Georgiy V. Girichev

Tautomeric Properties and Gas-Phase Structure of 3-Chloro-2,4-pentanedione

The tautomeric properties of alpha-chlorinated acetylacetone, 3-chloro-2,4-pentanedione CH3C(O)-CHCl-C(O)CH3, have been investigated by gas electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximations with different basis sets up to cc-pVTZ). Analysis of the GED intensities resulted in the presence of 100(2)% enol tautomer at 269(8) K. The following skeletal geometric parameters (rh1 values) of the molecule, which possesses Cs symmetry, were derived: r(C=C) = 1.378(3) A, r(C-C) = 1.450(3) A, r(C=O) = 1.243(3) A, r(C-O) = 1.319(3) A, r(O-H) = 1.001(4) A, r(C-Cl) = 1.752(4) A, angleC-C=C = 121.3(1.0) degrees , angleC=C-O = 119.9(1.2) degrees , angleC-C=O = 119.1(1.2) degrees . Due to very small contributions of the keto tautomer in alpha-chlorinated acetylacetone and its parent species, the effect of alpha-chlorination on tautomeric properties cannot be derived from experimental data. Quantum chemical calculations (B3LYP/6-31G**, B3LYP/cc-pVTZ, and MP2/cc-pVTZ) predict that alpha-chlorination of acetylacetone has no pronounced effect on the tautomeric properties. On the other hand, similar calculations for 1-chloro-1,3-butanedion, ClC(O)-CH2-C(O)CH3, demonstrate that chlorination in one beta position destabilizes the enol tautomer. In both chlorinated species the enol form is strongly preferred.

Molecular Structure and Conformations of para-Methylbenzene Sulfonamide and ortho-Methylbenzene Sulfonamide: Gas Electron Diffraction and Quantum Chemical Calculations Study

The molecular structure and conformational properties of para-methylbenzene sulfonamide (4-MBSA) and ortho-methylbenzene sulfonamide (2-MBSA) have been studied by gas electron diffraction (GED) and quantum chemical methods (B3LYP/6-311+G** and MP2/6-31G**). Quantum chemical calculations predict the existence of two conformers for 4-MBSA with the S-N bond perpendicular to the benzene plane and the NH2 group either eclipsing or staggering the S-O bonds of the SO2 group. Both conformers possess CS symmetry. The eclipsed form is predicted to be favored by DeltaE = 0.63 kcal/mol (B3LYP) or 1.00 kcal/mol (MP2). According to the calculations, the S-N bond in 2-MBSA can possess planar direction opposite the methyl group (phi(C2C1SN) = 180 degrees ) or nonplanar direction (phi(C2C1SN) approximately 60 degrees ). In both cases, the NH2 group can adopt eclipsed or staggered orientation, resulting in a total of four stable conformers. The nonplanar eclipsed conformer (C1 symmetry) and the planar eclipsed form (CS symmetry) are predicted to be favored. According to the GED analysis, the saturated vapor over solid 4-MBSA at T = 151(3) degrees C consists as mixture of the eclipsed (78(19) %) and staggered (22(19) %) forms. The saturated vapor over solid 2-MBSA at T = 157(3) degrees C consists as a mixture of the nonplanar eclipsed (69(11) %) and planar eclipsed (31(11) %) forms.

Tautomeric properties and gas-phase structure of acetylacetone.

The tautomeric and structural properties of acetylacetone, CH3C(O)CH2C(O)CH3, have been studied by gas-phase electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximation with different basis sets up to aug-cc-pVTZ). The analysis of GED intensities resulted in the presence of 100(3)% of the enol tautomer at 300(5) K and 64(5)% of the enol at 671(7) K. The enol tautomer possesses Cs symmetry with a planar ring and strongly asymmetric hydrogen bond. The diketo form possesses C2 symmetry. The experimental geometric parameters of both tautomeric forms are reproduced very closely by B3LYP/aug-cc-pVTZ and MP2/cc-pVTZ methods.

Molecular structure and bonding in octamethylporphyrin tin(II), SnN4C28H28

Gas-phase electron diffraction was applied for the molecular structure determination of octamethylporphyrin tin(II), SnN(4)C(28)H(28), at the temperature of 706(10) K. The molecule was found to possess C(4v) symmetry with the Sn atom 1.025(30) Å above the plane of the N atoms and the following main internuclear distances (r(h1), Å): Sn-N = 2.301(9), C(α)-N = 1.360(8), C(α)-C(β) = 1.453(4), C(α)-C(m) = 1.395(4), C(β)-C(CH3) = 1.498(4). Quantum chemical calculations, DFT (B3LYP, BP86, PBE, PBE0) with cc-pVDZ, cc-pVTZ and cc-pVQZ basis sets reproduce the experimental bond distances with accuracy within 0.03 Å. According to NBO(B3LYP/cc-pVTZ) analysis, the direct donation gives a prevailing contribution to Sn-N bonding, decreasing the net charge on Sn from formal +2 to +1.28. The substitution effects at the pyrrole rings are discussed. The ability of different theoretical methods to predict the structure of this compound is analyzed.

