Ab initio Study of the Keto-Enol Equilibriumof Malonaldehyde

 The mechanism of the keto-enol tautomerism of malonaldehyde was studied by ab initio methods using 6-21G^** and 6-311G^** basis functions at the HF level. Two separate mechanisms were examined: through-space proton transfer in the ω-shaped form and through-space proton transfer in a sickle-shaped form obtained from the ω form by rotation. The transition state structure of the ω form is non-planar, whereas that of the sickle form is planar. The sickle form is connected with a 2^nd order saddle, indicating that there should exist a lower energy barrier, i.e. that the through-bond mechanism may be preferred. The calculated energy barriers of keto-enol tautomerism for the sickle form is twice as high as those for the omega form.     

The Keto-Enol Equilibrium of Pentane-2,4-dione Studied by ab initio Methods

Summary.  The mechanism of the keto-enol interconversion of pentane-2,4-dione (trivial name: acetylacetone, acac) was examined at the restricted Hartree-Fock (HF) level and the DFT correlation functional BLYP method using the 6-311G^** basis, both included in the program GAUSSIAN 98. Two initial enol forms are considered: the omega and sickle forms, related by a rotation of 180° around the OC*CC bond. The study is restricted to the through-space transfer of the hydroxyl proton to C(2). The two geometry-optimized enol forms are planar; the geometry optimization of the diketone forms leads to the same non-planar structure, regardless of the starting enol geometry. The transition state of the through-space omega-enol→diketone conversion has also a non-planar structure, indicating that the hydroxyl proton moves outside of the CCC plane. The BLYP-calculated energy barrier of the forward (omega-enol→diketone) conversion is 245 kJ·mol⁻¹, that of the reverse (diketone→omega-enol) conversion 222 kJ·mol⁻¹; thus, an almost symmetric barrier, which is not thermally accessible, is defined. The energy barrier for the sickle-enol→diketone conversion is considerably lower (187 kJ·mol⁻¹), to access the sickle form from the more stable omega form, a rotation is needed (energy barrier: 88 kJ·mol⁻¹). The HF-calculated barriers are 1.3–1.4 times higher than those obtained with the BLYP method. Received July 6, 2000. Accepted (revised) September 8, 2000     

The Keto-Enol Equilibrium of Pentane-2,4-dione Studied by ab initio Methods

 The mechanism of the keto-enol interconversion of pentane-2,4-dione (trivial name: acetylacetone, acac) was examined at the restricted Hartree-Fock (HF) level and the DFT correlation functional BLYP method using the 6-311G^** basis, both included in the program GAUSSIAN 98. Two initial enol forms are considered: the omega and sickle forms, related by a rotation of 180° around the OC*CC bond. The study is restricted to the through-space transfer of the hydroxyl proton to C(2). The two geometry-optimized enol forms are planar; the geometry optimization of the diketone forms leads to the same non-planar structure, regardless of the starting enol geometry. The transition state of the through-space omega-enol→diketone conversion has also a non-planar structure, indicating that the hydroxyl proton moves outside of the CCC plane. The BLYP-calculated energy barrier of the forward (omega-enol→diketone) conversion is 245 kJ·mol⁻¹, that of the reverse (diketone→omega-enol) conversion 222 kJ·mol⁻¹; thus, an almost symmetric barrier, which is not thermally accessible, is defined. The energy barrier for the sickle-enol→diketone conversion is considerably lower (187 kJ·mol⁻¹), to access the sickle form from the more stable omega form, a rotation is needed (energy barrier: 88 kJ·mol⁻¹). The HF-calculated barriers are 1.3–1.4 times higher than those obtained with the BLYP method.     

