Quantum-chemical study on thermal transformations of urea in ethylene glycol

Thermal decomposition of urea in ethylene glycol with formation of isocyanic acid and ammonia was studied at the B3LYP/6-311++G(df,p) level of theory. The decomposition process is efficiently catalyzed by monomeric and dimeric forms of ethylene glycol. Ethylene glycol dimer formed via intermolecular hydrogen bonding is a stronger acid than the monomeric species, which is responsible for the higher catalytic activity of the former. Ethylene glycol associates efficiently catalyze addition of ammonium to isocyanic acid in the synthesis of ethylene carbonate.     

Quantum-chemical study of 1,3-dioxane complexes with two water molecules

Methods PM3, RHF/6-31G(d), and MP2/6-31G(d)//RHF/6-31G(d) were used in calculation of the energy of formation of five 1,3-dioxane complexes with two water molecules formed through hydrogen bonds. The study of the conformational properties of the most stable associate revealed two routes of the chair-chair conformation isomerization. It was shown that the difference between the minima on the potential energy surface in this gase increased, and the barriersto the interconversion decreased as compared to the calculated values for the isolated molecule of 1,3-dioxane.     

Quantum-chemical study of interaction of gases with molecular fragment of carbon surface

The atom-atom potential method has been applied in ranking industrial gases in their affinity for carbon surfaces. From an analysis of the Coulombic induction, and dispersion contributions to the total energy, and also an analysis of the repulsion potentials, it has been found that the energy of dispersion interaction is a factor with a substantial influence on the affinity of the molecules for the matrix.Sorption and Fine Inorganic Synthesis Branch, Institute of General and Inorganic Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 4, pp. 488–490, July–August, 1991. Original article submitted December 17, 1990.     

Quantum-chemical study of interaction of dipyrrolylmethenes with boron trifluoride and other lewis acids

The geometric parameters and energies of the products of donor-acceptor interaction of dipyrrolylmethenes with BF₃ and other inorganic Lewis acids were calculated by quantum-chemical methods. The bond nature and the energies of formation of the donor-acceptor complexes under consideration were analyzed. It was shown that the complexes with p-element fluorides are noticeably stabilized by hydrogen bonds involving the hydrogen atom of the NH group of dipyrrolylmethene and the nearest fluorine atom of a Lewis acid. Hydrogen bonding promotes further elimination of HF in the synthesis of boron fluoride complexes of dipyrrolylmethenes. The energy profile was calculated for the reaction of formation of the boron fluoride complex with dipyrrolylmethene through the intermediate donor-acceptor complex.     

Quantum-chemical analysis of the interaction of fragments

Some additional possibilities for analysis of the interaction of fragments using Bader’s theory were considered for the interaction of the complex [NbF₆]⁻ with the outer-sphere shell consisting of n alkaline metal cations (Na, K, Cs; n = 1–4).     

Quantum-chemical study of the effect of butadiene interaction with anionic active sites on the polymer microstructure

The electronic and geometric structure of the models for prereaction complexes of the anionic active sites for polymerization of butadiene have been calculated using a modified CNDO method: C₄H₄Li-cis-C₄H₆(I), C₄H₇Li-trans-C₄H₆(II), (C₄H₇Li)₂-cis-C₄H₆(III) and (C₄H₇Li)₂-trans-C₄H₆. The configuration of complexes I and II resulting from the total energy minimization points to the preferential C₄H₆ attack on the α-C atom of the monomeric active site (AS) leading to 1,4-units in polybutadiene. A more pronounced complexation effect observed with I as compared to II was taken into account when interpreting data on the preferential formation of cis-1,4-structure within macromolecules. The structure of models III and IV and also a decrease in the difference of the energy of interaction with C₄H₆ incorporated in these models, as compared to models I and II, indicate a decrease in the 1,4-cis-units content with increasing initiator concentration. Based on results of the present study, an evaluation was also made of the effect of the interaction between the living macromolecule aggregates and diene on the dissociation processes.     

Quantum-chemical study of allylstannanes

It has been shown by the MNDO method that in allyl compounds of tin, the atomic orbitals of the heteroatom interact with the -orbital through the bridge group, but interact with the ^*-orbital mainly through space. The position and intensity of the long-wave electronic transitions for methylvinylstannane should not depend on the conformation, with the ,- and , pseudo--conjugation effects being approximately identical, whereas for allylstannane, the ,-conjugation is considerably stronger. In molecules containing several allyl fragments bonded to heavy atoms, ,-conjugation is far weaker than ,-conjugation in polyenes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2071–2076, September, 1991.     

