Molecular Structure of Gaseous Oxopivalate Co(II): Electronic States of Various Multiplicities.

Synchronous electron diffraction/mass spectrometry was used to study the composition and structure of molecular forms existing in a saturated vapor of cobalt(II) oxopivalate at T = 410 K. It was found that monomeric complexes Co4O(piv)6 dominate in the vapor. The complex geometry possesses the C3 symmetry with bond lengths Co-Oc = 1.975(5) Å and Co-O = 1.963(5) Å, as well as bond angles Oc-Co-O = 111.8(3)°, Co-Oc-Co = 110.4(6)°, O-Co-O = 107.1(3)° in the central OcCo4 fragment and four OcCoO3 fragments. The presence of an open 3d shell for each Co atom leads to the possibility of the existence of electronic states of the Co4O(piv)6 complex with Multiplicities 1, 3, 5, 7, 9, 11, and 13. For them, the CASSCF and XMCQDPT2 calculations predict similar energies, identical shapes of active orbitals, and geometric parameters, the difference between which is comparable with the error of determination by the electron diffraction experiment. QTAIM and NBO analysis show that the Co-Oc and Co-O bonds can be attributed to ionic (or coordination) bonds with a significant contribution of the covalent component. The high volatility and simple vapor composition make it possible to recommend cobalt (II) oxopivalate as precursors in the preparation of oxide films or coatings in the CVD technologies. The features of the electronic and geometric structure of the Co4O(piv)6 complex allows for the conclude that only a very small change in energy is required for the transition from antiferromagnetically to ferromagnetically coupled Co atoms.

Dimer Rhenium Tetrafluoride with a Triple Bond Re-Re: Structure, Bond Strength.

Based on the data of the gas electron diffraction/mass spectrometry (GED/MS) experiment, the composition of the vapor over rhenium tetrafluoride at T = 471 K was established, and it was found that species of the Re2F8 is present in the gas phase. The geometric structure of the Re2F8 molecule corresponding to D4h symmetry was found, and the following geometric parameters of the rh1 configuration were determined: rh1(Re-Re) = 2.264(5) Å, rh1(Re-F) = 1.846(4) Å, α(Re-Re-F) = 99.7(0.2)°, φ(F-Re-Re-F) = 2.4 (3.6)°. Calculations by the self-consistent field in full active space approximation showed that for Re2F8, the wave function of the 1A1g ground electronic state can be described by the single closed-shell determinant. For that reason, the DFT method was used for a structural study of Re2X8 molecules. The description of the nature of the Re-Re bond was performed in the framework of Atom in Molecules and Natural Bond Orbital analysis. The difference in the experimental values of r(Re-Re) in the free Re2F8 molecule and the [Re2F8]2- dianion in the crystal corresponds to the concept of a triple σ2π4 (ReIV-ReIV) bond and a quadruple σ2π4δ2 (ReIII-ReIII) bond, respectively, which are formed between rhenium atoms due to the interaction of d-atomic orbitals. The enthalpy of dissociation of the Re2F8 molecular form in two monomers ReF4 (ΔdissH°(298) = 109.9 kcal/mol) and the bond energies E(Re-Re) and E(Re-X) in the series Re2F8→Re2Cl8→Re2Br8 molecules were estimated. It is shown that the Re-Re bond energy weakly depends on the nature of the halogen, while the symmetry of the Re2Br8 (D4d) geometric configuration differs from the symmetry of the Re2F8 and Re2Cl8 (D4h) molecules.

Cyclic Dimers of 4-n-Propyloxybenzoic Acid with Hydrogen Bonds in the Gaseous State.

