Ingvar Arnason

Conformational Properties of 1-Fluoro-1-silacyclohexane, C5H10SiHF: Gas Electron Diffraction, Low-Temperature NMR, Temperature-Dependent Raman Spectroscopy, and Quantum Chemical Calculations†

The molecular structures of axial and equatorial conformers of 1-fluorosilacyclohexane, C5H10SiHF, as well as the thermodynamic equilibrium between these species were investigated by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance, temperature-dependent Raman spectroscopy, and quantum chemical calculations (MP2, DFT, and composite methods). According to GED, the compound exists in the gas phase as a mixture of two conformers possessing the chair conformation of the six-membered ring and Cs symmetry and differing in the axial or equatorial position of the Si-F bond (axial ) 63(8) mol %/equatorial ) 37(8) mol %) at T ) 293 K, corresponding to an A value of –0.31(20) kcal mol. Density functional theory (DFT) calculations were employed to obtain the minimal energy path of the conformational inversion. The MP2, G3B3, and CBS-QB3 methods were also employed to calculate the equilibrium geometries and energies of the local minima in the gas phase and in solution. The gas-phase results are in good agreement with the experiment, whereas a combined PCM/IPCM(B3LYP/6-311G(d)) approach overestimates the stabilization of the axial conformer by 0.3-0.4 kcal mol in solution at 112 K. Temperature-dependent Raman spectroscopy in the temperature ranges of 210–300 K (neat liquid), 120–300 K (pentane solution), and 200–293 K (dichloromethane solution) also indicates that the axial conformer is favored over the equatorial one by 0.25(5), 0.22(5), and 0.28(5) kcal mol (∆H values), respectively.

A Conformational Study on Silacyclohexane. Comparison ofab initio (HF, MP2), DFT, and Molecular Mechanics Calculations. Conformational Energy Surface of Silacyclohexane

The structures and relative energies for the basic conformations of silacyclohexane 1 have been calculated using HF, RI-MP2, RI-DFT and MM3 methods. All methods predict the chair form to be the dominant conformation and all of them predict structures which are in good agreement with experimental data. The conformational energy surface of 1 has been calculated using MM3. It is found that there are two symmetric lowest energy pathways for the chair-to-chair inversion. Each of them consists of two sofa-like transition states, two twist forms with C1 symmetry (twist-C1), two boat forms with Si in a gunnel position (C1 symmetry), and one twist form with C2 symmetry (twist-C2). All methods calculate the relative energy to increase in the order chair < twist-C2 < twist-C1 < boat. At the MP2 level of theory and using TZVP and TZVPP (Si atoms) basis sets the relative energies are calculated to be 3.76, 4.80, and 5.47 kcal mol–1 for the twist-C2, twist-C1, and boat conformations, respectively. The energy barrier from the chair to the twisted conformations of 1 is found to be 6.6 and 5.7 kcal mol–1 from MM3 and RI-DFT calculations, respectively. The boat form with Si at the prow (Cs symmetry) does not correspond to a local minimum nor a saddle point on the MM3 energy surface, whereas a RI-DFT optimization under Cs symmetry constraint resulted in a local minimum. In both cases its energy is above that of the chair-to-twist-C1 transition state, however, and it is clearly not a part of the chair-to-chair inversion. Konformationen siliciumhaltiger Ringe. II Eine Konformationsuntersuchung des Silacyclohexans durch Vergleich von ab initio (HF, MP2), DFT- und Molekulmechanik-Rechnungen. Die Konformationshyperflache von Silacyclohexan. Strukturen und relative Energien der Grundkonformationen des Silacyclohexans 1 wurden nach HF-, RI-MP2-, RI-DFT- und MM3-Verfahren berechnet. Alle Verfahren fuhren zur Sesselform als vorherrschender Konformation, und alle berechneten Strukturen stimmen gut mit der experimentell gefundenen uberein. Die Konformationshyperflache von 1 wurde mit MM3 berechnet. Dabei wurden zwei symmetrische Wege niedrigster Energie fur die Inversion Sessel-Sessel gefunden. Beide enthalten zwei Sofa-artige Ubergangszustande, zwei Twistformen mit C1-Symmetrie (Twist-C1), zwei Bootformen mit Si an der Seite (C1-Symmetrie) und eine Twistform mit C2-Symmetrie (Twist-C2). Alle Methoden berechnen die relativen Energien der Konformationen in der Reihenfolge: Sessel < Twist-C2 < Twist-C1 < Boot. Berechnungen auf dem RI-MP2 Level der Theorie unter Verwendung der Basissatze TZVP und TZVPP (Si Atome) fuhren zu relativen Energiewerten von 3.76, 4.80 bzw. 5.47 kcal mol–1 fur die Twist-C2, Twist-C bzw. Boot-Konformationen. Die Energiebarriere vom Sessel zu den Twist-Konformationen von 1 betragt 6.6 bzw. 5.7 kcal mol–1 nach MM3- bzw. RI-DFT-Rechnungen. Auf der MM3-Energie-Hyperflache entspricht das Boot mit Si am Bug (Cs-Symmetrie) weder einem lokalen Minimum noch einem Sattelpunkt; jedoch fuhrt eine RI-DFT-Optimierung mit auf Cs festgelegter Symmetrie zu einem lokalen Minimum. Allerdings liegt die Energie in beiden Fallen uber der fur den Ubergangszustand der Umwandlung des Sessels zu Twist-C1. Deshalb ist das Cs-Boot offensichtlich auch nicht an der Inversion Sessel-Sessel beteiligt.

