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 equival ent to CCH3) shows a spectrum in which the contributions from the three che mically inequivalent carbons are clearly resolved and marked by distinct vi brational structure. This structure is well accounted for by ab initio theo ry. For 3,3,3-trifluoropropyne (HC equivalent to CCF3) and ethynylsulfur pe ntafluoride (HC equivalent to 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 conve ntional resonance structures; the vibrational broadening for the fluorinate d compounds can be understood in terms of the effects of the electronegativ e 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 charg e redistribution on ionization energies and acidities. In particular, these considerations make it possible to understand the apparent paradox that SF 5 and CF3 have much larder electronegativity effects on acidity than they h ave on carbon 1s ionization energies.