18c6 and the S4 lowest energy conformation of 12-crown-4, as well as other important conformations of both molecules, is made. It is concluded that the correlation energy is necessary to have an accurate energy order of the predicted conformations. A rationalization of the conformational energy order in terms of the hydrogen bonding and conformational dihedral angles is given. It is also suggested that to have a better energy order of the predicted conformations at the MM3 level, better empirical force fields corresponding to the hydrogen bond interactions are needed.     

An investigation of keto–enol tautomerism in N-vinyl-2-, and 3-pyrrolidinones using ab initio and density functional theory calculations

We investigated the protomeric tautomerism of N-vinyl-2-pyrrolidinone (NV2P) and N-vinyl-3-pyrrolidinone (NV3P) molecules, and considered the three neutral tautomers for NV2P (1a, 1b, 1c) and the five for NV3P (2a, 2b, 2c, 2d, 2e). Full geometry optimizations were carried out at the HF/6-31G**, HF/6-31+G** and B3LYP/6-31+G** levels in the gas phase and in water. Additionally, single-point MP2/6-31+G** calculations were performed on the HF/6-31+G** optimized geometries. The tautomerization processes in water (=78.54) were studied using the self-consistent reaction field theory. According to our estimations, the tautomerization of NV2P and NV3P molecules are not strongly influenced by the polar solvents, the improvement of the basis set quality and the electron correlation effects in the gas phase and in water. The calculated relative free energies (ΔG) predict that 1a and 2a are the energetically preferred tautomers in the gas phase and in water.     

Experimental and density functional theory and ab initio Hartree-Fock study on the vibrational spectra of 2-(4-fluorobenzylideneamino)-3-(4-hydroxyphenyl) propanoic acid

2-(4-Fluorobenzylideneamino)-3-(4-hydroxyphenyl) propanoic acid (4-FT) was synthesized through the reaction of 4-fluorobenzaldehyde and l-tyrosine in refluxing EtOH. The structure of 4-FT was verified by measuring 1H NMR, FTIR and Raman. The ground-state geometries were optimized at B3LYP/6-31G**, B3LYP/6-31G*, HF/6-31G** and HF/6-31G* levels without symmetry constrains. The vibrational wavenumbers of 4-FT were calculated at same levels. The scaled spectra using B3LYP methods, which are in a good agreement with the measured spectra, are superior to those calculated using HF methods.     

Experimental and density functional theory and ab initio Hartree-Fock study on the vibrational spectra of 2-(4-fluorobenzylideneamino)-3-mercaptopropanoic acid

2-(4-Fluorobenzylideneamino)-3-mercaptopropanoic acid (4-FC) was synthesized through the reaction of 4-fluorobenzaldehyde and l-cysteine in refluxing EtOH. Its structure was verified by (1)H NMR, FT-IR and Raman. The ground-state geometries were optimized at B3LYP/6-31G**, B3LYP/6-31G*, HF/6-31G** and HF/6-31G* levels without symmetry constrains, respectively. The vibrational wavenumbers of 4-FC were calculated at same level. The scaled theoretical spectra using B3LYP methods, which are in a good agreement with the experimental ones, are superior to those using HF methods.     

Three rotor potential energy scans, conformational equilibrium constants and vibrational analysis of 3-fluoro-1-propanol CH(2)FCH(2)CH(2)OH

The conformational stability and the three rotor internal rotations in 3-fluoro-1-propanol were investigated by the DFT-B3LYP/6-311+G** and the ab initio MP2/6-311+G** levels of theory. The calculated potential energy curves of the molecule at both levels of theory were consistent with complex conformational equilibria of about 12 minima, all of which were predicted to have real frequencies at both the B3LYP and the MP2 levels. The lowest energy minimum in the potential curves of 3-fluoro-1-propanol was predicted to correspond to the Gauche-gauche-trans (Ggt) conformer in excellent agreement with microwave and electron diffraction results. The equilibrium constants for the conformational interconversion of the molecule were calculated and found to correspond to an equilibrium mixture of about 33% Ggt, 14% Ggg1 and 13% Gg1g and about 43% Ggt, 12% Ggg1 and 10% Gg1g distribution by the B3LYP/6-311+G** and the MP2/6-311+G** calculations, respectively, at 298.15K. The vibrational frequencies of each molecule in its three stable forms were computed at B3LYP level and complete vibrational assignments were made based on normal coordinate calculations and comparison with experimental data of the molecule.     

