Valery F. Sidorkin

Ab initio study of medium effects on the geometries of the compounds of trigonal-bipyramidal silicon with coordination center ClSiC3O

Abstract For a number of (O–Si)chelate and zwitterionic intramolecular complexes with coordination center ClSiC 3 O, appreciable discrepancies between the experimental solid state geometry and those calculated at the HF/6-31G ∗ level for the gas phase and solution were found. The established regularity of changing the molecular structure of complexes in going from the gas phase to the solution, and the solid state corresponds to that expected from an analysis of the 29 Si NMR data and a physical model of the influence of medium on the geometry of hypervalent silicon compounds, developed earlier by the present authors.

Bifurcate Hydrogen Bonds. Interaction of Intramolecularly H-Bonded Systems with Lewis Bases

The structure and the hydrogen bonding in the systems formed by the intramolecularly H-bonded systems, namely, maltol (3-hydroxy-2-methyl-4-pyrone), 5, 2,4,6-trinitrophenol, 6, acetylacetone enol, 7, with Lewis bases, phosgene, 8, dioxane, 9, and DMSO, 10, have been studied by density functional theory (B3LYP) and MP2 using the 6-311G* basis set. The continuum solvent effect was simulated by IEF-PCM model. The hydrogen bond analysis using the atoms in molecules (AIM) method was applied by using the MP2(full)/6-311++G** electron density to establish the nature of the bifurcate hydrogen bond (BHB) in these systems as well as contributory factors for its stabilization. The nature of interaction in the intermolecular H-complexes formed by compounds 5- 7 with the Lewis bases 8- 10 was shown to depend on the strength of the intramolecular hydrogen bond O...H and the strength of the base. The critical values of the CO...H and NO...H angles for which the formation of BHB is possible, have been determined.

CAr–H···O Hydrogen Bonds in Substituted Isobenzofuranone Derivatives: Geometric, Topological, and NMR Characterization

Substituted isobenzofuranone derivatives 1a-3a and bindone 4 are characterized by the presence of an intramolecular C(Ar)-H···O hydrogen bond in the crystal (X-ray), solution ((1)H NMR and specific and nonspecific IEF-PCM solvation model combined with MP2 and B3LYP methods), and gas (MP2 and B3LYP) phases. According to geometric and AIM criteria, the C(Ar)-H···O interaction weakens in 1a-3a (independent of substituent nature) and in 4 with the change in media in the following order: gas phase > CHCl(3) solution > DMSO solution > crystal. The maximum value of hydrogen bond energy is 4.6 kcal/mol for 1a-3a and 5.6 kcal/mol for 4. Both in crystals and in solutions, hydrogen bond strength increases in the order 1a < 2a < 3a with the rising electronegativity of the ring substituents (H < OMe < Cl). The best method for calculating (1)H NMR chemical shifts (δ(calcd) - δ(expl) < 0.7 ppm) of hydrogen bonded and nonbonded protons in 1a-3a and 1b-3b (isomers without hydrogen bonds) is the GIAO method at the B3LYP level with the 6-31G** and 6-311G** basis sets. For the C-H moiety involved in the hydrogen bond, the increase of the spin-spin coupling constant (1)J((13)C-(1)H) by about 7.5 Hz is in good agreement with calculations for C-H bond shortening and for blue shifts of C-H stretching vibrations (by 55-75 cm(-1)).

Modelling of the pathway of SN2 intramolecular substitution at the silicon(IV) atom

Abstract Pathways of the reaction calculated by quantum chemical (MNDO) and Burgi-Dunitz methods were compared. The regularities found in the change in the geometric parameters of the ClSiC 3 O coordination centre are typical of a bimolecular nucleophilic substitution reaction at a tetrahedral atom and consistent with those observed experimentally in the series of dimethyl ( N -amidomethyl)chlorosilane derivatives of a hypervalent silicon atom. An analytic representation of the reaction pathway in l SiCl and l SiO coordinates is given. The experimental and theoretical points are shown to fit best onto the retention curve of the order of the sum of the axial bonds, determined within the range of strong correlations between the SiO and SiCl bonds. Perturbation of the principle of stereoelectronic selection of reaction pathways in the formation of (0Si) chelate cycles is demonstrated.

Correlation among the gas-phase, solution, and solid-phase geometrical and NMR parameters of dative bonds in the pentacoordinate silicon compounds. 1-Substituted silatranes.

Silatranes XSi(OCH2CH2)3N exhibit a good linear relationship between their experimental and calculated (IGLO and GIAO) values of the NMR chemical shifts of (15)N, δN, and the lengths of dative bonds Si←N, dSiN, determined in the gas phase (ED, CCSD), solutions (COSMO PBE0, B3PW91), and crystals (X-ray). An aggregate of the obtained data provides strong evidence that the gas-phase value of dSiN in MeSi(OCH2CH2)3N should be greater by ∼0.05 Å than that determined in the electron diffraction (ED) experiment (2.45 Å). Given this condition, a long-standing contradiction between the data of the structural (X-ray, ED) and NMR (15)N experiments for the molecules of 1-methyl- and 1-fluorosilatrane regarding the sensitivity of their coordination contact Si←N to the medium effect is resolved.