Preparation and characterization of (C5Me4Ph) TiCl3, the oxochloride complexes [(C5Me4Ph) TiCl2]2(μ-O) and [(C5Me4Ph)TiCl(μ-O)]3 and the oxo-complex[(C5Me4Ph)Ti]4(μ-O)6. The X-ray crystal structures of [(C5Me4Ph)TiCl2]2(μ-O) and [(C5Me4Ph)Ti]4(μ-O)6

Abstract The titanium compound (C5Me4Ph)TiCl3 (1) can be prepared by the reaction of (C5Me4Ph)SiMe3 with TiCl4. Hydrolysis of 1 in the presence of HNEt2 or Ag2O has allowed for the isolation of the oxochloride complexes [(C5Me4Ph)TiCl2]2(μ-O) (2) and [(C5Me4Ph)TiCl(μ-O)]3 (3), respectively. The NMR spectra of 3 are consistent with a nearly flat Ti3O3 ring having down—up—up or E-Z-Z arrangement of (C5Me4Ph)-ligands around the ring. The X-ray crystal structure of 2 (monoclinic, space group C2/c, a = 19.847(12), b = 9.089(5), c = 20.405(13)A, β = 124.06(5)°) consists of trans[(C5Me4Ph)TiCl2]2(μ-O) molecules of C2 symmetry with nearly linear Ti-O-Ti units (169.3(2)°). Reaction of 1 with an excess of H2O/HNEt2 results in the formation of the cxo-complex [(C5Me4Ph)Ti]4(μ-O)6 (4). The X-ray crystal structure of 4 (triclinic, space group P1¯, a = 11.861(5), b = 12.706(5), c = 18.218(8)A, α = 87.58(3)°, β = 89.00(3)°, γ = 86.60(3)°) reveals that the molecules of 4 have an adamantane-like Ti4O6 cage of nearly Td symmetry. The NMR spectra and molecular structures of 1, 2, 3 and 4 are discussed.

High‐Affinity, Selective σ Ligands of the 1,2,3,4‐Tetrahydro‐1,4′‐silaspiro[naphthalene‐1,4′‐piperidine] Type: Syntheses, Structures, and Pharmacological Properties

The 1′‐organyl‐1,2,3,4‐tetrahydrospiro[naphthalene‐1,4′‐piperidine] derivatives 1 a–4 a [for which organyl=benzyl (1 a), 4‐methoxybenzyl (2 a), 2‐phenylethyl (3 a), or 3‐methylbut‐2‐enyl (4 a)] are high‐affinity, selective σ1 ligands. The corresponding sila‐analogues 1 b–4 b (replacement of the carbon spirocenter with a silicon atom) were synthesized in multistep syntheses, starting from dichlorodivinylsilane, and were isolated as the hydrochlorides 1 b⋅HCl–4 b⋅HCl. Compounds 1 a⋅HCl–4 a⋅HCl and 1 b⋅HCl–4 b⋅HCl were structurally characterized by NMR spectroscopy (1H, 13C, 29Si) in solution, and the C/Si analogues 3 a⋅HCl and 3 b⋅HCl were studied by single‐crystal X‐ray diffraction. These structural investigations were complemented by computational studies. The σ1 and σ2 receptor affinities of the C/Si pairs 1 a/1 b–4 a/4 b were studied with radioligand binding assays. The σ1 receptor affinity of the silicon compounds 1 b–4 b is slightly higher than that of the corresponding carbon analogues 1 a–4 a. Because affinity for the σ2 receptor is decreased by the C/Si exchange, the σ1/σ2 selectivity of the silicon compounds is considerably improved, indicating that the C→Si switch strategy is a powerful tool for modulating both pharmacological potency and selectivity.

Molecular structure and conformational preferences of 1-chloro-1-silacyclohexane, CH2(CH2CH2)2SiH-Cl, as studies by gas-phase electron diffraction and quantum chemistry

The molecular structure of axial and equatorial conformer of the 1-chloro-1-silacyclohexane molecule, CH2(CH2CH2)2SiH-Cl, as well as thermodynamic equilibrium between these species were investigated by means of gas-phase electron diffraction and quantum chemistry on the MP2(full)/AUG-cc-PVTZ level of theory. According to electron diffraction data, the compound exists in the gas-phase as a mixture of conformers possessing the chair conformation of the six-membered ring and Cs symmetry and differing in the axial and equatorial position of the Si-Cl bond at 352 K. NBO analysis revealed that axial conformer of 1-chloro-1-silacyclohexane molecule is an example of the stabilization of the form that is unfavorable from the point of view steric effects and effects of conjugations and that stabilization is achieved due to electrostatic interactions.

