FT-IR, Raman and NMR Spectra, Molecular Geometry, Vibrational Assignments, ab initio and Density Functional Theory Calculations for Diethyl Phthalate

Phthalate acid esters (PAEs) possess endocrine disruptive effects and can produce reproductive and developmental toxicities. In this paper, both experimental and theoretical studies on FT-IR, Raman and 1H NMR spectra of diethyl phthalate (DEP) have been carried out. The geometrical structure of DEP was optimized at the HF/6-31G*, HF/6-311G**, B3LYP/6-31G*, and B3LYP/6-311G** levels, respectively. The harmonic vibrational frequencies, IR intensity, Raman activity and 1H NMR chemical shifts have been computed at the B3LYP/6-31G* and B3LYP/6-311G** levels. Anharmonic corrections to frequencies were obtained by means of second-order perturbation theory (PT2) at the B3LYP/6-31G* level. Based on potential energy distribution (PED), the vibrational assignments have also been performed. The theoretical calculation values were compared with the experimental observations and the results indicate they are in excellent agreement.     

吲哚分子振动光谱的密度泛函理论研究

用多种密度泛函理论(DFT)方法(BLYP/6-31G^*^*,B3LYP/6-31G^*^*,B3PW91/6-31G^*^*和SVWN/6-31G^*^*)对吲哚分子的平衡几何构型进行了优化。在优化构型的基础上计算了吲哚分子的谐力场、振动基频和红外光谱强度。计算得到的振动频率与实验值比较平均偏差对四种计算方法(BLYP/6-31G^*^*,P3LYP/6-31G^*^*,B3PW91/6-31G^*^*和SVWN/6-31G^*^*)分别为16.3,40.5,45.1和26.4cm^-^1。BLYP/6-31G^*^*理论力场被用于吲哚分子的简正坐标分析计算中。根据振动率的势能分布(PEDs)对此分子的振动基频进行了理论归属。     

硝酸甲与硝酸乙酯的结构、振动频率和热力学性质的密度函数 理论(DFT)研究

用密度函数理论(DFT)的BLYP和B3LYP方法,取6-31G,6-31G^*,6-31G^*^*,6-311G,6-311G^*和6-311G^*^*六种基组,对硝酸甲酯和硝酸乙酯的几何构型和红外振动频率进行了计算研究.结果表明,B3LYP方法在采用极化基组(6-31G^*,6-31G^*^*,6-311G^*和6-311G^*^*)时计算得到的结果均较好,适用于硝酸酯类化合物的研究.而BLYP方法无论采用何种基组均不适用;运用校正后的B3LYP/6-31G^*频率(校正因子0.975)计算得到的热力学性质(C^o~p,H^o和S^o)与实验结果较吻合。     

硝酸甲与硝酸乙酯的结构、振动频率和热力学性质的密度函数 理论(DFT)研究

用密度函数理论(DFT)的BLYP和B3LYP方法,取6-31G,6-31G^*,6-31G^*^*,6-311G,6-311G^*和6-311G^*^*六种基组,对硝酸甲酯和硝酸乙酯的几何构型和红外振动频率进行了计算研究.结果表明,B3LYP方法在采用极化基组(6-31G^*,6-31G^*^*,6-311G^*和6-311G^*^*)时计算得到的结果均较好,适用于硝酸酯类化合物的研究.而BLYP方法无论采用何种基组均不适用;运用校正后的B3LYP/6-31G^*频率(校正因子0.975)计算得到的热力学性质(C^o~p,H^o和S^o)与实验结果较吻合。     

