Conformational Properties of 1-Silyl-1-Silacyclohexane, C₅H₁₀SiHSiH₃: Gas Electron Diffraction, Low-Temperature NMR, Temperature-Dependent Raman Spectroscopy, and Quantum Chemical Calculations

Abstract

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.

Applications and validations of the Minnesota density functionals

“…Wallevik et al [88] studied the conformational properties of 1-silyl-1-silacyclohexane and found that M06-2X performs satisfactorily for the free energy difference of the axial and equatorial conformers, whereas B3LYP greatly overestimates the stability of the equatorial isomer. It is interesting that M06-2X agrees with experiment better than CCSD(T) or a doubly hybrid functional.…”

Molecular structure and conformational analysis of 3-methyl-3-silathiane by gas phase electron diffraction, FTIR spectroscopy and quantum chemical calculations

“…The conformational equilibrium between the chair-axial and chair-equatorial conformers of 1-X-1-silacyclohexanes C 5 H 10 SiHX is shifted toward the axial conformer for electronegative substituents (X ¼ Hal [1,2], CF 3 [3]), the axial predominance being increased by lowering the temperature. An exception is 1-silyl-1-silacyclohexane [4] for which the SiH 3 -axial conformer predominates in gas phase (GED), whereas in solution at low temperature (NMR) the ratio of the conformers is reversed. For X ¼ Me [5] or Ph [6,7], the equatorial conformers predominate with the ax/eq ratio of ca.…”

Conformational properties of 1-tert-butyl-1-silacyclohexane, C5H10SiH(t-Bu): gas-phase electron diffraction, temperature-dependent Raman spectroscopy, and quantum chemical calculations

“…Methyl [13][14][15] and phenyl [16] substituents were found to have positive A-values, albeit much lower in magnitude than the corresponding cyclohexane analogs [17][18][19]. Other substituents like CF 3 [20,21] and SiH 3 [22] were found to prefer the axial position contrary to their cyclohexane analogs. In a recent report, we have shown that all the halogenated 1-silacyclohexanes have a distinct preference for the axial position of the halogen atom contrary to their cyclohexane analogs [23].…”

“…The application of the method to this problem has been described in detail in one of the publications [22], therefore, only a brief description will be given here. Temperature-dependent Raman spectra of compounds are typically analyzed using the van't Hoff relation ln(A 1 / A 2 ) = -DH/RT ?…”

“…Auxiliary basis sets (CABS) in the form of cc-pVnZ-F12/OptRI were used in all calculations [51]. Similar CCSD(T)/CBS protocols have been used in studies on silacyclohexanes [15,22,23].…”