A reaction density functional theory study of solvent effects on keto-enol tautomerism and isomerization in pyruvic acid

It is important to study the solvent effect on keto-enol tautomerism that has applications in many areas of chemical engineering.In this work,we use a multiscale reaction density functional theory(Rx DFT)to study the keto-enol tautomerism and isomerization of pyruvic acid.The results show that both effects of solvation and water assistance could reduce the reaction barriers.The water molecule participates the reaction as a catalyst to accept/give the protons with forming a hexagonal ring in the transition state.As a result of this temporary and intermediate hexagonal ring,the solute configuration undergoes a small variation during the reaction,giving a diminished contribution to the intrinsic reaction free energy.The solvent distribution shows a local ordering behavior near the solute that also reduces the contribution of solvation effect to the reaction barrier.Water assistance plays a major role in both pre-reaction and postreaction process.In terms of the driving force for the reaction,the effects of both solvation and water assistance are important.

Assignment of terahertz vibrational modes of L-glutamine using density functional theory within generalized-gradient approximation

The crystal structure of L-glutamine is stabilized by a three-dimensional network of intermolecular hydrogen bonds.We utilize plane-wave density functional theory lattice-dynamics calculations within the generalized-gradient approximation（GGA）, Perdew–Burke–Ernzerhof（PBE）, PBE for solids（PBEsol）, PBE with Wu–Cohen exchange（WC）, and dispersion-corrected PBE, to investigate the effect of these intermolecular contacts on the absorption spectra of glutamine in the terahertz frequency range. Among these calculations, the solid-state simulated results obtained using the WC method exhibit a good agreement with the measured absorption spectra, and the absorption features are assigned with the help of WC. This indicates that the vibrational modes of glutamine were related to the combination of intramolecular and intermolecular motions, the intramolecular modes were dominated by rocking or torsion involving functional groups; the intermolecular modes mainly result from the translational motions of individual molecules, and the rocking of the hydrogenbonded functional groups.

Theory Study on Structures and Vibrational Frequencies of Pyruvic acid

Density functional theory BLYP (using Becke's and Lee-Yang-Pars's correlation functionals), ab initio Hartree-Fock (HF) and hybrid DFT/HF B3LYP calculations were carried out to study the structure and vibrational spectra of pyruvic acid. The scaled B3LYP/6-31G* frequencies correspond well with available experimental assignment of the functional vibrational modes and the mean absolut devation is only 12.3cm^(-1).

Molecular structure and vibrational spectra of phenobaraitone by density functional theory and ab initio hartree-Fock calculations

Quantum chemistry calculations have been performed using Gaussian03 program to compute optimized geometry,harmonic vibrational frequency along with intensities in IR and Raman spectra at RHF/6-31++G^(**)and B3LYP/6-31++G^(**)levels for phenobarbitone(C12H12N2O3)in the ground state.The scaled harmonic vibrational frequencies have been compared with experimental FT-IR and FT-Raman spectra.Theoretical vibrational spectra of the title compound were interpreted by means of potential energy distributions(PEDs)using MOLVIB program.A detailed interpretation of the infrared spectra of the title compound is reported.On the basis of the agreement between the calculated and observed results,the assignments of fundamental vibrational modes of phenobarbitone were examined and some assignments were proposed.The theoretical spectrograms for FT-IR and FT-Raman spectra of the title compound have been constructed.

Theoretical Study on the Structure and Vibrational Spectra for 4-methyl-3-penten-2-one

Density functional theory BLYP (using Becke's and Lee-Yang-Parr's correlation functionals), nb initio Hartree-Fock (HF) and hybrid DFT/HF B3LYP calculations were carried out to study the structure and vibrational spectra of 4-methyl-3-penten-2-one. The BLYP/6-31G* and scaled HF/6-31G* frequencies correspond well with each other and with available experimental assignment of the functional vibrational modes.

