电子亲和势法判断负离子聚合单体的活性

提出了利用不同取代基烯类单体的电子亲和势来判断其负离子聚合反应活性的方法.采用密度泛函理论的B3LYP/6-31G(d)方法优化了不同取代基烯类单体几何构型,在B3LYP/6-311++G(3df,2p)水平上计算了其电子亲和势.通过电子亲和势计算值与文献报道实验数据比较,表明本文采用的计算方法是比较可靠的.结合不同取代基烯类单体的电子亲和势的计算结果,通过与Q-e关系及取代基常数σ数据进行比较表明,电子亲和势可以用来判断不同单体负离子聚合反应的活性高低.     

C6FmH6－m (m＝1～6)结构及芳香性的理论研究

运用密度泛函理论方法B3LYP, 选取6-31G(d,p)和6-31G(d)两种基组对C₆F_mH_{6－ m}(m＝1～6)进行了几何优化, 并对优化结构运用B3LYP/6-31G(d,p)方法进行了键能计算, 选用B3LYP-GIAO/6-31＋＋G(d,p)方法进行核无关化学位移(Nucleus-Independent Chemical Shifts, NICS)的计算. 研究表明, 所研究的氟代苯的基态均呈平面几何结构, 6-31G(d,p)基组计算的键长、键角的结果与实验值更加吻合, 其芳香性都较苯的大, 且随取代F数目的增加而增大. 用NBO对分子总NICS及各键对NICS的贡献进行了分解, 结果显示, 氟的p_z孤对电子参与六元环π键的形成是使氟代苯分子芳香性变大的主要原因.     

2-(2-巯苯基)苯并噁唑分子内质子转移取代基电子效应的密度泛函理论研究

在B3LYP/6-31G(d,p)和TDB3LYP/6-31++G(d,p)//CIS/6-31G(d,p)水平上研究了2-(2-巯苯基)苯并噁唑及其衍生物基态和激发态分子内质子转移现象,并探讨取代基电子效应对分子内质子转移的影响,研究结果表明,在基态时,硫醇式异构体为优势构象,供电子取代基使基态分子内正向质子转移能垒(烯醇式→酮式)升高;而吸电子取代基则可降低能垒,有利于基态分子内质子转移并有助于硫酮式异构体的稳定.在激发态时,硫酮式结构为优势构象,所研究的2-(2-巯苯基)苯并噁唑化合物及衍生物均可以发生无能垒或低能垒(≤1.5kJ/mol)的激发态分子内质子转移.巯苯基部分是激发态失活的主要活性部分,供电子基团有利于激发态的质子转移,吸电子基团使激发态跃迁困难,不利于激发态的质子转移.     

氟代乙烯阳离子的理论研究

用B3LYP和MP2方法及6-31G(d, p)、6-31＋G(d, p)、6-311G(d, p)和6-311＋G(d, p) 基组，对六种氟代乙烯阳离子做了理论研究，优化了它们的基电子态的结构，计算了对应分子的垂直电离势（VIP）和绝热电离势(AIP).结果表明，与具有非平面结构的乙烯阳离子不同，六种氟代乙烯阳离子都只具有平面结构；与分子结构相比，离子结构的C－C键增长， C－F键缩短， CCF键角变小. 自然布居分析计算表明，这些离子的正电荷主要分布在与F原子相连的C原子和各H原子上. B3LYP/6-311＋G(d, p) 级别上计算的各分子的VIP和AIP值和实验值符合得很好. 使用含弥散基函数的基集可以明显提高这类分子的电离势的计算精度.     

含氮药物分子中氮a-位碳氢键离解能的理论研究

本文利用G3B3 和CBS-Q高精度理论方法检验了一系列胺类有机化合物中α-碳氢键离解能的实验测量值,在此基础上筛选出(U)BHandH/6-311++G(2df, 2p)//(U)B3LYP/6-31G(d)方法,发现其可以准确快速的预测氮α-碳氢键离解能。运用该方法研究了若干含氮药物分子,发现氮α-碳氢键离解能随药物分子结构发生明显变化。为了阐明其变化规律,系统研究单取代和双取代基效应,并解释了不同取代基效应的来源。     

OH-与CH2CIF反应的阴离子产物通道

理论研究了羟基负离子(OH-)与氟氯代甲烷(CH2CIF)反应的阴离子产物通道.分别在B3LYP/6-31+G(d,p)和B3LYP/6-311++G(2d,p)水平上得到反应势能面上各关键物种的优化构型,进而计算得到谐振频率和零点能.基于CCSD(T)/6-311+G(3df,3dp)水平的相对能量,描述了由质子转移和双分子亲核取代(SN2)过程生成各阴离子产物的途径.各阴离子产物途径势垒的计算结果表明质子转移过程是实验中的主要产物通道,与以往实验测量的结论相符.此外,计算还显示双分子亲核取代过程得到了非典型的阴离子产物,其中动力学效应可能会导致F-的生成.     

