弱氢键作用对硫脲水溶液拉曼光谱影响的密度泛函理论计算

将拉曼光谱和密度泛函理论(DFT)计算结合能够把拉曼谱峰和分子结构及分子间氢键作用之间的变化联系起来,反映分子周围的结构信息.本文通过理论计算水分子和硫脲形成复合物的拉曼光谱,并结合实验报道的拉曼光谱,探究硫脲基频对分子局域结构的依赖性.通过轨道分析,发现水分子的氢键作用可以引起硫脲的前线轨道对易,这将影响到硫脲的拉曼光谱性质.最后计算也表明在中性水溶液中由于大的正Gibbs自由能变,硫脲不易发生异构转变.     

丙氨酸晶体太赫兹振动光谱的实验和理论研究

利用太赫兹时域光谱和低频拉曼光谱仪研究了丙氨酸晶体在0.2-2.6 THz 范围内的太赫兹吸收和拉曼散射光谱. 研究表明: 在该低频范围有四个振动模式, 其中两个只具有拉曼活性, 其余两个同时具有红外和拉曼活性. 基于B3LYP杂化密度泛函的自洽场晶体轨道法对丙氨酸周期性结构进行了理论研究和光谱计算. 通过比较实验和理论结果, 指认了实验光谱特征峰所属的不可约表示. 通过理论计算得到的图形, 得出在此低频范围的振动模式主要包含分子间氢键的扭转和摇摆运动.     

p-π共轭分子的氨基面外弯曲振动模的拉曼光谱

拉曼光谱是一种用途广泛的无损分子检测技术,其能够提供化学物质的分子结构指纹信息.一种面外弯曲振动模被称作wagging振动,它的信号尤为特殊,其频率和强度都非常依赖于检测环境.以乙烯胺和苯胺为例,采用密度泛函理论计算研究了p-π共轭分子分别与水簇和银簇作用的平衡结构、成键作用和拉曼光谱.结果表明,弱相互作用,如分子与金属表面的弱吸附作用以及分子与水之间的氢键作用,均使氨基面外弯曲振动模(ωNH2)的拉曼信号发生显著的变化.考虑溶剂化效应后,氢键作用减弱,计算拉曼光谱趋于一致.通过进一步对电子结构的分析,解释了面外弯曲振动信号显著增强的原因,揭示了面外弯曲振动模与分子p-π共轭作用之间的关系.     

拉曼光谱研究CaCl2和MgCl2对水结构的影响

测试了CaCl2、MgCl2溶液(浓度小于1.0 mol•L-1)的OH伸缩振动区域的拉曼光谱.对所得到的拉曼光谱进行了计算机去卷积处理,并由此计算了不同溶液中水的四面体结构的百分数.研究表明,CaCl2、MgCl2对水中四面体结构有破坏作用，且CaCl2的破坏作用比MgCl2大.与17O核磁共振结果对比与分析,认为CaCl2、MgCl2虽然破坏水中的四面体结构,但通过促进含氢键数少的水分子形成氢键,故从总体上促进水的缔合结构.     

DFT方法研究基于脲及硫脲衍生物的阴离子受体

采用密度泛函理论(DFT)模拟了两个基于脲和硫脲衍生物的受体分子对卤素阴离子的识别过程.结构优化表明基于脲衍生物的受体分子1最稳定构象为"反反"构象,分子内部形成稳定的C~α-H…O=C分子内氢键;而基于硫脲衍生物的受体分子2,不能形成分子内氢键,最稳定构象为"反顺"构象.受体1、2与卤素阴离子F~-、Cl~-可形成稳定的双氢键复合物,在此过程中,受体2经历了由"反顺"构象到"反反"构象的异构化过程.结构和能量分析表明,1、2受体分子与F~-离子间的氢键强度远大于其与Cl~-离子间的氢键;另一方面,受体2与阴离子间的氢键明显强于受体1,这是由于硫脲基N-H键具有更强的酸性.此外,对受体分子、氢键复合物及去质子化产物的吸收光谱计算结果表明,受体与F~-离子作用可产生明显的吸收光谱红移,而与Cl~-离子的作用对光谱影响较小.     

