Hydrogen bond in 3-acetyl-4-hydroxycoumarin: X-ray diffraction study and quantum-chemical calculations

The proton transfer and the character of the strong intramolecular O--H...O hydrogen bond (O...O 2.442 ) in 3-acetyl-4-hydroxycoumarin were analyzed based on the results of X-ray diffraction study in the temperature range from 100 to 353 K and quantum-chemical B3LYP/6-31G(d,p) calculations. The barrier to proton transfer along the H-bond line is low (2 kcal mol^–1). However, no proton transfer was observed in the crystal at 100 K. Bader's topological analysis of the electron density distribution both in the crystal and in the isolated molecule demonstrated that the hydrogen bond corresponds to an intermediate type of interatomic interactions (E(r) < 0, ²(r) > 0 at the critical point (3, –1)).     

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*).     

Gas electron diffraction study of the geometric structure of triallylborane molecule

The main structural parameters of the triallylborane molecule having the C ₃ symmetry were determined by gas electron diffraction and quantum-chemical calculations at the MP2/6-31G(d,p) and B3LYP/6-31G(d,p) levels. The parameters calculated by the MP2/6-31G(d,p) method are in better agreement with the experimental data than those calculated by the B3LYP/6-31G(d,p) method.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 98–101, January, 2005.     

Matrix IR-spectroscopic and quantum-chemical study of difluorostannylene complexation with dinitrogen

Complexes of difluorostannylene with dinitrogen of composition 1∶1 and 1∶2 were stabilized in Ar matrix (12 K) and characterized by IR spectra. The bands at 588, 565, and 583, 557 cm⁻¹, respectively, were assigned to these complexes. Potential energy surfaces of the systems SnF₂+N₂ and SnF₂+2N₂ were studied by theab initio MP2/3-21G(d2)//HF/3-21G(d2) method using the basis set including polarization functions at Sn, F, and N atoms. Equilibrium structures of the complexes haveC _s andC _2v symmetry and correspond to coordination of lone electron pairs of nitrogen molecules with vacant p-AO of the carbenic center. The calculated complexation energies are equal to 4.6 and 8.9 kcal mol⁻¹, respectively. Based on results of quantum-chemical calculations an interpretation of the IR spectra of the complexes was given and it was shown that cycloaddition of SnF₂ to a triple N≡N bond with formation ofcyclo-SnF₂N₂ is energetically unfavorable. The absorption band belonging to SiF₄·N₂ complex in Ar matrix was detected and assigned. Dedicated to the memory of Academician M. E. Vol'pin timed to his 75th birthday. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1087–1093, June, 1998.     

Comparison of methods to estimate geometric and electronic properties on sulfur containing compounds

Summary Various spectroscopic properties have been calculated by CEPA-1 for NC₃NC, a molecule of potential interest to astrochemistry. According to CCSD(T) calculations with a basis set of 168 cGTOs, it is less stable than dicyanoacyetylene (NC₄N) by 108.5 kJ mol^–1. The equilibrium dipole moment is predicted to be 1.04 D (CCSD(T)/162 cGTO), with the negative end of the dipole at the terminal nitrogen site. The results of the present calculations are in excellent agreement with those of matrix-isolation IR spectroscopy carried out in the course of this work.     

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.     

Experimental and theoretical study of the reaction of difluorostannylene with methyl chloride. The first direct IR spectroscopic detection of a complex between a carbene analog and an alkyl halide in low temperature inert matrix

Complex between a carbene analog (SnF₂) and organo halide (CH₃Cl) was stabilized by a low-temperature (Ar, 12 K) matrix isolation technique and characterized by IR spectroscopy for the first time. The bands at 567 and 543 cm^–1 were assigned to this complex. The potential energy surface of the system SnF₂ + CH₃CI was studied byab initio MP2/ 3-21G(d)//HF/3-21G(d) and semiempirical PM3 methods. Calculations shown that the reaction between SnF₂ and CH₃C1 results in the formation of a donor-acceptor complex. The calculated energy of the complex formation is 14.2 kcal mol^–1 (ab initio) and 15.7 kcal mol^–1 (PM3). Quantum-chemical calculations were used to interpret the IR spectrum of the complex. Insertion of SnF₂ into the C-Cl bond with formation of CH₃SnF₂Cl is an energetically favored process but it requires surpassing of a high energetic barrier and does not occur under the experimental conditions. A complex of CH₃CI with H₂O codeposited in argon matrix was detected by IR spectroscopy for the first time.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1121–1128, May, 1996.     

