取代基效应对褐煤模型化合物离解焓影响的理论研究

为了探究褐煤热解过程中氧桥键C-O均裂这一重要反应, 选取α-O-4和β-O-4类结构单元作为褐煤模型化合物, 运用不同密度泛函计算了部分模型化合物中C-O的离解焓, 并以CBS-QB3作为理论基准值进行比较, 最后选取M05-2X进行离解焓计算. 结果显示, 对于选定的α-O-4和β-O-4类模型化合物, 其平均离解焓分别为51.0 kcal/mol和66.1 kcal/mol. 周围取代环境能显著影响C-O离解焓, 芳环上存在给电子基团(OH, OCH₃和CH₃)能降低C-O离解焓, 而吸电子基团COOH则能增加其离解焓. 然后深层次分析了取代基效应对C-O离解焓的影响. 此外, 分子内氢键的形成对离解焓也有很大的影响. C-O的离解焓与其键长没有特定的相关性, 不能简单的通过C-O键长来预测其离解焓.     

木质素二聚体模型化合物热解机理的量子化学研究

β-O-4连接是木质素主体结构单元之间的主要联结方式。采用密度泛函理论方法B3LYP,在6-31G (d, p)基组水平上,对β-O-4型木质素二聚体模型化合物(1-愈创木基-2-(2-甲氧基苯氧基)-1,3丙二醇)的热解反应机理进行了研究。提出了三种热解反应途径:C_β-O键均裂的后续反应、C_α-C_β键均裂的后续反应以及协同反应。对各种反应的反应物、产物、中间体和过渡态的结构进行了能量梯度全优化,计算了各热解反应途径的标准动力学参数。分析了各种主要热解产物的形成演化机理以及热解过程中温度对热解机理的影响。计算结果表明,C_β-O键的均裂反应和协同反应路径(1)和(3)是木质素二聚体热解过程中主要的反应路径,而C_α-C_β键的均裂反应和协同反应路径(2)和(5)是主要的竞争反应路径;热解的主要产物是酚类化合物如愈创木酚、1-愈创木基-3-羟基丙酮、3-愈创木基-3-羟基丙醛、愈创木基甲醛和乙烯等。在热解低温阶段协同反应是热解过程中的主要反应形式,而在高温阶段自由基均裂反应是热解过程的主要反应形式。     

木质素模化物紫丁香酚热解机理的量子化学研究

采用密度泛函理论方法B3LYP/6-31G++(d,p),对木质素模化物紫丁香酚的热解反应机理进行了量子化学理论研究。提出了三种可能的热解反应途径,对各种反应的反应物、产物、中间体和过渡态的结构进行了能量梯度全优化。计算了各热解反应途径的标准动力学参数,分析了各种主要热解产物的形成演化机理。键离解能计算结果表明,紫丁香酚中CH₃-O键的键离解能最小,各种键离解能的大小顺序为CH₃-O < O-H < CH₃O-C_aromatic < CH₂-H < HO-C_aromatic < C_aromatic-H。在反应路径(1)中,主要热解产物是3-甲氧基邻苯二酚,其形成反应的总能垒为366.6 kJ/mol;在反应路径(2)中主要热解产物是2-甲氧基-6-甲基苯酚,其形成反应的总能垒为474.8 kJ/mol;在反应路径(3)中形成邻甲氧基苯酚的总能垒很低,为21.4 kJ/mol,这表明,在连接甲氧基的碳原子上加氢后能够有效地降低木质素芳环模化物紫丁香酚去甲氧基反应的反应能垒。     

木质素β-O-4单元的光化学和电化学降解:醚键断裂反应的一种应用*

醚键断裂反应既是有机化学的重要教学内容,又在现实世界中具有广泛应用。木质素是一类重要的生物质资源,其降解核心为碳-氧键断裂反应体系,具有重要的研究价值。对木质素β-O-4单元利用光化学和电化学降解的最新科研进展进行总结,可为醚键断裂反应的教学提供应用案例,拓展学生科学视野。     

苯砜基羧酸酯类急性毒性的QSAR研究

采用密度泛函理论方法, 在B3LYP/6-31G(d)理论水平下, 计算了56种苯砜基羧酸酯类化合物的量子化学参数. 计算结果表明， 酯基连接的烷烃链亲水性越小, 毒性越大; 苯环连接的取代基亲水性越大, 毒性越大; 分子的体积越大, 毒性越小; 分子产生氢键的能力越大, 毒性越小; 分子最高占据轨道能量越高, 毒性越大.     