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.     

取代基对3(5)-(9-蒽基)吡唑光学性质的影响

使用密度泛函理论(DFT)B3LYP/6-31G(d)方法优化得到了3(5)-(9-蒽基)吡唑及其衍生物的基态(S₀)分子结构, 使用单激发组态相互作用(CIS)/6-31G(d)方法优化得到这些分子的第一单重激发态(S₁)的几何结构, 并使用含时密度泛函理论(TD-DFT)B3LYP/6-311++G(d,p)方法计算了它们的吸收和发射光谱. 计算结果表明, 与3(5)-(9-蒽基)吡唑相比, 无论取代基是吸电子基团还是供电子基团, 衍生物的吸收和发射峰均发生红移, 并且当取代基―R=―BH₂, ―CCl₃, ―CHO, ―NH₂时衍生物有较长的吸收波长和发射波长.     

取代基对醌的结构及电子转移能力的影响

用半经验的AM1,abinitio(HF/3-21G,HF/6-31G^*),及密度泛函[B3LYP/6-31G^*,B3LYP/6-311G(d,p)]方法对1,4-苯酯及其阴离子的结构与性质进行了研究。在此基础上用AM1及BLYP/6-31G(d,p)方法对泛醌Q~0,用AM1方法对泛醌Q~1,泛醌Q~2及其阴离子的结构与性质进行了研究。研究结果表明,由于取代基的影响,泛醌的环平面发生了扭曲,环上原子不再位于同一平面上。结构的变化导致了电子分布发生了变化,并由此导致了偶极矩随6位上取代基的增大而增大。随着6位支链的增长,泛醌的电离势逐渐减小而电子亲和势逐渐增大,泛醌传递电子的能力增强。在上述研究的基础上,对1,4-苯醌及泛醌的自交换电子转移反应过程进行了过程,计算了反应的内重组能,溶剂重组能及电子转移速率常数。结果表明内重组能的大小与取代基的长短无直接关系,而与得电子前后醌的结构变化程度密切相关。除高频振动的变化外,低频振动的变化对内重组能也有显著的贡献。计算所得1,4-苯醌自交换电子转移反应速率常数与实验值相符得很好。     

OH-与CH2ClF反应的阴离子产物通道

理论研究了羟基负离子(OH^-)与氟氯代甲烷(CH₂ClF)反应的阴离子产物通道. 分别在B3LYP/6-31+G(d,p)和B3LYP/6-311++G(2d,p)水平上得到反应势能面上各关键物种的优化构型, 进而计算得到谐振频率和零点能. 基于CCSD(T)/6-311+G(3df,3dp)水平的相对能量, 描述了由质子转移和双分子亲核取代(S_N2)过程生成各阴离子产物的途径. 各阴离子产物途径势垒的计算结果表明质子转移过程是实验中的主要产物通道, 与以往实验测量的结论相符. 此外, 计算还显示双分子亲核取代过程得到了非典型的阴离子产物, 其中动力学效应可能会导致F^-的生成.     

Imidoylketene-alpha-oxoketenimine and alpha-oxoketene-alpha-oxoketene rearrangements. 1,3-shifts of substituted phenyl groups

1,3-phenyl shifts interconvert imidoylketenes 1 and alpha-oxoketenimines 2 and, likewise, alpha-oxoketenes 3 automerize by this 1,3-shift. These rearrangements usually take place in the gas phase under conditions of flash vacuum thermolysis. Energy profiles calculated at the B3LYP/6-31G(d,p) and B3LYP/6311 + G(3df,2p)//B3LYP/6-31G(d,p) levels demonstrate that electron donating substituents (D) in the migrating phenyl group and electron withdrawing ones (W) in the non-migrating phenyl group, can stabilise the transition states TS1 and TS2 to the extent that activation barriers of ca. 100 kJ mol(-1) or less are obtained; i.e. enough to make these reactions potentially observable in solution at ordinary temperatures. The calculated transition state energies DeltaG(TS1) show an excellent correlation with the Hammett constants sigma(p)(W) and sigma(p+)(D).     

