Contemplation on Some Prismatic Polynitrogen Structures – A DFT Treatment

Polynitrogen compounds, are rare molecules having only nitrogen atoms. In recent years, they have been considered as promising candidates of clean (green) high energy density materials. They possess high energy content and their sole decomposition product is N₂. Presently some prismatic polynitrogen structures (N₆ – N₁₄) are considered within the limitations of density functional theory at the levels of B3LYP/6‐311++G(d,p) and B3LYP/cc‐PVTZ. The calculations reveal that they are all highly endothermic but stable. Certain quantum chemical properties, IR and UV/Vis spectra are reported. Homolytic bond cleavage of top rings are considered. Then, the transition state and activation energies, and also homolytic bond dissociation energies for the top rings have been calculated at the level of UB3LYP/6‐311++G(d,p). Also NICS(0) values have been calculated at the level of B3LYP/6‐311++G(d,p). The results indicate that N₆ and N₁₀ have aromatic and nonaromatic top (and also base) rings, respectively. All the rest of the structures have antiaromatic rings (all the structures have antiaromatic N₄ rings).     

Parquet compounds on the basis of eight- and twelve-membered structure blocks: Quantum-chemical study

Annelated structures of cyclooctatetraene and cyclododecahexene and their expanded lowmolecular derivatives were studied using DFT B3LYP method with 6-311G(d,p), 6-311(df,p), 6-311+G(3df) basis sets. Monomer systems including planar antiaromatic cycles are stable in the singlet state, whereas the corresponding triplet states are significantly destabilized. The stability of nonstandard planar forms with antiaromatic cycles is provided by steric factors and the effects of π-electron interaction between the central and peripheral fragments.     

Reaction of ketenyl radical with acetylene: a promising route for cyclopropenyl radical

The reaction of the ketenyl radical (HCCO) with acetylene (C(2)H(2)) is very relevant to the oxygen-acetylene flames and fuel-rich combustion process for nitrogen-containing compounds. Unfortunately, except for several rate constant measurements, the mechanism is completely unknown for this reaction. In this paper, detailed theoretical investigations are performed for the HCCO + C(2)H(2) reaction at the G3B3 level using the B3LYP/6-31G(d), B3LYP/6-311++G(d,p), and QCISD/6-31G(d) geometries. The exclusive fragmentation channel is the formation of the cyclopropenyl radical (c-C(3)H(3)) and carbon monoxide (CO) via the chainlike OCCHCHCH and three-membered ring OC-cCHCHCH intermediates. Thus, the mass spectroscopic peak of C(3)H(3)(+) in a previous experiment can be explained. The calculated overall reaction barrier is 4.4, 4.4, and 5.3 kcal/mol at the G3B3//B3LYP/6-31G(d), G3B3//B3LYP/6-311++G(d,p), and G3B3//QCISD/6-31G(d) levels, respectively. The title reaction may provide an effective route for generating the long-sought cyclopropenyl radical in the laboratory, which has been the long-standing subject of numerous theoretical studies as the simplest cyclic conjugate radical, and its bulky derivatives were already known. Future experimental investigations for the HCCO + C(2)H(2) reaction are greatly desired to test the predicted fragmentation channel. The implication of the present study in combustion and interstellar processes is discussed.     

Aromatic behavior of three membered rings

The aromatic behavior of three membered ring compounds has been analyzed by mean of GAUSSIAN94 theoretical calculations at B3LYP//6-311++G(2d,p) level. Diamagnetic exaltation values obtained from magnetic susceptibilities as well as NICS show the expected aromatic character of the cyclopropyl cation, but the correspondent anion also shows aromatic behavior. This unexpected result is analyzed as well as the character of the other three membered ring derivatives.     

甲醛与乙腈相互作用的理论研究

在HF/6-311G(d,p)、 MP2/6-311G(d,p)和B3LYP/6-311G(d,p)水平上,对H2CO和CH3CN以及设计的4种结构H2CO…CH3CN复合物等进行几何全优化和振动频率计算,排除振动频率为负值的非局域极小点结构,并对稳定的环状构型复合物结合能进行基组重叠误差校正和零点振动能校正.分子间相互作用的能量分解分析显示,静电能在H2CO...CH3CN相互作用能量中占主导地位,电荷转移能居第二位.     