The gas-phase structure of octaphenyloctasilsesquioxane Si8O12Ph8 and the crystal structures of Si8O12(p-tolyl)8 and Si8O12(p-ClCH2C6H4)8

The equilibrium molecular structure of octaphenyloctasilsesquioxane Si(8)O(12)Ph(8) in the gas phase has been determined by electron diffraction. It was found to have D(4) point-group symmetry, with Si-O bond lengths of 1.634(15)-1.645(19) A, and a narrow range [147.5(45)-149.8(24) degrees] of Si-O-Si angles. The structures of Si(8)O(12)(p-tolyl)(8) and Si(8)O(12)(p-ClCH(2)C(6)H(4))(8) have been determined by X-ray diffraction and are found to have Si(8)O(12) cages significantly distorted from the symmetry found for Si(8)O(12)Ph(8) in the gas phase. Thus, Si-O-Si angles range between 144.2(2)-151.64(16) degrees for Si(8)O(12)(p-tolyl)(8), and between 138.8(2)-164.2(2) degrees for Si(8)O(12)(p-ClCH(2)C(6)H(4))(8). These three structures show how much a Si(8)O(12) cage may be distorted away from an ideal structure, free from intermolecular forces, by packing forces in a crystalline lattice.

IR Spectra of N,N′-Ethylene-Bis(salicylaldiminates) and N,N′-ethylene-Bis(acetylacetoniminates) of Ni(II), Cu(II), and Zn(II)

Vibrational spectra of N,N′-ethylene-Bis(salicylaldiminaates) and N,N′-ethylene-Bis(acetylacetoniminates) of nickel (II), copper (II), and zinc (II) are studied experimentally (IR spectroscopy, 400–4000 cm−1) and theoretically (B3LYP), band assignment is given, and the distribution of potential energy of normal vibrations in internal coordinates is studied. Differences between vibrational spectra of the complexes are discussed. Thermodynamic functions of gas-phase complexes corresponding to temperatures of 298 and 800 K are calculated.

Ab Initio Structure Investigation of the Enol Forms of β-Diketones RCOCH2COR (R = H, CH3, CF3)

The geometrical parameters, force field parameters, and vibrational frequencies of the enol forms of β-diketones RCOCH2COR (R = H, CH3, CF3) were calculated using the ab initio MO LCAO SCF method involving broad bases of Cartesian Gaussian functions. For acetylacetone (R = CH3), the potential surface of rotation of the two methyl groups was investigated in detail, and the minimal energy route of the transfer of the hydroxyl proton inside the chelate ring was calculated. For hexafluoroacetylacetone (R = CF3) and malonic dialdehyde (R = H), calculations were carried out only for stationary points on the potential energy surface. For all the geometrical configurations under study, the relative energies were refined with inclusion of electron correlation in terms of second-, third-, and fourth-order Möller–Plesset perturbation theory; for molecules with R = H, CH3, the configuration interaction procedure taking account of all single and double excitations was additionally employed. The experimental and theoretical data are compared with those available in the literature. The substituent effect on the geometrical and electronic structure of the chelate ring is studied.

Tautomeric and Conformational Properties of Benzoylacetone, CH3-C(O)-CH2-C(O)-C6H5: Gas-Phase Electron Diffraction and Quantum Chemical Study

Tautomeric and structural properties of benzoylacetone, CH(3)-C(O)-CH(2)-C(O)-C(6)H(5), have been studied by gas-phase electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximation with different basis sets up to aug-cc-pVTZ). Analysis of GED intensities resulted in the presence of 100% enol tautomer at 331(5) K. The existence of two possible enol conformers in about equal amounts is confirmed by both GED and quantum chemical results. In both conformers the enol ring possesses C(s) symmetry with a strongly asymmetric hydrogen bond. The experimental geometric parameters are reproduced very closely by the B3LYP/cc-pVTZ method.

The molecular structure of selenium dibromide as determined by combined gas-phase electron diffraction-mass spectrometric experiments and quantum chemical calculations.

The vapour over solid SeBr(4) at 10 degrees C was investigated with a combined gas-phase electron diffraction/mass spectrometric (GED/MS) method. The composition of the vapour derived from the mass spectra (43% SeBr(2), 56.7% Br(2) and 0.3% Se(2)Br(2)) was in agreement with the composition obtained from the analysis of the simultaneously recorded GED intensities (41(3)% SeBr(2), 59(3)% Br(2)). The GED study results in the following geometric parameters (r(g), angle(g) values with total uncertainties): Se-Br = 2.306(5) A and Br-Se-Br = 101.6(6) degrees . Most quantum chemical approximations (B3LYP, MP2, CCSD and CCSD(T) with relativistic effective core potentials and cc-pVTZ as well as aug-cc-pVTZ basis sets for the outer shells) overestimate the Se-Br bond length by 0.01 to 0.03 A. All methods reproduce the bond angle correctly, except for the B3LYP method. Gas phase vibrational frequencies estimated from experimental vibrational amplitudes agree well with those measured by Raman spectroscopy in acetonitrile solutions. All computational methods overestimate vibrational frequencies, especially that for the symmetric stretch vibration, by about or 8 to 13%.

Structure and Energetics of β–Diketonates. XI. Molecular Structure of Sc(thd)3 from Gas–Phase Electron Diffraction Data

Electron–diffraction and mass–spectrometric studies of saturated vapor of scandium tris–dipivaloyl–methanate showed that at 135(5)°, the vapor contains only monomeric Sc(thd)3, whose structural parameters ra, rg, and rα were determined. The internuclear distances in the chelate ring were found to be rαSc=O) = 2.066(5)Å, rα(O=C) = 1.272(3)Å, and rα(C=Cr) = 1.385(3)Å. The ScO6 coordination polyhedron has a D3 symmetry configuration close to a regular antiprism. The angle of rotation of the O=O=O trigonal faces relative to their position in a regular prism is 25.7(1.5)°.