Effect of the counterion on the steric and electronic structure of pyrylium cation

The effect of counterions on the stabilization of aromatic pyrylium cation, a heterocyclic analog of benzene, was examined by ab initio [MP2(full)/6-311++G**] method and density functional theory [B3LYP/6-311++G(d,p)(6d,10f)]. Structures of the charge-transfer molecular complexes formed by interaction of pyrylium cation with counterion are predicted. The principal role of the counterion in the stabilization of the pyrylium cation aromatic system is demonstrated. The stabilization results from both electron density transfer and covalent bonding.     

NMR spectra, GIAO and charge density calculations of five-membered aromatic heterocycles

The B3LYP/6-31+G(d) molecular geometry optimized structures of 17 five-membered heterocycles were employed together with the gauge including atomic orbitals (GIAO) density functional theory (DFT) method at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p) and B3LYP/6-311+G(2d,p) levels of theory for the calculation of proton and carbon chemicals shifts and coupling constants. The method of geometry optimization for pyrrole (1), N-methylpyrrole (2) and thiophene (7) using the larger 6-311++G(d,p) basis sets at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,p) and B3LYP/cc-pVTZ levels of theory gave little difference between calculated and experimental values of coupling constants. In general, the (1)H and 13C chemical shifts for all compounds are in good agreement with theoretical calculations using the smaller 6-31 basis set. The values of nJHH(n=3, 4, 5) and rmnJ(CH)(n=1, 2, 3, 4) were predicted well using the larger 6-31+G(d,p) and 6-311++G(d,p) basis sets and at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,2p) levels of theory. The computed atomic charges [Mülliken; Natural Bond Orbital Analysis (NBO); Merz-Kollman (MK); CHELP and CHELPG] for the B3LYP/6-311++G(d,p) geometry optimized structures of 1-17 were used to explore correlations with the experimental proton and carbon chemical shifts.     

Thermochemical properties of molecules and cations of MgNnHm (n=1,2 and m=1–6): enthalpies of formation, proton affinities and ionization energies

Ab initio molecular orbital calculations are reported for small neutral molecules and cations containing magnesium, nitrogen and hydrogen. Structures have been optimized using gradient techniques at B3LYP/6-31+G(d) and at MP2(full)/6-311++G(d,p). Single-point calculations are reported at QCISD(T)(full)/6-311++G(2df,p) and at CCSD(T)(full)/6-311++G(2df,p) levels using geometries optimized at MP2(full)/6-311++G(d,p). Standard enthalpies of formation at 298 K have been calculated at these two higher levels of theory. Other thermochemical properties calculated include ionization energies and proton affinities. The binding enthalpies of ammonia to Mg⁺, MgNH₂⁺ and MgNH₃⁺ are also reported.     

Theoretical studies on the intramolecular hydrogen bond and tautomerism of 8-mercaptoquinoline in the gaseous phase and in solution using modern DFT methods

A theoretical quantum chemical study of the intramolecular hydrogen bonding interactions in 8-mercaptoquinoline has been carried out. Special attention has been paid to the rotation of S-H bond and intramolecular proton-transfer reactions. Therewith, the B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,2p), MPW1K/6-311++G(d,p), MPW1K/6-31+G(2d,2p), BH&HLYP/6-311++G(d,p), and G96LYP/6-311++G(d,p) methods have been used. By means of the Onsager and PCM reaction field methods, the effects of solvent on hydrogen-bond energies, conformational equilibria, rotational barriers, and tautomerism in aqueous solution have been studied. These simulations were done at the MPW1K/6-311++G(d,p) and B3LYP/6-311++G(d,p) levels. Natural-bond orbital analysis has been performed to study the intramolecular hydrogen bond (IHB) in the gaseous phase and in aqueous medium. The stability of forms under consideration in solution does not coincide with that in the gaseous phase, underlining a great importance of the electrostatic influence of solvent. Double-proton transfer in the prototropic tautomerization of 8-mercaptoquinoline, one water molecule complex in the gaseous phase and in solution, has been systematically studied. The double-proton transfer occurs concertedly and synchronously. The water-assisted tautomerization is kinetically less, but thermodynamically more favorable, compared to that of the single-proton transfer. As in the case with single-proton transfer, for water-assisted reaction, the tautomerization energies and barrier heights decrease with the increase in dielectric constant, which implies faster and more complete tautomerization of 8-mercaptoquinoline in a polar solvent.     