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.     

Quantum-Chemical Study of Spin Transitions in Bimetallic Fe–M Complexes (M = Co,Ni, Cu,Zn) with the 1,10-Phenanthroline Linker

The structure, energies, and magnetic properties of electromeric forms of binuclear Fe–M complexes (M = Co, Ni, Cu, Zn) with a 1,10-phenanthroline-based linker have been studied by the DFT UB3LYP*/6-311++G(d,p) method. Studying the spin-state switching mechanisms has demonstrated that all the compounds under consideration are capable of undergoing spin crossover at the iron ion. In solutions of complexes with cobalt and nickel bis-chelates, a competing process accompanied by the change in magnetic characteristics—configurational isomerism—is possible.     

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.     

On the stability of large [4n]annulenes

[reaction: see text] The stabilization energies (B3LYP/6-31G) of planar [4n]annulenes, evaluated by a new indene-isoindene isomerization method (see Abstract graphic), reveal that all 4n pi-electron rings larger than the energetically unfavorable cyclobutadiene are only slightly destabilized by the pi-electron interactions. Cyclooctatetraene prefers the "tub" conformation because of strain effects. Generally, the antiaromatic character of the larger systems with 4n pi-electrons is revealed best by their magnetic properties rather than by their energies.     

Ab initio and density functional theory studies of the structure,gas-phase acidity and aromaticity of tetraselenosquaric acid

Results of ab initio self-consistent-field (SCF) and density functional theory (DFT) calculations of the gas-phase structure,acidity (free energy of deprotonation,G0) and aro-maticity of tetraselenosquaric acid (3,4-diselenyl-3-cy-dobutene-1,2-diselenone,H2C4Se4) are reported.The global minimum found on the potential energy surface of tetraselenosquaric acid presents a planar conformation.The ZZ iso-mer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very dose in energy.The optimized geometric parameters exhibit a bond length equalization relative to reference compounds,cyclobu-tanediselenone,and cydobutenediselenol.The computed aromatic stabilization energy (ASE) by homodesmotic reaction is -77.4 (MP2(fu)/6 - 311 G //RHF/6 - 311 G) and - 54.8 kJ/mol (B3LYP/6 - 311 G //B3LYP/6 -311 G).The aromaticity of tetraselenosquaric add is indicated by the calculated diamagnetic susceptibility exaltation (A) - 19.13 (CSGT(IGAEM) - RHF/6 - 311 G// RHF/6-     

1,2-二硫方酸的气相酸性和芳香性

在ＲＨＦ／６－３１１Ｇ、ＲＨＦ／６－３１１＋Ｇ　和Ｂ３ＬＹＰ／６－３１１＋Ｇ　水平,优化得到１,２－二硫方酸（３,４．二羟基－３－环丁烯－１,２－二硫酮）三种平面构象异构体的平衡几何构型．进一步用ＭＰＺ（ｆｕｌｌ）／６－３１１＋Ｇ　、／／ＲＨＦ／６－３１１＋Ｇ　方法计算三种异构体的单点能量,发现ＺＺ型异构体是能量最低构象,且ＺＺ和ＺＥ型能量非常接近．用优化的最稳定构象ＺＺ型异构体,在ＲＨＦ／６－３１１Ｇ　／／ＲＨＦ／６－３１１Ｇ　、ＲＨＦ／６．３１１＋Ｇ　／／ＲＨＦ／６－３１１＋Ｇ　、ＭＰＺ（ｆｕｌｌ）／６．３１１＋Ｇ　／／ＲＨＦ／６－３１１＋Ｇ　和Ｂ３ＬＹＰ／６－３１１＋Ｇ　斤Ｂ３ＬＹＰ／６－３１１＋Ｇ　水平,计算其气相酸性（△Ｇ＿２９８）,并用同键反应方法在同样水平计算其芳香性稳定化能．用基团加和法（ｇｒｏｕｐｉｎｃｒｅｍｅｎｔａｐｐｒｏａｃｈ）在ＲＨＦ／６．３１１＋Ｇ　／／ＲＨＦ／６－３１１＋Ｇ　和Ｂ３ＬＹＰ／６－３１１＋Ｇ　／／Ｂ３ＬＴｈ／６－３１１＋Ｇ　水平计算其磁化率增量（　）．计算结果指出,标题化合物的键长发生了平均化,芳香性稳定化能和磁化率增量均为负值,表明它具有芳香性,实现了标题化合物芳香     