密度泛函理论研究温度和取代基对4-亚芳基亚氨基-1,2,4-三唑-3(2H)-酮气相热分解的影响

采用密度泛函理论(DFT)方法, 在B3LYP/6-31＋＋G(d,p)水平上研究了4-亚芳基亚氨基-1,2,4-三唑-3(2H)-酮及其类似物的气相热分解反应. 从热力学性质、几何结构参数、自然电荷分布、温度效应等角度探讨不同取代基对标题物热分解反应的影响. 结果表明: R (R＝H, CH2CH2CN)取代基对反应影响很小; 而苯环上Y (Y＝MeO, Me, H, Cl和NO2)取代基对活化自由能(ΔG≠)、键长(ΔR≠)、键角(Δα≠)和自然电荷(Δq≠)变化的影响与Hammet常数(σ)呈现线性关系ΔP≠＝ρσ＋C (P＝G, R, α和q, r＞0.95). 给电子基团使得相应的活化自由能降低, 键长变短, 键角变大, 自然电荷增加; 吸电子反之.     

单金属锰、铁卟啉对空气氧化α-蒎烯的催化作用及取代基效应

以α-蒎烯为底物,空气作氧化剂,研究了苯环上带有不同取代基团的单锰卟啉和单铁卟啉对空气氧化α-蒎烯的催化作用及反应中的取代基效应.结果表明,单锰卟啉及单铁卟啉催化氧化α-蒎烯得到双键和烯丙位一、二级碳氢键的氧化产物,没有烯丙位三级碳氢键的氧化产物.环氧化合物是主要产物,而且氧化产物的产率随时间的变化呈较好的线性关系.随着苯环上meso位取代基Cl,CH3,OCH3和OH的供电子能力的增强,锰卟啉和铁卟啉对α-蒎烯的催化活性逐渐减弱,α-蒎烯的转化率逐渐降低.锰卟啉和铁卟啉的催化反应表观速率常数k与环外苯基上的取代基特性常数σ均呈良好的线性关系,Hammett关系式分别为lnk=1.2168σ-7.9968,lnk=0.6251σ-8.2426;线性相关系数分别为0.9507和0.9715.     

高位阻烷烃的C-C和C-H键的键离解能

本文使用ONIOM-G3B3的方法计算了一系列高位阻烷烃的C-C和C-H键离解能。研究还测定了它们的几何参数,如键长,键角,分子体积等,它们中的绝大多数分子目前还没有被合成。这些几何参数表征了位阻效应对键离解能产生的影响。研究确定了键离解能的迅速减小和分子体积的增大之间的一些关系。这些关系可以帮助使用理论方法预测很多高位阻化合物的合成。     

利用电子效应解释取代基的位置及种类对芳香化合物化学性质的影响

苯环上的取代基可以通过两种途径对官能团传递电子效应。一是通过共轭体系传递,其特点是：由于交替极化,取代基的电子效应（共轭效应（C）和诱导效应（I））主要影响到处于其邻位或对位的官能团电子云密度的变化,而对处于其间位的官能团影响很小;二是通过苯环的σC-C键向官能团传递电子效应,此时取代基仅能通过诱导效应产生影响。     

超分子体系中的分子识别研究(Ⅻ)──环糊精及环糊精双核铜配合物对4-取代苯酚的分子识别

用分光光度滴定法在25.0℃时测定了不同pH值下α-,β-,γ-环糊精以及1mol/LNaOH水溶液中α-和β-环糊精双核铜配合物与4-取代苯酚形成超分子配合物的稳定常数.化学计量法表明,主体环糊精及环糊精双核铜配合物与客体4-取代苯酚形成了1：1的超分子配合物.从主-客体间的尺寸关系、pH值、多点识别和诱导契合作用等因素讨论了环糊精及环糊精双核铜配合物对客体4-取代苯酚的分子识别机制.结果表明,β-环糊精双核铜配合物对4-取代苯酚具有特殊的键合能力和分子选择性。     

Electron distribution in 3,4-dimethylanisole molecule according to quantum-chemical calculations

Calculations of two forms of 3,4-dimethylanisole molecule with full geometry optimization are performed by AM1, RHF/6-31G(d), and MP2/6-31G(d) methods. The form with cis location of H₃C-O and C¹–C⁶ bonds is somewhat more preferable energetically than that with the trans location. By the RHF/6-31G(d) method, this molecule is also calculated with fixed rotation angles of methoxy group around the C-O bond. It is shown that, in the first form, the negative charge on the C⁶ atom is considerably smaller than on C², while in the second form the charges are practically equal, which does not confirm the concerted effects of steric and electronic factors in reactions of 3,4-dimethylanisole with methyl isocyanate and isobutyraldehyde. The charge difference is due to the effect of methyl group in the 3-position of the aromatic ring and to the orientation of the methoxy group toward geminal C-C bonds of the ring.     

