Pathways of the reaction of nucleophilic addition of ammonia to formaldehyde in the gas phase and in the complex with formic acid:ab initio calculations

The gradient pathways of the reaction of nucleophilic addition of ammonia to formaldehyde were calculated for free molecules and in the NH₃...H₂CO...HC(O)OH complex by theab initio RHF/6-31G^**, MP2(fc)/6-31G^**, and MP2(full)/6-311++G^** methods. Both reactions proceed concertedly. In the first case, the reaction successively passes through two transitional states with an energy barrier exceeding 35 kcal mol⁻¹. In the case of the complex with formic acid, the reaction follows a conventional pathway, although its activation barrier calculated by the RHF/6-31G^** and MP2(fc)/6-31G^** methods decreases to 12.6 and 3.8 kcal mol⁻¹, respectively. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 13–20, January, 1998.     

Pathways of the reactions of nucleophilic addition of H2O and HF molecules to formaldehyde in the gas phase and in the complex with formic acid:ab initio calculations

The gradient pathways of the reactions of nucleophilic addition of H₂O and HF molecules to formaldehyde in the gas phase and in the XH…H₂CO…HC(O)OH complex (X=OH, F) were calculated by theab initio RHF/6-31G^**, MP2(fc)/6-31G^**, and MP2(full)/6-311++G^** methods. Both reactions proceed concertedly. The formation of H-bonded bimolecular pre-reaction complexes is the initial stage of the gas-phase reactions; at the same time, no indications of the formation of stable π-complexes were found on the potential energy surfaces of systems under study. The calculated energy barriers to the gasphase reactions exceed 40 kcal mol⁻¹, while those to reactions in the complex XH…H₂CO…HC(O)OH (X=OH, F) become more than halved. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2146–2154, November, 1998.     

Methane and nitrous oxide concentration and emission flux of Yangtze Delta plain river net

Methane (CH4) and nitrous oxide (N2O) saturation concentration and gas-water interface emission flux in surface water of the Yangtze Delta plain river net were investigated in summer at representative sites including the upper reaches of the Huangpu River and the rivers in the Chongming Island. The results show that the CH4 concentration in river water ranged from 0.30±0.03 to 6.66±0.14 μmol.L-1, and N2O concentration ranged from 13.8±2.33 to 435±116 nmol.L-1. River surface water had a very high satura- tio...     

Isomerism in OBe3F3 + cation: anab initio study

Ab initio MP2/6-31G^*//HF/6-31G^*+ZPE(HF/6-31G^*) calculations of the potential energy surface in the vicinity of stationary points and the pathways of intramolecular rearrangements between low-lying structures of the OBe₃F₃ ⁺ cation detected in the mass spectra of μ₄-Be₄O(CF₃COO)₆ were carried out. Ten stable isomers with di- and tricoordinate oxygen atoms were localized. The relative energies of six structures lie in the range 0–8 kcal mol⁻¹ and those of the remaining four structures lie in the range 20–40 kcal mol⁻¹. Two most favorable isomers, aC _2v isomer with a dicoordinate oxygen atom, planar six-membered cycle, and one terminal fluorine atom and a pyramidalC _3v isomer with a tricoordinate oxygen atom and three bridging fluorine atoms, are almost degenerate in energy. The barriers to rearrangements with the breaking of one fluorine bridge are no higher than 4 kcal mol⁻¹, except for the pyramidalC _3v isomer (∼16 kcal mol⁻¹). On the contrary, rearrangements with the breaking of the O−Be bond occur with overcoming of a high energy barrier (∼24 kcal mol⁻¹). A planarD _3h isomer with a tricoordinate oxygen atom and linear O−Be−H fragments was found to be the most favorable for the OBe₃H₃ ⁺ cation, a hydride analog of the OBe₃F₃ ⁺ ion; the energies of the remaining five isomers are more than 25 kcal mol⁻¹ higher. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 420–430, March, 1999.     

