Interaction of anions with perfluoro aromatic compounds

The complexes formed by a variety of anions with perfluoro derivatives of benzene, naphthalene, pyridine, thiophene, and furan have been calculated using DFT (B3LYP/6-31++G**) and MP2 (MP2/6-31++G** and MP2/6-311++G**) ab initio methods. The minimum structures show the anion interacting with the pi-cloud of the aromatic compounds. The interaction energies obtained range between -8 and -19 kcal mol(-1). The results obtained at the MP2/6-31++G** and MP2/6-311++G** levels are similar. However, the B3LYP/6-31++G** results provide longer interaction distances and smaller interaction energies than do the MP2 results. The interaction energies have been partitioned using an electrostatic, polarization, and van der Waals scheme. The AIM analysis of the electron density shows a variety of topologies depending on the aromatic system considered.     

Theoretical study of light-metal tetrahydroaluminates

Ab initio calculations of the structural, energetic, vibrational, and magnetic characteristics of the lowest-lying structures for isolated molecules and ions of light-metal tetrahydroaluminates (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 molecular characteristics have been analyzed in various related series of these compounds. The results are compared with the data on analogous light-metal tetrahydroborates calculated at the same levels of theory. The differences in structure and stability between analogous hydroborate and alanate complexes are examined. The economical approximation B3LYP/6-311++G**//B3LYP/6-311+G** has been shown to adequately reproduce the results obtained at the higher level of theory CCSD(T)/6-311++G**//MP2/6-31G* even though it requires considerably shorter CPU times and smaller amounts of memory.     

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.     

Computational treatment of the microsolvation of neutral and zwitterionic forms of alanine

B3LYP/6-31++G** and MP2/6-31++G**//B3LYP/6-31++G** calculations are reported for the structures of neutral alanine–(H₂O)_n and zwitterionic alanine–(H₂O)_n clusters where n = 2–10. Optimized geometries and energies were obtained. In general, with an increasing number of water molecules, the hydrated zwitterionic form becomes more thermodynamically stable. In the presence of six or seven water molecules, the energetics indicate that the two forms may coexist. Eight water molecules are sufficient to computationally guarantee the reported experimental observation of zwitterionic dominance in solution.     

Basis set effects on calculated geometries: 6-311++G** vs. aug-cc-pVDZ

The effect of basis sets on MP2 and CCSD calculated geometries has been investigated for a series of small organic molecules with two to four nonhydrogen atoms. Whereas 6-311++G** usually leads to satisfactory structures, this was not true with aug-cc-pVDZ, although it uses more basis functions than the former set. In addition, the smaller 6-311++G** invariably leads to lower calculated energies than aug-cc-pVDZ. With a given basis set, CCSD gives more satisfactory calculated geometries than MP2, but with a much greater computation time. For the compounds in this report, B3LYP/6-311++G** is quite effective in giving satisfactory calculated geometries at a relatively small computational cost.     

A theoretical study on the intermolecular interaction of energetic system-Nitromethane dimer

Three optimized geometries of nitromethane dimer have been obtained at the HF/6-31G level.Dimer binding energies have been corrected for the basis set superposition error (BSSE) and the zero point energy.Computed results indicate that the cyclic structure of (CH3NO2)2 is the most stable of three optimized geometries,whose corrected binding energyis 17.29 kJ mol-1 at the MP4SDTQ/6-31G//HF/6-31G level.In the optimized structures of nitromethane dimer,the inter-molecular hydrogen bond has not been found; and the charge-transfer interaction between CH3NO2 subsystems is weak; and the correlation interaction energy makes a little contribution to the intermolecular interaction energy of the dimer.     

Pi-pi interaction in pyridine

pi-pi Interaction in pyridine dimer and trimer has been investigated in different geometries and orientations at the ab initio (HF, MP2) and DFT (B3LYP) levels of theory using various basis sets (6-31G, 6-31G, 6-311++G) and corrected for basis set superposition error (BSSE). While the HF and DFT calculations show the pyridine dimer and the trimer to be unstable with respect to the monomer, the MP2 calculations show them to be clearly stable, thus emphasizing the need to include electron correlation while determining stacking interaction in such systems. The calculated MP2/6-311++G binding energy (100% BSSE corrected) of the parallel-sandwich, antiparallel-sandwich, parallel-displaced, antiparallel-displaced, T-up and T-down geometries for pyridine dimer are 1.53, 3.05, 2.39, 3.97, 1.91, 1.47 kcal/mol, respectively. The results show the antiparallel-displaced geometry to be the most stable. The binding energies for the trimer in parallel-sandwich, antiparallel-sandwich, and antiparallel-displaced geometry are found to be 3.18, 6.14, and 8.04 kcal/mol, respectively.     