Vibrational frequencies of hydrazoic acid and methyl azide: density functional theory study

Harmonic vibrational frequencies of HN3 and CH3N3 molecules and their several isotopomers are calculated using HF, MP2 and five popular density functional theory (DFT) methods. On the basis of the comparison between calculated and experimental results, assignments of fundamental vibrational modes are examined. HF and MP2 results are in bad agreement with experimental values. Of the five DFT methods, BLYP reproduces the observed fundamental frequencies the most satisfactorily. Two hybrid DFT methods are found to yield frequencies generally higher than the observed fundamental frequencies. The results indicate that BLYP calculation is a very promising approach for understanding the observed spectral features.

Study on lattice vibrational properties and Raman spectra of Bi_2Te_3 based on density-functional perturbation theory

We present a variational density-functional perturbation theory （DFPT） to investigate the lattice dynamics and vibra- tional properties of single crystal bismuth telluride material. The phonon dispersion curves and phonon density of states （DOS） of the material were obtained. The phonon dispersions are divided into two fields by a phonon gap. In the lower field, atomic vibrations of both Bi and Te contribute to the DOS. In the higher field, most contributions come from Te atoms. The calculated Born effective charges and dielectric constants reveal a great anisotropy in the crystal. The largest Born effective charge generates a significant dynamic charge transferring along the c axis. By DFPT calculation, the greatest LO-TO splitting takes place in the infrared phonon modes and reaches 1.7 THz in the Brillouin zone center. The Raman spectra and peaks corresponding to respective atomic vibration modes were found to be in good agreement with the experimental data.

Density Functional Study on the Vibrational Frequencies of Hydrazoic Acid and Methyl Azide

Harmonic vibrational frequencies of hydrazoic acid and methyl azide are calculated using HF. MP2 methods and five popular density functional (DFT) methods and compared with experimental results. Of these seven methods, BLYP reproduces the observed frequencies most satisfactorily, while the results in HF and MP2 are worse. These indicate the BLYP calculation is a very promising approach for understanding the observed spectral features.

Density functional study on chirospectra of hydrogen-bonded systems X^-(H_2O)_3(X=F,Cl,Br,I)

This paper calculates the molecular structures, infrared, Raman, circular dichroism spectra and optical rotatory powers of some hydrogen-bonded supramolecular systems as a cyclic water trimer, （H2O）3 and its pyramidal halide complexes, X- （H2O）3 （X= F, Cl, Br, I） with the gradient-corrected density functional theory method at the B3LYP/6- 311＋＋G（2d,2p） and B3LYP/Aug-cc-pVTZ levels. It finds that the complexation of halide anions with the water trimer can efficiently modulate the chirally optical behaviors. The calculated vibrational circular dichroism spectrum illuminates that the vibrational rotational strength of S（＋） （H2O）3 mostly originates from the O-H rocking modes, whereas chirality of S（-）-X-（H2O）3 （X = F, Cl, Br, I） has its important origin in the O-H stretching modes. The calculated optical rotatory power demonstrates that S（＋） （H2O）3 and S（＋）-F-（H2O）3 are positively chiral, whereas S（-）-X-（H2O）3 （X=Cl, Br, I） are negatively chiral. With the polarizable continuum model, calculated bulk solvent effect in the solvents water and carbontetrachloride and argon shows that the positive chirality of S（＋）-（H2O）3 is enhanced and the negative chirality of S（-）-X-（H2O）3 （X=Cl, Br, I） and the positive chirality of S（＋）-F-（H2O）3 are reduced with an augmentation of the solvent dielectric constant.