A comprehensive study of saturated vapors of 4-n-propyloxybenzoic acid (POBA) by gas electron diffraction (GED) and mass spectrometric (MS) methods supplemented by quantum chemical (QC) calculations was carried out for the first time. An attempt was made to detect dimeric forms of the acid in the gaseous state. It has been established that at the temperature of GED experiment, vapor over a solid sample contains up to 20 mol.% of cyclic dimers with two O-H...O hydrogen bonds. The main geometrical parameters of gaseous monomers and dimers of POBA are obtained. The distance r(O…O) = 2.574(12) Å in the cyclic fragment of the gaseous dimer is close to that in the crystal structure (2.611 Å). In the mass spectrum of the POBA recorded the ions of low intensity with a mass exceeding the molecular mass of the monomer were detected. The presence of ions, whose elemental composition corresponds to the dissociative ionization of the dimer, confirms the results of the GED experiment on the presence of POBA dimers in the gas state. The results of GED studies of acetic acid, benzoic acid, and POBA were compared. It is shown that the COOH fragment saves its geometric structure in monomers, as well as the COOH...HOOC fragment with two hydrogen bonds in dimers of different acids. The intermolecular interaction energy in considered acid dimers was estimated using QC calculations (B97D/6-311++G **). The significant value of last (>84 kJ/mol) is the reason for the noticeable presence of dimers in the gas phase.

Molecular Structure, Vibrational Spectrum and Conformational Properties of 4-(4-Tritylphenoxy)phthalonitrile-Precursor for Synthesis of Phthalocyanines with Bulky Substituent.

By DFT method with B3LYP, PBE, CAM-B3LYP, and B97D functionals, it was found that the molecule 4-(4-tritylphenoxy)phthalonitrile (TPPN) has four conformers. The geometric structure, vibrational frequencies, electronic characteristics, and thermodynamic functions of conformers, as well as the structure and energy of transition states, were determined. IR spectrum of TPPN film contains vibrational bands belonging to different conformers. The assignment of bands was performed basing the distribution of normal vibration energy on internal coordinates. A synchronous electron diffraction/mass spectrometric experiment was performed to determine the structure of conformers in a saturated TPPN vapor. The elemental composition of the ions recorded in the mass spectrum indicates the thermal stability of TPPN at least up to T = 200 °C. The difference in the structure of tetrasubstituted metal phthalocyanines, which can be synthesized from different TPPN conformers, has been shown.

Gas-Phase Structures of Potassium Tetrakis(hexafluoro- acetylacetonato) Lanthanide(III) Complexes [KLn(C5 HF6 O2 )4 ] (Ln=La, Gd, Lu).

The molecular structures of potassium tetrakis(hexafluoroacetylacetonato)lanthanide(III) complexes [KLn(hfa)4 ] (Ln=La, Gd, Lu; hfa=C5 HF6 O2 ,) were studied by synchronous gas-phase electron diffraction/mass spectrometry (GED/MS) supported by quantum-chemical (DFT/PBE0) calculations. The compounds sublime congruently and the vapors contain a single molecular species: the heterobinuclear complex [KLn(hfa)4 ]. All molecules are of C1 symmetry with the lanthanide atom in the center of an LnO8 coordination polyhedron, while the potassium atom is coordinated by three ligands with formation of three K-O and three K-F bonds. One hfa ligand is not bonded to the potassium atom. Topological analysis of the electron-density distributions confirmed the existence of ionic-type K-O and K-F bonding. The structures of the free [KLn(hfa)4 ] molecules are compared with those of the related compounds [KDy(hfa)4 ] and [KEr(hfa)4 ] in their crystalline state. The complex nature of the chemical bonding is discussed on the basis of electron-density topology analyses.

The Effect of Intramolecular Hydrogen Bond Type on the Gas-Phase Deprotonation of ortho-Substituted Benzenesulfonic Acids. A Density Functional Theory Study.

