Sergey A. Shlykov

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.

1,3-Dimethyl-3-silapiperidine: Synthesis, Molecular Structure, and Conformational Analysis by Gas-Phase Electron Diffraction, Low Temperature NMR, IR and Raman Spectroscopy, and Quantum Chemical Calculations

The first Si-H-containing azasilaheterocycle, 1,3-dimethyl-3-silapiperidine 1, was synthesized, and its molecular structure and conformational properties were studied by gas-phase electron diffraction (GED), low temperature NMR, IR and Raman spectroscopy and quantum chemical calculations. The compound exists as a mixture of two conformers possessing the chair conformation with the equatorial NMe group and differing by axial or equatorial position of the SiMe group. In the gas phase, the SiMe(ax) conformer predominates (GED: ax/eq = 65(7):35(7)%, ΔG = 0.36(18) kcal/mol; IR: ax/eq = 62(5):38(5)%, ΔG = 0.16(7) kcal/mol). In solution, at 143 K the SiMe(eq) conformer predominates in the frozen equilibrium (NMR: ax/eq = 31.5(1.5):68.5(1.5)%, ΔG = -0.22(2) kcal/mol). Thermodynamic parameters of the ring inversion are determined (ΔG(‡) = 8.9-9.0 kcal/mol, ΔH(‡) = 9.6 kcal/mol, ΔS(‡) = 2.1 eu). High-level quantum chemical calculations (MP2, G2, CCSD(T)) nicely reproduce the experimental geometry and the predominance of the axial conformer in the gas phase.

Structure and Energetics of β-Diketonates. XII. Structure of Lanthanide tris-Dipivaloylmethanates for Er(thd)3 Used as an Example

A simultaneous electron diffraction and mass spectroscopic study of saturated vapors of erbium tris-dipivalylmethanate has revealed that at 136(5)°C, the vapor consists solely of Er(thd)3 molecules. Electron diffraction data may be described by two alternative models (of C_3 and D_3 symmetry), for which ra, rg, and rα structural parameters have been determined. D3 symmetry is recognized to be preferable for free Er(thd)3 molecules. The main structural parameters of the model are rα (Er − O) 2.218(5), rα (O − C) 1.279(5), rα (C − Cr) 1.404(6), rα (C − Ct) 1.512(3), rα(Ct − Cm) 1.542(5), rα (rm Cm− H) 1.804(4) Å, The ErO 75.0(0.4)°. The ErO6 coordination polyhedron has a structure close to an antiprism. A rotational angle of the O–O–O trigonal face relative to the position in a regular prism is 20.7(0.8)°. Possible reasons for the differences in the structure of Er(thd)3 molecules in the gas phase and crystal are discussed.

The structure of a thiadiazole-containing expanded heteroazaporphyrinoid determined by gas electron diffraction and density functional theory calculations.

The molecular structure of a macrocycle with a 24-membered ring, a thiadiazole-containing expanded heteroazaporphyrinoid, has been, for the first time, directly characterised by a synchronous gas electron diffraction and mass spectrometric experiment and DFT calculations; the molecule has the equilibrium structure of C(3h) symmetry with a planar macrocycle.

The joint gas electron diffraction and mass spectrometric study of GeI4(g) + Ge(s) system. Molecular structure of germanium diiodide

Abstract A mass spectrometric study was made of the GeI 4 (g) + Ge(s) system at 400–780 K. The conditions at which GeI 2 is the main molecular species were determined. The electron diffraction patterns were recorded at 653 K with simultaneous mass spectrometric control of the vapour composition. The experimental molecular scattering intensity function was represented using the bond distances, force constants, and the concentrations of the molecular species present in the vapour. It was established that GeI 2 has a non-linear geometry with r α (GeI) = 254.0(5) pm and a valence angle of 102.1(1.0)°. The correlations between the mean bond energies, the bond distances and the valence force constants of the germanium dihalides and tetrahalides are discused.

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.

Synthesis and Conformational Analysis of 3-Methyl-3-silatetrahydropyran by GED, FTIR, NMR, and Theoretical Calculations: Comparative Analysis of 1-Hetero-3-methyl-3-silacyclohexanes

3-Methyl-3-silatetrahydropyran 1 was synthesized and its molecular structure and conformational behavior was studied by gas-phase electron diffraction (GED), FTIR, low temperature (1)H and (13)C NMR spectroscopy, and by theoretical calculations (DFT, MP2). Two conformers, 1-ax and 1-eq, were located on the potential energy surface. In the gas phase, a slight predominance of the axial conformer was determined, with the ratio 1-ax:1-eq = 54(9):46(9) (from GED) or 53:47 or 61:39 (from IR). In solution, LT NMR spectroscopy at 103 K gives the ratio 1-ax:1-eq = 35:65 (-ΔG°103 = 0.13 kcal/mol). Simulation of solvent effects using the PCM continuum model or by calculation of the corresponding solvent-solute complexes allowed us to rationalize the experimentally observed opposite conformational predominance of the conformers of 3-methyl-3-silatetrahydropyran in the gas phase and in solution. Comparative analysis of the effect of heteroatom in 1-hetero-3-methyl-3-silacyclohexanes on the structure, stereoelectronic interactions, and relative energies of the conformers is done.

Molecular structure and conformational properties of N -cyclohexylpiperidine as studied by gas-phase electron diffraction, mass spectrometry, IR spectroscopy and quantum chemical calculations

Abstract Conformational properties and molecular structure of N-cyclohexylpiperidine (NCHP) were studied by quantum chemical calculations (DFT-B3LYP and MP2), gas-phase electron diffraction (GED–MS) and IR spectroscopy. NCHP may exist in at least eight conformeric forms that differ by their axial and/or equatorial positions and the relative orientation of the two rings. Two of these conformers were found to dominate in the gas phase at room temperature: The most stable conformer, I, has both rings in equatorial positions and oriented perpendicularly relative to each other. The less abundant conformer, II, has the rings in equatorial position and nearly parallel equatorial to each other. Structural parameters are given for these conformers.Graphical AbstractThe experimental and theoretical methods indicate that conformers with equatorial position of the cyclohexane ring relative to the piperidine ring are more stable in a gas and liquid phases

Structure and Energy Studies of β-Diketonates. X. Molecular Structure of Sc(aa)3 According to Gas-Phase Electron Diffraction Data

Synchronous electron diffraction and mass spectrometric investigation of the saturated vapor of scandium tris-acetylacetonate was carried out. It was stated that at 155(5)°C the vapor contains only monomeric Sc(aa)3 molecules, whose structural parameters ra, rg, and rα were determined. The internuclear distances in the chelate ring were found to be rα(Sc–O) = 2.076(4) Å, rα(O–C) = 1.271(3) Å, rα(C–Cr) = 1.386(4) Å. The coordination polyhedron ScO6 has a nearly antiprism structure of D3 symmetry. The angle of rotation of the O–O–O trigonal faces relative to their position in a regular prism is 26.8(1.1)°.

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.