A3型Corrole中位取代基对其β位^1H-NMR的影响

采用密度泛函(DFT)方法对八个中位有不同取代基的A3型corrole进行了几何结构优化和核磁共振(NMR)理论计算.几何优化结果显示5,10,15-三苯基corrole的两个环内NH异构体的中位苯环空间排布方向不同.此外,虽然A3型corrole两个环内NH异构体A和B的能量相差不大,但A和B分子的Boltzman分布概率却有较大的不同,且受取代基影响很大. 因而在计算NMR时必须对A和B异构体的理论化学位移做Boltzman统计平均处理.NMR计算结果表明,在B3LYP/6.311+G(2d,p)//B3LYP/6-31G(d,p)水平下可以得到较为合理的corrole的1H-NMR化学位移结果.β-位氢的化学位移与取代基Hammett常数σ-p的大小成正比关系.此外,由于corrole大环的低对称性,取代基对不同位置β-位氢的NMR影响程度不同,其β-位氢化学位移的大小和顺序与中位取代基的电子效应和corrole的立体结构因素有关.     

A3型Corrole中位取代基对其β位1H-NMR的影响

采用密度泛函(DFT)方法对八个中位有不同取代基的A3型corrole进行了几何结构优化和核磁共振(NMR)理论计算. 几何优化结果显示5,10,15-三苯基corrole的两个环内NH异构体的中位苯环空间排布方向不同. 此外, 虽然A3型corrole两个环内NH异构体A和B的能量相差不大, 但A和B分子的Boltzman分布概率却有较大的不同, 且受取代基影响很大. 因而在计算NMR时必须对A和B异构体的理论化学位移做Boltzman统计平均处理. NMR计算结果表明, 在B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d,p)水平下可以得到较为合理的corrole的1H-NMR化学位移结果. β-位氢的化学位移与取代基Hammett常数σ+P的大小成正比关系. 此外, 由于corrole大环的低对称性, 取代基对不同位置β-位氢的NMR影响程度不同, 其β-位氢化学位移的大小和顺序与中位取代基的电子效应和corrole的立体结构因素有关.     

紫杉醇的核磁共振谱及其分子几何构型的从头算研究

应用规范不变原子轨道GIAO法在HF/6-31G水平计算了MM3、PM3和HF/STO-3G三种最优分子构型下紫杉醇的δ1H-NMR和δ13C-NMR.对NMR化学位移计算值与实验值的对比和误差的统计和分析表明,采用HF/STO-3G优化的几何构型计算得到的化学位移误差最小,因此这一构型与实际分子最为接近.从理论构型得到4-OAc与3′-H的距离为0.2663nm,4-OAc与3′-Ph的距离为0.3104nm,4-OAc与2-OCOPh的距离为0.3707nm,支持了Williams等关于紫杉醇分子内存在NOE(nuclearoverhausereffect)效应及Velde等关于在极性溶剂中4-OAc与3′-Ph、2-OCOPh形成分子内疏水簇的观点.     

紫杉醇的核磁共振谱及其分子几何构型的从头算研究

应用规范不变原子轨道GIAO法在HF/6 31G水平计算了MM3、PM3和HF/STO 3G三种最优分子构型下紫杉醇的δ 1H NMR和δ 13C NMR.对NMR化学位移计算值与实验值的对比和误差的统计和分析表明,采用HF/STO 3G优化的几何构型计算得到的化学位移误差最小，因此这一构型与实际分子最为接近.从理论构型得到4 OAc与3′ H的距离为0.266 3 nm，4 OAc与3′ Ph的距离为0.310 4 nm，4 OAc与2 OCOPh的距离为0.370 7nm，支持了Williams等关于紫杉醇分子内存在NOE (nuclear overhauser effect)效应及Velde等关于在极性溶剂中4 OAc与3′ Ph、2 OCOPh形成分子内疏水簇的观点.     

活泼氢的核磁共振氢谱

核磁共振氢谱是有机化合物结构表征中最常使用的波谱方法之一,提供了有机化合物质子的化学位移、积分面积和耦合裂分等信息。常见的活泼氢是与氧、氮和硫共价相连的氢原子,存在着快速交换机制,与碳上的氢有显著的差异。在不同条件下活泼氢化学位移不固定、峰形多变并且耦合裂分情况复杂。本文探讨了如何通过核磁共振氢谱解析活泼氢,培养学生对谱图进行观察、分析以及结构推导的能力。     

聚丁二烯的核磁共振二维谱研究

<正> 聚丁二烯分子中有三种微观结构,顺式-1,4,反式-1,4和-1,2。这三种结构单元的含量和序列长度对聚丁二烯的物性有很大影响。异构单元的含量可用IR和氢谱测定。序列分布可用氢谱和碳谱测定。已经有许多工作致力于聚丁二烯碳谱的谱图解析。因为各作者使用不同的经验式或半经验式,统计方法也不一样,结论不尽相同。最近Sato等     