Structural factors have been identified that determine the gas-phase acidity of ortho-substituted benzenesulfonic acid, 2-XC6H4-SO3H, (X = -SO3H, -COOH, -NO2, -SO2F, -C≡N, -NH2, -CH3, -OCH3, -N(CH3)2, -OH). The DFT/B3LYP/cc-pVTZ method was used to perform conformational analysis and study the structural features of the molecular and deprotonated forms of these compounds. It has been shown that many of the conformers may contain anintramolecular hydrogen bond (IHB) between the sulfonic group and the substituent, and the sulfonic group can be an IHB donor or an acceptor. The Gibbs energies of gas-phase deprotonation ΔrG0298 (kJ mol-1) were calculated for all compounds. It has been set that in ortho-substituted benzenesulfonic acids, the formation of various types of IHB is possible, having a significant effect on the ΔrG0298 values of gas-phase deprotonation. If the -SO3H group is the IHB donor, then an ion without an IHB is formed upon deprotonation, and the deprotonation energy increases. If this group is an IHB acceptor, then a significant decrease in ΔrG0298 of gas-phase deprotonation is observed due to an increase in IHB strength and the A- anion additional stabilization. A proton donor ability comparative characteristic of the -SO3H group in the studied ortho-substituted benzenesulfonic acids is given, and the ΔrG0298 energies are compared with the corresponding values of ortho-substituted benzoic acids.

Accurate equilibrium structure of 3-aminophthalimide from gas electron diffraction and coupled-cluster computations and diverse structural effects due to electron density transfer.

For the first time, the molecular structure of 3-aminophthalimide has been determined by the gas electron diffraction (GED) method supported by a mass-spectrometric analysis of the gas phase and results of quantum-chemical computations up to coupled-cluster level of theory, CCSD(T). The semiexperimental equilibrium structure, rsee, has been derived from the GED data by taking into account harmonic and anharmonic vibrational corrections estimated from the quantum-chemical force field (up to cubic terms). High accuracy structures have been exploited for the observation of fine structural effects arising due the presence of the electron-donating amino group and the formation of a hydrogen bond. Natural bond orbital (NBO) analysis and quantum theory of atoms in molecules (QTAIM) have been applied to explain these effects.

Ligand Coordination in Bis(ß-diketonato) d Metals: The Mn(II) Case of D2 h versus D2 d Symmetry.

The structure of free manganese(II) bis-acetylacetonate [Mn(acac)2] was determined experimentally by gas-phase electron diffraction. The vapor at 197(5) °C is composed of a single conformer of Mn(acac)2 in D2 d symmetry with a central structural motif of an elongated MnO4 tetrahedron with a Mn-O distance ( re) of 2.035(5) Å and a bond angle in chelate rings (∠O-Mn-O) of 89.4(6)°. This result contradicts the predicted planar structure of the MO4 moiety ( D2 h) with a short Mn-O distance ( re) of 1.771 Å from a MC(5i5)QDPT2 calculation. From these findings and by comparison with data from the literature, we conclude that in general in bis(ß-diketonato) coordination compounds of d metals there is a perpendicular arrangement of the two ligands for d0, d5, and d10 coordination centers and a planar arrangement ( D2 h) for others.

The Structure of Mn(acac)3 -Experimental Evidence of a Static Jahn-Teller Effect in the Gas Phase.

The structure of free manganese(III) tris(acetylacetonate) [Mn(acac)3 ] was determined by mass-spectrometrically controlled gas-phase electron diffraction. The vapor of Mn(acac)3 at 125(5) °C is composed of a single conformer of Mn(acac)3 in C2 symmetry with the central structural motif of a tetragonal elongated MnO6 octahedron and 47(2) mol % of acetylacetone (Hacac) formed by partial thermal decomposition of Mn(acac)3 . Three types of Mn-O separations have been refined (rh1 =2.157(16), 1.946(5), and 1.932(5) Å. We have found no indication for a significant deviation of the -C-C-C-O-Mn-O- six-membered rings from planarity, which is observed in the solid state.

Gas-phase structure of 1,8-bis[(trimethylsilyl)ethynyl]anthracene: cog-wheel-type vs. independent internal rotation and influence of dispersion interactions.

