Study of solid-state catalytic isotope exchange of hydrogen inl-hydroxyproline under the action of spillover tritium

Reaction of high-temperature solid-state catalytic isotope exchange (HSCIE) of hydrogen in L-hydroxyproline was studied byab initio quantum-chemical calculations. A one-center synchronous mechanism of isotope exchange between the amino acid and the H₃O⁺ model acidic center was considered. The structures of transition states of the reaction and the activation energies were determined. Relative reactivity of the C−H bonds in the hydroxyproline molecule under conditions of HSCIE was studied. The results obtained are in agreement with experimental data on the stereoselectivity and regioselectivity of the HSCIE reaction,viz., the lower the calculated activation energy of isotope exchange, the larger the portion of hydrogen substituted by tritium in a given position of the amino acid molecule. The enhancement of the reactivity under conditions of solid-state isotope exchange can be associated with additional interaction between the exchanging H atoms and the electron-donor O and N atoms of the amino acid molecule in transition state. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1056–1060, June, 1999.     

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.     

Quantum-chemical study of supramolecular complexes (DPyEt)n(AgNO3)m

The electronic structures and energies of formation of supramolecular complexes of dipyridylethylene with AgNO₃ were calculated by the semiempirical AM1/d method, at the Hartree—Fock level, and by the density functional theory (B3LYP/6-31G*).     

Binary molecular complexes of silicon tetrafluoride with water,methanol, and dimethyl ether. Quantum-chemical study

The molecular structures, the energies of complex formation, and the vibrational spectra of the binary molecular complexes of SiF₄ with water, methanol, and dimethyl ether were calculated by the ab initio MP2 method with the basis sets up to 6-311++G(2d,2p). In the complexes, which have been detected previously by IR spectroscopy in low-temperature (12—15 K) inert matrices, the five-coordinate Si atom is in a distorted trigonal-bipyramidal environment, which is formed through the donor-acceptor interaction of the O atom with the Si atom and is additionally stabilized by the H...F hydrogen bonds.     

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.     

Quantum-Chemical Study of Interactions between the Hydroxonium Cation and the Monomethyl Mercury Cation in Aqueous Media

The potential surface of the reaction between the polyhydrated monomethyl mercury cation and the hydroxonium cation is considered. An type reaction, resulting in solvate-separated products (mercury dication and methane molecule), was found to be thermodynamically probable. The thermal balance of the reaction is 132.3 kJ/mole. The activation energy of the limiting stage of the process is 56.7 kJ/mole.     

Anab initio MO study on the direct and migratory reaction mechanism for the formation of HF in H + CIF system

Anab initio MO method has been used to calculate the potential energy surface for the formation of HF when H reacts with C1F. The various transition states possibly related to forming HF were optimized. An IRC calculation starting from the collinear F-centered transition state was performed and energies of a series of points with bending configurations were also calculated to determine the direct reaction path leading to HF product. Another IRC calculation starting from the H-centered transition state was run to determine the migratory path to forming HF. By doing so, the Polanyi’s assumption that it would involve the direct and a migratory mechanisms for the formation of HF when H reacts with CIF has been verified theoretically. Project supported by the National Natural Science Foundation of China.     

Thermodynamic and kinetic parameters of elementary steps in gas-phase hydrolysis of SiF4. Quantum-chemical and FTIR spectroscopic studies

The energies and thermodynamic parameters of elementary steps in the proposed mechanism of silicon tetrafluoride hydrolysis in the gas phase were calculated by the ab initio quantum-chemical method (MP4//MP2/6-311G(2d,2p)) and the density functional theory (B3LYP/6-311G(2d,2p)). The proposed mechanism of gas-phase hydrolysis involves the formation of mono- and dihydroxy derivatives, hexafluorodisiloxane (SiF₃OSiF₃), and linear and cyclic siloxane polymers with the chain length from three to six Si—O and difluorosilanone units. According to the calculations, all reactions considered are endothermic and are characterized by positive Gibbs free energies. The initial hydrolysis steps can be presented with a high accuracy by two parallel processes: formation of trifluorohydroxysilane (SiF₃OH) and SiF₃OSiF₃. These are the most thermodynamically favorable among all reaction channels. The transition states of these elementary steps were found and their kinetic parameters were estimated (G = 132 and 147 kJ mol^–1, respectively). The calculation results were verified using FTIR spectroscopy of a mixture of gas-phase SiF₄ and water vapor. The comparison of the theoretical (absolute) intensities of bands in the IR spectra and integral absorption coefficients in the experimental IR spectrum made it possible to calculate the equilibrium concentrations of the reactants and equilibrium constants of elementary steps of formation of SiF₃OH and SiF₃OSiF₃, which agree with the theoretical values. The role of different derivatives in deep hydrolysis and possibilities of experimental detection of particular intermediates in the gas phase were discussed.     