Mono-, di-, and tricoordinated phosphorus attached to a N-N unit: an experimental and theoretical study

N,N',N'-[Tris(trimethylsilyl)]hydrazino-diphenylphosphane, (TMS)2 N-(TMS)N-PPh2 (1), and N,N',N'-[tris(trimethylsilyl)]hydrazino-phenyl(chloro)phosphane, (TMS)2 N-(TMS)N-P(Cl)Ph2 (2), were obtained in the reaction of bis-[lithium-tris(trimethylsilyl)hydrazide] with Ph(n)PCl(3 - n) (n = 1, 2). The structure and bonding of both species are discussed on the basis of experimentally observed (X-ray, Raman, NMR, and MS) and theoretically obtained data (B3LYP/6-31G(d,p), NBO analysis). Oxidation with sulfur and selenium results in the formation of (TMS)2 N-(TMS)N-P(S)Ph2 (4), (TMS)2 N-(TMS)N-P(Se)Ph2 (5), (TMS)2 N-(TMS)N-P(S)Ph(Cl) (6), and (TMS)2 N-(TMS)N-P(Se)Ph(Cl) (7). Moreover, the thermal decomposition of N,N',N'-[tris(trimethylsilyl)]hydrazine-dichlorophosphane, (TMS)2 N-(TMS)N-PCl2 (3) and the reaction with magnesium have been investigated. The formation and molecular structure of the novel MgCl2(THF)2 x 2Mg[(TMS)NP(O)2 N(TMS)2](THF) (8) salt containing the hitherto unknown (TMS)NP(O)2 N(TMS)2(2-) anion are discussed. DFT calculations (B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p)) are used to evaluate the bonding, ground-state structures, and energy landscape for the different isomers of 3: the thermodynamics and kinetics of the successive elimination of chlorotrimethylsilane (TMS-Cl) resulting in the formation of covalent azide analogues such as TMS-PNN or TMS-NNP.     

COS与O2反应的密度泛函理论研究

用量子化学密度泛函理论（DFT）方法，对COS与O2的反应进行了理论研究．在UB3LYP／6—31G^＊，UB3LYP／6—311＋＋G^＊＊水平上，优化了反应势能面上各驻点（反应物、产物、中间体和过渡态）的几何构型，在UB3LYP／6—31G^＊水平上通过内禀反应坐标（IRC）计算和振动分析，对过渡态进行了确认．在CCSD（T）／6—311＋＋G（2d，2p）水平上进行了单点能量计算，并确定了反应机理．研究结果表明，反应主要产物为CO2和SO.     

Conformational stability, the spectroscopic (FT-IR and UV), first order hyperpolarizability, NBO analysis, HOMO and LUMO analysis of 6,8-diphenylimidazo[1,2-α]pyrazine molecule by ab initio HF and density functional methods

The conformational analysis of 6,8-diphenylimidazo[1,2-α]pyrazine molecule (abbreviated as 68DIP) was performed by using B3LYP/6-31G(d) level of theory to find the most stable form. Two staggered stable conformers were observed on the torsional potential energy surface. The equilibrium geometry, bonding features and vibrational frequencies of 68DIP have been investigated by using the DFT (B3LYP) and HF methods for the lowest energy conformer. The first order hyperpolarizability (β(total)) of this molecular system and related properties (β, μ, <α> and Δα) are calculated using HF/6-311++G(d,p) and B3LYP/6-311++G(d,p) methods based on the finite-field approach. Stability of the molecule arising from hyperconjugative interactions, charge delocalization and C-H?N intramolecular hydrogen-bond-like weak interaction has been analyzed using natural bond orbital (NBO) analysis by using B3LYP/6-311++G(d,p) method. The results show that electron density (ED) in the σ* and π* antibonding orbitals and second order delocalization energies E((2)) confirm the occurrence of intramolecular charge transfer (ICT) within the molecule. UV-vis spectrum of the compound was recorded and electronic properties, such as HOMO, LUMO energies, excitation energies and wavelength were performed by TD-DFT/B3LYP, CIS and TD-HF methods by using 6-311++G(d,p) basis set. Finally, the calculation results were applied to simulated infrared spectra of the title compound which show good agreement with observed spectra.     

An exploration of conformational search of leucine molecule and their vibrational spectra in gas phase using ab initio methods

An extensive exploration of the conformational space has been carried out to characterize all possible gas phase structures of leucine. A total of 324 unique trial structures for canonical leucine were generated by considering all possible combinations of single bond rotamers. All trial structures were optimized at the B3LYP/6-311G* level of the DFT method. A total of 77 unique and stationary canonical conformers were found. Further, 15 most stable conformers were reoptimized at B3LYP/6-311++G** level and their respective relative energies, vertical ionization energies, hydrogen bonding patterns, rotational constants and dipole moments were calculated. A single point energy calculations for leucine conformers have also been done at both B3LYP/6-311++G(2df, p) and MP2/6-311++G(2df, p) levels. The good agreement between our estimates of rotational constants for two most stable conformers and available experimental measurements supports the reliability of the B3LYP/6-311++G** level of theory for describing the conformational behavior of leucine molecule. The proton affinity and gas phase basicity were also determined. Using the statistical approach, conformational distributions at various temperatures have also been performed and analyzed. Vibrational spectra were also calculated. It is also observed that zwitterions of leucine are not stable in gas phase.     

