The structure of 1-chlorosilatrane: An ab initio molecular orbital and a density functional theory study

The molecular geometries of the 1-chloro-, 1-fluoro-, 1-methyl-, and 1-hydrogenosilatranes were fully optimized by the restricted Hartree-Fock (HF) method supplemented with 3-21G, 3-21G(d), 6-31G(d), and CEP-31G(d) basis sets; by MP2 calculations using 6-31G(d) and CEP-31G(d) basis sets; and by GGA-DFT calculations using 6-31G(d5) basis set with the aim of locating the positions of the local minima on the energy hypersurface. The HF/6-31G(d) calculations predict long (>254 pm) and the MP2/CEP calculations predicted short (∼225 pm) equilibrium Si(SINGLE BOND)N distances. The present GGA-DFT calculations reproduce the available gas phase experimental Si(SINGLE BOND)N distances correctly. The solid phase experimental results predict that the Si(SINGLE BOND)N distance is shorter in 1-chlorosilatrane than in 1-fluorosilatrane. In this respect the HF results show a strong basis set dependence, the MP2/CEP results contradict the experiment, and the GGA-DFT results in electrolytic medium agree with the experiment. The latter calculations predict that 1-chlorosilatrane is more polarizable than 1-fluorosilatrane and also support a general Si(SINGLE BOND)N distance shortening trend for silatranes during the transition from gas phase to polar liquid or solid phase. The calculations predict that the ethoxy links of the silatrane skeleton are flexible. Consequently, it is difficult to measure experimentally the related bond lengths and bond and torsion angles. This is the probable origin of the surprisingly large differences for the experimental structural parameters. On the basis of experimental analogies, ab initio calculations, and density functional theory (DFT) calculations, a gas phase equilibrium (r_e) geometry is predicted for 1-chlorosilatrane. The semiempirical methods predict a so-called exo minimum (at above 310 pm Si(SINGLE BOND)N distance); however, the ab initio and GGA-DFT calculations suggest that this form is nonexistent. The GGA-DFT geometry optima were characterized by frequency analysis. © 1996 by John Wiley & Sons, Inc.     

Solvated CH5+ in liquid superacid

The transition states for methane activation in liquid superacid have been studied by experimentally determined secondary kinetic deuterium isotope effects (SKIEs) and computational chemistry. For the first time, the SKIEs on hydrogen/deuterium exchange of methane have been measured by using the methane isotopologues in homogeneous liquid superacid (2HF/SbF5). To achieve high accuracy of the SKIEs, the rate constants for pairs of methane isotopologues were simultaneously measured in the same superacid solution by using NMR spectroscopy. Density functional theory (DFT) and high-level ab initio methods have been employed to model possible intermediates and transition states, assuming that the superacids involved in the exchange reactions are H2F+ ions solvated by HF. Only the unsolvated superacid H2F+ is found to be strong enough to protonate methane, yielding the methonium ion solvated by HF as a potential energy minimum. In contrast, the (HF)x-solvated H2F+ superacids (x = 1-4) do not appear to be strong enough to yield stable solvated methonium ions. However, such ions show up as parts of the transition states of the exchange in which the methonium ions are solvated by (HF)x. The calculated DFT activation barrier is in good agreement with that experimentally observed.     

Determination of the overlapping pK(a) values of chrysin using UV-vis spectroscopy and ab initio methods