Thermodynamic and structural aspects of the interaction of l-histidine with urea and dimethylformamide in water

The solubility of l-histidine in water and aqueous solutions of dimethylformamide (DMF) at 25°C was determined. The thermal effects of solution of l-histidine in aqueous solutions of urea and DMF were measured. The standard enthalpies, free energies, and entropies of solution and transfer of the amino acid from water to aqueous solutions of amides were calculated. It was shown that the interaction of l-histidine with urea is attractive, and that with DMF, repulsive, with the change in the free energy determined by the enthalpy term in both cases. The enthalpy-entropy compensation taking place to a certain extent in the interaction of l-histidine and l-phenylalanine with amide was revealed. This effect is the most pronounced in the interaction of predominantly hydrophobic DMF and l-phenylalanine.     

Hydrogen-bonding interaction of urea with DNA bases: A density functional theory study

This work deals with the interaction between urea and DNA bases (adenine, thymine, guanine, and cytosine). The optimized geometries, binding energies, and harmonic vibrational frequencies are calculated using the DFT/B3LYP functional combined with the 6–31+G(d,p) basis set. Their interactions are studied aiming to understand more about the nature of the intercalation binding forces between urea and DNA. Fourteen stable complexes are found on the potential energy surface. The structures are cyclic; they are stabilized by NH...O/N and CH...O interactions. The binding energies range from −19.9 kJ·mol⁻¹ to −74.0 kJ·mol⁻¹. The obtained formation energies indicate that Urea:G and Urea:C are more favorable than Urea:T and Urea:A. In addition, the Atoms in Molecules theory is performed to study the hydrogen bonds in the complexes.     

Quantum-chemical study of spectral and luminescent properties and photolysis of phenol and its complexes with water

The spectral and luminescent properties of phenol and the photoinduced O-H bond cleavage reaction, depending on complexation and excitation energy, were considered using the INDO method. It was shown that the formation of H-bonded complexes with water does not exert a noticeable effect on the spectral-luminescent properties of phenol. A decrease in the fluorescence quantum yield with increasing excitation energy is associated with the enhancement of the photoreaction rather than a change in the dynamics of intramolecular photophysical processes. The photolysis of phenol proceeds via the predissociation scheme. Complexation results in the enhancement of population of photodissociative singlet and triplet sates.     

Quantum-chemical study of alkyloxybenzenes hydrophobicity

Equilibrium states of alkyloxybenzenes (1-methyl-3,5-dioxybenzene C₁-AOB, 1-butyl-3,5-dioxybenzene C₄-AOB, and 1-hexyl-3,5-dioxybenzene C₆-AOB) and their complexes with water (H₂O-C _n -AOB, n = 1, 3, or 6) have been optimized by means of the DFT/ROB3LYP method. For various conformations, intermolecular energy profiles, and thermodynamic parameters of C _n -AOB interaction with water have been calculated. The dissociation energy of the alkyloxybenzenes-water complexes has been calculated in the frame of the DFT/6-31G(2p,2d) method, and the alkyloxybenzenes hydrophobicity has been estimated.     

Quantum-chemical study of endohedral fullerenes

State-of-the-art in quantum-chemical research on endohedral fullerenes is considered. Separate chapters are devoted to complexes with noble gases, main group elements, and methals and their nitrides. The structures and the mechanisms of formation and decomposition of these compound sare highlighted. Applicability of various quantum-chemical methods for endohedral fullerene complexes is discussed. Problems associated with the use of the ROHF and UHF methods are revealed. The DFT method with hybrid functionals and effective core potentials for heavy atoms is shown to hold the greatest promise.     

Quantum-chemical simulation of interaction of polycaproamide with a lithium chloride solution in dimethylacetamide

Quantum-chemical calculations of interaction between lithium chloride and dimethylacetamide (DMAc) and between the polycaproamide model fragment CH₃NHCO(CH₂)₅NHCOCH₃ and a lithium chloride solution in DMAc were performed. The software package GAMESS with the MINI basis set was used in the calculations. Models of the solution included 2 LiCl molecules and 8–16 DMAc molecules. All of these models suggest three potential energy minimums corresponding to three stable structures that differ in the relative arrangement of lithium and chloride ions. A decrease in the amount of solvent in the system leads to transition from Li⁺(DMAc)₄Cl^- to Li⁺...Cl^-(DMAc)₃ and then to the (LiCl)₂(DMAc)₂ species, which crystallizes to form the 1: 1 crystal solvate. The mechanism of dissolution of polycaproamide in DMAc containing lithium chloride was refined.     

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.