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.     

甲醛与甲酸相互作用的从头算研究

在MP2／6—31G(d)和MP2(FC)／6—311 G(d，p)水平上，对H2CO和HCOOH以及设计的4种构型H2CO…HCOOH复合物等进行几何全优化计算，经振动频率分析，确认它们为势能面上的稳定驻点．然后在MP2／6—311 G(2df，2p)水平上进行单点能计算和基组重叠误差(BSSE)校正以获得相互作用能，并利用自然键轨道理论探讨H2CO和H(X)OH相互作用的本质。     

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.     

Mechanism for the addition of carbenoid CH2ClLi to formaldehyde

Ab initio HF/6-31G calculations have been performed for the addition mechanism of carbenoid CH₂ClLi with formaldehyde in tetrahydrofuran. An early complex of formaldehyde with CH₂ClLi is first formed with quite exothermic effect. Only a little activation energy of 14.6 kJ/mol is needed for the complex developing into the product through a transition state with coplanar bicyclic structure. In this process, the eletrophilic attack of carbonyl carbon of formaldehyde is more active than the nucleophilic attack of carbon of carbenoid. The exothermal effect of this addition process isupto 216.5kJ/mol.     

Ab initio calculations of methionines and their protonated forms

Ab initio calculations of molecular and electronic structures of neutral molecules and protonated forms of methionine and its derivatives in the gaseous phase were carried out by the Hartree-Fock method using the 6–31G^* basis set with full geometry optimization. Proton affinities of methionine (1), methionine sulfoxide (2), and methionine sulfone (3) were calculated for different modes of coordination of the proton. The results of calculations demonstrated that in protonated forms of 1 and 3, bonding between the proton and the N atom is most favorable, while in protonated form of 2, bonding between the proton and the O atom of the SO group is most favorable. The proton affinities of the amino acids are as follows: 223.2 (1), 241.2 (2), and 221.5 (3) kcal mol⁻¹,i.e., methionine sulfoxide 2 exhibits the highest proton affinity in the series of the amino acids under consideration. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1487–1490, August, 1998.     

Four-proton migrations in associates of two molecules of formic acid with two molecules of water or hydrogen fluoride

The mechanisms of proton transfer in associates of two molecules of formic acid with two molecules of water or hydrogen fluoride were studied usingab initio (SCF/6-31G^**) method. Cooperative (concerted, or one-step) four-proton transfer occurs in the associates studied. The structures of the transition states are in complete agreement with the previously proposed concept of stereochemical correspondence for cooperative reactions. The calculated energy barriers to cooperative proton transfer in the associates investigated are 32.9 and 24.2 kcal mol^–1, respectively.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2636–2640, November, 1996.     

Hydration and protonation ofO-vinylacetoxime in the gas phase: anab initio study

Ab initio HF/6–31G^* calculations ofO-vinylacetoxime monohydrates and cations were performed. Each conformer forms two stable H-complexes with participation of N and O atoms. The former have planar heavy-atom skeletons, whereas the water molecule in the latter is located above the plane of the proton-acceptor complex. The complexes stabilized by N...HO and O...HO bonds have different dipole moments and frequencies of the OH stretching vibrations. The most energetically favorable cation is formed by adding a proton to the C_β atom of the vinyl group ofO-vinylacetoxime. Theap,ap-conformer (ap is antiperiplanar) of this cation is 6.5 and 34.9 kcal mol⁻¹ more stable than the onium cations with the NH⁺ and OH⁺ fragments, respectively, and is characterized by polarization and appreciable lengthening of the N−O and C=C bonds. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 597–600, April, 2000.     