Deposition of Diamondlike Carbon Film and Mass Spectrometry Measurement in CH4/N2 RF Plasma

The deposition of diamondlike carbon (DLC) film and the measurements of ionic species by means of mass spectrometry were carried out in a CH₄/N₂ RF (13.56 MHz) plasma at 0.1 Torr. The film deposition rate greatly depended on both CH₄/N₂ composition ratio and RF power input. It was decreased monotonically as CH₄ content decreased in the plasma and then rapidly diminished to negligible amounts at a critical CH₄ content, which became large for higher RF power. The rate increased with increasing RF power, reaching a maximum value in 40% CH₄ plasma. The predominant ionic products in CH₄/N₂ plasma were NH⁺ ₄ and CH₄N⁺ ions, which were produced by reactions of hydrocarbon ions, such as CH⁺ ₃, CH⁺ ₂, CH⁺ ₅, and C₂H⁺ ₅ with NH₃ molecules in the plasma. It was speculated that the production of NH⁺ ₄ ion induced the decrease of C₂H⁺ ₅ ion density in the plasma, which caused a reduction in higher hydrocarbon ions densities and, accordingly, in film deposition rate. The N⁺ ₂ ion sputtering also plays a major role in a reduction of film deposition rate for relatively large RF powers. The incorporation of nitrogen atoms into the bonding network of the DLC film deposited was greatly suppressed at present gas pressure conditions.     

The C−NO2 bond dissociation energies of some nitroaromatic compounds: DFT study

The C−NO₂ bond dissociation energies in nitrobenzene; 3-amino-nitrobenze; 4-amino-nitrobenze; 1,3-dinitrobenzene; 1,4-dinitrobenzene; 2-methyl-nitrobenzene; 4-methyl-nitrobenzene; and 1,3,5-trinitrobenzene nitroaromatic molecules, are computed using B3LYP, B3PW91, B3P86 three-parameter hybrid Density Functional Theory (DFT) methods in conjunction with 6-31G^** basis set. By comparing the computed energies and experimental ones, it is found that B3P86/6-31G^** is not capable of predicting the satisfactory bond dissociation energy (BDE). The BDEs computed with both B3LYP/6-31G^** and B3PW91/6-31G^** for the nitroaromatic molecules are closer to the experimental ones than those obtained with B3P86/6-31G^**. But, when compared with the experimental one, the BDE from the B3LYP/6-31G^** has the maximum deviation, which is completely outside our desired target accuracy for chemical predictions (less than 2.00 kcal mol⁻¹). Therefore, we suggest B3PW91/6-31G^** method as a reliable method of computing the BDE for removal of the nitrogen dioxide group in the nitroaromatic compounds. In addition, the C−NO₂ BDEs for 2,4,6-trinitrotoluene (TNT), triaminotrinitrobenzene (TATB), diaminotrinitrobenzene (DATB), and picramide are studied with B3PW91/6-31G^** method.     

Theoretical study on the reaction mechanism of (CH<Subscript>3</Subscript>)<Subscript>3</Subscript>CO<Superscript>.</Superscript> radical with NO

The reaction mechanism of (CH₃)₃CO^. radical with NO is theoretically investigated at the B3LYP/6-31G* level. The results show that the reaction is multi-channel in the single state and triplet state. The potential energy surfaces of reaction paths in the single state are lower than that in the triple state. The balance reaction: (CH₃)₃CONO⇔(CH₃)₃CO^.+NO, whose potential energy surface is the lowest in all the reaction paths, makes the probability of measuring (CH₃)₃CO^. radical increase. So NO may be considered as a stabilizing reagent for the (CH₃)₃CO^. radical.     

Tetraalkylammonium Ions in Aqueous and Non-aqueous Solutions

Property data for tetraalkylammonium cations, [H(CH₂) _n ]₄N⁺, are reviewed. They pertain to the isolated cations and their transfer from the gas phase into aqueous solutions. Various properties of these cations in aqueous and non-aqueous solutions and data for their transfer between these are also reviewed. Emphasis is placed on the dependence of data on the length n of the alkyl chains rather than on the absolute values. Most of the data are available only for the first four members of the series. The properties of the isolated ions increase linearly with the chain length. Molar enthalpies of formation of the gaseous and aqueous cations, and absolute standard molar enthalpies of hydration, are derived. Standard molar entropies of dissolution of several salts in water are obtained from their solubilities and enthalpies of solution. The molar entropies of the crystalline iodides of the first four members of the series then give the standard partial molar entropies of the aqueous cations and their molar entropies of hydration. The standard partial molar volumes in aqueous and non-aqueous solutions are quite linear with n and in non-aqueous solutions the molar volume hardly depends on the nature of the solvent. On transfer from water to non-aqueous solvents the volume of Me₄N⁺ suffers some shrinkage, that of Et₄N⁺ appears to be unaffected, but from Pr₄N⁺ onwards an increasing expansion takes place. This unexpected result is tentatively explained by hydrophobic intra-molecular association of pairs of alkyl chains in aqueous solutions, resulting in a tightening of the structure. The transfer of the R₄N⁺ cations from water into non-aqueous solvents is governed by a large positive entropy change, outweighing the smaller positive enthalpy change. The transport properties of the aqueous R₄N⁺ cations are non-linear with n. A major impediment to movement is thus the sticking of the water molecules to the ice-like hydrophobic hydration sheaths of the larger cations. The number of water molecules affected by the hydrophobic cations is open to widely differing estimates resulting from various approaches, and constitute an open issue.     