Influence of the H/F replacement on the homoaromaticity of homotropylium ion: a GIAO/DFT theoretical study

The problem of homoaromaticity in mono-, di- and polyfluorinated- homotropylium cations is addressed by the B3LYP/6-311++G** DFT method. The energetic, structural and magnetic criteria are used for this purpose. They convincingly show that the ground state equilibrium species are aromatic, or in other words that the homoaromaticity is preserved by the (poly)fluorination. In contrast, a considerable decrease in the aromatic stabilization is observed in the transition structures (TS). According to the NICS(0) index, they vary form strongly antiaromatic, via weakly and non-aromatic to slightly aromatic transition states. However, the hierarchy of the aromaticity in fluorinated homotropylium ions predicted by NICS(0) is completely unrelated to that obtained by using the energy criterion assuming a kinetic definition of aromaticity. On the other hand the latter is closely related to geometric parameters of the equilibrium and transition structures.     

Corannulene as a Lewis base: computational modeling of protonation and lithium cation binding.

A computational modeling of the protonation of corannulene at B3LYP/6-311G(d,p)//B3LYP/6-311G(d,p) and of the binding of lithium cations to corannulene at B3LYP/6-311G(d,p)//B3LYP/6-31G(d,p) has been performed. A proton attaches preferentially to one carbon atom, forming a sigma-complex. The isomer protonated at the innermost (hub) carbon has the best total energy. Protonation at the outermost (rim) carbon and at the intermediate (bridgehead rim) carbon is less favorable by ca. 2 and 14 kcal mol(-)(1), respectively. Hydrogen-bridged isomers are transition states between the sigma-complexes; the corresponding activation energies vary from 10 to 26 kcal mol(-)(1). With an empirical correction obtained from calculations on benzene, naphthalene, and azulene, the best estimate for the proton affinity of corannulene is 203 kcal mol(-)(1). The lithium cation positions itself preferentially over a ring. There is a small energetic preference for the 6-ring over the 5-ring binding (up to 2 kcal mol(-)(1)) and of the convex face over the concave face (3-5 kcal mol(-)(1)). The Li-bridged complexes are transition states between the pi-face complexes. Movement of the Li(+) cation over either face is facile, and the activation energy does not exceed 6 kcal mol(-)(1) on the convex face and 2.2 kcal mol(-)(1) on the concave face. In contrast, the transition of Li(+) around the corannulene edge involves a high activation barrier (24 kcal mol(-)(1) with respect to the lowest energy pi-face complex). An easier concave/convex transformation and vice versa is the bowl-to-bowl inversion with an activation energy of 7-12 kcal mol(-)(1). The computed binding energy of Li(+) to corannulene is 44 kcal mol(-)(1). Calculations of the (7)Li NMR chemical shifts and nuclear independent chemical shifts (NICS) have been performed to analyze the aromaticity of the corannulene rings and its changes upon protonation.     

Theoretical Study on Vibrational Spectra,Detonation Properties and Pyrolysis Mechanism for Cyclic 2-Diazo-4,6-dinitrophenol

Based on the full optimized molecular geometrical structures at the DFT-B3LYP/6-311+G** level, there exists intramolecular hydrogen bond interaction for cyclic 2-diazo-4,6-dinitrophenol. The assigned infrared spectrum is obtained and used to compute the thermodynamic properties. The results show that there are four main characteristic regions in the calculated IR spectra of the title compound. The detonation velocities and pressures are also evaluated by using Kamlet-Jacobs equations based on the calculated density and condensed phase heat of formation. Thermal stability and the pyrolysis mechanism of 2-diazo-4,6-dinitrophenol are investigated by calculating the bond dissociation energies at the B3LYP/6-311+G** level.     