乙腈溶剂中芳酰基硫脲对F~-识别作用的理论研究

采用混合密度泛函B3LYP方法,在6-31G**水平上和URHF溶剂半径中优化乙腈溶剂中四种芳酰基硫脲及其氟离子复合物的几何构型,从几何结构参数、电荷布居、前线轨道、结合能以及热力学参数等角度探讨复合物形成过程中主客体间的相互作用,结果表明,芳酰基硫脲在乙腈溶液中识别氟离子是一个自发过程,识别作用主要是硫脲氮氢与氟离子的氢键作用。受体分子识别氟离子能力与取代基密切相关,以N-对硝基苯基-N’-对硝基苯甲酰基硫脲氟离子复合物最稳定。在基态结构的基础上用相同的理论水平计算了乙腈溶剂中四种受体分子及氟离子复合物的电子吸收光谱。结果表明,有取代基的芳酰基硫脲识别氟离子后的光谱行为变化大,有利于使用比色法检测阴离子。光谱理论计算值很好地反映了实测值,这为实验室开展工作提供了依据。     

密度泛函理论研究银上吸附对巯基吡啶的SERS化学增强效应

基于密度泛函理论计算和拉曼光谱理论分析,我们研究了对巯基吡啶(4MPY)分子的拉曼光谱和其在银上的表面增强拉曼光谱(SERS),并进一步探讨了SERS与界面吸附结构、异构化、质子化和氢键作用以及低能激发态的关系。首先,我们对两种分子异构体的相对稳定性和拉曼光谱进行了理论分析。在此基础上,进而研究了该分子与不同银簇作用时的拉曼光谱,结果表明,4MPY以巯基硫与银簇作用形成强的Ag―S键,导致拉曼光谱的线型不依赖于所选银簇的大小。接着我们考虑了吡啶氮端作用的两种情况。(1)当4MPY-银簇复合物同时以吡啶氮与水簇或水合质子簇形成氢键时,结果表明吡啶环的部分振动频率随氢键和质子化发生蓝移。(2)当考虑吡啶氮与银簇作用时,吡啶环三角畸变振动发生蓝移。上述情况不仅解释了实验观测的振动频率变化,而且表明了化学环境改变对相对拉曼强度的影响。最后,我们计算了当对巯基吡啶分子以单端或双端与银簇作用,在考虑激发光与低能激发态的能量匹配时,拉曼光谱强度与低能激发态的关系。计算结果表明,在双端吸附构型下,与吡啶氮成键的银簇受激发产生电荷转移态,不仅导致吡啶环v_(12)、v_1和v_(8a)振动的拉曼信号增强,而且选择性地增强吡啶环C―H面内对称弯曲振动v9a的拉曼信号。     

类水滑石主体层板与客体CO2-3、H2O间的超分子作用

通过构建类水滑石双层计算模型,采用混合密度泛函B3LYP/6-31G(d)//B3LYP/3-21G方法计算类水滑石(LDHs-CO3-yH2O)的结构与能量,探讨LDHs限域空间中客体阴离子及水分子的分布形态以及主客体超分子作用.计算结果表明,客体阴离子与水分子以平行层板的方式存在于水滑石层间.主客体发生作用时,CO2-3的HOMO轨道向层板的LUMO轨道转移电子.所形成的LDHs-CO3主客体作用要强于LDHs-F以及LDHs-Cl.与其离子交换性能相一致.水滑石去水结构(LDHs-CO3)水合过程,氢键作用较静电作用更占优势,并且layer-water型氢键要强于anion-water型氢键.此外,水合能计算表明LDHs水合具有一定的饱和量.     

类水滑石主体层板与客体CO2-3、H2O间的超分子作用

通过构建类水滑石双层计算模型, 采用混合密度泛函B3LYP/6-31G(d)//B3LYP/3-21G方法计算类水滑石(LDHs-CO3-yH2O)的结构与能量, 探讨LDHs限域空间中客体阴离子及水分子的分布形态以及主客体超分子作用. 计算结果表明, 客体阴离子与水分子以平行层板的方式存在于水滑石层间. 主客体发生作用时, CO2-3的HOMO轨道向层板的LUMO轨道转移电子. 所形成的LDHs-CO3主客体作用要强于LDHs-F以及LDHs-Cl, 与其离子交换性能相一致. 水滑石去水结构(LDHs-CO3)水合过程, 氢键作用较静电作用更占优势, 并且layer-water型氢键要强于anion-water 型氢键. 此外, 水合能计算表明LDHs水合具有一定的饱和量.     