核磁共振法测定高聚物的结构—核磁共振二维谱

介绍了核磁共振(NMR)波谱,尤其是核磁共振二维谱(2D NMR)在最近几年内的进展。内容包括怎样用NMR测定高聚物的序列分布、共聚物结构、聚合物的混溶性、聚合物液晶的取向度、分子量、短链度和长链支化度、凝聚态高聚物的多相结构等。     

聚醚型化合物的核磁共振碳谱解析

论述了以环氧乙烷、环氧丙烷为代表的聚醚型化合物的核磁共振碳谱特点及其立构规整性特征，并通过对主链碳谱和末端基碳谱的解析，讨论了聚合反应机理和聚合物组成。     

6α和6β-羟基雄酮的电子光谱和核磁共振谱的理论研究

在B3LYP/6-31G(d)水平下,优化6α和6β-羟基雄酮的几何构型.用TD-B3LYP/6-31G(d)方法,计算它们的电子吸收光谱;采用规范不变原子轨道GIAO法,计算它们的1H-NMR和13C-NMR化学位移值.结果表明,6β-羟基雄酮比6α-羟基雄酮稳定;6α-羟基雄酮的最长吸收波长比6β-羟基雄酮的要长;6α和6β-羟基雄酮的部分原子的NMR值具有明显差别.     

Theoretical DFT and experimental Raman and NMR studies on thiophene, 3-methylthiophene and selenophene

The results of extended MO calculations using density functional theory (DFT) approximation supported by experimental Raman, ¹H and ¹³C NMR studies on thiophene are reported. Raman spectra of liquid thiophene were re-examined and the performance of a hybrid B3PW91 density functional was compared with the ab initio restricted Hartree–Fock (RHF) method. With the basis sets of the 6-311++G^** quality, the DFT calculated bond lengths, dipole moments and harmonic vibrations were predicted in a very good agreement with available experimental data.

Additionally, the results on thiophene were extended by calculations on 3-methylthiophene and selenophene. In this case, a significant change in geometry and charge distribution in thiophene ring due to a methyl group substituent or replacement of sulphur by selene atom was observed.

A linear correlation between the predicted harmonic vibrational frequencies (scaled using SQM method) and experimental ones for thiophene, selenophene and 3-methylthiophene was shown. The theoretically calculated spectra have satisfactorily reproduced the available experimental spectra for thiophene and selenophene.     

Quantum mechanical calculations of conformationally relevant 1H and 13C NMR chemical shifts of N-, O-, and S-substituted calixarene systems

QM GIAO calculations of (13)C and (1)H chemical shift values of the ArCH(2)Ar group in N-, O-, and S-substituted calixarene systems were performed with a hybrid DFT functional MPW1PW91 and 6-31G(d,p) basis set. A good reproduction of experimental data was obtained for some representative calixarenes and for a series of simplified calixarene models. This allowed the derivation of chemical shift surfaces versus phi and chi dihedral angles. The applicability of chemical shift surfaces in the study of calixarene conformational features is illustrated.     

A theoretical investigation on the isomerism and the NMR properties of thiosemicarbazones

Hybrid density functional theory calculations at the mPW1PW91/6-31+G(d,p) level of theory have been used to investigate the optimized structures and other molecular properties of five different series of thiosemicarbazones. The investigated compounds were obtained from acenaphthenequinone, isatin and its derivatives, and alloxan. The focus of the study is the isomerism and the NMR characterization of these thiosemicarbazones. It was found that only one isomer is expected for thiosemicarbazones and methylthiosemicarbazones, while for dimethylthiosemicarbazones, two isomers are possible. All investigated thiosemicarbazones exhibit a hydrazinic proton that is highly deshielded and resonates far downfield in the proton NMR spectra. This proton is a part of a characteristic sixmembered ring, and its NMR properties are a result of its strong, intermolecular hydrogen bond. The relationships between the calculated ¹H and ¹³C NMR chemical shifts and various geometric parameters are reported.      

Solid‐State NMR and DFT Combined for the Surface Study of Functionalized Silicon Nanoparticles

Silicon nanoparticles (NPs) serve a wide range of optical, electronic, and biological applications. Chemical grafting of various molecules to Si NPs can help to passivate their reactive surfaces, “fine‐tune” their properties, or even give them further interesting features. In this work, ¹H, ¹³C, and ²⁹Si solid‐state NMR spectroscopy has been combined with density functional theory calculations to study the surface chemistry of hydride‐terminated and alkyl‐functionalized Si NPs. This combination of techniques yields assignments for the observed chemical shifts, including the contributions resulting from different surface planes, and highlights the presence of physisorbed water. Resonances from near‐surface ¹³C nuclei were shown to be substantially broadened due to surface disorder and it is demonstrated that in an ambient environment hydride‐terminated Si NPs undergo fast back‐bond oxidation, whereas long‐chain alkyl‐functionalized Si NPs undergo slow oxidation. Furthermore, the combination of NMR spectroscopy and DFT calculations showed that the employed hydrosilylation reaction involves anti‐Markovnikov addition of the 1‐alkene to the surface of the Si NPs.     

