Can Substituted Cyclopentadiene Become Aromatic or Antiaromatic?

Cyclopentadiene derivatives with electronegative (F, Cl) or electropositive (H(3)Si, Me(3)Si) bis-5,5-substituents were studied at the B3LYP/6-311G* level of theory. It was found that there is no special stabilization or destabilization for any of the derivatives; the energetic effects that were previously attributed to aromatic stabilization or antiaromatic destabilization are the result of interactions in the reference systems. A nucleus-independent chemical shift (NICS) scan study at the HF-GIAO/6-311+G* theoretical level of these and similar derivatives suggest that they all show different magnitudes of diamagnetic ring current. None of the derivatives shows a paramagnetic ring current. Thus, cyclopentadienes are neither aromatic nor antiaromatic. It is also concluded that a diamagnetic ring current is perhaps necessary but certainly not a sufficient condition for aromaticity. The NICS scan procedure describes the type of ring current in the system, whereas a single isotropic NICS value (i.e., NICS(1)) may wrongly assign the type of ring current. It is shown that neither NICS(1) nor the NICS scan procedure can be used as a single aromaticity criterion.     

A comparative theoretical study of dipeptide solvation in water

Molecular dynamics studies have been performed on the zwitterionic form of the dipeptide glycine-alanine in water, with focus on the solvation and electrostatic properties using a range of theoretical methods, from purely classical force fields, through mixed quantum mechanical/molecular mechanical simulations, to fully quantum mechanical Car-Parrinello calculations. The results of these studies show that the solvation pattern is similar for all methods used for most atoms in the dipeptide, but can differ substantially for some groups; namely the carboxy and aminoterminii, and the backbone amid NH group. This might have implications in other theoretical studies of peptides and proteins with charged -NH(3) (+) and -CO(2) (-) side chains solvated in water. Hybrid quantum mechanical/molecular mechanical simulations successfully reproduce the solvation patterns from the fully quantum mechanical simulations (PACS numbers: 87.14.Ee, 87.15.Aa, 87.15.He, 71.15.Pd).     

2-(2-甲基烯丙基)-3-(3-甲基-1,2-丁二烯基)环己-2-烯酮重排生成八元环化合物反应机理的理论研究

采用密度泛函理论B3LYP/6-31G(d,p)方法研究了2-(2-甲基丙烯基)-3-(3-甲基-1,2-丁二烯基)环己-2-烯酮重排生成八元环化合物在气相中的反应机理. 考虑了两条可能的反应途径: 途径1包含5个过程, 途径2包含两个过程. 从能量上看, 两条途径的决定速度步均是[1,5]氢迁移. 采用自洽反应场极化连续模型(PCM模型)和极化导体模型(CPCM模型)研究了反应体系在甲苯溶液中的溶剂效应. 结果表明, 气相和溶液中途径2均是较优途径, 并且甲苯对该反应的溶剂化效应不明显. 理论研究结果与实验观察结果一致, 并很好地解释了有关实验现象.     

(MN)nHm(M=Ga,In; n=1-4; m=1, 2)团簇的结构与稳定性

采用密度泛函理论(DFT)的B3LYP方法, 在6-31G**和Lanl2dz水平上分别对(MN)nHm(M=Ga, In; n=1-4; m=1, 2)进行了优化和振动频率计算. 得到了上述团簇的最稳定构型、H原子的结合能以及它们的能隙. 结果表明, (MN)nH(M=Ga, In; n=1-4)的基态构型均为双重态, (MN)nH2(M=Ga, In; n=1-4)的基态构型均为单重态; 当氢的个数为1时, 加在N原子上比加在M(M=Ga, In)原子上稳定, 如有N3单元, 那么加在N3单元两侧的构型是相同的, 且它是最稳定的; 当氢的个数为2时, 除n=1外, 分别加在两个N原子上的构型是最稳定的, 如有N3单元, 那么分别加在N3单元分离最远的两个N原子的构型是最稳定的. GaNH、(GaN)3H 和InNH的结合能和能隙都很大, 说明这些团簇都有很高的稳定性.     

亚甲基蓝与鲱鱼精DNA相互作用的光谱法研究

以吖啶橙(AO)作为光谱探针, 采用UV和荧光光谱等方法研究了亚甲基蓝(MB)与鲱鱼精DNA的作用机制. 确定了在低浓度MB时, MB与DNA以嵌插方式作用; 而在高浓度MB时, MB与DNA之间为混合作用方式. 结合比n(MB)∶n(DNA)＝10∶1, 结合常数 ＝2.46×10⁵ L•mol^－1, MB-DNA复合物的表观摩尔吸光系数ε＝5.70×10⁶ L•mol^－1•cm^－1. 同时研究了酸度和温度等对MB与DNA相互作用的影响, 热力学研究推导了MB结合DNA为焓驱动反应.     

N,N-二甲基乙酰胺与醇分子间相互作用的理论研究

采用密度泛函理论在B3LYP/6-311＋G*水平上对乙醇、丙醇、丁醇、戊醇与N,N-二甲基乙酰胺形成的1∶1氢键复合物进行计算研究. 结果表明: 醇与N,N-二甲基乙酰胺形成的复合物存在强的氢键, 表现为羰基氧原子的孤对电子与醇羟基反键σ轨道的相互作用. 振动分析显示, 分子间C＝O…H—O氢键的形成使C＝O和H—O伸缩振动频率明显红移. 溶剂对氢键产生较大的影响, 随着溶剂极性的增加, 复合物氢键有蓝移趋势.     