The overlapping pK(a) values of 5,7-dihydroxyflavone (chrysin) in EtOH-water solutions were determined by means of a UV-vis spectroscopic method that uses absorbance diagrams, at constant ionic strength (0.050 M) and temperature (25.0+/-0.1 degrees C). It was observed that the pK(a) values increase when the polarity-polarizability and solvation abilities of the reaction medium decrease. In order to calculate the pK(a1) and pK(a2) of chrysin in pure water, various relationships between the determined pK(a) and properties of solvents (relative permittivity, alpha-parameter of Taft and parameter Acity), are proposed. Moreover, with the aim of explaining the first pK(a1) value obtained, the molecular conformations and solute-solvent interactions of the 7(O(-))chrysinate monoanion were also investigated, using ab initio methods. Several ionization reactions and equilibria in water, which possesses a high hydrogen-bond-donor ability, are proposed. These reactions and equilibria constituted the necessary theoretical basis to calculate the first acidity constant of chrysin. The HF/6-31G(d) and HF/6-31+G(d) methods were used for calculations. Tomasi's method was used to analyze the formation of intermolecular hydrogen bonds between the 7(O(-))chrysinate monoanion and water molecules. It was proposed that in alkaline aqueous solutions the monoanion of chrysin is solvated with one water molecule. The agreement between the experimental and theoretical pK(a1) values provides good support for the acid-base reactions proposed in this paper.     

丙酮酸分子结构与振动光谱的密度泛函理论研究

用密度泛函方法BLYP、B3LYP和从头算Hartree-Fock(HF)方法在6-31G*基组水平上对丙酮酸分子的几何结构（甲基的重叠式和交错式两种构象）和振动光谱分别进行了优化和计算,并给出了各种频率所对应的红外强度及拉曼活性,对光谱进行了指认。结果表明：在丙酮酸分子的两种构象中,重叠式比较稳定*B3LYP计算得到的构型参数与实验结果比较一致;在振动频率的计算中,BLYP未标度力场所计算的非CH3伸缩振动基频预测值和实验值的平均绝对偏差为10.4cm-1;而HF标度力场的平均绝对偏差为17.9cm-1。说明两者的结果与实验观测频率比较吻合,但B3LYP的频率计算值偏差（38.3cm-1）较大。根据振动频率的势能分布和红外光谱强度对此分子的振动基频进行了理论归属。     

Determination of the overlapping pKa values of resorcinol using UV-visible spectroscopy and DFT methods

In this paper we determine the overlapping pK(a) values of resorcinol in water, applying a UV-Vis spectroscopic method that uses absorbance diagrams. On the other hand, in order to explain the pK(a) values obtained, we also investigate the molecular conformations and solute-solvent interactions of the resorcinate anions, using ab initio and density functional theory methods. Several ionization reactions and equilibria in protic solvents, which possess a high hydrogen-bond-donor capability, are proposed. The mentioned reactions and equilibria constituted the indispensable theoretical basis to calculate the acidity constants of resorcinol. Basis sets at the HF/6-31 + G(d) and B3LYP/6-31 + G(d) levels of theory were used for calculations. Tomasi's method was used to analyze the formation of intermolecular hydrogen bonds between the resorcinate anions and water molecules. In this way, it was determined that in alkaline aqueous solutions the monoanion and dianion of resorcinol are solvated with two and four molecules of water, respectively. The agreement between the experimentally determined pK(a) values and those reported in the literature demonstrates the applicability and accurateness of the spectroscopic method here used. On the other hand, the agreement between the experimental and theoretically calculated pK(a) values provides solid support for the acid-base reactions proposed in this work.     

Accurate ab initio pair potentials between helium and the heavier group 2 elements

Interactions between the heavier Group 2 metals (Ca, Sr, and Ba) and helium were studied using the well-tempered model core potential method. Accurate pair potentials, calculated at the coupled-cluster level of theory with very large basis sets, were used in bound state calculations. Three bound rovibrational states were found for each complex. The pair-potential parameters were used to predict how each of the metal atoms would be solvated by a helium nanodroplet. The Ca atom is not fully solvated by the droplet and the interaction between the helium and the metal decreases from Ca to Ba. This agrees with the experimental observation that the spectra of these atoms in a nanodroplet are intermediate between the spectra of the free atoms and the spectra in liquid helium.     