喹唑啉酮互变异构的理论计算

采用密度泛函理论B3LYP/6-311+G(d,p)方法,计算并考察了喹唑啉酮进行结构互变的质子迁移过程的两种可能途径:(a)分子内质子迁移,(b)水助质子迁移.结果表明,途经b所需要的能垒小,氢键在降低反应能垒方面起重要作用.     

Solvent optimization and conformational flexibility effects on 1H and 13C NMR scaling factors

The effects of including (a) implicit solvent in geometry optimizations, (b) conformationally flexible molecules in test sets, and (c) empirical dispersion D3(BJ) on scaling factors for predicting ¹H and ¹³C NMR chemical shifts were explored. Scaling factors with optimizations performed in the gas phase and with a Polarizable Continuum Model (PCM) solvent model were obtained for 12 organic solvents, including 2,2,2-trifluroethanol and chlorobenzene, for which scaling factors have been developed for the first time. Scaling factors for aromatic solvents were split into primary and secondary scaling factors to account for CH–π effects. Including empirical dispersion D3(BJ) did not lead to significant improvement.     

2-巯基吡啶质子迁移过程的理论研究

采用密度泛函理论,在B3LYP/6-311G**基组水平上,计算并考察了2-巯基吡啶分子硫醇式结构和硫酮式结构进行结构互变质子迁移过程中的4种可能途径:(a)分子内质子迁移,(b)水助质子迁移,(c)同种二聚体双质子迁移和(d)异种二聚体间双质子迁移.计算结果表明,途经c所需要的活化能最小(9.73 kJ.mol-1,逆反应则为55.28 kJ.mol-1),而过程a所需要的活化能最大(106.02 kJ.mol-1),途径b和d的活化能居中间(分别为32.05和15.91 kJ.mol-1).研究还表明,氢键在降低反应活化能方面起着重要的作用.     

双环-HMX结构和性质的理论研究

在DFT-B3LYP/6-311G*水平上, 计算研究了高能化合物四硝基四氮杂双环辛烷(双环-HMX) α和β两种异构体的结构和性质. 比较分子对称性、分子内氢键和环张力等几何参数以及分子总能量和前线轨道能级等电子结构参数, 发现α比β稳定. 分子中N—N键较长, N—N键集居数较小, 预示该键为热解和起爆的引发键. 基于简谐振动分析求得IR谱频率和强度. 运用统计热力学方法求得200～1000 K温度的热力学性质. 以非限制性半经验PM3方法探讨其热解机理, 求得各反应通道的过渡态和活化能, 发现热解始于侧链N—NO₂键的均裂. 还从理论上预测了该化合物的密度、爆速和爆压, 有助于寻求高能量密度材料(HEDM).     

HCN消除反应机理的理论研究

采用密度泛函理论方法从HCN氧化和水解两个方面研究了HCN消除反应机理,并考虑了HCN的直接消除反应(途径Ⅰ和途径Ⅱ)和CuO上的HCN消除反应(途径Ⅲ和途径Ⅳ)。途径Ⅰ为HCN与2个O2分子生成CO2、NO和H原子;途径Ⅱ为HCN与1个O2分子和1个H2O分子生成 CO2和NH3;途径Ⅲ为CuO上HNCO水解为CO2和NH3;途径Ⅳ为CuO上HCN水解为CO和NH3。研究发现,途径III速控步骤的活化自由能垒为157.32 kJ/mol,比途径Ⅱ中HNCO水解降低12.34 kJ/mol;比途径Ⅳ降低了63.8 kJ/mol。可见,HNCO是HCN净化过程中的重要中间体,CuO的加入降低了反应能垒,促进了HCN消除。     

Theoretical and experimental 13C and 15N NMR investigation of guanylhydrazones in solution

Experimental and theoretical ¹⁵N and ¹³C NMR data for the three nitrobenzaldehyde guanylhydrazones are reported. The theoretical data were obtained using sequential molecular dynamics/quantum mechanics methodology for the calculation of flexible molecules in a condensed phase, followed by the use of the GIAO/DFT method with the 6–311G** basis set. The experimental ¹⁵N chemical shifts for the guanylhydrazones are compared with the calculated shifts. Copyright © 2003 John Wiley & Sons, Ltd.