对苯二甲酸二苯酯类液晶化合物光谱和二阶非线性光学性质的理论研究

在密度泛函理论B3LYP/6-31G*水平上计算对苯二甲酸二苯酯类液晶化合物分子的几何结构、振动光谱、电子光谱和非线性光学性质, 分析讨论端接基对其光谱与非线性光学性质的影响. 结果表明, 端接基的引入对该类分子的几何结构影响不大. 烷氧基的链长对分子振动光谱的影响很小, 端基引入CN时, C＝O的伸缩振动频率蓝移9 cm^－1. TD-DFT计算表明, 最大吸收光谱源于分子中HOMO→LUMO的p→p*跃迁, 对应的最大吸收波长值在313～375 nm之间, 属于紫外区. 端接强供电子基团可以提高分子的二阶非线性光学性质.     

New insights into the polymerization of methyl methacrylate initiated by rare-earth borohydride complexes: a combined experimental and computational approach

Polymerization of methyl methacrylate (MMA) initiated by the rare-earth borohydride complexes [Ln(BH(4))(3)(thf)(3)] (Ln=Nd, Sm) or [Sm(BH(4))(Cp*)(2)(thf)] (Cp*=eta-C(5)Me(5)) proceeds at ambient temperature to give rather syndiotactic poly(methyl methacrylate) (PMMA) with molar masses M(n) higher than expected and quite broad molar mass distributions, which is consistent with a poor initiation efficiency. The polymerization of MMA was investigated by performing density functional theory (DFT) calculations on an eta-C(5)H(5) model metallocene and showed that in the reaction of [Eu(BH(4))(Cp)(2)] with MMA the borate [Eu(Cp)(2){(OBH(3))(OMe)C=C(Me)(2)}] (e-2) complex, which forms via the enolate [Eu(Cp)(2){O(OMe)C=C(Me)(2)}] (e), is calculated to be exergonic and is the most likely of all of the possible products. This product is favored because the reaction that leads to the formation of carboxylate [Eu(Cp)(2){OOC-C(Me)(=CH(2))}] (f) is thermodynamically favorable, but kinetically disfavored, and both of the potential products from a Markovnikov [Eu(Cp)(2){O(OMe)C-CH(Me)(CH(2)BH(3))}] (g) or anti-Markovnikov [Eu(Cp)(2){O(OMe)C-C(Me(2))(BH(3))}] (h) hydroboration reaction are also kinetically inaccessible. Similar computational results were obtained for the reaction of [Eu(BH(4))(3)] and MMA with all of the products showing extra stabilization. The DFT calculations performed by using [Eu(Cp)(2)(H)] to model the mechanism previously reported for the polymerization of MMA initiated by [Sm(Cp*)(2)(H)](2) confirmed the favorable exergonic formation of the intermediate [Eu(Cp)(2){O(OMe)C=C(Me)(2)}] (e') as the kinetic product, this enolate species ultimately leads to the formation of PMMA as experimentally observed. Replacing H by BH(4) thus prevents the 1,4-addition of the [Eu(BH(4))(Cp)(2)] borohydride ligand to the first incoming MMA molecule and instead favors the formation of the borate complex e-2. This intermediate is the somewhat active species in the polymerization of MMA initiated by the borohydride precursors [Ln(BH(4))(3)(thf)(3)] or [Sm(BH(4))(Cp*)(2)(thf)].     

四唑负离子与脒正离子复合物稳定性的理论研究

采用密度泛函B3LYP/6-31+G(d,p) 方法和自洽反应场极化连续模型(PCM)研究四唑负离子与脒类(乙脒或苄脒)正离子形成的两种形式的复合物(末端和侧端)及甲酸根负离子与脒类(乙脒或苄脒)正离子形成的复合物在气相和二甲亚砜（DMSO）溶剂中的稳定性。在气相中,四唑-乙脒和四唑-苄脒复合物的相互作用能(∆E)末端分别比侧端的大3.56和3.72 kJ/mol,表明末端复合物稍占优势。甲酸与乙脒和苄脒的复合物的相互作用能(∆E)分别比四唑与乙脒和苄脒的复合物的大59.35和58.99 kJ/mol,表明脒与甲酸形成复合物时相互作用更强。溶剂DMSO的作用使得所有复合物的相互作用能变小,但脒与四唑的相互作用仍比脒与甲酸的弱。前者的结合常数与后者的相比只有1/315（乙脒）和1/218（苄脒）,这与实验结果相一致。     

吡咯和呋喃气相质子化过程的密度泛函理论研究

Density functional theory and ab initio calculations have been used to determine structures and stabilities of the protonated aromatics species AH^＋ and AH2^2＋ （A=pyrrole, furan）. Possible mechanisms and relative energetics for protonation of pyrrole and furan by H3O^＋ and AH^＋ in the gas phase have been explored. Calculations show that the Cα-protonated species was the most stable structure for AH^＋, and the protonated AH^＋ might accommodate the second proton to yield AH2^2＋ if the free proton was available. The gas-phase H3O^＋ could protonate pyrrole and furan with significant exothermiCity and almost without barrier. The proton transfer from AH4^＋ to pyrrole and furan has a barrier ranging from 33.5 to 39.3 kJ/mol in the gas phase.