Theoretical Studies on the Intermolecular Interactions of Aza-calix[2]arene[2]-triazines with RDX

Six fully optimized structures of the aza-calix[2]arene[2]-triazines/RDX supramo-lecular complexes have been obtained at the DFT-B3LYP/6-311++G** level,and the corresponding intermolecular interactions have been investigated using the B3LYP,mPWPW91 and MP2 methods at the 6-311++G** level,respectively.The natural bond orbital(NBO) and atoms in molecules(AIM) analyses have been performed to reveal the origin of interactions.To our interest,the result indicates that the strongest interaction is up to -22.34 kJ/mol after basis set superposition error(BSSE) and zero point energy(ZPE) correction at the MP2/6-311++G** level.Furthermore,the intermolecular interactions between aza-calix[2]arene[2]-triazines with the substituted amidos and RDX are stronger than those of other complexes.Thus,the complexes with amidos can be used as the candidates to increase the stability of explosive and eliminate the explosive wastewater.     

A quantum-chemical study of the LiH + LiH⋅− ⇄ LiH2− + Li⋅ reaction

Parts of the potential energy surface of the title process and related processes have been investigated at the SCF /6-31G **, SCF /6-31++G **, and MP 2/6-31++G ** levels. The investigated reaction is exothermic (?6.23 kcal/mol, MP 4/6-31++G **//MP 2/6–31++G** level, ZPE included): A linear intermediate radical anion, Li? H? Li? H??, is significantly stabilized with respect to LiH + LiH?? (?38.74 kcal/mol, the same level as above). The BSSE at MP 2/6–31++G **//MP 2/6–31++G ** amounts to 1.8 kcal/mol. The title process seems to be suitable for experimental study in molecular beams.     

Theoretical Study on Intermolecular Interactions and Thermodynamic Properties of Nitroamine Dimers

Ab initio self-consistent field(SCF) and Mφller-Plesset correlation correction methods employing 6-31G^** basis set have been applied to the optimizations of nitroamine dimers.The binding energies have been corrected for the basis set superposition error (BSSE) and the zero-point energy.Theree optimized dimers have been obtained.The BSSE corrected binding energy of the most stable dimer is predicted to be -31.85kJ/mol at the MP4/6-31G^**//MP2/6-31G^** level.The energy barriers of the Walden conversion for -NH2 group are 19.7kJ/mol and 18.3kJ/mol for monomer and the most stable dimer,respectively.The molecular interaction makes the internal rotation around N1-N2 even more difficult.The thermodynamic properties of nitroamine and its dimers at different temperatures have been calculated on the basis of vibrational analyses.The change of the Gibbs free energy for the aggregation from monomer to the most stable dimer at standard pressure and 298.2 K is predicted to be 14.05kJ/mol.     

An ab initio and density functional theory study of keto-enol equilibria of hydroxycyclopropenone in gas and aqueous solution phase

Keto-enol tautomerism in hydroxycyclopropenone (2-hydroxy-2-cyclopropen-1-one) has been studied using ab initio methods, the B3LYP functional of density functional theory, as well as complete basis set (CBS-QB3 and CBS-APNO) and G3 methods. Absolute and relative energies were calculated with each of the methods, whereas computations of geometries and harmonic frequencies for hydroxycyclopropenone and 1,2-cyclopropanedione were computed in the gas phase but were limited to HF, MP2 and CCSD levels of theory, and the B3LYP functional, in combination with the 6-31++G** basis set. Using the MP2/6-31++G** gas phase optimized structure, each species was then optimized fully in aqueous solution by employing the polarizable continuum model (PCM) self-consistent reaction field approach, in which HF, MP2 and B3LYP levels of theory were utilized, with the same 6-31++G** basis set. In both gas and aqueous solution phases, the keto form is higher in energy for all of the model chemistries considered. The presence of the solvent, however, is found to have very little effect on the bond lengths, angles and harmonic frequencies. From the B3LYP/6-31++G** Gibbs free energy, the keto-enol tautomeric equilibrium constant for 2-hydroxy-2-cyclopropen-1-one <==> 1,2-cyclopropanedione is computed to be K(T)(gas) = 2.35 x 10(-6), K(T)(aq) = 5.61 x 10(-14). It is concluded that the enol form is overwhelmingly predominant in both environments, with the effect of the solvent shifting the direction of equilibrium even more strongly in the favor of hydroxycyclopropenone. The almost exclusive nature of this species is attributed to stabilization resulting from aromaticity. Confirmation is provided by comparison of the simulated vibrational spectra of hydroxycyclopropenone with the measured infrared spectrum in an argon matrix.     