Ab initio study of some peroxides. HOOH, CH3OOH and, CH3OOCH3

The geometries of HOOH, CH₃OOH, and CH₃OOCH₃, were optimized with different basis sets (3-21G, 6-31G^*(*) and D95^**) at different levels of theory (HF, MP2, MP4, and CI). HF/3-21G optimizations result in planar trans conformations for all three peroxides. HF/6-31G^** calculations predict skew conformations for HOOH and CH₃OOH, but a planar trans struture for CH₃OOCH₃. For the larger basis set the calculated bond lengths, especially the O-O bonds, are too short. Optimizations for HOOH including electron correlation at the MP2, MP3, MP4, CI, and CCD level improve the agreement for bond lengths and the OOH angle, but result in dihedral angles Which are too large by 3– 8°. In the case of CH₃OOCH₃, similar calculations at the MP2 and CI level predict planar trans structures instead of the experimentally observed skew conformation. On the other hand, MP4 single point calculations at MP2 optimized parameters result in a correct skew structure. For all three peroxides a computationally “economic” method, i.e., single point calculations at MP2 or MP4 level with HF/3-21G optimized parameters, result in close agreement between calculated and experimental structures.     

Self consistent tight binding model for dissociable water

We report results of development of a self consistent tight binding model for water. The model explicitly describes the electrons of the liquid self consistently, allows dissociation of the water and permits fast direct dynamics molecular dynamics calculations of the fluid properties. It is parameterized by fitting to first principles calculations on water monomers, dimers, and trimers. We report calculated radial distribution functions of the bulk liquid, a phase diagram and structure of solvated protons within the model as well as ac conductivity of a system of 96 water molecules of which one is dissociated. Structural properties and the phase diagram are in good agreement with experiment and first principles calculations. The estimated DC conductivity of a computational sample containing a dissociated water molecule was an order of magnitude larger than that reported from experiment though the calculated ratio of proton to hydroxyl contributions to the conductivity is very close to the experimental value. The conductivity results suggest a Grotthuss-like mechanism for the proton component of the conductivity.     

Ab initio and DFT studies on the mechanism of the 1,3-dipolar cycloaddition reaction between nitrone and sulfonylethene chloride

The molecular mechanism for the 1,3-dipolar cycloaddition of nitrone with sulfonylethene chlorides has been studied using ab initio and DFT methods at the HF, MP2 and B3LYP levels together with the 6-31G* basis set. Relative rates, stereo and regioselectivity, have been analysed and discussed. For this cycloaddition four reactive channels associated with the formation of two pairs of diastereoisomeric regioisomers have been characterized. Analysis of the geometries of the corresponding transition structures shows that the cycloaddition takes place along a concerted but asynchronous mechanism. Activation energies as asynchronicity are dependent on the computation level. Thus, while HF calculations gave large barriers, MP2 calculations tend to underestimate them. DFT calculations gave reasonable values. These 1,3-dipolar cycloadditions present an endo stereoselectivity while the meta regioselectivity depends on the computational level. Thus, while HF and DFT calculations predict meta path, in agreement with the experimental results, MP2 calculation predict ortho regioselectivity. The frontier molecular orbitals analysis shows that the reaction is controlled by the (HOMO_dipole–LUMO_{dipolarophile}) interaction in agreement with the charge transfer analysis carried out at the transition structures. Inclusion of diffuse functions at the B3LYP/6-31+G* level increases the energy barriers about 4 kcal/mol, giving a similar endo/meta selectivity. Solvent effects have been taken into account, by means of self-consistent reaction field.     