The reaction mechanism of the gas‐phase thermal decomposition kinetics of neopentyl halides: A DFT study

The kinetics and mechanisms of the gas‐phase elimination reactions of neopentyl chloride and neopentyl bromide have been studied by means of electronic structure calculations using density functional methods: B3LYP/6‐31G(d,p), B3LYP/ 6‐31++G(d,p), MPW1PW91/6‐31G(d,p), MPW1PW91/6‐31++G(d,p), PBEPBE/6‐31G(d,p), PBEPBE /6‐31++G(d,p). The reaction channels that account in products formation have a common first step involving a Wagner‐Meerwein rearrangement. The migration of the halide from the terminal carbon to the more substituted carbon is followed by beta‐elimination of HCl or HBr to give two olefins: the Sayzeff and Hoffmann products. Theoretical calculations demonstrated that these eliminations proceed through concerted asynchronous process. The transition state (TS) located for the rate‐determining step shows the halide detached and bridging between the terminal carbon and the quaternary carbon, while the methyl group is also migrating in a concerted fashion. The TS is described as an intimate ion‐pair with a large negative charge at the halide atom. The concerted migration of methyl group provides stabilization of the TS by delocalizing the electron density between the terminal carbon and the quaternary carbon. The B3LYP/6‐31++G(d,p) allows to obtain reasonable energies and enthalpies of activation. The nature of these reactions is examined in terms of geometrical parameters, electron distribution, and bond order analysis. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Solid Phase Derivatizations in HPLC: Borohydride/Silica Reductions for Carbonyl Compounds

Abstract

Sodium borohydride adsorbed onto silica gel has now been utilized for on-line, pre-and post-analytical column chemical derivatizations via reductions of various organic carbonyl compounds. These on-line reactions have been performed using normal phase HPLC conditions, involving conventional silica gel packings, organic mobile phases, and commercially available HPLC equipment and instrumentation. This approach for on-line HPLC derivatizations has been evaluated for a large number of organic carbonyl compounds, at a variety of temperatures for aldehydes and ketones. The overall rates of such carbonyl reductions via sodium borohydride/silica are sufficiently different as a function of temperature of the reaction to permit for compound/class identifications. Analyte identification can be both qualitative and quantitative, even wherein an analyte co-elutes with a non-carbonyl compound. In-house prepared borohydride/silica gel reactors can be characterized via standard iodine titration procedures along with inductively coupled plasma (ICP) elemental boron analysis. These normal phase derivatization approaches have been applied to certain standard vitamins, and to cinnamaldehyde found in a commercial spice product. Such approaches to on-line HPLC derivatizations     

Combined computational and experimental studies of the mechanism and scope of the retro-Nazarov reaction

Density functional theory calculations (B3LYP/6-31+G) demonstrate that conjugating and electron-donating substituents at carbons three and four of a cyclopentenyl oxyallylic cation should have a rate-accelerating effect on the retro-Nazarov reactions of these species. The retro-Nazarov reaction of these intermediates is predicted to exhibit significant torquoselectivity when carbon three is substituted with a methoxy and a methyl group. Experimental studies show that oxyallylic cations can undergo effective retro-Nazarov reactions when two alkyl and one aryl/vinyl groups are on carbons three and four. An equal number of alkyl substituents or a single aryl substituent is not effective in promoting the reaction. Interestingly, a single alkoxy substituent at carbon three is sufficient for the retro-Nazarov reaction to occur. The methodology developed was used in a total synthesis of the natural product turmerone.     

Regioselective Reduction of 1H-1,2,3-Triazole Diesters

Regioselective reactions can play pivotal roles in synthetic organic chemistry. The reduction of several 1-substituted 1,2,3-triazole 4,5-diesters by sodium borohydride has been found to be regioselective, with the C(5) ester groups being more reactive towards reduction than the C(4) ester groups. The amount of sodium borohydride and reaction time required for reduction varied greatly depending on the N(1)-substituent. The presence of a β-hydroxyl group on the N(1)-substituent was seen to have a rate enhancing effect on the reduction of the C(5) ester group. The regioselective reduction was attributed to the lower electron densities of the C(5) and the C(5) ester carbonyl carbon of the 1,2,3-triazole, which were further lowered in cases involving intramolecular hydrogen bonding.     