Ab initio studies of aspartic acid conformers in gas phase and in solution

Systematic and extensive conformational searches of aspartic acid in gas phase and in solution have been performed. For the gaseous aspartic acid, a total of 1296 trial canonical structures and 216 trial zwitterionic structures were generated by allowing for all combinations of internal single-bond rotamers. All the trial structures were optimized at the B3LYP/6-311G* level and then subjected to further optimization at the B3LYP/6-311++G** level. A total of 139 canonical conformers were found, but no stable zwitterionic structure was found. The rotational constants, dipole moments, zero-point vibrational energies, harmonic frequencies, and vertical ionization energies of the canonical conformers were determined. Single-point energies were also calculated at the MP2/6-311++G** and CCSD/6-311++G** levels. The equilibrium distributions of the gaseous conformers at various temperatures were calculated. The proton affinity and gas phase basicity were calculated and the results are in excellent agreement with the experiments. The conformations in the solution were studied with different solvation models. The 216 trial zwitterionic structures were first optimized at the B3LYP/6-311G* level using the Onsager self-consistent reaction field model (SCRF) and then optimized at the B3LYP/6-311++G** level using the conductorlike polarized continuum model (CPCM) SCRF theory. A total of 22 zwitterions conformers were found. The gaseous canonical conformers were combined with the CPCM model and optimized at the B3LYP/6-311++G** level. The solvated zwitterionic and canonical structures were further examined by the discrete/SCRF model with one and two water molecules. The incremental solvation of the canonical and zwitterionic structures with up to six water molecules in gas phase was systematically examined. The studies show that combining aspartic acid with at least six water molecules in the gas phase or two water molecules and a SCRF solution model is required to provide qualitatively correct results in the solution.     

Structure of the boronic acid dimer and the relative stabilities of its conformers

Despite the widespread use of boronic acids in materials science and as pharmaceutical agents, many aspects of their structure and reactivity are not well understood. In this research the boronic acid dimer, [HB(OH)(2)](2), was studied by second-order M?ller-Plesset (MP2) perturbation theory and coupled cluster methodology with single and double excitations (CCSD). Pople split-valence 6-31+G*, 6-311G**, and 6-311++G** and Dunning-Woon correlation-consistent cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis sets were employed for the calculations. A doubly hydrogen-bonded conformer (1) of the dimer was consistently found to be lowest in energy; the structure of 1 was planar (C(2h)) at most computational levels employed but was significantly nonplanar (C(2)) at the MP2/6-311++G** and CCSD/6-311++G** levels, the result of an intrinsic problem with Pople-type sp-diffuse basis functions on heavy atoms. The dimerization energy, enthalpy, and free energy for the formation of (1) from the exo-endo conformer of the monomer were -10.8, -9.2, and +1.2 kcal/mol, respectively, at the MP2/aug-cc-pVTZ level. Several other hydrogen-bonded conformers of the dimer were local minima on the potential energy surface (PES) and ranged from 2 to 5 kcal/mol higher in energy than 1. Nine doubly OH-bridged conformers, in which the boron atoms were tetracoordinated, were also local minima on the PES, but they were all greater than 13 kcal/mol higher in energy than 1; doubly H-bridged structures proved to be transition states. MP2 and CCSD results were compared to those from the BLYP, B3LYP, OLYP, O3LYP, PBE1PBE, and TPSS functionals with the 6-311++G** and aug-cc-pVTZ basis sets; the PBE1PBE functional performed best relative to the MP2 and CCSD results. Self-consistent reaction field (SCRF) calculations predict that boronic acid dimerization is less favorable in solution than in vacuo.     

Theoretical Studies on Intermolecular Hydrogen-bond Interactions between Hexamethylenetetramine and Nitric Acid

The structures of the complexes generated by hexamethylenetetramine and nitric acid have been fully optimized by B3LYP method at the 6-311++G** and aug-cc-pVTZ levels. The intermolecular hydrogen-bonding interactions have been calculated by the B3LYP/6-311++G**, B3LYP/aug-cc-pVTZ, MP2(full)/6-311++G** and CCSD(T)/6-311++G** methods, respectively. The NBO (nature bond orbital), AIM (atom in molecule), temperature effect and solvation effect have been analyzed to reveal the origin of the interactions. The results indicate that the stable hydrogen-bonded complexes could be generated by hexamethylenetetramine and nitric acid. The interactions follow the order of (a)>>(e)>(b)>(c)>(d)>(f)>(g). The C-N bonds which are adjacent to the methylene involving the hydrogen bonds tend to break in the chemical reaction. Due to the exothermic process, low temperature is conducive to the formation of the composition, which tallies with the experimental result.     