乙醇-水分子团簇C2H5OH(H2O)n (n=1-9)稳定结构的量子化学研究

采用密度泛函理论B3LYP方法, 在B3LYP/6-311++G(2d,2p)//B3LYP/6-311++G(d,p)基组水平上对乙醇-水分子团簇(C₂H₅OH(H₂O)_n (n=1-9))的各种性质进行研究, 如: 优化的几何构型、结构参数、氢键、结合能、平均氢键强度、自然键轨道(NBO)电荷分布、团簇的生长规律等. 结果表明, 从二维(2-D)环状结构到三维(3-D)笼状结构的过渡出现在n=5的乙醇-水分子团簇中. 此外, 利用团簇结合能的二阶差分、形成能、能隙等性质, 发现在n=6时乙醇-水分子团簇的最低能量结构稳定性较好, 可能为幻数结构. 最后, 为了进一步探讨氢键本质, 将C₂H₅OH(H₂O)_n (n=2-9)最低能量结构的各种性质与纯水分子团簇(H₂O)_n (n=3-10)比较, 结果表明前者与后者中的水分子之间氢键相似.     

FC(O)O自由基与NO2反应的量子化学研究

用量子化学DFT, MP2, G3和G3MP2方法对FC(O)O自由基与NO₂的反应机理进行了理论研究. 优化了反应势能面上各驻点的几何结构, 通过内禀反应坐标(IRC)计算和振动分析, 确认了反应中的过渡态, 并用过渡态理论(TST)计算了相关反应的速率常数.     

Protonated nitro group: structure, energy and conjugation

Structure of protonated nitro compounds was investigated by calculations at the levels MP2(FC)/6-311++G(2d,2p)//MP2(FC)/6-311++G(2d,2p)(nitromethane and reference compounds) or B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p)(nitrobenzene and its 18 meta- and para-substituted derivatives). The group NO2H+ reveals many similarities with the isoelectronic group CO2H as the preferred conformation, conformational equilibrium, and stabilization by interaction (resonance) within the group quantified by means of isodesmic reactions. However, there is a difference in the interaction with donor groups (for instance in 4-nitroaniline) that is much stronger with NO2H+ than with CO2H. This interaction may be called resonance and may be described by standard resonance formulas, but these formulas predict only partially the geometry and cannot explain the great interaction energy.     

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.     

类锗烯H2C=GeLiCl与RH(R=F,OH, NH2)的插入反应

采用DFT B3LYP和QCISD方法研究了不饱和类锗烯H2C=GeLiCl与RH(R=F, OH, NH2)的插入反应. 在B3LYP/6-311+G(d,p)水平上优化了反应势能面上的驻点构型. 结果表明, H2C=GeLiCl与HF、H2O 或NH3发生插入反应的机理相同. QCISD/6-311++G(d,p)//B3LYP/6-311+G(d,p)计算的三个反应的势垒分别为173.53、194.48和209.05 kJ·mol-1, 反应热分别为60.18、72.93和75.34 kJ·mol-1. 相同条件下发生插入反应时, 反应活性顺序都是H—F>H—OH>H—NH2.     

Mass spectrometric characterization of 4-oxopentanoic acid and gas-phase ion fragmentation mechanisms studied using a triple quadrupole and time-of-flight analyzer hybrid system and density functional theory

4-Oxopentanoic acid was characterized experimentally by electrospray ionization using a triple quadrupole and time-of-flight analyzer hybrid system. This compound was chosen as a model substance for small organic compounds bearing an acetyl and a carboxyl group. Collision-induced dissociation experiments at different activation energies were performed to elucidate possible fragmentation pathways. These pathways were also studied on the theoretical level using density functional theory (DFT) B3LYP/6-311++G(3df,3pd)//B3LYP/6-31+G(d)+ZPVE calculations. CO2 ejection from the [M-H](-) anion of 4-oxopentanoic acid was observed and the fragmentation pathway studied by DFT reveals a new concerted mechanism for CO2 elimination accompanied by an intramolecular proton transfer within a pentagonal transition state structure. Successive elimination of water and CO from the [M-H](-) anion of 4-oxopentanoic acid was also observed. A rearrangement in the primary deprotonated ketene anion produced after water elimination was found on the theoretical level and leads to CO elimination from the primary product anion [M-H-H2O](-). Energy diagrams along the reaction coordinates of the fragmentation pathways are presented and discussed in detail. Mulliken charge distributions of some important structures are presented.     