Study of the solid-state hydrogen isotope exchange ofl-alanine

The solid-state reaction of isotope exchange ofl-alanine (l-Ala) with spillover-hydrogen activated on a Rh(Pd)-supported catalyst was studied. The reactivity of the carbon atoms and the activation energies of isotope exchange of the hydrogen at the C(2) and C(3) atoms of thel-Ala molecule were determined using tritium NMR. Theab initio calculations of the activation energy of a model reaction between the alanine molecule and a hydroxonium cation were carried out. The mechanism and plausible structures of the transition states of this reaction were proposed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 757–762, April. 1997.     

“Spectroscopic” Calculation of CH and NH Bond Dissociation Energies for a Series of Primary Amines

The CH and NH bond dissociation energies were calculated by the spectroscopic method for a series of primary amines. The energies were calculated from the fundamental absorption bands in an anharmonic approximation using the Morse harmonic basis set. The results are compared with data obtained from ab initio calculations in different basis sets and experimental values. It is shown that, unlike quantum-chemical calculations, the spectroscopic method correctly reproduces the effect of the molecular structure on the CH and NH bond dissociation energies in amines.Original Russian Text Copyright © 2004 by L. A. Gribov, I. A. Novakov, A. I. Pavlyuchko, V. V. Korolkov, and B. S. Orlinson__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 6, pp. 999–1007, November–December, 2004.     

Theoretical calculation of the low laying electronic states of the molecular ion KH

Using an ab initio method the potential energy has been calculated for the 25 lowest molecular states of symmetries ²Σ⁺, ²Π, ²Δ for the molecular ion KH⁺. The calculation is based on nonempirical pseudopotentials and parameterized ℓ-dependent polarization potentials. Gaussian basis sets have been used for both atoms. The spectroscopic constants for 18 electronic sates have been calculated by fitting the calculated energy values to a polynomial in terms of the internuclear distance R. Through the canonical functions approach the eigenvalue E_v, the abscissas of the corresponding turning points (R_min and R_max) and the rotational constants B_v have been calculated up to 24 vibrational levels for the considered bound states. The comparison of the present results with those available in literature shows a very good agreement.     

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.     

Structure and isomerization of heterapentalene compounds containing hypervalent bonds with central nitrogen,phosphorus, or arsenic atom

The possibility for a specific type of isomerization (electromorphism) to occur in conjugated bicyclic organic compounds containing Group V elements was studied by the ab initio (RHF/6-31G**, MP2(full)/6-31G**), and DFT (B3LYP/6-31G**) methods. Compounds 2 (X = N, P, As) were found to exist in a monocyclic planar form with intramolecular donor-acceptor N...O coordination (X = N) and as aromatic heterapentalene structures with hypervalent O--X--O bonds (X = P, As). According to calculations, no isomerization of planar heteroaromatic structures into pyramidal ones occurs. The strength of the O--X--O hypervalent bond and the aromaticity of heterapentalene structures 2 with ten -electrons increase on going from X = N to X = P. Correct estimation of these effects requires the inclusion of electron correlation.     