Ab initio Study of Malonaldehyde Rotamers

Summary.  Malonaldehyde rotamer geometries were optimized using ab initio calculations at the HF level with STO-3G^** and 6-21G^** basis sets. The most stable rotamer is the ω-shaped one with cyclic structure and intramolecular hydrogen bond. The most unstable rotamer is that obtained by rotation of the ω-rotamer around the CO single bond by 180° due to the loss of the additional stabilization contributed by the intramolecular H-bond. The energy barriers separating the different rotamers vary between 13 and 233 kJċmol⁻¹. The structure of the transition states is non-planar with rotation angles varying between 72 and 98°. Received January 18, 1999. Accepted (revised) August 4, 1999     

Ab initio Study of Malonaldehyde Rotamers

 Malonaldehyde rotamer geometries were optimized using ab initio calculations at the HF level with STO-3G^** and 6-21G^** basis sets. The most stable rotamer is the ω-shaped one with cyclic structure and intramolecular hydrogen bond. The most unstable rotamer is that obtained by rotation of the ω-rotamer around the CO single bond by 180° due to the loss of the additional stabilization contributed by the intramolecular H-bond. The energy barriers separating the different rotamers vary between 13 and 233 kJċmol⁻¹. The structure of the transition states is non-planar with rotation angles varying between 72 and 98°.     

Multiple bond migration with participation of a protophilic agent

The pathways of migration of the multiple bond in propene and propyne molecules involving the hydroxide ion were investigated by theab initio (RHF/6-31+G^* and MP2/6-31+G^*) methods. Stationary points corresponding to stable complexes between the molecules under study and the hydroxide ion and between corresponding carbanions and water molecule were found on the potential energy surfaces of the proton transfer reactions. In the presence of hydroxide ion, migration of the multiple bond can occur by an “intramolecular” mechanism of the proton transfer involving the proton of hydroxide ion bound in the complex with propene or propyne molecule. For the propene system, such a mechanism seems to be quite realistic and more preferable energetically than a traditional two-stage mechanism with a passage of the proton into the medium. For the system with the triple bond, an equal expenditure of energy is required to follow any mechanism (without taking into account the effects of solvation and the interaction with a cation), whereas the formation of the stable [H₂C=C=CH·H₂O]⁻ complex can prevent further transformations. For Part 1, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 35–41, January, 1999.     

Intramolecular proton transfer of serine in aqueous solution. Mechanism and energetics

Serine amino acid in aqueous solution is theoretically studied at the B3PW91/6-31+G^** level using a dielectric continuum solvent model. Neutral and zwitterionic structures in the gas phase and in solution are described and the proton-transfer mechanism is discussed. A neutral conformation in which the carboxyl hydrogen atom is already oriented toward the amino group seems to be the absolute energy minimum in the gas phase and the most stable neutral form in solution. The absolute energy minimum in solution is a zwitterionic form. The energy barrier for proton transfer is predicted to be very small, in particular when zero-point-energy contributions are added. Our calculations allow the dynamic aspects of the ionization mechanism to be discussed by incorporating nonequilibrium effects. Received: 28 June 1999 / Accepted: 13 October 1999 / Published online: 14 March 2000     

2-羟基咪唑在气相和水中的异构平衡和质子迁移的从头算计算和Monte Carlo模拟

采用从头算方法在MP2/6-31＋G^*水平上研究了2-羟基咪唑分子在孤立分子和一水合物的异构体的相对稳定性和可能的质子迁移反应, 分析了一个水分子的参与对2-羟基咪唑分子异构体的相对稳定性和质子迁移速率的影响, 采用Monte Carlo模拟方法研究了反应体系在水溶液中反应的溶剂化效应. 结果表明: 2-羟基咪唑分子的孤立分子和一水合物的最稳定异构体相同, 都为酮式. 直接质子迁移反应在水溶液中活化能垒有轻微增加, 但产物能量得到降低; 水助催化质子迁移反应在水溶液中的活化能垒和产物能量都得到明显降低. 综合气相和水相的计算结果, 2-羟基咪唑水助催化的质子迁移反应较易进行, 且在水溶液中进行容易, 可以很容易被实验观察到.     