Geometry of intramolecular silicon complexes: an ab initio estimation of the sensitivity to the effect of medium. (Aroyloxymethyl)trifluorosilanes

The effect of medium on the geometry of the (benzoyloxymethyl)trifluorosilane (1) molecule was studied by the HF and MP2 methods with the 6-31G(d), 6-311G(d), and 6-311+G(d,p) basis sets, as well as using the Onsager SCRF model, the PCM approach, and the data of X-ray diffraction study. Molecule 1 has a low complexation energy (5.4 kcal mol–1 according to MP2(Full)/6-31G(d)+ZPE calculations and ∼6.8 kcal mol–1 according to IR spectroscopy data), while its geometric parameters are the least sensitive to the effects of medium among all hypervalent silicon compounds studied to date. Nevertheless, the results obtained revealed a pronounced deformation of the Si←O coordination bond in 1 on going from the gas phase to the polar solution and crystal. This serves as a theoretical substantiation of the hypothesis that substantial changes in the IR and NMR spectral characteristics of the (O—Si)-chelate (aroyloxymethyl)trifluorosilanes upon variation of external factors are due to geometric reasons. Analysis of the electron density distribution according to Bader indicates that the Si←O bond in molecule 1 can be treated as a covalent bond of high polarity.

Analysis of the Hypersensitivity of the 29Si NMR Chemical Shift of the Pentacoordinate Silicon Compounds to the Temperature Effect. N-(Silylmethyl)acetamides

Theoretical investigation of the phenomenon of hypersensitivity of the (29)Si NMR chemical shift, δ, in the pentacoordinate silicon compounds to the temperature effect has been performed by the example of N-(silylmethyl)acetamides MeC(O)NMeCH2SiX3 (X = Me, 1; OMe, 2; F, 3) and MeC(O)NMeCH2SiMe2F (4) with the use of experimental dynamic NMR (DNMR) (29)Si data. It is based on the following: (i) the analysis of the potential energy surface of molecules 1-4 in polar solvents and the energetics of interconversion between their possible isomeric forms; (ii) the calculations of δ at different temperatures taking into account the dependence of the dielectric constant (ε) of the medium on T, and (iii) the isolation of dynamic, geometrical, and polar contributions to the temperature drift of δ. The results obtained allowed us to give a consistent explanation of the DNMR (29)Si spectra of acetamides 1-4 and to elucidate the nature of an unusual effect of T on δ.

Stereoelectronic effects and the problem of the choice of model compounds for organic derivatives of a pentacoordinated silicon atom (taking silatranes as an example)

Anomeric effects at the silicon atom in silatranes XSi(OCH2CH2)3N and model trimethoxysilanes XSi(OMe)3 (X=H, Me, SiH3, F, Cl) containing a tetracoordinated silicon atom were studied by the MNDO and AM1 methods. Unlike silatranes, the nature of substituent X pronouncedly affects the type and degree of stereoelectronic effects on the values of X−Si−O−C dihedral angles, X−Si bond lengths, and proton affinities of trimethoxysilanes. The results obtained indicate the necessity of modifying the set of spectral and structural indications of pentacoordination of the anomeric silicon atom established previously and observing the principle of conformational similarity for compared compounds. Bicyclic organosilicon ethers XSi(OCH2)3CH were proposed to be used as models of corresponding silatranes.

QUANTUM-CHEMICAL STUDY OF PENTACOORDINATED SILICON COMPOUNDS : SI-SUBSTITUTED (O-SI)DIMETHYL(N-ACETYLACETAMIDOMETHYL)SILANES

The mechanisms and self-consistency of the changes in geometric parameters, charge distribution, orbital structure and energy of the “soft” coordination center XSiC3O in Si-substituted (O−Si)dimethyl(N-acetylacetamidomethyl)silanes upon variation of the nature of the substituent X (X=OCOCH3, OCOCF3, F, Cl, Br, I) have been established by the PM3 method. They have been found to be in complete correspondence with the experimental data and the hypervalent bonding model. In contrast to the prediction of the standard donor-acceptor complex model, a decrease in the ionization potential of the donor upon the formation of the 3c−4e XSi←O bond has been demonstrated. Significant shortcomings of the PM3 method in quantitative estimations of the complexation energy and the gas-phase geometry have been pointed out.

Electrochemical oxidation and radical cations of structurally non-rigid hypervalent silatranes. Theoretical and experimental studies

Using 18 silatranes XSi(OCH2 CH2 )3 N (1) as examples, the potentials of electrochemical oxidation E0 of the hypervalent compounds of Si were calculated for the first time at the ab initio and DFT levels. The experimental peak potentials Ep (acetonitrile) show an excellent agreement (MAE=0.03) with the MP2//B3PW91 calculated E0 (C-PCM). Radical cations of 1 reveal a stretch isomerism of the N→Si dative bond. Localization of the spin density (SD) on the substituent X and the short (s) coordination contact Si⋅⋅⋅N (dSiN <2.13 Å) along with the high five-coordinate character of Si are typical for the first isomer 1+.(s) , whereas the second one, 1+.(l) , has a longer (l) Si⋅⋅⋅N distance (dSiN >3.0 Å), the four-coordinate Si and the SD localized on the silatrane nitrogen atom Ns . The vertical model of adiabatic ionization (1→1+.(s) or 1→1+.(l) ) was developed. It allows, in accordance with an original experimental test (electrooxidation of 1 in the presence of ferrocene), a reliable prediction of the most probable pathways of the silatrane oxidation. The reliable relationships of E0 (1) with the strength characteristics of the dative contact N→Si were revealed.