取代基对木质素三聚体模型化合物醚键均裂反应影响的密度泛函理论研究

利用密度泛函理论M062X/6-31++G(d,p)方法,对27种具有不同取代基(甲基、羟甲基和甲氧基)的木质素三聚体模型化合物的Cα-O和Cβ-O键均裂解离能进行了理论计算,探究了不同位置取代基对醚键解离能的影响规律。结果表明,当R2或R3位氢原子仅有一个被甲氧基取代时,Cβ-O键解离能变化很小;当R2、R3位氢原子均被甲氧基取代时,Cβ-O键解离能明显降低;且R4、R5位甲氧基能强化R2、R3位甲氧基对Cβ-O键解离能的降低程度,而不受R1位取代基的影响。当R4、R5位氢原子相继被甲氧基取代时,Cα-O键解离能逐渐降低,且R2、R3位甲氧基也能强化R4、R5位甲氧基对Cα-O键解离能的降低程度。当R1位氢原子相继被甲基、羟甲基取代时,Cα-O键解离能逐渐升高,然而R2、R3位甲氧基会弱化R1位甲基、羟甲基对Cα-O键解离能的升高程度;R1位甲基不会影响Cβ-O键解离能,羟甲基却能明显提高Cβ-O键解离能。     

Silver-mediated aminophosphinoylation of propargyl alcohols with aromatic amines and H-phosphine oxides leading to α-aminophosphine oxides

A new silver mediated aminophosphinoylation of propargyl alcohols with aromatic amines and Hphosphine oxides for the construction of a-aminophosphine oxides has been developed.The C-N and C-P bond could be efficiently formed in one pot operation via sequential C-C and C-O bond cleavage of propargylic alcohols.This present methodology,which not only provides a simple and alternative strategy for the synthesis of α-aminophosphine oxides,but also opens a new window for the cleavage reactions of propargyl alcohols via dealkynalation coupling.     

Photodecomposition profiles of beta-bond cleavage of phenylphenacyl derivatives in the higher triplet excited states during stepwise two-color two-laser flash photolysis

Photochemical properties of p-phenylphenacyl derivatives (PP-X) having C-halide, C-S, and C-O bonds in the lowest (T 1) and higher (T n ) triplet excited states were investigated in solution by using single-color and stepwise two-color two-laser flash photolysis techniques. PP-Xs (X = Br, SH, and SPh) undergo beta-bond dissociation in the lowest singlet excited states (S 1) while the C-X bonds of other PP-Xs are stable upon 266-nm laser photolysis. The T 1(pi,pi*) states of PP-X were efficiently produced during 355-nm laser photolysis of benzophenone as a triplet sensitizer. Triplet PP-Xs deactivate to the ground state without photochemical reactions. Upon 430-nm laser photolysis of the T 1 states of PP-X (X = Br, Cl, SH, SPh, OH, OMe, and OPh), decomposition of PP-X in the T n states was found. On the basis of the changes in the transient absorption, quantum yields (Phi dec) of the decomposition of PP-X in the T n states were determined, while bond dissociation energies (BDE) of the C-X bonds were calculated by computations. According to the relationship between the Phi dec and BDE values, it was shown that the decomposition of PP-X in the T n state is due to beta-cleavage of the corresponding C-X bond, and that the state energy of the reactive T n for the C-O bond cleavage differs from that for the C-halide and C-S bond cleavage. The reaction profiles of the C-X bond cleavage of PP-X in the T n states were discussed.     