Molecular structure and conformational preferences of 1-bromo-1-silacyclohexane, CH2(CH2CH2)2SiH-Br, as studies by gas-phase electron diffraction and quantum chemistry

The molecular structure of axial and equatorial conformer of the 1-bromo-1-silacyclohexane molecule, CH2(CH2CH2)2SiH-Br, as well as thermodynamic equilibrium between these species are investigated by means of gas-phase electron diffraction and quantum chemistry on the MP2(full)/SDB-AUG-cc-PVTZ level of theory. It is revealed that according to electron diffraction data, the compound exists in the gasphase as a mixture of conformers possessing the chair conformation of the six-membered ring and Cs symmetry and differing in the axial and equatorial position of the Si-Br bond (ax. = 80(5) mol %, eq. = 20(7) mol %) at 352 K, that corresponds to the value of A = (Gax○ − Geq○) = −0.82(32) kcal/mol. It is found that observed data agree well with theoretical ones. Using Natural Bond Orbital (NBO) analysis it is revealed that axial conformer of 1-bromo-1-silacyclohexane molecule is an example of the stabilization of the form that is unfavorable from the point of view of steric effects and effects of conjugations. It is concluded that stabilization is achieved due to electrostatic interactions.

Gas phase structures, energetics, and potential energy surfaces of disilacyclohexanes.

The molecular structures of 1,4-, 1,3-, and 1,2-disilacyclohexanes (denoted as 14, 13, and 12, respectively) were investigated by means of gas electron diffraction (GED). Each molecule was found to possess a chair as the most stable conformation in the gas phase, the point group being C(2h), C(s), and C(2), respectively. Experimental GED structures are in good agreement with theoretical calculations (MP2/cc-pVTZ and B3LYP/cc-pVTZ). A qualitative ring strain analysis suggests 14 to be the most stable and 12 the least stable of the parent disilacyclohexanes. Relative energy calculations with the G4 model chemistry protocol, on the other hand, predict 13 to be the most stable isomer, 5.9 and 14.2 kcal/mol more stable than 14 and 12, respectively. The enhanced stability of 13 compared to 14 is in agreement with an analysis on endocyclic bond lengths and bond polarities. The heats of formation (G4 calculations) are predicted to be -12.3, -18.1, and -3.9 kcal/mol for 14, 13, and 12, respectively. The potential energy surface (PES) and the lowest energy path for the chair-to-chair inversion have been calculated for each isomer. In addition to the two chair forms in each case and some half-chair or sofa-like transition states (four in the case of 14, and two in the case of 13), there are two twist forms found as stationary points on the PES of 14, six twist and six boat forms on the PES of 13, and four twist and six boat forms on the PES of 12.

First synthesis of the three isomeric parent disilacyclohexanes. An improved preparation of methylene di-Grignard.

Ring closure of 1,5-dibromo-1,5-disilapentane with methylene di-Grignard was the key step in the preparation of the parent 1,3-disilacyclohexane. For that purpose, the preparation of methylene di-Grignard has been improved and simplified. The successful synthesis of the isomeric 1,2- and 1,4-disilacyclohexanes is also reported.

Conformations of silicon-containing rings. Part 3. Relative conformational energies of alkylated 1,3,5-trisilacyclohexanes as calculated from quantum chemical and molecular mechanics methods☆

Abstract The relative energies of the basic conformations for five series of Si-alkylated derivatives of 1,3,5-trisilacyclohexane 1 with methyl, ethyl, i -propyl and t -butyl alkyl groups have been calculated using quantum chemical (QC) methods (HF and RI-DFT) and the MM3 force field. The results were compared to those from previous NMR investigations. It was found that the QC methods predict for all 1-mono-, cis -1,3-di and cis–cis -1,3,5-tri-alkylated 1 the chair(eq) to be the lowest energy conformation. The QC methods also predict a preference for twisted conformation in the cases of trans -1,3-di- and cis–trans -1,3,5-tri- t -butylated 1 . The QC results confirm earlier predictions from the NMR data. In contrast, MM3 fails to calculate relative conformational energies properly. The reason for its failure is discussed.

Molecular Structure of 1,2-Bis(trifluoromethyl)-1,1,2,2-tetramethyldisilane in the Gas, Liquid, and Solid Phases: Unusual Conformational Changes between Phases

The molecular structure of 1,2-bis(trifluoromethyl)-1,1,2,2-tetramethyldisilane has been determined in three different phases (solid, liquid, and gas) using various spectroscopic and diffraction techniques. Both the solid-state and gas-phase investigations revealed only one conformer to be present in the sample analyzed, whereas the liquid phase revealed the presence of three conformers. The data have been reproduced using computational methods and a rationale is presented for the observation of three conformers in the liquid state.