The gas-phase structure of 1,8-bis[(trimethylsilyl)ethynyl]anthracene (1,8-BTMSA) was determined by a combined gas electron diffraction (GED)/mass spectrometry (MS) experiment as well as by quantum-chemical calculations (QC). DFT and dispersion corrected DFT calculations (DFT-D3) predicted two slightly different structures for 1,8-BTMSA concerning the mutual orientation of the two -C-C[triple bond, length as m-dash]C-SiMe3 units: away from one another or both bent to the same side. An attempt was made to distinguish these structures by GED structural analysis. To probe the structural rigidity, a set of Born-Oppenheimer molecular dynamics (BOMD) calculations has been performed at the DFT-D level. Vibrational corrections Δr = ra - re were calculated by two BOMD approaches: a microcanonically (NVE) sampled ensemble of 20 trajectories (BOMD(NVE)) and a canonical (NVT) trajectory thermostated by the Noose-Hoover algorithm (BOMD(NVT)). In addition, the conventional approach with both, rectilinear and curvilinear approximations (SHRINK program), was also applied. Radial distribution curves obtained with models using both MD approaches provide a better description of the experimental data than those obtained using the rectilinear (SHRINK) approximation, while the curvilinear approach turned out to lead to physically inacceptable results. The electronic structure of 1,8-BTMSA was investigated in terms of an NBO analysis and was compared with that of the earlier studied 1,8-bis(phenylethynyl)anthracene. Theoretical and experimental results lead to the conclusion that the (trimethylsilyl)ethynyl (TMSE) groups in 1,8-BTMSA are neither restricted in rotation nor in bending at the temperature of the GED experiment.

Accurate Determination of Equilibrium Structure of 3-Aminophthalonitrile by Gas Electron Diffraction and Coupled-Cluster Computations: Structural Effects Due to Intramolecular Charge Transfer.

For the first time, the molecular structure of 3-aminophthalonitrile with unique electronic properties has been determined by the gas electron diffraction (GED) method supported by a mass spectrometric analysis of the gas phase. Moreover, it has been optimized at the high-level quantum-chemical coupled-cluster theory, CCSD(T), in conjunction with the triple-ζ basis set. The equilibrium structure has been determined from the GED data taking into account harmonic and anharmonic vibrational corrections estimated from the quantum-chemical force field (up to cubic terms). The computed CCSD(T) structure has been corrected for the core-core and core-valence electron-correlation effects estimated at the MP2 level and extrapolated to the basis set of quadruple-ζ quality. A remarkable agreement between the experimental and theoretical equilibrium structural parameters (bond lengths and angles) points to a high accuracy of both the molecular structure and applied theories. The high accuracy of structure computations and experimental determination allows the observation of structural changes due to the intramolecular charge transfer predicted by natural bond orbital (NBO) calculations. According to the NBO analysis, the amino group is the electron-donating substituent, whereas the nitrile groups are able to withdraw the π electrons from the benzene ring. Noticeable variations in the structural parameters are also explained by the interaction of σ → π* orbitals of the nearest C≡N and N-H bonds.

1,8-Bis(phenylethynyl)anthracene - gas and solid phase structures.

1,8-Bis(phenylethynyl)anthracene (1,8-BPEA) was synthesized by a twofold Kumada cross-coupling reaction. The molecular structure of 1,8-BPEA was determined using a combination of gas-phase electron diffraction (GED), mass spectrometry (MS), quantum chemical calculations (QC) and single-crystal X-ray diffraction (XRD). Five rotamers of the molecule with different orientations of phenylethynyl groups were investigated by DFT calculations. According to these, molecules of C2 symmetry with co-directional rotation of the phenylethynyl groups are predicted to exist in the gas phase at 498 K. This was confirmed by a GED/MS experiment at this temperature. The bonding of this conformer was studied and described in terms of an NBO-analysis. Dispersion interactions in the solid state structure and in the free molecule are discussed. In the solid this symmetry is broken; the asymmetric unit of the single crystal contains 3.5 molecules and a herringbone packing motif of π-stacked dimers and trimers. The π-stacking in the dimers is between the anthracene units, and the trimers are linked by π-stacking between phenyl and anthracene units. The interaction between these stacks can be described in terms of σ(C-H)π interactions.

Structures and intriguing conformational behavior of 1- and 2-naphthalenesulfonamides as determined by gas-phase electron diffraction and computational methods.