PCM study of the solvent and substituent effects on bond dissociation energies of the O?NO Bond—A DFT study

A PCM continuum model, at the B3LYP, B3P86, and B3PW91 three‐parameter hybrid DFT methods with 6‐311G** basis set, is used to study the bond dissociation energies (BDEs) of benzyl nitrites. Compared the computed results with the experimental values, it is noted that B3PW91 functional is the best method to compute the BDEs of benzyl nitrites. The solvent and substituent effects on the BDEs of the O? NO bond are analyzed, and it is shown that the BDE of the O? NO bond decreases with the increment of the Hammett constants of substituent groups on benzene for benzyl nitrites except C₆H₅CH₂O? NO. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Radical synthesis of tetrameric lignin model compound

The lack of suitable lignin model compound limits the understanding of the characteristics of lignin, and hence hinders the efficient utilization of this kind of bioresource. A tetramer phenolic lignin model compound composed of 5-5, α-O-4 and β-5 linkages was prepared by a two-step of free radical reaction with hydrogen peroxide/horseradish peroxidase and S₂O₈^2- /Fe²⁺ as the initiator. Compared with enzymatic process, this synthetic process gives a higher yield of 33.8% within a shorter time. HRMS and ¹³C NMR spectroscopy show that synthesized model compound contains phenylpropane structure linked by 5-5, α-O-4 and β-5 bonds, which can mimic some chemical characteristics of lignin.     

Equilibrium acidities and homolytic bond dissociation energies of acidic C-H bonds in alpha-arylacetophenones and related compounds

The equilibrium acidities (pK(AH)s) and the oxidation potentials of the congugate anions [E(ox)(A(-))s] were determined in dimethyl sulfoxide (DMSO) for eight ketones of the structure GCOCH(3) and 20 of the structure RCOCH(2)G, (where R = alkyl, phenyl and G = alkyl, aryl). The homolytic bond dissociation energies (BDEs) for the acidic C-H bonds of the ketones were estimated using the equation BDE(AH) = 1.37pK(AH) + 23.1E(ox)(A(-)) + 73.3. While the equilibrium acidities of GCOCH(3) were found to be dependent on the remote substituent G, the BDE values for the C-H bonds remained essentially invariant (93.5 +/- 0.5 kcal/mol). A linear correlation between pK(AH) values and [E(ox)(A(-))s] was found for the ketones. For RCOCH(2)G ketones, both pK(AH) and BDE values for the adjacent C-H bonds are sensitive to the nature of the substituent G. However, the steric bulk of the aryl group tends to exert a leveling effect on BDEs. The BDE of alpha-9-anthracenylacetophenone is higher than that of alpha-2-anthracenylacetophenone by 3 kcal/mol, reflecting significant steric inhibition of resonance in the 9-substituted system. A range of 80.7-84.4 kcal/mol is observed for RCOCH(2)G ketones. The results are discussed in terms of solvation, steric, and resonance effects. Ab initio density functional theory (DFT) calculations are employed to illustrate the effect of steric interactions on radical and anion geometries. The DFT results parallel the trends in the experimental BDEs of alpha-arylacetophenones.     

Photo- and thermochromic spirans. 15. Effect of structural factors on the length of the Cspiro-O bond

Calculations of a large number of compounds that contain a 2H-pyran fragment were made by means of the MINDO/3 method. A correlation between the magnitude of the barrier to opening of the pyran ring and the length of the C_spiro-O bond was noted. Structural factors that determine the length of the C_spiro-O bond were discovered. Spiropyran structures that have extremal lengths of the C_spiro-O bonds were predicted on the basis of the correlations obtained.See [30] for communication 14.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 747–752, June, 1984.     

Calculations of Cotton Effects in the Vacuum UV Region in the Non-Anomeric Part of Pyranoses: Correlations With the Absolute Stereochemistries

ABSTRACT

A graphic method of Drude equations was applied to calculate the Cotton effects below 180 nm for a series of 1, 5-anhydro-D-alditols I-IX. The calculations, taking the vacuum CD data of Johnson et al. into account, gave a Cotton effect at ca. 140 nm region (Band 2) and a popular band (Band 1) due to the n-σ transition at ca.180 nm. Band 2, probably due to the σ-electron transition of the C-O or C-C bond, could be correlated with the helical structures of vicinal C-O and C-O bonds (C-O/C-O) and C-O and C-C bonds (C-O/C-C) providing a new additivity rule to calculate intensity (K2) using the vicinal C—O/C-O and C-O/C-C units.     

Ruthenium-catalyzed conversion of sp3 C-O bonds in ethers to C-C bonds using triarylboroxines

Catalytic conversion of unreactive sp(3) C-O bonds in alkyl ethers to C-C bonds is described. Alkyl ethers bearing 2- or 4-pyridyl groups were coupled with triarylboroxines in the presence of a ruthenium catalyst. Triarylboroxines bearing a variety of functional groups including electron-withdrawing and -donating groups can be used for the reaction. No additional base was required for the coupling with the organoboron reagents, and base-sensitive groups can be tolerated. The reaction is considered to proceed via dehydroalkoxylation followed by addition of triarylboroxines to form C-C bonds.