Effect of substituents (NH2, OH, F) on structures and stability of lithofluorosilylenoids

Isomeric structures and energies of three kinds of lithofluorosilylenoids, R₂SiLiF (R = NH₂, OH, F) were studied using theab initio molecular orbital theory. The calculations show that thermal stability of the three-membered ring structures of these three kinds of silylenoids decreases in the order of substituents NH₂ > OH > F because of the conjugation between NH₂, OH or F and Si atom. The interaction of substituents R with Li atom makes R₂SiLiF have a structure with two Li-A-Si-F (A = N, O, F) four-membered rings, which is the most stable of the isomers of each of three kinds of silylenoids and whose stability decreases in the order of substituents F > OH > NH₂. Inductive effect of substituents influences the thermal stability of the linear structure of silylenoids.     

Ab initio study of the complexes of first-row transition-metal ions with CH, CH2, and CH3

The geometries and bonding characteristics of the complexes of the first-row transition-metal ions with CH, CH₂ and CH₃ were investigated byab initio molecular orbital theory. MCH⁺ and MCH₂ ⁺ are linear and coplanar, respectively. Both of them are with obvious treble or double bond characteristics, but these multiple bonds are mostly “imperfect”. The calculated bond dissociation energies of , and are mostly close to the experimental values, and appear in similar periodic trends from Sc to Zn. Project supported by the National Natural Science Foundation of China (Grant No. 29170070).     

Na2O-P2O5系晶体微结构形态的拉曼光谱研究及其ab initio计算

本文采用激光拉曼光谱仪测量了磷酸钠二元系(1-x)Na₂O·xP₂O₅(x=0.25,0.33,0.50,1.0)几种晶体的拉曼光谱,比较并解释了随化学组成而变化微结构单元的拉曼振动模。同时用Gaussian 98W量子化学软件从头计算了这几种化合物的拉曼光谱。实验和计算均表明,磷酸盐晶体的基本结构单元为磷氧四面体[PO₄],并且晶体中磷氧四面体的伸缩模振动频率与连接中心磷原子的桥氧数密切相关,随桥氧数增加而升高。此外还解释了模拟图谱与实验谱差异的原因。     

Possible intramolecular rearrangements of O-vinyloximes initiated by N—O bond dissociation: a quantum-chemical analysis

The potential surface of conformational transitions of O-vinylacetoxime was studied and the regions of starting states for possible isomeric transformations with the N—O bond dissociation as the limiting stage were recognized. The activation parameters and heat effects of intramolecular rearrangement O-vinylacetoxime iminoacetaldehyde were evaluated. Transition-state structure of the rearrangement was identified.     

An accurate three-dimensional potential energy surface for the He-Na<Subscript>2</Subscript> complex

An accurate three-dimensional potential energy surface (PES) for the He-Na₂ van der Waals complex was calculated at the coupled cluster singles-and-doubles with noniterative inclusion of connected triple (CCSD(T)) level of theory. A mixed basis set, aug-cc-pVQZ for the He atom and cc-pCVQZ for the sodium atom, and an additional (3s3p2d1f) set of midbond functions were used. The computed interaction energies in 819 configurations were fitted to a 96-parameter analytic potential model by least squares fitting. The PES has two shallow wells corresponding to the T-shaped structure and the linear configuration, which are located at 12.5a ₀ and 14 a ₀ with depths of 1.769 and 1.684 cm⁻¹, respectively. The who potential energy surface exhibits weak anisotropy. Based on the fitted PES, state-to-state differential cross sections were calculated. Supported by the Natural Science Foundation of Anhui Educational Committee (Grant No. 2006kj072A) and the Natural Science Foundation of Anhui Province (Grant No. 070416236)     

IR Spectra of Paracetamol and Phenacetin. 1. Theoretical and Experimental Studies

IR spectra of paracetamol and phenacetin have been measured for powder crystals of these compounds and for their solutions in chloroform and dimethylsulfoxide. Ab initio calculations of their equilibrium geometry and vibrational spectra were carried out for spectrum interpretation. Differences between the experimental IR spectra of solutions and crystalline samples have been analyzed. Variations of molecular structure from the isolated state to molecular crystal were estimated based on the difference between the optimized molecular parameters of free molecules and the experimental bond lengths and angles evaluated for the crystal forms of the title compounds. The role of hydrogen bonds in the structure of molecular crystals of paracetamol and phenacetin is investigated, and spectral ranges with maximal intermolecular interactions are determined.