Error of Atomic Charges Derived from Electrostatic Potential

The purpose of this article is to show that CHELP, CHELPG, and Merz and Kollman undergo error for the charge on atoms of HCOO^– (H₂O) _n for n = 1 6. We also demonstrate that the CHELP, CHELPG, and Merz and Kollman show error for the tendency toward change in the charges on carbons for CH₃NH⁺ ₃ (CH₃)₂NH⁺ ₂ (CH₃)₃NH⁺ (CH₃)₄N⁺.     

Hydration of the carboxylate group: Anab initio molecular orbital study of acetate-water complexes

The hydration of the carboxylate group in the acetate anion has been investigated by performingab initio molecular orbital calculations on selected conformers of complexes with the form CH₃CO₂ ^– ·nH₂O·mH₂O, wheren andm denote the number of water molecules in the first and second hydration spheres around the carboxylate group, andn + m 7. The results of RHF/6–31G^* optimizations for all the complexes and MP2/6–31+G^** optimizations for several one-water complexes are reported. The primary consequence of hydration on the structure of the acetate anion is a decrease in the length of the C-C bond. Enthalpy and free energy changes calculated at the MP2/6–31+G^** and MP2/6–311+ +G^** levels are reported for the reactions CH₃CO₂ ^– + [H₂O] _P CH₃CO₂ ^– ·nH₂O ·mH₂O where [H₂O] _P is a water cluster containingp water molecules andp=n+m 7. The calculations show that conformers with the lowest enthalpy change on complex formation are often not those with the lowest free energy change, due to a greater entropic loss in complexes with tighter and more favorable enthalpic interactions. Hydrogen bonding of six water molecules directly to the carboxylate group in CH₃CO₂ ^– is found to account for approximately 40% of the enthalpy change and 37% of the free energy change associated with bulk solvation.     

The 6-31G++ basis set: An economical basis set for correlated wavefunctions

The 6-31G ⁺⁺ basis set is described. This basis set is very similar to the existing 6-31G ^** set but is somewhat smaller through the use of five (rather than six) second-order Gaussians (d functions) and has polarization function exponents optimized for correlated rather than Hartree–Fock wavefunctions. The performance of 6-31G ⁺⁺ is compared with that of the 6-31G ^** and 6-31G ^** basis sets through calculation of the geometries and atomization energies for the set of molecules LiH, FH, H₂O, NH₃, CH₄, N₂, CO, HCN, and HCCH.     

Intramolecular nucleophilicS N 2 substitution at the tetrahedral carbon atom: Anab initio study

Pentacoordination of carbon atom in bicyclic organic compounds of the pentalene type was studied by theab initio RHF/6-31G^** and MP2(full)/6-31G^** methods. It was shown that intramolecularS _N 2 reactions with energy barriers within the energy scale of NMR spectroscopy can occur in systems in which a linear orientation of the attacking and leaving groups is realized. The barrier to the intramolecular nucleophilic substitution reaction in 2,3-dihydro-3-formylmethylenefuran is 36.9 (RHF) and 27.7 kcal mol⁻¹ (MP2) and decreases to 16.4 and 19.4 kcal mol⁻¹, respectively, in the case of diprotonation at the O atoms in this system. For model pentalene type compounds containing electron-deficient B atoms in the ring, theab initio calculations predict a further decrease in the barrier height (down to less than 10 kcal mol⁻¹). Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 7, pp. 1246–1256, July, 1999.     

Reexamination of the C2H4F potential surface. the status of the classical 2-fluoroethyl cation: a local minimum or transition structure?

