Dissociative Photoionization of 1-Halogenated Silacyclohexanes: Silicon Traps the Halogen.

The threshold photoelectron spectra and threshold photoionization mass spectra of 1-halogenated-1-silacyclohexanes, for the halogens X = F, Cl, Br, and I, have been obtained using synchrotron vacuum ultraviolet radiation and photoelectron photoion coincidence spectroscopy. As confirmed by a similar ionization onset and density functional theory molecular orbitals, the ionization to the ground state is dominated by electron removal from the silacyclohexane ring for X = F, Cl, and Br, and from the halogen lone pair for X = I. The breakdown diagrams show that the dissociative photoionization mechanism is also different for X = I. Whereas the parent ions decay by ethylene loss for X = F to Br in the low-energy regime, the iodine atom is lost for X = I. The first step is followed by a sequential ethylene loss at higher internal energies in each of the compounds. It is argued that the tendency of silicon to lower bond angles stabilizes the complex cation in which C2H4 is η2-coordinated to it, and which precedes ethylene loss. Together with the relatively strong silicon-halogen bonds and the increased inductive effect of the silacyclohexane ring in stabilizing the cation, this explains the main differences observed in the fragmentation of the halogenated silacyclohexane and halogenated cyclohexane ions. The breakdown diagrams have been modeled taking into account slow dissociations at threshold and the resulting kinetic shift. The 0 K appearance energies have been obtained to within 0.08 eV for the ethylene loss for X = F to Br (10.56, 10.51, and 10.51 eV, respectively), the iodine atom loss for X = I (10.11 eV), the sequential ethylene loss for X = F to I (12.29, 12.01, 11.94, and 11.86 eV, respectively), and the minor channels of H loss for X = F (10.56 eV) and propylene loss in X = Cl (also at 10.56 eV). The appearance energies for the major channels likely correspond to the dissociative photoionization reaction energy.

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.

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.

Conformational properties of 1-silyl-1-silacyclohexane, C(5)H(10)SiHSiH(3): gas electron diffraction, low-temperature NMR, temperature-dependent Raman spectroscopy, and quantum chemical calculations (&).

The molecular structure of axial and equatorial conformers of 1-silyl-silacyclohexane, C(5)H(10)SiHSiH(3), and the thermodynamic equilibrium between these species were investigated by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance (DNMR), temperature-dependent Raman spectroscopy, and quantum chemical calculations (CCSD(T), MP2 and DFT methods). According to GED, the compound exists as a mixture of two conformers possessing the chair conformation of the six-membered ring and C(s) symmetry and differing in the axial or equatorial position of the SiH(3) group (axial = 57(7) mol %/equatorial = 43(7) mol %) at T = 321 K. This corresponds to an A value (free energy difference = G(axial) - G(equatorial)) of -0.17(15) kcal mol(-1). A low-temperature (13)C NMR experiment using SiD(4) as a solvent resulted in an axial/equatorial ratio of 45(3)/55(3) mol % at 110 K corresponding to an A value of 0.05(3) kcal mol(-1), and a DeltaG(#) value of 5.7(2) kcal mol(-1) was found at 124 K. Temperature-dependent Raman spectroscopy in the temperature range of 210-300 K of the neat liquid, a THF solution, and a heptane solution indicates that the axial conformer is favored over the equatorial one by 0.26(10), 0.23(10), and 0.22(10) kcal mol(-1) (DeltaH values), respectively. CCSD(T)/CBS and MP2/CBS calculations in general predict both conformations to have very similar stability and are, thus, in excellent agreement with the DNMR result but in a slight disagreement with the GED and Raman results. Two DFT functionals, that account for dispersion interactions, M06-2X/pc-3 and B2PLYP-D/QZVPP, deviate from the high-level coupled cluster and MP2 calculations by only 0.1 kcal mol(-1) on average, whereas B3LYP/pc-3 calculations greatly overestimate the stability of the equatorial conformer.

Conformational properties of six-membered heterocycles: accurate relative energy differences with DFT, the importance of dispersion interactions and silicon substitution effects.