Structure and stability of VO2+ in aqueous solution: a Car-Parrinello and static ab initio study

Quantum chemical calculations have been carried out to get some insight concerning the effects of temperature and solvent acidity on the structure and stability of solvated VO2+ as the elementary chemical unit involved in the nucleation of vanadophosphates. First, because some recent theoretical studies have suggested a tendency of density functional theory (DFT) to favor lower coordination numbers for such systems, static calculations have been performed on [VO2(H2O)(4-n)]+.nH2O (n=0-2) conformers at the MP2 and DFT level of theory, using two different combinations of basis sets. The results of two pure-GGA (BP86 and PBEPBE), two hybrid-GGA (PBE1PBE and mPWPW91), and two hybrid-meta-GGA (mPW1B95 and B1B95) functionals were analyzed on these systems. The comparison of the results indicates that the stability differences between the two methodologies are resolved when hydration energy is taken into account, provided that some amount of HF exchange is introduced in the DFT calculations. In a second step, Car-Parrinello simulations have been carried out starting from VO2(H2O)4+ surrounded by water molecules. The calculations at 300 K show the natural tendency of VO2(H2O)4+ to decompose to VO2(OH)2- and the requirements to work with an already acidified medium to be able to investigate the coordination sphere of VO2+ for an extended period of time. Under such conditions, we have obtained a clear preference for a five-coordinated vanadium. The molecular dynamics simulations performed at 500 K starting from hydrated VO2+ in a protonated medium found VO(OH)3 to be the most stable structure, whereas this ideal candidate for oxolation reactions is expected to be a very minor species at room temperature.     

离子液中溶剂化电子的结构及其演化动力学

本文综述了离子型液体介质中过剩电子的结构、存在状态及其时间演化动力学特征。基于从头算分子动力学模拟及计算结果,重点阐述了咪唑型、吡啶型、碱金属离子型熔盐氯化物离子液中与过剩电子溶剂化密切相关的溶剂化能量学、结构特征、可能的存在状态以及态-态转化稳态动力学机制,分析了此类离子型介质中电子高效传导的内在本质及离子液组成离子的重要作用。阳离子的最低未占轨道组成的导带结构是离子液中过剩电子的溶剂化态及其稳定性的决定因素,任何能影响或改变其导带结构的因素均能显著影响过剩电子溶剂化。但快速的态-态转化及电子迁移并不明显取决于其组成离子扩散动力学,而是敏感地受离子液涨落所控制。这种基于溶剂化电子的迁移模式构成了此类离子型介质甚至其它液态介质中电子转移的新途径。     

Infrared spectra of the OH+ and H2O+ cations solvated in solid argon

Infrared spectra of various OH+ and H2O+ isotopomers solvated in solid argon are presented. The OH+ and H2O+ cations were produced by co-deposition of H2O/Ar mixture with high-frequency discharged Ar at 4 K. Detailed isotopic substitution studies confirm the assignments of absorptions at 3054.9 and 3040.0 cm(-1) to the antisymmetric and symmetric H-O-H stretching vibrations of H2O+ and 2979.6 cm(-1) to the O-H stretching vibration of OH+. The frequencies of H2O+ solvated in solid argon are red-shifted, whereas the frequency of OH+ is blue-shifted with respect to the gas-phase fundamentals. On the basis of previous gas-phase studies and quantum chemical calculations, the OH+ and H2O+ cations solvated in solid argon may be regarded as the OH+-Ar5 and H2O+-Ar4 complexes isolated in the argon matrix.     

Insights into the post-translational methylation of arginine from studies of guanidinium-water nanodroplets

Highly solvated methylated guanidinium ions ([Me nGuan(H2O)m](+); n=0-4 and 6; m=1->50), which model the various post-translationally methylated states of arginine, were generated in the electrospray ionization (ESI) source of an unmodified commercial tandem mass spectrometer. The dehydration processes of highly solvated [Me nGuan(H2O)21](+) ions were monitored by using energy-dependent ESI MS. These data, together with supporting calculations, provide a detailed picture of the interplay of methylation and hydration, and both the calculations and the experimental evidence suggest a specific structural basis for the observed effects. The ramifications for biochemical binding events that are controlled by the post-translational methylation of arginine to give dimethylarginines are discussed.     