Si~2Cl~6分子的内旋转能垒和振动基频的理论研究

用从头算方法HF/6-31G^*^*和密度函方法B3LYP/6-31G^*^*,对Si~2Cl~6分子的平衡几何构型进行优化,优化的结果与实验结果吻合得较好.并用上述两种不同的方法计算Si~2Cl~6分子的内旋转能垒,结果分别为8.786和6.694kJ/mol,其中DFT方法的计算结果与实验结果4.18kJ/mol吻合得较好.对Si~2Cl~6分子的振动基频进行计算.用HF/6-31G^*^*SQM力场所计算的频率理论值与实验值的平均误差为7.3cm^-^1,用B3LYP/6-31G^*^*未标度的力场所计算的频率理论值与实验值的平均误差为6.0cm^-^1.该密度泛函方法(B3LYP/~6-31G^*^*)的理论计算值比用HF/6-31G^*^*标度后的SQM力场计算的频率与实验值(除Si--Si键扭转振动基频之外的11条振动基频)吻合得更好.并给出了Si--Si键扭转振动基频的预测值。     

Si~2Cl~6分子的内旋转能垒和振动基频的理论研究

用从头算方法HF/6-31G^*^*和密度函方法B3LYP/6-31G^*^*,对Si~2Cl~6分子的平衡几何构型进行优化,优化的结果与实验结果吻合得较好.并用上述两种不同的方法计算Si~2Cl~6分子的内旋转能垒,结果分别为8.786和6.694kJ/mol,其中DFT方法的计算结果与实验结果4.18kJ/mol吻合得较好.对Si~2Cl~6分子的振动基频进行计算.用HF/6-31G^*^*SQM力场所计算的频率理论值与实验值的平均误差为7.3cm^-^1,用B3LYP/6-31G^*^*未标度的力场所计算的频率理论值与实验值的平均误差为6.0cm^-^1.该密度泛函方法(B3LYP/~6-31G^*^*)的理论计算值比用HF/6-31G^*^*标度后的SQM力场计算的频率与实验值(除Si--Si键扭转振动基频之外的11条振动基频)吻合得更好.并给出了Si--Si键扭转振动基频的预测值。     

具有D~6~h对称性的C~3~6及其衍生物电子结构的密度泛函研究

运用密度泛函方法,比较不同水平的基组(HF/6-311+G^*;B3LYP/6-31G^*;B3LYP/6-311+G^*)对具有D~6~h对称性的C~3~6分子进行构型优化的结果,并分析其几何结构、电子结构、稳定性等性质;采用基组B3LYP/6-31G^*对H@C~3~6,Li@C~3~6,Na@C~3~6,K@C~3~6分子进行构型全优化,分析了不同内嵌原子对其几何结构、电子结构、稳定性等性质的影响;首次在B3LYP/6-311+G^*水平上,对C~3~6H~6,C~3~6H~1~2,X@C~3~6(X=H,Li,Na,K)几何构型及电子结构进行研究并得到其稳定性规律。     

水溶液中铵离子pKa值的理论计算

在HF/6-31+G*和B3LYP/6-31+G*水平上, 采用导体极化连续模型(CPCM)及UAKS孔穴计算了11种铵离子在水溶剂中的溶剂化自由能, 与实验值相比较, 平均误差和标准偏差分别为0.17, 12.04和0.96, 10.96 kJ/mol. 结合B3LYP/6-31+G*水平上的11种铵离子气相质子转移反应自由能, 得到了水溶剂中的绝对pKa值, 计算结果与实验数据吻合得很好, 相应的平均误差和标准偏差分别为0.05, 1.50和0.45, 1.40 pKa单位. 可见, 采用CPCM-UAKS模型能够较为精确地计算铵离子型化合物的绝对pKa值.     

Theoretical Study on Hydrogen Interaction between 5-Fluorouracil and Glycine

The geometries,electronic structure,IR spectrum and other properties of hydrogen interaction between 5-fluorouracil and glycine were studied at the B3LYP/6-31+G* level.Single point energy calculations were executed at the B3LYP/6-311++G** and B3LYP/aug-cc-pvdz levels,and natural bond orbital (NBO) analysis was carried out at the B3LYP/6-31+G* level.Finally,the hydrogen bonds were discussed via AIM electronic density topology analysis.     