三元水溶液体系中氢键作用与分子结构的拉曼光谱研究

采用拉曼光谱研究了水-乙腈-二甲基亚砜三元水溶液体系中氢键作用对分子结构的影响.结果表明,在三元水溶液中乙腈C≡N键的电子云向碳原子发生偏移,二甲基亚砜中S=O双键的电子云向硫原子发生偏移;在三元体系中乙腈和二甲基亚砜与水形成氢键时存在明显的竞争关系,出现乙腈分子和二甲基亚砜分子共用1个水分子形成复合物的情况,并且随着水含量的增加,共用水分子的情况逐渐消失.     

Vibrational spectroscopic study on the quantum chemical model and the X-ray structure of gallic acid,solvent effect on the structure and spectra

Infrared, Raman and far-IR spectra of gallic acid were recorded both in crystalline and in its dry form. Solvent effect of water was studied. The molecular and crystal structures were determined by single crystal X-ray diffraction. A complex system of intermolecular interactions was revealed. Optimized geometries, vibrational frequencies and infrared intensities were calculated utilizing the post-HF DFT method with the Becke3LYP functional and the 6-31G* basis set. Normal coordinate analysis was carried out. The results of the calculations were applied to simulate the infrared and Raman spectra and the full assignment of the acquired spectra is presented.     

The effect of intermolecular dipole-dipole interaction on Raman spectra of polyconjugated molecules: density functional theory simulations and mathematical models

In this work, we analyze the effect of intermolecular dipole-dipole interactions on Raman spectra of polyconjugated molecules. In particular, the behavior of push-pull polyenes has been studied. By means of density functional theory (DFT) calculations on isolated molecules and dimers, we have found that both the frequencies and intensities of the strongest Raman lines (R mode) are strongly influenced by intermolecular interactions. The results have been rationalized within the effective conjugation coordinate (ECC) theory developed in the past. The calculations for different configurations have also shown that the Raman spectra are sensible to different intermolecular geometries, thus implying a possible application of vibrational spectroscopy to the study of supramolecular properties of polyconjugated systems. The comparison with the available experimental spectra confirms the results obtained with the DFT computations. Finally, a very simple mathematical model is proposed for the prediction of the Raman frequencies of interacting systems. From the knowledge of just a few quantities for the isolated molecule and of some geometrical parameters, an estimate of the frequency of the dimers can be obtained. Despite its simplicity, this model gives results in very good agreement with DFT calculations carried out explicitly on dimers in several different arrangements.     

Vibrational spectra and chemical quantum calculations for 2-adamantylamino-5-nitropyridine crystals--a novel material for laser Raman converters

Polycrystalline infrared and polarized FT-Raman spectra have been measured for 2-adamantylamino-5-nitropyridine, a novel organic material for laser Raman converters. The assignment of IR and Raman bands is given on the basis of DFT calculations. The spectroscopic studies have not indicated the presence of any significant intermolecular interactions in the crystal structure of this compound. The lines observed in the stimulated Raman spectrum of this crystal are assigned to the respective molecular vibrations.     

DFT vibrational calculations of rhodamine 6G adsorbed on silver: analysis of tip-enhanced Raman spectroscopy

The tip-enhanced near-field Raman (TERS) bands of Rhodamine 6G (R6G), that we reported earlier [Chem. Phys. Lett. 2001, 335, 369.], are assigned on the basis of density-functional theory (DFT) calculations at the 6-311++G(d,p) level. The Raman and infrared intensities as well as frequencies of the vibrational modes are used for band assignments. These vibrational modes, in combination with characterization of resonant electronic transitions using time-dependent DFT calculations, predict spectral changes in resonant Raman and surface-enhanced resonant Raman scatterings of R6G. Moreover, the TERS spectra of R6G are analyzed in detail, where interactions between the tip and R6G molecules and their enhancement mechanisms are discussed. Finally, we propose a novel Raman spectroscopy technique capable of detecting molecular vibrations at sub-nanometer scale.     