Conformation and tautomerism of the cimetidine molecule: a theoretical study

The cimetidine molecule conformation and tautomer stability was studied at the ab initio HF/6-31G** level and for single point energies at the MP2/6-31G** level. The most stable N3-H cimetidine tautomer was found to be more stable than the most stable N1-H tautomer by ca. 3.7 and 5.0 kcal/mol, at the HF/6-31G** and MP2/6-31G**//HF/6-31G** level, respectively. At the HF/6-31G** level, the most stable N3-H and 1-H forms are stabilized by the intramolecular N3′-H?N1 hydrogen bond and N1-H?N4′, respectively. However, when the correlation effects are included at the MP2/6-31G**//HF/6-31G** level, the most stable N3-H and N1-H tautomers appeared to be folded forms without hydrogen bonds.     

Synthesis, spectroscopic characterization, crystal structures, theoretical studies, and antibacterial evaluation of two novel N-phosphinyl ureas

Abstract  

Two novel N-phosphinyl ureas containing different substituents were synthesized and characterized by ¹H, ¹³C, and ³¹P NMR, IR, UV, mass spectroscopy, and elemental analysis. The crystal structures of these compounds were determined by X-ray crystallography. The structure of one compound exhibits the presence of two independent forms of the molecule with equal occupancy in the lattice and theoretical data reveal the same stabilization energies for these conformers. The title molecules have anti conformation with respect to the C=O and P=O bonds, whereas the other compound shows syn configuration. Quantum chemical calculations were applied to clarify this conformational behavior. Furthermore, the molecular geometry and vibrational frequencies of the new derivatives in the ground state were calculated by using the Hartree–Fock (HF) and density functional method (B3LYP) with 6-31+G** and 6-311+G** basis sets and compared with experimental values. The new derivatives were additionally tested in view of their antibacterial properties.     

FT-IR and FT-Raman investigation, computed vibrational intensity analysis and computed vibrational frequency analysis on m-Xylol using ab-initio HF and DFT calculations

The FT-IR and FT-Raman spectra of m-Xylol molecule have been recorded using Bruker IFS 66V spectrometer in the range 4000-100cm(-1). The molecular geometry and vibrational frequencies in the ground state are evaluated using the Hartree-fock (HF) and B3LYP with 6-31+G (d, p), 6-31++G (d, p) and 6-311++G (d, p) basis sets. The computed frequencies are scaled using a suitable scale factors to yield good agreement with the observed values. The HF and DFT analysis agree well with experimental observations. Comparison of the fundamental vibrational frequencies with calculated results by HF and B3LYP methods indicate that B3LYP/6-311++G (d, p) is superior to HF/6-31+G (d, p) for molecular vibrational problems. The complete data of this title compound provide some useful information for the study of substituted benzenes. The influences of Methyl groups on the geometry of benzene and its normal modes of vibrations have also been discussed.     

Theoretical and experimental NMR study of protopine hydrochloride isomers

The 1H and 13C NMR chemical shifts of cis- and trans-protopinium salts were measured and calculated. The calculations of the chemical shifts consisted of conformational analysis, geometry optimization (RHF/6-31G** method) and shielding constants calculations (B3LYP/6-31G** method). Based on the results of the quantum chemical calculations, two sets of experimental chemical shifts were assigned to the particular isomers. According to the experimental results, the trans-isomer is more stable and its population is approximately 68%.     

Energetic analyses of chair and boat conformations of maleimide substituted cyclohexane derivatives

An analysis of the conformational preferences of two maleimide substituted cyclohexane derivatives proposed as scaffolds for HIV-1 fusion inhibitors is presented. Hybrid Low Mode-Monte Carlo (1:1) conformational searches using seven different force fields were performed in combination with the GBSA(water) solvent model. Low energy structures identified in this way were subjected to geometry optimization on the B3LYP/6-31G** surface. Solvent effects were included in the quantum calculation using the self-consistent reaction field model for water. Quantum results indicate that the 1,3,5-maleimide functionalized 1,3,5-methyl cyclohexane is more stable in the boat conformation, whereas 1,3,5-maleimide functionalized cyclohexane adopts the expected chair conformation with equatorial arms. None of the force fields studied was able to predict the unexpected preference for the boat conformation of 1,3,5-maleimide functionalized 1,3,5-methyl cyclohexane. Comparison of low energy and experimental structures was also performed.     

DFT法研究3-羟基丙烯醛的双键旋转异构反应机理

利用密度泛函理论(DFT)分别在B3LYP/6-31G**和B3LYP/6-311＋＋G**的计算水平上优化了基态3-羟基丙烯醛分子在双键旋转异构反应过程中的平衡态以及过渡态的几何构型，分析了反应过程中键参数的变化，计算了该反应的内禀反应坐标(IRC)，发现在重排反应途径上存在一个四元环骨架的中间体.通过振动分析对平衡态和过渡态进行了确认，并得到了零点能.计算结果表明，基态3-羟基丙烯醛分子的双键旋转异构反应经过两步完成，第一步反应位垒稍高，第二步反应位垒较低，存在着发生重排反应的可能性.