Double proton migrations in the associates of formic acid with nitrous,nitric, orthophosphoric,and sulfuric acids

Mechanisms of the proton transfer in dimeric associates of formic acid with nitrous, nitric, orthophosphoric, and sulfuric acids were studied by theab initio (HF/6-31G^**) method. The mechanism of the cooperative (concerted or one-step) proton transfer was shown to occur in all cases. The calculated activation barriers of the proton transfer reactions for the associates investigated are equal to 19.9, 14.2, 13.3, and 10.7 kcal mol^–1, respectively.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2184–2189, September, 1996.     

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.     

Hydrogen Bonding Properties of the Complexes of Formaldehyde and its Derivatives with HF and HCl

Summary.  The complexes of formaldehyde and some of its derivatives with HF and HCl were investigated at HF/6-311 + +G^** and MP2/6-311 + +G^** levels of theory. Interaction energies were corrected for the basis set superposition error (BSSE). The full optimizations of dimers and monomers were performed during calculations. The Bader theory of atoms-in-molecules (AIM) was also applied for the localization of bond critical points (BCP) and for the calculation the electron densities and their Laplacians at these points. The relationships between H-bond energy and parameters obtained from calculations were also studied. Received June 29, 2001. Accepted (revised) October 29, 2001     

An ab initio Study of the Rotamers and Rotations of Propane-1,3-dial by DFT and SCF Calculations

Summary.  Eight planar rotamers of the enol form of malonaldehyde were considered at the HF (Hartree-Fock) and DFT (density functional theory) levels with 6-311G^** and D95^** (Dunning/Huzinaga full double-ζ) basis sets with the aim to establish the most stable of them and to find the energy barriers of their conversions. The results show that the rotamer with an intramolecular hydrogen bond is the most stable one. High energy barriers were ascertained for the conversions including rotations around a CC double bond. Most of the reactions connected with breaking of the hydrogen bond display strongly asymmetric energy barriers. Their transition states were determined as first-order saddle points because of one imaginary frequency in the IR spectrum related with a negative energy gradient. Received July 6, 2000. Accepted (revised) September 7, 2000     

Tautomerism of xanthine: The second-order MØller-Plesset study

Four 9H and four 7H tautomers of DNA base xanthine were studied by the ab initio LCAO-MO method at the MP2/6-311G^**//HF/6-31G^** and MP2/6-31G^**//HF/6-31G^** approximations. All calculated structures are minima at the HF/6-31G^** potential energy surface with the dioxo 7H tautomer (A1) being the global minimum. The second most stable tautomer, dioxo-9H (B1) is by 9 kcal/mol less stable. For the A1 B1 transition the calculated MP2 energy gap corresponds to the equilibrium constant of 2 × 10^–7. Therefore, only the major tautomeric form A1 is predicted to be detectable in the gas phase. The 7H and 9H groups of tautomers are discussed separately. Within both groups, the dioxo form (A1-7H, B1-9H) is the most stable one and is succeeded by the 2-dihydroxy (A2, B2) form. However, while the energy difference between A1 and A2 is 10 kcal/mol, the energy difference between B1 a B2 is only 2 kcal/mol. The effect of polar environment was estimated by the SCRF method, using a spherical cavity, at the HF/6-31G^** level. These calculations did not change the gas phase stability order of the tautomers. However, the energy difference between A1 and B1 decreased from 9 kcal/mol at the HF/6-31G^** level to 4 kcal/mol at the SCRF HF/6-31G^** level.     

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.     