Structure and vibrational frequencies of 2-butanimine

The conformational behavior and structural stability of 2-butanimine were investigated by utilizing ab initio calculations with 6-311++G** basis set at HF, MP2, B3LYP and BLYP levels. The vibrational frequencies of 2-butanimine were computed. Complete vibrational assignments were made on the basis of normal coordinate calculations for stable conformer of the molecule. HF results without scaled quantum mechanical (SQM) force field procedure considered are in bad agreement with experimental values. Of the two DFT methods, BLYP reproduces the observed fundamental frequencies most satisfactorily with the mean absolute deviation of the non-CH stretching modes less than 21.3 cm(-1). The results indicate that BLYP calculation is a very promising approach for understanding the observed spectral features.     

硝酸氯在冰表面上反应的研究

对硝酸氯与一，二，三个水分子的反应作为硝酸氯在冰表面上反应的模型进行了理论研究．MP2     

Experimental (FTIR and FT-Raman) and ab initio and DFT study of vibrational frequencies of 5-amino-2-nitrobenzoic acid

The FTIR and FT-Raman spectra of 5-amino-2-nitrobenzoic acid (ANB) have been recorded in the region 400-4000cm(-1). The observed frequencies were assigned to different modes of vibrations on the basis of fundamental, combination and overtones. The geometry has been optimized with complete relaxation on the potential energy surface at HF, MP2 and B3LYP level of theories using 6-311++G(d,p) basis set and compared with the crystal data. The possible hydrogen bond interaction has been estimated taking a model compound. Further harmonic vibrational frequency calculations have been carried out at HF and B3LYP levels and the scaled values were in good agreement with majority of the experimental observations. The theoretically constructed spectra coincide satisfactorily with those of experimental spectra.     

Density functional theory study of vibrational spectra of acridine and phenazine

Density functional theory (DFT) calculations using Becke's exchange in conjunction with Lee-Yang-Parr's correlation functionals (BLYP), Becke's three-parameter hybrid DFT/HF method using Lee-Yang-Parr's correlation functionals (B3LYP) and ab initio Hartree-Fock (HF) method have been carried out to investigate the structure and vibrational spectra of acridine and phenazine. Structural parameters obtained by B3LYP/6-31G* geometry optimization are in good agreement with available experimental data. The raw BLYP non-CH stretching frequencies approximate the experimental results much better than the HF results with the mean absolute deviation about 16 cm(-1). The scaled B3LYP frequencies are more reliable than that of the BLYP and HF methods with the mean absolute deviation about 17 cm(-1). On the basis of the comparison between calculated and experimental results, assignments of fundamental vibrational modes are examined. Also the structure and vibrational frequencies are compared with those of anthracene, pyridine and benzene to study the similarities and differences.     

Extraction of tetra-alkylammonium bromides into 1,2-dichloroethane + alkane mixtures

The influence of an inert diluent on the extractability of tetradecyltrimethyl-, tetradecyl-triethyl-, decyltriethyl- and tetra-amylammonium bromides into 1,2-dichloroethane + n-alkane mixtures at 25 degrees has been determined. On the basis of the experimental data it is suggested that the bromide moiety of the ion-pair is solvated in the organic phase by 5-6 molecules of 1,2-dichloroethane.     

Basis set and method dependence in quantum theory of atoms in molecules calculations for covalent bonds

The influence of various small- and medium-size basis sets used in Hartree-Fock (HF) and density functional theory (DFT)/B3LYP calculations on results of quantum theory of atoms in molecules based (QTAIM-based) analysis of bond parameters is investigated for several single, double, and triple covalent bonds. It is shown that, in general, HF and DFT/B3LYP methods give very similar QTAIM results with respect to the basis set. The smallest 6-31G basis set and DZ-quality basis sets of Dunning type lead to poor results in comparison to those obtained by the most reliable aug-cc-pVTZ. On the contrary, 6-311++G(2df,2pd) and in a somewhat lesser extent 6-311++G(3df,3pd) basis sets give satisfactory values of QTAIM parameters. It is also demonstrated that QTAIM calculations may be sensitive for the method and basis set in the case of multiple and more polarized bonds.