Density functional methods were evaluated in their ability to predict relative conformational energies of a test set of monosubstituted cyclohexanes and six-membered heterocycles. It is shown that while popular density functionals like B3LYP are unreliable for predicting accurate conformational energies for the axial/equatorial equilibrium of monosubstituted cyclohexanes, 1-silacyclohexanes and tetrahydropyrans, density functionals that take into account dispersion interactions like M06-2X and B2PLYP-D result in energy differences close to CCSD(T)/CBS results. Using the M06-2X density functional, we have then investigated the conformational properties of a large number of monosubstituted silacyclohexanes, with the number of silicon atoms ranging from 1 to 6. Our calculations suggest remarkably different conformational properties when compared to cyclohexane. The carbon/silicon exchange in a cyclohexane ring often has systematic, yet counterintuitive effects on the conformational properties. Dispersion interactions are shown to be especially important for accurate relative energy calculations of polysilacyclohexanes.

Conformational Properties of 1-Halogenated-1-Silacyclohexanes, C5H10SiHX (X = Cl, Br, I): Gas Electron Diffraction, Low-Temperature NMR, Temperature-Dependent Raman Spectroscopy, and Quantum-Chemical Calculations.

The molecular structures of axial and equatorial conformers of cyclo-C5H10SiHX (X = Cl, Br, I) as well as the thermodynamic equilibrium between these species was investigated by means of gas electron diffraction, dynamic nuclear magnetic resonance, temperature-dependent Raman spectroscopy, and quantum-chemical calculations applying CCSD(T), MP2, and DFT methods. According to the experimental and calculated results, all three compounds exist as a mixture of two chair conformers of the six-membered ring. The two chair forms of Cs symmetry differ in the axial or equatorial position of the X atom. In all cases, the axial conformer is preferred over the equatorial one. When the experimental uncertainties are taken into account, all of the experimental and theoretical results for the conformational energy (Eaxial - Eequatorial) fit into a remarkably narrow range of -0.50 ± 0.15 kcal mol-1. It was found by NBO analysis that the axial conformers are unfavorable in terms of steric energy and conjugation effects and that they are stabilized mainly by electrostatic interactions. The conformational energies for C6H11X and cyclo-C5H10SiHX (X = F, Cl, Br, I, At) were compared using CCSD(T) calculations. In both series, fluorine is predicted to have a lower conformational preference (cyclohexane equatorial, silacyclohexane axial) than Cl, Br, and I. It is predicted that astatine would behave very similarly to Cl, Br, and I within each series.

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.

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.

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.

Relative energy and structural differences of axial and equatorial 1-fluoro-1-silacyclohexane.

The rotational spectra of the main isotopomer, of the (29)Si and of all (13)C isotopologues of axial and equatorial forms of 1-fluoro-silacyclohexane have been measured by conventional (only main species) and molecular beam Fourier transform microwave spectroscopy. r(0) and partial r(s) structures are given separately for the two forms. The main structural differences are discussed. From dipole moments and relative intensity measurements, a slight preference (E(Eq) - E(Ax) = 42 +/- 24 cm(-1)) for the axial conformer was found. The rotational spectra of some, the most intense, vibrational satellites have also been measured. They belong to the ring-puckering motions.

Conformations of silicon-containing rings. 5. Conformational properties of 1-methyl-1-silacyclohexane: gas electron diffraction, low-temperature NMR, and quantum chemical calculations.

The molecular structure of 1-methyl-1-silacyclohexane 3 has been determined by gas electron diffraction (GED). The conformational preference of the methyl group was studied experimentally in the gas phase (GED) and in solution (low-temperature (13)C NMR) and by quantum chemical calculations (HF, MP2, and B3LYP with 6-31G basis sets and mPW1PW91/6-311G(2df,p)). Both experimental methods result in a preference of the equatorial position of the methyl group, 68(7)% in the gas phase at 298 K and 74(1)% in solution at 110 K. The calculations predict 68-73% equatorial conformer at room temperature. From coalescence temperatures, Gibbs free energies of activation for ring inversion DeltaG++ (eq --> ax) = 5.81(18) and DeltaG++ (ax --> eq) = 5.56(18) kcal mol(-1) were derived. The calculated values for DeltaG++ (eq --> ax) are 5.92 (B3LYP) and 5.84 kcal mol(-1) (mPW1PW91).