Ab initio calculations of activation energy of the reaction of hydrogen exchange on strongly acidic centers

Ab initio calculations of fragments of the potential energy surfaces of hydrogen exchange reactions between H₂, CH₄, and alanine molecules and the H₃O⁺ ion were performed by the restricted Hartree-Fock method, at the second-order Møller-Plesset level of perturbation theory, and by the method of coupled clusters using the 6–31G^* and aug-cc-pVDZ basis sets. The one-center synchronous mechanism of hydrogen exchange reaction was studied and the activation energies and structures of transition states were determined. It was found that the geometric parameters of the H₂ and CH₄ molecules in the transition states are close to those of the H₃ ⁺ and CH₅ ⁺ ions. The higher the proton affinity of the reacting molecule in the reaction studied the lower the activiation energy of hydrogen exchange. The one-center mechanism studied can be used to describe the high-temperature solid-state catalytic isotope exchange (HSCIE) reaction. The results ofab initio calculations of synchronous hydrogen exchange between the H₃O⁺ ion and hydrogen atoms in different positions of the alanine molecule are in good agreement with experimental data on the regioselectivity and stereoselectivity of the HSCIE reaction with spillover-tritium.     

FTIR and FT-Raman spectra, assignments, ab initio HF and DFT analysis of xanthine

The molecular vibrations of xanthine were investigated in polycrystalline sample, at room temperature by Fourier transform infrared (FTIR) and FT-Raman spectroscopies. The spectra of the molecule have been recorded in the regions 4000-50 cm(-1) and 3500-100 cm(-1), respectively. Theoretical information on the optimized geometry, harmonic vibrational frequencies, infrared and Raman intensities were obtained by means of ab initio Hartree-Fock (HF) and density functional theory (DFT) gradient calculations with complete relaxation in the potential energy surface using 6-311++G(d,p) basis set. The vibrational frequencies which were determined experimentally from the spectral data are compared with those obtained theoretically from ab initio and DFT calculations. A close agreement was achieved between the observed and calculated frequencies by refinement of the scale factors. The infrared and Raman spectra were also predicted from the calculated intensities. Thermodynamic properties like entropy, heat capacity, zero point energy have been calculated for the molecule. Unambiguous vibrational assignment of all the fundamentals was made using the potential energy distribution (PED).     

An ab initio CI study of the ground and excited states of p-quinodimethane

Configuration interaction (CI) studies of the ground, electronically excited singlet and triplet states and of the ionized states (cations) are reported for p- quinodimethane (p-xylylene). The calculated ionization potentials are compared with the experimental photoelectron spectrum for the low-energy ionization region. The two high-energy low-intensity flanks of the second and third band observed in the photoelectron spectrum are assigned to be due to the two non-Koopmans' cation states, ascribing to shake-up ionizations.The calculated singlet-singlet and singlet-triplet excitation energies are compared with previous semiempirical MO results and experimental data.     

Quantum-chemical gas-phase calculations on the protonation forms oftrans- andcis-urocanic acid

The neutral, cationic, and anionic structures of both prototropic tautomers oftrans- andcis-urocanic acid [(E)- and (Z)-3-(1H-imidazol-4(5)-yl)propenoic acid, respectively] were studied by using semiempirical andab initio gas-phase calculations. Potential energy surfaces of the structures were calculated by using the semiempirical AM1 method, and the geometries corresponding to global minima on these surfaces were optimized up to the MP2/6-31G^* level of theory. The calculated protonation forms of each urocanic acid isomer have a planar molecular structure due to a delocalized -electron system, and all of them prefer thes-trans conformation with respect to the bond between the imidazole and the propenoic acid moieties. Thecis-urocanic acid structures are stabilized by an intramolecular hydrogen bond. The chargedcis-urocanic acid isomers have a lower molecular energy than the correspondingtrans-isomers, whereas the neutral molecules have, after inclusion of thermodynamic corrections, approximately the same energy. The cationic urocanic acid structures have about 2500 kJ mol^–1 lower energy than the anionic ones and about 1000 kJ mol^–1 lower energy than the neutral ones. The nonzwitterionic forms of the neutral urocanic acid isomers have about 200 kJ mol^–1 lower energy than the zwitterionic ones. These energy differences are explained by the proton affinities of the imidazole and the propenoic acid moieties of the urocanic acid structures.     