Experimental (FT-IR and FT-Raman), electronic structure and DFT studies on 1-methoxynaphthalene

In this work, FT-IR and FT-Raman spectra of 1-methoxynapthalene (C(11)H(10)O) have been reported in the regions 4000-400 cm(-1) and 3500-100 cm(-1), respectively. Density functional method (DFT) has been used to calculate the optimized geometrical parameters, atomic charges, vibrational wavenumbers and intensity of the vibrational bands. The vibrational frequencies have been calculated and scaled values are compared with experimental FT-IR and FT-Raman spectra. The structure optimizations and normal coordinate force field calculations are based on density functional theory (DFT) method with B3LYP/3-21G, B3LYP/6-31G, B3LYP/6-31G(d,p) and B3LYP/6-311++G(d,p) basis sets. The complete vibrational assignments of wavenumbers are made on the basis of potential energy distribution (PED). The optimized geometric parameters are compared with experimental values of naphthoic acid. The results of the calculation shows excellent agreement between experimental and calculated frequencies in B3LYP/6-311++G(d,p) basis set. The effects due to the substitutions of methyl group and carbon-oxygen bond are also investigated. A study on the electronic properties, such as excitation energies and wavelengths, were performed by time-dependent DFT (TD-DFT) approach. HOMO and LUMO energies are calculated that these energies show charge transfer occurs within the molecule.     

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.     

Neutral homoaromaticity in some heterocyclic systems

[reaction: see text] Neutral homoaromaticity has been evaluated in heterocyclic systems related to the bicyclo[3.2.1]octane skeleton with replacement of CH(2) at C-2 in bicyclo[3.2.1]octa-3,6-diene with X = BH, AlH, Be, Mg, O, S, PH, NH (12); replacement of CH at C-3 in bicyclo[3.2.1]octa-3,6-dien-2-yl anion with PH, S, NH, O (13); and replacement at C-2 and C-3 with N and O (14). Stabilization energies (SE) are evaluated using density functional theory and homodesmotic equations at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) level for series 12-14. Stabilization energies are compared with diamagnetic susceptibility exaltations, Lambda, CSGT-B3LYP/6-31G(d)//B3LYP/6-31G(d), and nucleus-independent chemical shifts (NICS), GIAO-B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d). Analysis of frontier orbitals and geometries, B3LYP/6-31G(d)//B3LYP/6-31G(d), and proton affinities of 2-azabicyclo[3.2.1]octa-3,6-diene, pyrrole, and related model systems, B3LYP/6-311+G(2d,2p)//B3LYP/6-31G(d), provide complementary evidence supporting the division of the substrates evaluated into antihomoaromatic (12, X = BH, AlH, and Be), nonhomoaromatic (12, X = O, S, NH, PH), and homoaromatic (13, X = S, PH, NH, O and 14 X = ON), with 12 (X = Mg) appearing as transitional between anti- and nonhomoaromatic.     

On the aromatic character of the heterocyclic bases of DNA and RNA

Studies based on ab initio optimized geometries (at B3LYP/6-311+G** and MP2/6-311+G** levels) and on experimental structures retrieved from the Cambridge Structural Database (CSD) reveal that the nucleobases constituting DNA and RNA differ significantly in their aromatic character, as shown by the geometry-based index of aromaticity HOMA that ranges from 0.466 for thymine to 0.917 for adenine, based on B3LYP/6-311+G** calculations, and 0.495-0.926, respectively, if based on the MP2/6-311+G** level. Aromaticity of the bases decreases markedly with an increase of the number of double-bond C=X (X = N, O) substituents at the rings. H-bonds involving C=O groups in Watson-Crick pairs cause an increase of the aromatic character of the rings.     

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.     

Hydrogen peroxide clusters: the role of open book motif in cage and helical structures

Hartree-Fock (HF) calculations using 6-31G*, 6-311++G(d,p), aug-cc-pVDZ, and aug-cc-pVTZ basis sets show that hydrogen peroxide molecular clusters tend to form hydrogen-bonded cyclic and cage structures along the lines expected of a molecule which can act as a proton donor as well as an acceptor. These results are reiterated by density functional theoretic (DFT) calculations with B3LYP parametrization and also by second-order M?ller-Plesset perturbation (MP2) theory using 6-31G* and 6-311++G(d,p) basis sets. Trends in stabilization energies and geometrical parameters obtained at the HF level using 6-311++G(d,p), aug-cc-pVDZ, and aug-cc-pVTZ basis sets are similar to those obtained from HF/6-31G* calculation. In addition, the HF calculations suggest the formation of stable helical structures for larger clusters, provided the neighbors form an open book structure.