Relative Energy and Structural Differences of Axial and Equatorial 1-Fluoro-1-silacyclohexane

Abstract

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.

Intrinsic conformational preferences of and an anomeric‐like effect in 1‐substituted silacyclohexanes

“…(To our knowledge this work presents the first determination of Δ E for 4 and 5 .) According to the work of Favero et al 14, the axial preference in 1‐fluorosilacyclohexane is 0.12 ± 0.07 kcal mol −1 , which is nearly identical with our ZPVE corrected best estimate 0.13 kcal mol −1 for 1 (0.09 kcal mol −1 without the ZPVE correction). For 1‐methylsilacyclohexane and 1‐chlorosilacyclohexane, the quantitative agreement is not quite as impressive.…”

“…Through additional experiments and computations, the axial and equatorial chair conformations of 1‐methylsilacyclohexane were shown to be isoenergetic (0.0 ± 0.2 kcal mol −1 ) 13. In a separate work, the same group examined the relative energy between axial and equatorial conformations of 1‐fluorosilacyclohexane with microwave spectroscopy and computational calculations 14. They determined that, like Cl, the F substituent has a slight preference for the axial conformation by 0.12 ± 0.07 kcal mol −1 .…”

Conformational analysis of 3‐methyl‐3‐silathiane and 3‐fluoro‐3‐methyl‐3‐silathiane

“…In the last few years Arnason et al . intensively investigated the conformational equilibria, steric effects of substituents and stereoelectronic interactions in 1‐X‐1‐silacyclohexanes (X = H,3 Me,4, 5 F,6, 7 CF 3 ,8 SiH 3 9, cf. Scheme ) by various physico‐chemical methods such as gas‐phase electron diffraction, dynamic nuclear magnetic resonance, microwave spectroscopy, temperature‐dependent Raman spectroscopy and quantum chemical calculations.…”

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

“…A number of monosubstituted 1-silacyclohexanes have been studied by experimental and theoretical methods in the last few years in order to explore the effect that silicon substitution has on the axial/equatorial equilibrium of the cyclohexane ring. [8][9][10][11][12][13] Because the synthesis and accurate measurement of these air-sensitive molecules are difficult, an attractive alternative is to explore the conformational properties by theoretical calculations, DFT being a computationally feasible option. The energy difference between axial and equatorial conformers of a monosubstituted cyclohexane or heterocycle can be very small, depending on both the ring system and substituent, and thus the reliability of the QM method used is important.…”