Proton migrations in associates of two molecules of formic acid with one molecule of hydrazine or hydrogen peroxide

The mechanisms of the proton transfer in associates of two molecules of formic acid with one molecule of hydrazine or hydrogen peroxide were studied usingab initio (SCFj6-31G^**) method. The mechanism of cooperative (concerted, one-step) four-proton transfer is realized in the associate with the hydrazine molecule. The proton transfer occurs stepwisevia an intermediate in the associate with a hydrogen peroxide molecule. The calculated activation barriers to the proton transfer in the associates investigated are 34.7 kcal mol^–1 and 27.1 kcal mol^–1, respectively.Translated fromlzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2631–2635, November, 1996.     

Analysis of the electron density redistribution in the course of nucleophilic addition reactions of H− and F− to acetylene and methylacetylene molecules according toab initio calculations

Energy characteristics and peculiarities of variation of structural parameters along the minimum energy pathways (MEP) calculated earlier of six reactions of nucleophilic addition of H^– and F^– to acetylene and methylacetylene have been analyzed. The electronic mechanism of the reactions, the character of the electron density redistribution, and its relation with the changes in structural parameters have been discussed. It has been found for all six reactions that the structural reorganization of an alkyne + Nu system is completed before the barriers. However, the increase in the alkyne multiple bond length and changes in electronic characteristics for the reactions with F^– (endothermic reactions) take place before the barrier (late transition state) and for the reaction with H^– (exothermic reactions), after the barrier (early transition state).Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2373–2377, December, 1995.     

Pentacoordination of boron, carbon, aluminum, and silicon atoms in organic compounds: Anab initio study

Pentacoordination of boron, carbon, aluminum, and silicon atoms in bicyclic organic compounds of the pentalene type was studied using theab initio RHF/6-31G^** and MP2(full)/6-31G^** methods. It was shown that the ability of the atom to form pentacoordinate structures increases on going from B to Al and from C to Si atom,i.e. as the number of the element of Groups IIIA and IVA of the periodic system increases. At the same time, the reverse tendencies are observed in the 2nd and 3rd periods of the periodic system,viz., the ability of the atom to form pentacoordinate structures increases on going from C to B and from Al to Si atom. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1238–1245, July, 1999.     

Vibrational and electronic spectra of benzophenone in different phase states: Ab initio calculations and experiment

An ab initio method has been used to perform quantum mechanical calculations of the formation energy of different conformers of benzophenone: planar molecule, twisted molecule, planar molecule dimer, twisted molecule dimer; electronic and vibrational spectra of these conformers were also obtained. An assessment of the medium (solvent) influence on the optimal geometry, dipole moment and stability of different forms of benzophenone was performed in the self-consistent reaction field approximation. It is shown that the twisted conformer is more stable than the planar one (the difference of free energies is 32 kJ/mol for free molecules) and it becomes even more stable with the increase in solvent polarity. The calculated electronic and vibrational spectra agree well with the experimental data and properly reflect the complication of the vibrational spectrum when passing from the gaseous phase to the condensed state of benzophenone. The difference between spectral properties of the two dimer forms allows their identification from the spectra and qualitative explanation of the observed peculiarities of phosphorescence of the amorphous phase of benzophenone by the stabilization of different conformers.     

Quantum-chemical analysis of the nucleophilic substitution of the bromine atom by the cyano group at the sp-hybridized carbon atom in the methylbromoacetylene molecule

This paper reports on our quantum-chemical analysis of the nucleophilic substitution of the bromine atom by the cyano group in the reaction of methylbromoacetylene with copper cyanide. According to calculations, the reaction can form a four-membered ring containing a copper atom.     