Dynamic mechanical thermal analysis transitions of partially and fully substituted cellulose fatty esters

The main transitions of cellulose fatty esters with different degrees of substitution (DSs) were investigated with dynamic mechanical thermal analysis. Two distinct main relaxations were observed in partially substituted cellulose esters (PSCEs). They were attributed to the glass‐transition temperature and to the chain local motion of the aliphatic substituents. The temperatures of both transitions decreased when DS or the number of carbon atoms (n) of the acyl substituent increased. Conversely, all the transitions of fully substituted cellulose esters occurred within a narrow temperature range, and they did not vary significantly with n. This phenomenon was explained by the formation of a crystalline phase of the fatty substituents. The presence of few residual OH groups in PSCEs was responsible for a large increase in the storage bending modulus, and it eliminated the effect of n on damping. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 281–288, 2003     

Theoretical studies on the elimination of hydrogen fluoride from alkyl fluoride and its substituent effect

The reaction mechanism of the α, α and α, β elimination of hydrogen fluorides from alkyl fluorides has been studied theoretically. For fluoroethane as a reactant, the transition state (TS) optimized at the level of the 6-31G** basis set shows that the α, β elimination proceeds via a four membered-ring TS with a barrier height 64.6 kcal/mol, while the α, α elimination, via a three-membered ring TS with a 83.7 kcal/mol barrier. Four substituents, CH₃, CN, F, and NH₂, were used to investigate the substituent effect of elimination by using the 3-21G basis set. The calculated barriers show that NH₂-substituted alkyl fluorides favor both the α, α and α, β elimination and these two reactions would be expected to proceed simultaneously. © 1996 John Wiley & Sons, Inc.     

Catalyzing the Erlenmeyer Plöchl reaction: organic bases versus sodium acetate

An evaluation of organic base versus sodium acetate as the base in the Erlenmeyer Plöchl reaction was performed. Aldehyde substituents play an important role in these reactions. Organic bases afford fast reactions, but other side reactions were observed. Alternatively, in the presence of solvent, sodium acetate could be used in catalytic amounts rather than the typical stoichiometric quantities. This Letter reports the comparative results for the use of organic bases versus sodium acetate in Erlenmeyer Plöchl reactions.     

Substituent Effects on Site Selectivity (C=C vs C=N) in Heterocumulene-Heterodiene [4 + 2] Cycloadditions: Density Functional and Semiempirical AM1 Molecular Orbital Calculations

The effect of substituents on the site selectivity (C=C vs C=N) in the [4 + 2] cycloaddition between heterocumulenes (ketene imines) 2a-g with heterodienes (acroleines 9a-n and 4-acylfuran-2,3-diones 1a-d) is treated by semiempirical AM1 molecular orbital and density functional calculations using Becke's three-parameter hybrid method (B3LYP/6-31G). For some reactions calculations were also done at the B3LYP/6-31+G level of theory. For reaction of the oxa 1,3-dienes with ketene imines unsubstituted at the terminal carbon invariably addition across the C=C heterocumulene double bond has a lower activation energy than addition across the C=N double bond. Substitution of methyl or especially phenyl groups at the ketene imine C-terminus leads to a reversal of the respective activation energies. Incorporation of the oxa 1,3-diene system into the heterocyclic dione 1 substantially enhances the reactivity ( approximately 10 kcal mol(-1) lower activation energies) as compared to similarly substituted acroleins. At the DFT level of theory all reactions are found to proceed via a concerted asynchronous mechanism.     

Kinetics of the reaction of sodium arylthiolates with nitro-carboxybenzyl halide derivatives

The rate constants for the reactions of 4-halomethyl-3-nitrobenzoic acids, the nonnitro derivatives, and their ethyl esters with arylthiolates were measured at different temperatures. It was found that the retardation in rate constants compared to benzyl halides is due to the electrostatic repulsion between the electronegative substituents (COO⁻ and/or NO₂) in the substrates and thiolate ions. Good correlations between log k₂ values of the acids and carbon basicities of thiolates were found while log k₂ values of the esters show good straight lines with Hammett σ constants, pk_a, and carbon basicities of arylthiolates. © 1996 John Wiley & Sons Inc