Raman dispersion and intermolecular interactions in unsubstituted thiophene oligomers

We present a critical analysis of the Raman spectra of unsubstituted oligothiophenes and rediscuss the well-known Raman dispersion of conjugated systems explicitly considering intermolecular interactions. Temperature-dependent Raman spectra and DFT calculations for dimers of different chain lengths show that the effect of intermolecular interactions on the frequency and intensity of carbon-carbon (CC) stretching modes is non-negligible. This effect has not been considered in previous works and might explain many spectral features of this class of compounds which are not completely interpreted by the usual models. Both intensities and frequencies are significantly affected by intermolecular interactions showing that molecular self-organization should be taken into account in future spectroscopic studies of conjugated molecules. In particular, the interactions among molecules cause an upward shift of the frequency of the R mode (amplitude mode) which can explain the lack of frequency dispersion with conjugation length of oligothiophenes, as experimentally observed for solid-state samples at room temperature.     

Binding interactions and Raman spectral properties of pyridine interacting with bimetallic silver-gold clusters.

The binding interactions between pyridine and bimetallic silver-gold clusters are investigated using density functional theory (DFT). The binding energies of pyridine-bimetallic cluster complexes indicate that the bonding depends strongly on the binding site (Au or Ag atom) and bonding molecular orbitals in a given configuration. The donation of the lone-pair electrons of the nitrogen of pyridine to an appropriate unoccupied orbital of each metal cluster plays an important role. The low-lying excited states and charge-transfer states of four stable complexes of interest are calculated on the basis of a time-dependent DFT method. In nonresonance Raman scattering processes, the influence of binding interactions on the relative Raman intensity of totally symmetric pyridine vibrational modes is discussed. These calculated relative Raman intensities are compared with observed surface-enhanced Raman spectra of pyridine adsorbed on silver-gold alloy surfaces.     

Vibrational spectra and structures of urazole and 4-methylurazole: DFT calculations of the normal modes in aqueous solution and in the solid state,and the influence of hydrogen bonding

Solid-state IR and Raman as well as aqueous solution state Raman spectra are reported for urazole, 4-methylurazole and their deuterated derivatives. DFT calculations, at the B3-LYP/cc-pVTZ level, established that the structures and vibrational spectra of the molecules can be interpreted using models with hydrogen-bonded water molecules, in conjunction with the polarizable continuum solvation method. The vibrational spectra were computed at the optimised molecular geometry in each case, enabling normal coordinate analysis, which yielded satisfactory agreement with the experimental IR and Raman data. Computed potential energy distributions of the normal modes provided detailed vibrational assignments. Solid-state pseudopotential-plane-wave DFT calculations, using the PW91 functional were also carried out, reflecting the importance of intermolecular hydrogen bonding in the solid state.     

Hybrid organic semiconductors including chalcogen atoms in pi-conjugated skeletons. Tuning of optical, redox, and vibrational properties by heavy atom conjugation

In this work, the interactions between heteroatoms (S, Se, and Te) and conjugated skeletons are analyzed. The study is carried out by using electronic absorption and fluorescence spectroscopies, electrochemistry, vibrational Raman spectroscopy, and theoretical calculations in the framework of DFT and TD-DFT theories. Optical spectra are described in terms of one-electron promotions between orbitals around the energy gap. Electrochemistry, in the framework of the Koopman's approach, is also interpreted. The vibrational Raman spectra are assigned to molecular modes and the evolution changing the heteroatom is addressed and an effective tuning of these properties is found. Part of this modulation is associated with local electronic interactions depending on the relative S, Se, and Te electronegativities. Unconventional long-range heteroatom-heteroatom interactions have been proposed which arise from the existence of effective pi-conjugated channels. The molecular level understanding of structure-property relationships in these organic/inorganic semiconductors are of great interest in the interdisciplinary area of material science.