Alkylhydrazides and their protonated forms: Physical methods andab initio calculations

The basicity of hydrazides of the highest aliphatic carboxylic acids RC(O)NHNH₂ (R = C_nH_2n+1,n = 5-12) has been studied by potentiometric titration, and IR and¹H NMR spectroscopy.Ab initio Hartree-Fork calculations using the 6–31G^* basis set with full optimization of geometry were carried out on the simplest acy1hydrazines and their possible protonated forms. Based on these calculations, and the 1R and¹H NMR spectra, the tautomerism of alkylhydrazides and the structures of their protonated forms are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2645–2649, November, 1996.     

Molecular structure,conformational mobility,vibrational spectra,and thermochemistry of peroxyacetic acid: an ab initio and density functional study

The structure of the peroxyacetic acid (PAA) molecule and its conformational mobility under rotation about the peroxide bond was studied by ab initio and density functional methods. The free rotation is hindered by the trans-barrier of height 22.3 kJ mol^–1. The equilibrium molecular structure of AcOOH (C _s symmetry) is a result of intramolecular hydrogen bond. The high energy of hydrogen bonding (46 kJ mol^–1 according to natural bonding orbital analysis) hampers formation of intermolecular associates of AcOOH in the gas and liquid phases. The standard enthalpies of formation for AcOOH (–353.2 kJ mol^–1) and products of radical decomposition of the peroxide — AcO^· (–190.2 kJ mol^–1) and AcOO^· (–153.4 kJ mol^–1) — were determined by the G2 and G2(MP2) composite methods. The O—H and O—O bonds in the PAA molecule (bond energies are 417.8 and 202.3 kJ mol^–1, respectively) are much stronger than in alkyl hydroperoxide molecules. This provides an explanation for substantial contribution of non-radical channels of the decomposition of peroxyacetic acid. The electron density distribution and gas-phase acidity of PAA were determined. The transition states of the ethylene and cyclohexene epoxidation reactions were located (E _a = 71.7 and 50.9 kJ mol^–1 respectively).     

Geometry of intramolecular silicon complexes: an ab initio estimation of the sensitivity to the effect of medium. (Aroyloxymethyl)trifluorosilanes

The effect of medium on the geometry of the (benzoyloxymethyl)trifluorosilane (1) molecule was studied by the HF and MP2 methods with the 6-31G(d), 6-311G(d), and 6-311+G(d,p) basis sets, as well as using the Onsager SCRF model, the PCM approach, and the data of X-ray diffraction study. Molecule 1 has a low complexation energy (5.4 kcal mol^–1 according to MP2(Full)/6-31G(d)+ZPE calculations and 6.8 kcal mol^–1 according to IR spectroscopy data), while its geometric parameters are the least sensitive to the effects of medium among all hypervalent silicon compounds studied to date. Nevertheless, the results obtained revealed a pronounced deformation of the SiO coordination bond in 1 on going from the gas phase to the polar solution and crystal. This serves as a theoretical substantiation of the hypothesis that substantial changes in the IR and NMR spectral characteristics of the (O—Si)-chelate (aroyloxymethyl)trifluorosilanes upon variation of external factors are due to geometric reasons. Analysis of the electron density distribution according to Bader indicates that the SiO bond in molecule 1 can be treated as a covalent bond of high polarity.     

Crystal structure and ab initio calculations of a cyano-carbamimidic acid ethyl ester

The structure of one tautomer (amine form) of cyano-carbamimidic acid ethyl ester or (amino-ethoxy-methylidene)aminoformonitrile (CAS: 13947-84-7) was determined by single crystal X-ray diffraction. Ab initio quantum chemical calculations at the B3LYP, MP2 and G3 levels were performed to investigate the stability and the formation of the different tautomers and conformers. The calculations indicate that the amine form is the more stable tautomer, showing a high degree of electron conjugation. The most stable amine conformer located by the calculations corresponds to the crystallized structure. On the contrary, in the less stable imine form, the conjugation is separated by a N2–C2 single bond.