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.     

Structure and Internal Rotation of Molecules in Chlorodifluoroacetic Acid

The potential energy profile of an isolated CF₂ClCOOH molecule with a CF₂Cl group rotating around the C–C bond was determined by the Hartree–Fock method using the 6-31G(d) basis set. Barriers to internal rotation were estimated for this molecule; its geometrical parameters were found for the equilibrium and transition states that are due to the torsion potential with unequal wells. Crystal effect on CF₂Cl reorientations in solid chlorodifluoroacetic acid has been evaluated.     

Quantum-chemical study of the singlet potential energy surface of the nitrene—dioxygen system

The potential energy surfaces of the HN—O₂ and PhN—O₂ systems were calculated by the MP2 and B3LYP methods. The mechanism of photooxidation of azides was refined. Photooxidation produces the nitrene—O₂ adducts with dioxaziridine and non-cyclic structures. The parameters of IR spectra of the adducts were calculated. The rearrangement of dioxaziridine to a nitro compound is likely a reason for chemiluminescence accompanying the photooxidation of azides.     

Molecular structure of 6-(N,N-dimethylamino)fulvene from a joint gas-phase electron diffraction and quantum-chemical study

The molecular structure of 6-(N,N-dimethylamino)fulvene was studied by gas-phase electron diffraction and quantum-chemical methods (HF/6-31G(d), MP2/3Z, MP2/4Z, density functional theory with the B3LYP/6-31G(d) and PBE/3Z functionals). Pronounced flattening of the nitrogen atom and equalization of the intracyclic C—C bonds were found to be a consequence of the electron delocalization in the molecule.     

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.     

Electronic structure and reactivity of S—S dications

The electronic structure and reactivity of some S—S dications were studied at the MP2/6-31G* level of theory. The results obtained indicate a stepwise electrophilic addition of disulfonium dication moiety to the double C=C bond to be the preferable mechanism.     

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).     

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.     

Vibrational spectra of trinitromethane derivatives

Spectroscopic parameters of trinitromethane derivatives RC(NO₂)₃ (R = F, Cl, Br, I, NC, NF₂, N₃) were determined. Vibrational frequencies and modes were calculated and the assignment of experimental spectra was performed. Spectral features due to the mutual influence of the C(NO₂)₃ group and other atomic groups and particular atoms were revealed.     

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.     

Structure and stability of closo-hexaboranes and their heteroanalogs

The molecular and electronic structures of closo-hexaboranes B₆H₆ ^2–, B₆H₇ ^–, and B₆H₈ and closo-heterohexaboranes XYB₄H₄ (X = Y = CH, N; X = BH, Y = CH^–, N^–, NH, O) were studed by the ab initio (MP2(full)/6-311+G**) and density functional (B3LYP/6-311+G**) methods. The bridging H atoms in closo-hexaboranes B₆H₇ ^– and B₆H₈ can undergo facile low-barrier migrations around the boron cage (the barrier heights are about 10—15 kcal mol^–1). All heteroboranes having octahedron-like structures with hypercoordinated N and O atoms are rather stable and can be the subject of synthetic research efforts.     

Proton affinities of fluoro derivatives of benzene,toluene, and<Emphasis Type="Italic">m</Emphasis>-xylene from<Emphasis Type="Italic">ab initio</Emphasis> MP2 calculations

The proton affinity (PA) energies of fluoro derivatives of benzene, toluene, andm-xylene were obtained fromab initio MP2-FC/6-31G^* calculations and compared with experimental results. Protonated forms of the molecules, resulting from different ways of proton addition, were studied. Relative concentrations of isomeric arenonium ions were calculated and compared with the results of NMR studies on arenonium ions in solutions. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1847–1849, November, 2000.