HOCl…HCOCl复合物的结构和电子性质

在DFT-B3LYP/6-311++G**水平上求得HOCl+HCOCl复合物势能面上的四种稳定构型(S1, S2, S3和S4). 其中, 在复合物S1和S3中, HOCl单体的5H原子作为质子供体, 与HCOCl单体中作为质子受体的1O原子相互作用, 形成红移氢键复合物; 在复合物S4中, HOCl单体的7Cl原子作为质子供体, 与HCOCl单体中作为质子受体的1O原子相互作用, 形成红移卤键复合物; 而在复合物S2中, 同时存在2C—3H…6O蓝移氢键和4Cl…5O相互作用. 在MP2/6-311++G**水平上计算的单体间的相互作用能考虑了基组重叠误差(BSSE)和零点振动能(ZPVE)校正, 其值在-5.05与-14.76 kJ·mol-1之间. 采用自然键轨道理论(NBO)对两种单体间相互作用的本质进行了考查, 并通过分子中原子理论(AIM)分析了复合物中氢键和卤键键鞍点处的电子密度拓扑性质.     

2-丁基-四氢噻吩亚砜13C-NMR的理论研究

在RHF/6-31G和B3LYP/6-31G水平上对顺式(Cis-)与反式(Trans-)2-丁基-四氢噻吩亚砜(BTHTO)进行几何优化,应用规范不变原子轨道法(GIAO)在6-31G、6-31+G、6-31++G和6-31+G(2d,p)水平上计算了Cis-和Trans-BTHTO的13C-NMR,对13C-NMR谱进行了归属。结果表明,BTHTO噻吩五元环的稳定构象呈半椅式,Cis-和Trans-BTHTO中与硫原子直接碳原子13C-NMR的显著差异主要是由于空间构型不同引起分子的静电势场对相应碳原子的屏蔽作用不同所致。     

The use of the hybrid density functional theory (DFT) approach for calculation of vibrational spectra: nitromethane

The molecular geometry of nitromethane was optimized and its force field and vibrational spectrum were calculated by the BECKE3LYP method. The accuracy of optimization of the geometry of MeNO₂ obtained by this method using the 6–311G(d,p) and 6–311++G(d,p) basis sets is not poorer than that obtained at the second-order Møller-Plesset level of perturbation theory (MP2). The vibrational frequencies of nitromethane and its d₁, d₂, and d₃ isotopomers obtained by the BECKE3LYP method are in much better agreement with the experimental data than those calculated at the MP2 level using the same basis set. The average absolute error of calculations performed without the use of any scaling factors is ～2% for frequencies; the maximum deviation is ～4%.     

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.     

Structures and Electronic Properties of HOCl···HCOCl Complexes

B3LYP/6-311++G** and MP2/6-311++G** calculations were used to analyze the interaction between hypochlorous acid (HOCl) and formyl chloride (HCOCl). The results showed that there were four equilibrium geometries (S1, S2, S3, and S4) optimized at B3LYP/6-311++G** level, and all the equilibrium geometries were confirmed to be in stable states by analytical frequency calculations. Complexes S1 and S3 use the 5H atom of HOCl as proton donor and the terminal 1O atom of HCOCl as acceptor to form red shift hydrogen bond systems. However, the blue-shifted hydrogen bond (2C-3H···6O) coexists with 4Cl···5O interaction in structures S2. As for S4, it uses the 7Cl atom of HOCl as proton donor and the terminal 1O atom of HCOCl as acceptor to form red shift halogen bond system. Interaction energies between monomers in the four complexes corrected with basis set superposition error (BSSE) and zero-point vibrational energy (ZPVE) lie in the range from −5.05 to −14.76 kJ·mol⁻¹ at MP2/6-311++G** level. The natural bond orbital (NBO) and atoms in molecules (AIM) theories have also been applied to explain the structures and the properties of the complexes.     

气相中O3与HSO自由基间的氢键复合物

气相中O3与HSO自由基之间的相互作用及其反应在大气化学中非常重要.在DFT-B3LYP/6-311++G**和MP2/6-311++G**水平上求得O3+HSO复合物势能面上的稳定构型,B3LYP方法得到了三种构型(复合物Ⅰ,Ⅱ和Ⅲ),而MP2方法只能得到一种构犁(复合物Ⅱ).在复合物Ⅰ和Ⅲ中,HSO单元中的1H原子作为质子供体.与O3分子中的端基O原子作为质子受体相互作用,形成红移氢键复合物;而在复合物Ⅱ中,虽与复合物Ⅰ和Ⅲ中具有相间的质子供体和质子受体,却形成了蓝移氢键复合物.B3LYP/6-311++G**水平上计算的单体间相互作用能的计算考虑了基组重甍误差(BSSE)和零点振动能(ZPVE)校正,其值在-3.37到-4.55 kJ·mol-1之间.采用自然键轨道理论(NBO)对单体间相互作用的本质进行了考查,并通过分子中原子理论(AIM)分析了三种复合物中氢键的电子密度拓扑性质.