According to ab initio molecular orbital calculations carried out with full geometry optimization at the MP2/6–31G^** level, the classical 2-fluoroethyl cation, FCH₂CH₂⁺, is a transition structure for H-scrambling in CH₃CHF⁺. Single point MP4/6–31G^** calculations at the optimized geometries predict the cyclic ethylene fluoronium ion to lie 24.2 kcal mol⁻¹ above CH₃CHF⁺ and 5.4 kcal mol⁻¹ below the 2-fluoroethyl cation. ΔG‡ for ring opening of the cyclic fluoronium ion at -60° is estimated to be ca 15 kcal mol⁻¹. This barrier is largely attributable to the powerful negative fluorine hyperconjugation in the transition state as described by Hoffmann and coworkers. When electron correlation effects are ignored a qualitatively different potential surface is obtained on which the 2-fluoroethyl cation is calculated to be a local minimum separated from the stable 1-fluoroethyl cation by an H-bridged transition state.     

Molecular structure of nickel(II) and copper(II) N,N′-ethylene-bis(acetylacetoneiminates) MO2N2C12H18 according to gas-phase electron diffraction data and quantum-chemical calculations

The molecular structure of nickel(II) and copper(II) N,N′-ethylene-bis(acetylacetoneiminates), NiO₂N₂C₁₂H₁₈ and CuO₂N₂C₁₂H₁₈, at 442(5) K and 425(5)K, respectively. Both molecules have C ₂ symmetry with a nearly planar MN₂O₂ coordination site and internuclear distances r _h1(M-O) = 1.862(10)/1.923(17) Å and r _h1(M-N) = 1.879(10)/1.947(18) Å for Ni(acacen) and Cu(acacen), respectively. The structure of free molecules is close to the structure of molecules in crystal. The DFT/3LYP quantum-chemical calculations (CEP-31G and 6-31G* basis sets) gave a molecular structure that agreed satisfactorily with the one found in experiment. The low-spin ¹ A and high-spin ³ A states of the Ni(acacen) molecule were considered. It was found that a change in multiplicity caused significant changes in the geometrical and electronic structure of the MN₂O₂ coordination site. As shown by experiment and calculations for the NiO₂N₂C₁₂H₁₈ molecule, the low-spin ¹ A state is the ground state. The internal rotation of CH₃(CN) and CH₃(CO) methyl groups was studied by the 3LYP/CEP-31G method. It was shown that steric hindrances led to a high rotation barrier of the CH₃(CN) group.     

Vertical proton affinities of CH2O and CH2OH+ in their ground singlet,excited triplet and ionized doublet states

Vertical proton affinities were calculated with closed and open shell direct SCF-MO methods for the ground, excited triplet and ionized doublet states of CH₂O and CH₂OH⁺.The computed gas phase basicity of CH₂O follows the order: CH₂O(¹ A ₁) > CH₂O*(³ A ₁ or ³ A ₂) > CH₂O⁺(² B ₂ or ² B ₁).     

A theoretical study of the reaction of Ti+ with propane

Detailed quartet and doublet potential energy surfaces for the Ti⁺ + C₃H₈ → TiC₃H₆⁺ + H₂ and Ti⁺ + C₃H₈ → TiC₂H₄⁺ + CH₄ elimination reactions have been studied using density functional theory with B3LYP functional and ab initio coupled cluster CCSD(T) methods. Several H₂ elimination and CH₄ elimination reaction paths have been examined including the IRC following. In particular, the mechanisms involving, respectively, the H₂TiC₃H₆⁺ and CH₃TiHC₂H₄⁺ intermediates have been studied. Contribution to the Mark S. Gordon 65th Birthday Festschrift Issue.     

Ab initio calculations of the structure and harmonic force fields for the amine forms of dinitramine and methyldinitramine. Vibrational spectra and their interpretation using a scaling procedure

Ab initio geometries and vibrational spectra have been calculated for the amine structures of dinitramine and methyldinitramine, HN(NO₂)₂ and CH₃N(NO₂)₂. It is shown at the RHF and MP2 levels with the use of the 6-31G^* and 6-31G^** basis sets that these molecules have different symmetries in their equilibrium states,C _sandC ₁ respectively. The quantum chemical RHF/6-31G^* force fields were scaled with the set of transferable factors previously obtained by the authors to assign the available experimental vibrational bands and predict the positions of bands for the unmeasured spectral regions. Some common patterns of the geometrical parameters, vibrational spectra, and force fields of the simplest nitramines are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2135–2147, November, 1995.The authors are grateful to the Russian Foundation for Basic Research (Project No. 93-03-4410) and to the International Science Foundation (Grant No. MQXOOO) for financial support of works fulfilled at the Department of Chemistry, M. V. Lomonosov Moscow State University. The authors also acknowledge the support of the Scientific Technical Program Universities of Russia.     

