Gas-phase structures of ethynyl- and vinyl pentafluorosulfur derivatives: an electron diffraction and microwave spectroscopy study

Torsional splitting and the four-fold barrier to internal rotation: The rotational spectra of vinylsulfur pentafluoride

Vinylsulfur pentafluoride (VSPF), a molecule with a four-fold internal rotor, -SF₄, has been studied with high resolution Fourier transform microwave spectroscopy. We believe that this is the first report of resolved four-fold internal rotation. As such, we have presented the tools needed to understand and analyze such a problem. These include debugging the ERHAM computer program necessary to fit the spectra and the free rotor to high barrier correlation diagram necessary to understand the torsional states of the four-fold rotor. The A, E, and B torsional state rotational transitions are well resolved and assigned. Spectroscopic transitions of four isotopologues of VSPF, H₂C=CH-SF₅, the normal isotopologue, and the singly substituted ³⁴S and ¹³C isotopologues were measured and assigned. Contrary to expectation, the A torsional state could not be fit with only a semi-rigid Hamiltonian. The barrier to internal rotation, V ₄, is found to be 227 cm^-1. Ab initio calculations at the MP2 aug-cc-pVQZ level of theory and basis set were performed and the results of this calculation are compared to our experimental results.

Chemical insights from high-resolution X-ray photoelectron spectroscopy and ab initio theory: propyne, trifluoropropyne, and ethynylsulfur pentafluoride

High-resolution carbon 1s photoelectron spectroscopy of propyne (HC triple bond CCH3) shows a spectrum in which the contributions from the three chemically inequivalent carbons are clearly resolved and marked by distinct vibrational structure. This structure is well accounted for by ab initio theory. For 3,3,3-trifluoropropyne (HC triple bond CCF3) and ethynylsulfur pentafluoride (HC triple bond CSF5), the ethynyl carbons show only a broad structure and have energies that differ only slightly from one another. The core-ionization energies can be qualitatively understood in terms of conventional resonance structures; the vibrational broadening for the fluorinated compounds can be understood in terms of the effects of the electronegative fluorines on the charge distribution. Combining the experimental results with gas-phase acidities and with ab initio calculations provides insights into the effects of initial-state charge distribution and final-state charge redistribution on ionization energies and acidities. In particular, these considerations make it possible to understand the apparent paradox that SF5 and CF3 have much larger electronegativity effects on acidity than they have on carbon 1s ionization energies.