Structural peculiarities and the possibility of the existence of small water cluster anions (H2O) n − withn≤4

Structures of (H₂O) _n ⁻ anions withn≤4 were optimized at the UHF/4-31++G^** level and their stability was estimated at the MP2/4-31++G^** level. The trimer anion has a chain-like structure while the tetramer anion can exist either in a chain-like or a cyclic configuration. In the dimer anion and in the chain-like anions, the excess electron density is localized on the terminal water molecule, an acceptor of the H-bond proton. In the cyclic anion, it is uniformly distributed over the free hydrogen atoms. All considered anions have energy values higher than those of the corresponding neutral oligomers. The detachment of an electron from the anions should proceed with the liberation of energy. However, trimer and larger anions are stable against dissociation into individual water molecules and a free electron. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 41–46, January, 1997.     

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.     

Molecular structures of acetylene derivatives of tin

The geometry and force fields of the bis(trimethylstannyl)acetylene molecule (a conformer withD _3d symmetry corresponding to a minimum of the total energy of the molecule) were calculated by the RHF and MP2(fc) methods. The effective core potential in SBK form with the optimized 31G^* valence basis set was employed in the case of Sn atoms. The 6–31G^** and 6–311G^** basis sets were used for carbon and hydrogen atoms. Vibrational spectra of the light and perdeuterated isotopomers of bis(trimethylstannyl)acetylene were interpreted using the procedure of scaling the quantum-chemical force fields. For Part 5, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 616–626, April, 2000.     

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.     

Reentrant behavior of poly(vinyl alcohol)–borax semidilute aqueous solutions

 The reentrant behavior of Poly(vinyl alcohol) (PVA)–borax aqueous semidilute solutions with a PVA concentration of 20 g/l and borax concentrations varies from 0.0 to 0.20 M was investigated using dynamic light scattering (DLS) and dynamic viscoelastic measurements. Two (fast and slow modes) and three (fast, middle, and slow) relaxation modes of PVA semidilute aqueous solutions without and with the presence of borax, respectively, were observed from DLS measurements. The fast and middle relaxation modes were q ²-dependent (q is the scattering vector) characteristic of diffusive behavior; however, the slow modes were q ³-dependent, characteristic of intraparticle dynamics. The experimental results showed that the slow relaxation mode dominates the DLS relaxation. The DLS slow mode relaxation time, τ_s, and the viscoelastic modulus G′(ω) and G′′(ω) data had a similar trend and demonstrated reentrant behavior as the borax concentration was increased from 0.0 to 0.20 M, i.e. τ_s, G′(ω), and G′′(ω) fluctuated with increasing borax concentration. The excluded-volume effect of polymers, charge repulsion among borate ions bound on PVA molecules, and intermolecular cross-linking didiol–borate complexation caused an expansion of the polymer chain; however, the screening effect of free Na⁺ ions on the negative charge of the borate ions bound on PVA and intramolecular cross-linking didiol–borate complexation led to a shrinkage of the polymer chain. The reentrant behavior was the consequence of the balance between expansion and shrinkage of the PVA–borate complex. Received: 26 March 1999/Accepted in revised form: 3 September 1999     

Theoretical study on keto–enol tautomerism and isomerization in pyruvic acid

Isomerization and tautomerism of 12 isomers of pyruvic acid including 4 keto and 8 enol forms were studied at the MP2 and B3LYP levels of theory using 6‐311++G(2df,p) basis set, separately. Activation energy (E_a), imaginary frequency (υ), and Gibbs free energy (ΔG^#) of the considered isomerization and tautomerism reactions were calculated. Interconversion of the enol forms proceeds through two paths: (i) proton transfer and (ii) internal rotation. Activation energies for the proton transfer paths were in the range of 125–145 kJ/mol and for the internal rotation paths were in the range of 5–45 kJ/mol. Keto–enol tautomerism of pyruvic acid proceeds only through proton transfer route and their activation energies were in the range of 200–300 kJ/mol. Effect of microhydration on the transition state structures and activation energies was also investigated. It was found that the presence of a water molecule catalyzes the isomerization and tautomerism reactions of pyruvic acid so that the activation energies decrease. © 2013 Wiley Periodicals, Inc.