Theoretical study on reactions of cyclic-N3 with NO, NO2 and Cl2

In this article, we report our detailed mechanistic study on the reactions of cyclic-N₃ with NO, NO₂ at the G3B3//B3LYP/6-311+G(d) and CCSD(T)/aug-cc-pVTZ//QCISD/6-311+G(d)+ZPVE levels; the reactions of cyclic-N₃ with Cl₂ was studied at the G3B3//B3LYP/6-311+G(d) and CCSD(T)/aug-cc-pVTZ//QCISD/6-31+G(d)+ZPVE levels. Both of the singlet and triplet potential-energy surfaces (PESs) of cyclic-N₃ + NO, cyclic-N₃ + NO₂ and the PES of cyclic-N₃ + Cl₂ have been depicted. The results indicate that on singlet PESs cyclic-N₃ can undergo the barrierless addition–elimination mechanism with NO and NO₂ forming the respective dominant products N₂ + ¹cyclic-NON and ¹NNO(O) + N₂. Yet the two reactions on triplet PESs are much less likely to take place under room temperature due to the high barriers. For the cyclic-N₃ + Cl₂ reaction, a Cl-abstraction mechanism was revealed that results in the product cyclic-N₃Cl + Cl with an overall barrier as high as 14.7 kcal/mol at CCSD(T)/aug-cc-pVTZ//QCISD/6-31+G(d)+ZPVE level. So the cyclic-N₃ radical could be stable against Cl₂ at low temperatures in gas phase. The present results can be useful for future experimental investigation on the title reactions.     

反式2-甲基-2-丁烯酸甲酯与臭氧反应机理的计算研究

采用G3B3方法构建反式2-甲基-2-丁烯酸甲酯与O3反应体系以及后续Criegee自由基有、无水分子参与下异构化反应的势能面剖面.结果表明,反式2-甲基-2-丁烯酸甲酯与O3反应首先生成一个稳定的五元环中间体,此中间体按断键位置不同后续裂解反应存在两条路径,分别生成产物P1(CH3CHOO+CH3OC(O)C(CH3)O)和P2(CH3CHO+CH3OC(O)C(CH3)OO).利用经典过渡态理论(TST)并结合Wigner矫正模型计算了200-1200 K温度区间内标题反应的速率常数kTST/W.计算结果显示,294 K时,该反应速率常数为7.55×10-18cm3molecule-1s-1,与Bernard等对类似反应所测实验值非常接近.生成的Criegee自由基(CH3CHOO和CH3OC(O)C(CH3)OO)可分别与水分子发生α-加成及β-氢迁移反应,其中Criegee自由基与水的α-加成反应较其与水的β-氢迁移反应具有优势.另外与无水分子参与CH3CHOO和CH3OC(O)C(CH3)OO异构化反应相比,水分子的参与使得异构化反应较为容易进行.     

Novel Methods of Synthesis of Dibenzo[3n]crown-n and Their Cation Binding Studied by Fluorescence Spectroscopy. Part V

The dibenzo[3n]crown-n were synthesised starting from bis[2-(o-hydroxyphenoxy)ethyl]ether obtained from bis[2-(o-formylphenoxy)ethyl]ether via Baeyer-Villiger oxidation in H₂O₂/CH₃COOH in a good yield. The cyclic condensation ofbis[2-(o-hydroxyphenoxy)ethyl]etherwith tri- and tetraethylene glycol bisdichlorides andthe bisditosylate of pentaethylene glycol in DMF/Me₂CO₃ afforded the large cyclic ethers of dibenzo[21]crown-7, dibenzo[24]crown-8 and dibenzo[27]crown-9. The structures were analysed with IR, ¹H NMR, ¹³C NMR and low-resolution mass spectroscopy methods. The Na⁺, K⁺, Rb⁺ and Cs⁺ cations' recognition of the molecules were conducted withsteady-state fluorescence spectroscopy. The 1:1 association constants, K_a, in acetonitrile were estimated. Dibenzo[21]crown-7 was the best both for K⁺ and Rb⁺ binding but showed too small an effect on Cs⁺. Dibenzo[24]crown-8 exhibited the binding power in the order of Rb⁺ > K⁺ > Na⁺ > Cs⁺. However, dibenzo[27]crown-9 displayed marked binding with only K⁺ but not with Rb⁺ or with Cs⁺ cations probably due to the heavy atom effect of fluorescence quenching.