特殊氢方法预测丙氨酸-α-四肽构象稳定性

使用B3LYP/6-31G*方法优化得到了丙氨酸-α-四肽分子的48个稳定构象.定义了丙氨酸-α-多肽中的特殊氢原子,对构象中与特殊氢原子有关的主要非键相互作用进行分析,提出了使用与特殊氢原子有关的非键相互作用来预测多肽构象的相对稳定性,并将之称为特殊氢方法.基于丙氨酸-α-二肽分子的5个构象和三肽分子的7个构象,确定了特殊氢方法中的与特殊氢原子有关的非键相互作用参数.利用特殊氢方法预测丙氨酸-α-四肽分子的48个构象的相对稳定性,与B3LYP/6-31G*方法的相对能量比较,得到了满意的结果.特殊氢方法得到的相对能量(Y)和B3LYP/6-31G*方法得到的相对能量(X)的线性相关方程为Y=0.9296X 2.2041,相关系数R=0.9532,标准偏差为3.0kJ/mol,偏差绝对值≤4.18kJ/mol的构象占87.5%.     

联吡啶构象异构的理论研究

采用HF/6-31G*方法,对6种联吡啶构造异构体进行了构象分析.之后采用B3LYP/6-311G**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,并且寻找构象异构化过渡态.采用自洽反应场极化导体模型溶剂理论,探讨了水溶剂对优势构象异构体的几何结构和能量的影响.     

Conformational study of the structure of 12-crown-4-alkali metal cation complexes

A conformational search was performed for the 12-crown-4 (12c4)-alkali metal cation complexes using two different methods, one of them is the CONFLEX method, whereby eight conformations were predicted. Computations were performed for the eight predicted conformations at the HF/6-31+G*, MP2/6-31+G*//HF/6-31+G*, B3LYP/6-31+G*, MP2/6-31+G*//B3LYP/6-31+G*, and MP2/6-31+G* levels. The calculated energies predict a C4 conformation for the 12c4-Na+, -K+, -Rb+, and -Cs+ complexes and a C(s) conformation for the 12c4-Li+ complex to be the lowest energy conformations. For most of the conformations considered, the relative energies, with respect to the C4 conformation, at the MP2/6-31+G*//B3LYP/6-31+G* are overestimated, compared to those at the MP2/6-31+G* level, the highest level of theory considerd in this report, by 0.2 kcal/mol. Larger relative energy differences are attributed to larger differences between the B3LYP and MP2 optimized geomtries. Binding enthalpies (BEs) were calculated at the above-mentioned levels for the eight conformations. The agreement between the calculated and experimental BEs is discussed.     

An Ab Initio Molecular Orbital Study of Valence Bond Isomers of Silabenzene and Phosphabenzene

Ab initio molecular orbital calculation at HF/6-31G*, HF/6-31G**, HF/6-311G**, HF/6-311++G**, RMP2-FC/6-31G*, and B3LYP/6-31G* levels of theory for geometry optimization and MP4(SDQ)/6-31G* for a single point total energy calculation are reported for silabenzene ( 7 ), phosphabenzene ( 8 ) and 16 valence bond isomers of silabenzene and phosphabenzene ( 9-24 ). The calculated energy difference (19.78 kcal mol m 1 ) between silabenzene and the most stable valence bond isomer of silabenzene (1-silabenzvalene, 9 ) is much smaller than the difference (73.60 kcal mol m 1 ) between benzene and benzvalene ( 2 ). The energy difference between phosphabenzene and the most stable valence bond isomer of phosphabenzene (1-phosphabenzvalene, 17 ) is calculated to be 43.29 kcal mol m 1 .     

