Morpholine sulfur trifluoride: Vibrational spectra, conformational properties and crystal structure

Analysis of the vibrational spectra and quantum chemical calculations for morpholine sulfur trifluoride, SF3N(CH2CH2)2O, demonstrate that the morpholine ring possesses chair conformation and occupies an equatorial position of the trigonal bipyramidal SF3 group. The compound exists in the gas phase as a mixture of two conformers. The conformer with the SF3 group in equatorial orientation relative to the six-membered morpholine ring is slightly more stable than the axial conformer and the equilibrium is surprisingly temperature independent. MP2 and B3LYP calculations with cc-pVTZ basis sets predict free energy differences ΔG° = G°ax − G°eq of +0.18 and +0.40 kcal/mol, respectively. At temperatures below 12 °C the title compound crystallizes and the structure of the equatorial conformer, which is the only form present in the crystal, was determined by X-ray diffraction. At 131(1) K a phase transition occurs and the low and high temperature forms were studied at 100 and 150 K, respectively.

The Ar-matrix IR spectrum of morph-SF3 confirms the equilibrium of the equatorial and axial conformers.Figure optionsDownload as PowerPoint slideHighlights
► Morph-SF3 exists in the gas phase as a mixture of two conformers: equatorial and axial.
► Ar matrix IR spectra show an unexpected temperature independent conformational equilibrium.
► The equatorial conformer is the only form present in the crystal (X-ray crystallography).