Steric and electronic structure of complexes of pyrylium and thiopyrylium cations with borabenzene anion

The coordination of pyrylium and thiopyrylium cations with borabenzene was studied by DFT [B3LYP/6-311++G(d,p)] calculations. The structures of charge-transfer molecular complexes formed by the aromatic ions were predicted. The stabilization is due both to electron density transfer and to covalent bonding.     

Effect of methyl groups substitution on the strength of intramolecular hydrogen bonding of naphthazarin: DFT and NBO studies

A density functional theory (DFT) study-based method B3LYP/6-311++G** was carried out to investigate the methyl groups substitution effect on the structure and the strength of intramolecular hydrogen bonding in naphthazarin (NZ) (5,8-dihydroxy-1,4-naphthoquinone). The full geometry optimization of molecular structures, the difference between the energies of hydrogen-bonded and non-hydrogen-bonded rotamers, and the proton chemical shift of the hydroxyl groups in NZ and its methyl substituents obtained at the B3LYP/6-311++G** level. The vibrational frequencies of all samples and their deuterated analogues were calculated at the same theoretical level. The ¹H chemical shifts for NZ and its methyl substituents were computed at the B3LYP/6-311++G** level using the gauge-including atomic orbital method. Furthermore, in order to investigate the changes in bond order, electron density, electron delocalization, and steric effects caused by methyl substituents, natural bond orbital analysis were carried out at the B3LYP/6-311++G** level. After comparing these effective parameters in methyl substituents with those of their parent, NZ, we concluded that, in general, intramolecular hydrogen bonding strength increases by substituting methyl groups in the different positions of NZ.     

Theoretical study on the mechanism of the gas-phase reaction of diborane(3) anion with carbon disulfide

The complex potential energy surface of the gas-phase reaction of HB(H)BH- with CS2 to give three low-lying products [B2H3S]- + CS, [BH2CS]- + HBS, and [BH3CS] + BS-, involving nine [B2H3CS2]- isomers and 12 transition states, has been investigated at the CCSD(T)/6-311++G(d,p)/B3LYP/6-311++G(d,p) level. Our calculations are in harmony with the recent experimental and theoretical results, and reveal some new bonding and kinetic features of this reaction system. Our theoretical results may help the further identification of the products [BH2CS]- + HBS and [BH3CS] + BS- and may provide useful information on the chemical behaviors of other electron-deficient boron hydride anions.     

Theoretical study of light-metal tetrahydroborates

Ab initio calculations of the structural, energetic, vibrational, and magnetic characteristics of the lowest-lying structures for isolated molecules and ions of light-metal tetrahydroborates (Li, Na, Be, Mg, and Al) have been performed by the perturbation theory (MP2), quadratic configuration interaction (QCISD(T)), coupled cluster (CCSD(T)), and density functional theory (B3LYP) methods using the 6-31G*, 6-31G**, 6-311+G**, and 6-311++G** basis sets. The trends in the behavior of the structural parameters, the energies of different decomposition pathways, barriers to internal rotation of BH₄ groups, normal mode frequencies, magnetic shielding constants, and spin density distribution (in radicals) have been analyzed in various related series of these compounds. The results obtained by ab initio methods and at the DFT level are compared. The economical approximation B3LYP/6-311++G**//B3LYP/6-311+G** adequately reproduces the results obtained at the higher level of theory CCSD(T)/6-311++G*s*//MP2/6-31G* even though it requires considerably shorter CPU times and smaller amounts of memory.     

类硅烯H2C＝SiLiBr与RH (R＝F, OH, NH2)的插入反应

采用DFT B3LYP和QCISD方法研究了类硅烯H2C＝SiLiBr与RH (R＝F, OH, NH2)的插入反应. 在B3LYP/6- 311＋G(d,p)水平上优化了反应势能面上的驻点构型. 结果表明, H2C＝SiLiBr与HF, H2O或NH3发生插入反应的机理相同. QCISD/6-311＋＋G(d,p)//B3LYP/6-311＋G(d,p)计算的三个反应的势垒分别为148.62, 164.42和165.07 kJ&#8226;mol－1, 反应热分别为－69.63, －43.02和－28.27 kJ&#8226;mol－1. 相同条件下发生插入反应时, 反应活性都是H—F＞H—OH＞H—NH2.