The saturated vapors of 1- and 2-naphthalenesulfonamides (1-NaphSA and 2-NaphSA) were studied by the gas-phase electron diffraction/mass-spectrometric method at 413(9) and 431(9) K. According to quantum chemical calculations (DFT/B3LYP and MP2 with cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis set) 1-NaphSA possesses four conformers with different orientations of the SO2NH2 fragment relative to the naphthalene frame and eclipsed or staggered orientation of the N-H and S═O bonds, whereas 2-NaphSA possesses only two conformers with different orientations of the N-H and S═O bonds. It was experimentally established that vapors over 1-NaphSA and 2-NaphSA exist predominantly (up to 75 mol %) of low-energy conformers of C1 symmetry in which the C-S-N planes deviate from perpendicular orientation relative to the naphthalene skeleton with near eclipsed orientation of the N-H and S═O bonds of the SO2NH2 fragment. The following geometrical parameters (Šand degrees) of the dominant conformers were derived: r(h1)(C-H) = 1.089(4), r(h1)(C-C)av = 1.411(3), r(h1)(C-S) = 1.761(10), r(h1)(S-O)av = 1.425(3), r(h1)(S-N) = 1.666(10), ∠C-C1-C = 119.8(2), ∠C1-S-N = 104.5(22), C9-C1-S-N = 69.5(30) for 1-NaphSA; r(h1)(C-H) = 1.083(5), r(h1)(C-C)av = 1.411(3), r(h1)(C-S) = 1.780(7), r(h1)(S-O)av = 1.427(4), r(h1)(S-N) = 1.668(6), ∠C-C2-C = 123.0(3), ∠C2-S-N = 103.6(19), C1-C2-S-N = 110(10) for 2-NaphSA. The connection between nonequivalence of the C-C bonds in the naphthalene frame and spatial orientation of the substituents SO2NH2 is discussed. Transition states between conformers and enantiomers were determined.

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(ß) = 1.453(4), C(α)-C(m) = 1.395(4), C(ß)-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 structures of tellurium(IV) halides in the gas phase and as solvated molecules.

The structures of molecular tellurium tetrafluoride and tellurium tetrachloride were determined by a combination of gas-phase electron diffraction, mass spectrometry and quantum chemical calculations. The combined GED/MS experiments showed no evidence of decomposition of TeF(4) and TeCl(4). No ions of oligomeric (dimeric, trimeric, etc.) or any other composition were found in the mass spectra. The monomeric molecules possess a pseudo trigonal bipyramidal structure (C(2v) symmetry) with the equatorial Te-X distances being shorter than the axial ones. The fluorine atoms are bent away from the lone pair resulting in X(eq)-Te-X(eq) and X(eq)-Te-X(ax) bond angles smaller than 120 and 90 degrees, respectively. The structure of solvates TeF(4) (THF)(2), TeF(4) (dioxane) TeF(4) (DME)(2), TeF(4)(Et(2)O) TeF(4)(toluene), TeCl(4)(CH(3)CN)(2), TeCl(4)(DME)(2) and TeCl(4)(dioxane) were determined by X-ray diffraction. The structures of tellurium tetrafluoride solvates are strongly influenced by the choice of the solvent molecules. Monomeric TeF(4) units were obtained with THF, DME and dioxane whereas fluoride bridged coordination polymers were formed using diethyl ether or toluene. All tellurium tetrachloride solvates studied contain monomeric TeCl(4) units with coordinated solvent molecules. Coordination numbers range from four in the gas phase to eight in the TeF(4) dimethoxyethane solvate. Geometric parameters of the TeX(4) molecules in the crystal, solvates and gas phase were compared. DFT, MP2, CCSD, CCSD(T) methods were applied for calculation of geometric and vibrational characteristics of free TeX(4) molecules (X = F, Cl). The pseudorotation barriers were estimated and an NBO analysis was performed. It was shown that both, GED and theoretical, quantitative results are in agreement with the qualitative results of the VSEPR model.