肉桂酸衍生物的太赫兹光谱及弱相互作用分析

利用太赫兹时域光谱技术(THz-TDS)测试了对羟基肉桂酸(PCA)、反式-2-羟基肉桂酸(OCA)和4-氟肉桂酸(4-FCA)三种肉桂酸的衍生物(CADs)在0.5~3.5 THz范围内的吸收峰。基于密度泛函理论(DFT),借助振动模式自动关联判定方法(VMARD)对三种样品THz吸收峰的来源进行了指认。最后采用分子力场能量分解法(EDAFF)分析了分子体系的弱相互作用力形式,通过VMD绘制原子着色图进行了可视化分析,并研究了其原子在分子体系中对弱相互作用力的贡献类型和强弱。研究结果表明,基于THz-TDS、DFT与VMARD、EDA-FF方法结合不但能有效地辨别同分异构体及结构近似的有机分子,而且也能为其生化功能揭示提供有价值的参考数据。

First-principles studies of the vibrational properties of amorphous carbon nitrides

Raman spectra of amorphous carbon nitride films （a-C：N） resemble those of typical amorphous carbon （a-C）, and no specific features in the spectra are shown due to N doping. The present work provides a correlation between the microstructure and vibrational properties of a-C：N films from first principles. The six periodic model structures of 64 atoms with various mass densities and nitrogen contents are generated by the liquid-quench method using Car-Parinello molecular dynamics. By using Raman coupling tensors calculated with the finite electric field method, Raman spectra are obtained. The calculated results show that the vibrations of C=N could directly contribute to the Raman spectrum. The similarity of the Raman line shapes of N-doped and N-free amorphous carbons is due to the overlapping of C=N and C=C vibration bands. In addition, the origin of characteristic Raman peaks is also given.

丙酮酸分子结构与振动光谱的密度泛函理论研究

用密度泛函方法ＢＬＹＰ、Ｂ３ＬＹＰ和从头算 Ｈａｒｔｒｅｅ－Ｆｏｃｋ （ＨＦ）方法在 ６－３１Ｇ基组水平 上对丙酮酸分子的几何结构（甲基的重叠式和交错式两种构象）和振动光谱分别进行了优化和计 算，并给出了各种频率所对应的红外强度及拉曼活性，对光谱进行了指认。结果表明：在丙酮酸分子 的两种构象中，重叠式比较稳定。Ｂ３ＬＹＰ计算得到的构型参数与实验结果比较一致；在振动频率的 计算中，ＢＬＹＰ标度力场所计算的非ＣＨ３伸缩振动基频预测值和实验值的平均绝对偏差为 １０．４ｃｍ－１；而ＨＦ标度力场的平均绝对偏差为１７．９ｃｍ－１。说明两者的结果与实验观测频率比较吻 合，但Ｂ３ＬＹＰ的频率计算值偏差（３８．３ ｃｍ－１）较大。根据振动频率的势能分布和红外光谱强度对此 分子的振动基频进行了理论归属。

去氢抗坏血酸分子振动光谱的理论研究

采用RHF,MP2,DFT(B3LYP)方法,以6-311++G**为基组研究了去氢抗坏血酸分子(DHA)的平衡几何构型和振动光谱.计算结果表明,采用RHF,B3LYP以及MP2方法优化得到的几何结构以及频率值是一致的.采用B3LYP/6-311++G**计算了DHA分子平衡构型下的谐振动力场、振动频率和振动强度.使用Wilson的GF矩阵方法对DHA分子进行了简正坐标分析,依据所得的势能分布对DHA分子的振动基频进行了合理的理论归属.

L-抗坏血酸的太赫兹波时域光谱研究

文章从理论分析和实验研究两方面探讨了L-抗坏血酸分子在0.2～2.4 THz波段的光谱特性。应用密度泛函理论的Becke-3-Lee-Yang-Parr（B3LYP）方法计算了L-抗坏血酸分子在THz波段的振动吸收谱，同时利用THz时域光谱系统（THz-TDS）测得了L-抗坏血酸在此波段的吸收谱和折射率谱。根据理论计算结果，借助于Gaussian View软件对L-抗坏血酸的THz吸收谱进行了指认。最后将L-抗坏血酸分子和市售维生素C药片的THz吸收谱进行了比较。研究表明，L-抗坏血酸分子在THz波段存在吸收峰，理论计算结果与实验结果符合较好，且这些吸收是由分子内和分子间振动共同引起的。与市售维生素C药片的吸收谱比较表明，L-抗坏血酸的吸收峰在维生素C药片的太赫兹吸收谱中均有反映。研究结果有助于进一步理解L-抗坏血酸的生物作用，为应用THz-TDS技术进行L-抗坏血酸的定量分析提供了依据。