An Ab Initio Molecular Orbital Study of Isophosphinines

Ab initio molecular orbital calculation at HF/6-31G*, HF/6-31G**, HF/6-311G**, HF/6-311++G**, RMP2-FC/6-31G*, and B3LYP/6-31G* levels of theory for geometry optimization and MP4(SDQ)/6-31G* for a single-point total energy calculation are reported for phosphinine and 13 isophosphinines 7-19 . Isomers 7-11 with an allenic system are calculated to be 8-18 kcal mol m 1 more stable than structures 12-17 with an acetylenic moiety. The calculated energy difference (66.19 kcal mol m 1 ) between phosphinine and the most stable isophosphinine (1-phospha-1,2,4-cyclohexatriene, 10 ) is smaller than the difference (78.96 kcal mol m 1 ) between benzene and the most stable isobenzene (cyclohexa-1,2,4-triene, 2 ). The isophosphinines 18 and 19 , with a butatriene moiety, are calculated to be the least stable isomers.     

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.     

电场极化对碱基对质子转移和电子传递的影响

在B3LYP/6-31+G**//HF/6-31+G**水平上研究了在不同电场极化环境下碱基对A-T的几何构型和电子结构. 通过碱基对的氢键和结合能的变化讨论了碱基对间的质子转移, 进一步利用密度泛函理论结合非平衡态格林函数方法研究了通过碱基对的电子输运行为. 结果表明, 在0.6～2.0 V的偏压下, 由T向A方向的电子传递更易进行, 表现了微弱的整流行为.     

Probing furanose ring conformation by gas-phase computational methods: energy profile and structural parameters in methyl beta-D-arabinofuranoside as a function of ring conformation

The potential energy surface of methyl beta-D-arabinofuranoside (3) has been studied by ab initio molecular orbital (HF/6-31G) and density functional theory (B3LYP/6-31G) calculations via minimization of the 10 possible envelope conformers. The partial potential energy surface identified that the global minimum and lowest energy northern conformer was E(2). In the HF calculations, (2)E was the most stable southern conformer, while the density functional theory methods identified (4)E as the local minimum in this hemisphere. Additional calculations at higher levels of theory showed that the B3LYP-derived energies of many of the envelope conformers of 3 are dependent upon the basis set used. It has also been demonstrated that B3LYP/6-31+G//B3LYP/6-31G single point energies are essentially the same as those obtained from full geometry optimizations at the B3LYP/6-31+G level. The northern and southern minima of the B3LYP/6-31+G surface are, respectively, the E(2) and (2)E conformers. The B3LYP/6-31G geometries were used to study the relationship between ring conformation and various structural parameters including bond angles, dihedral angles, bond lengths, and interatomic distances.     

Computational studies of the structure and vibrational spectra of hexafluorodisilane

Ab initio calculations predict that D_3d symmetry of Si₂F₆ is more stable than D_3h symmetry. The calculated potential barrier to internal rotation was 0.77, 0.73 and 0.78 kcal/mol using HF/6-31G*, B3LYP/6-31G* and MP2/6-31G* methods respectively, which was in good agreement with the experimental value between 0.51±0.10 and 0.73±0.14 kcal/mol. The optimized geometries, harmonic force fields, infrared intensities, Raman activities, and vibrational frequencies are reported for D_3d symmetry of Si₂F₆ from HF/6-31G* and B3LYP/6-31G*. A normal coordinate analysis was carried out. The average error between the scaled DFT frequencies obtained from the B3LYP/6-31G* calculation and observed frequencies was 4.2 cm⁻¹ and the average error between the scaled HF and observed frequencies was 2.2 cm⁻¹.     

Theoretical studies on the conformations of selenamides

Ab initio HF/6-31+G*, MP2/6-31+G*, B3LYP/6-31+G* level calculations have been performed on HSe-NH₂ to estimate the Se-N rotational barriers and N-inversion barriers. Two conformers have been found withsyn andanti arrangement of the NH₂ hydrogens with respect to Se-H bond. The N inversion barriers in selenamide are 1.65, 2.47, 1.93 kcal/mol and the Se-N rotational barriers are 6.58, 6.56 and 6.12 kcal/mol respectively at HF/6-31+G*, MP2/6-31+G* and B3LYP/6-31+G* levels respectively. The n_N →Σ *_Se-H negative hyperconjugation is found to be responsible for the higher rotational barriers.