The molecular structure, equilibrium conformation and barrier to internal rotation in decachloroferrocene, Fe(η-C5Cl5)2, determined by gas electron diffraction.

The molecular structure of decachloroferrocene has been determined by gas electron diffraction supported by quantum chemical calculations. The equilibrium conformation has staggered ligand rings and D(5d) symmetry. The barrier to internal rotation is, however, only 0.8(2) kJ mol⁻¹. This barrier is so low that even at room temperature the vast majority of molecules in the gas phase would have sufficient thermal energy to undergo virtually non-hindered internal rotation. While the eclipsed equilibrium conformation of unsubstituted ferrocene is determined by attractive dispersion interaction between the two cyclopentadienyl ligands, the staggered equilibrium conformation of Fe(η-C5Cl5)2 is due to steric repulsion between Cl atoms at different rings. The ligands are non-planar: the C-Cl bonds are bent 3.7(3)° out of the plane of the C5 ring away from the metal atom. The Fe-C, C-C and C-Cl bond distances (r(a)) are: 205.0(4) pm, 143.4(3) pm and 170.2(4) pm respectively.

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%.

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.

Gas-phase structure and conformational properties of 3,3,6,6-tetramethyl-1,2,4,5-tetroxane.

The geometric structure and conformational properties of 3,3,6,6-tetramethyl-1,2,4,5-tetroxane (diacetone diperoxide) have been studied by gas electron diffraction and quantum chemical calculations (MP2 and B3LYP methods with 6-31G(d,p) and 6-311+G(2df,p) basis sets). The molecule possesses a chair conformation with C2h symmetry and the following geometric parameters for the six-membered ring (rh1 values) have been determined: O-O = 1.463(5) A, C-O = 1.432 (4) A, O-C-O = 108.2(7) degrees, C-O-O = 107.7(4) degrees, phi(C-O-O-C) = 63.7(4) degrees, and phi(O-O-C-O) = -63.9(4) degrees. A small contribution of less than 3.5% of a twist conformer with C2 symmetry cannot be excluded. Quantum chemical calculations predict a contribution between 1 and 2%. Additional calculations for the parent compound 1,2,4,5-tetroxane (diformaldehyde diperoxide) demonstrate that methyl substitution at the carbon atoms has a minor effect on the ring geometry but a strong effect on the conformational properties. Methyl substitution reduces the energy difference between twist and chair conformers by more than 5 kcal/mol.

Unexpected conformational properties of 1-trifluoromethyl-1-silacyclohexane, C5H10SiHCF3: gas electron diffraction, low-temperature NMR spectropic studies, and quantum chemical calculations.

The molecular structure of axial and equatorial conformers of 1-trifluoromethyl-1-silacyclohexane, (C5H10SiHCF3), as well as the thermodynamic equilibrium between these species was investigated by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance (DNMR) spectroscopy, and quantum chemical calculations (B3LYP, MP2, and CBS-QB3). According to GED, the compound exists as a mixture of two Cs symmetry conformers possessing the chair conformation of the six-membered ring and differing in the axial or equatorial position of the CF3 group (axial=58(12) mol%/equatorial=42(12) mol%) at T=293 K. This result is in a good agreement with the theoretical prediction. This is, however, in sharp contrast to the conformational properties of the cyclohexane analogue. The main structural feature for both conformers is the unusually long exocyclic bond length Si--C 1.934(10) A. A low-temperature 19F NMR experiment results in an axial/equatorial ratio of 17(2) mol%:83(2) mol% at 113 K and a DeltaG (not equal) of 5.5(2) kcal mol-1. CBS-QB3 calculations in the gas-phase and solvation effect calculations using the PCM(B3LYP/6-311G*) and IPCM(B3LYP/6-311G*) models were applied to estimate the axial/equatorial ratio in the 100-300 K temperature range, which showed excellent agreement with the experimental results. The minimum energy pathways for the chair-to-chair inversion of trifluoromethylsilacyclohexane and methylsilacyclohexane were also calculated using the STQN(Path) method.