苯分子振动光谱的密度泛函理论研究

用 5种常用密度泛函理论法 ,MP2和HF两种方法计算苯分子的谐振频率 ,同实验结果进行了比较 .HF、MP2两种方法的结果同实验数值相差很大 .在 5种DFT法中 ,BLYP得到结果最好 ,再现了实验观测的频率 .在确定可测光谱的研究上 ,表明了BLYP计算是一个很有前途的方法 .

三聚氰胺结构和热力学性质的密度泛函理论研究

采用Materials Studio软件中的DMol3模块对三聚氰胺的结构和性质进行了理论研究.得到了分子的几何构型、振动频率、各原子上的电荷分布、热力学、以及Fukui指数和前线分子轨道.计算结果表明:三聚氰胺分子中的C原子易得电子,是亲核试剂进攻点,而胺基上的N原子因含孤对电子是亲电反应中心.

基于密度泛函理论的菲分子结构与光谱研究

选用密度泛函理论(DET)中的B3LYP方法,在6-311++G(d,p)下对菲分子结构进行优化,计算了其振动频率、极化率及热力学参数,对比了菲分子实测光谱图,首次对其振动频率进行了完全归属。此外,分析并讨论了其前线分子轨道、分子静电势和密立根布局,获得了HOMO-LUMO能隙、分子静电势分布、原子电荷分布等与分子性质密切相关的重要数据,为后续其他多环芳烃分子的光谱检测技术及其光谱和电子结构的分析提供了理论基础。

尿素分子振动光谱的密度泛函理论研究

用密度泛函理论方法 BLYP/6-31 1 ++G( 2 df ,2 pd)对尿素分子的平衡几何构型进行了优化 ,并计算了该分子的谐力场 .使用 Wilson的 GF矩阵方法 ,对尿素分子的振动基频进行了理论研究 .根据振动频率的势能分布对此分子的振动基频进行了理论归属 。

槲皮素分子结构和振动光谱的理论研究

本文采用密度泛函理论(DFT)方法研究了槲皮素分子的平衡几何构型和振动光谱。在B3LYP水平上,以6-311++G**为基组计算了其平衡构型下的谐振动力场﹑振动频率和振动强度。使用Wilson的GF矩阵方法对其进行了简正坐标分析,依据所得的势能分布(PEDs)对槲皮素分子的振动基频进行了合理的理论归属。

1-乙基-3-甲基咪唑阳离子与天冬酰胺阴离子的相互作用

利用密度泛函理论B3LYP方法,在6-311+G(d,p)水平上,对1-乙基-3-甲基咪唑阳离子[Emim]+与天冬酰胺阴离子[Asn]-形成的氨基酸离子液体气态阴阳离子对([Emim][Asn])进行理论研究.通过几何结构优化和频率分析得到势能面上的五个稳定构型.[Emim]+和[Asn]-之间能够形成较强的氢键相互作用,零点能校正后的能量在-373.96至-326.28kJ·mol-1之间.其稳定化能主要来源于[Asn]-中羰基O的孤对电子lp(O)与[Emim]+中C—H反键轨道σ*(C—H)之间的相互作用:lp(O)→σ*(C—H).红外光谱特征和自然布居分析(NPA)计算表明咪唑阳离子中参与形成氢键的C—H键振动的红移值、阴阳离子间的电荷转移与氢键相互作用能成正比关系.分子中的原子(AIM)理论分析得到[Emim]+和[Asn]-之间的氢键相互作用以静电作用为主.通过计算结果初步探讨影响氨基酸离子液体玻璃化温度Tg的结构因素.