Chemical Insights from High-Resolution X-ray Photoelectron Spectroscopy and ab Initio Theory:  Propyne, Trifluoropropyne, and Ethynylsulfur Pentafluoride

Abstract

High-resolution carbon 1s photoelectron spectroscopy of propyne (HCtCCH 3 ) 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 (HCtCCF 3 ) and ethynylsulfur pentafluoride (HCtCSF 5 ), 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 SF 5 and CF 3 have much larger electronegativity effects on acidity than they have on carbon 1s ionization energies.

Carbon 1s photoelectron spectroscopy of 1-pentyne conformers

“…Three peaks are well resolved and, according to the resonance model shown in Scheme 1, one would expect C1 to have the lowest ionization energy, C3 the highest and the other three C atoms to constitute the central main peak of the spectrum. This is similar to the situation in propyne [44], where C1 is found to have the lowest ionization energy and C3 the highest.…”

Core‐ionized and core‐excited states of macromolecules

“…The first set contains only hydrocarbon while the second contains molecules involving heteroatoms (O or N). The experimental data are taken form references [47] and [48]. When several experimental data are available, the two extreme values are given.…”

“…The ionization energy is computed with various levels of theory (B3LYP/6-311ϩϩG** and PBE0/6-311ϩϩG**) and with various localization criteria (PM or BF at the B3LYP/6-311ϩϩG** level). The accuracy of the calculations is checked against experimental data [47,48]. Readily, one can see that the PBE0 functional performs a much better job that the B3LYP exchange-correlation functional.…”

Conformational impact of pentafluorosulfanylation on acyclic aliphatic molecules

“…Conceptually an analog of a trifluoromethyl (CF 3 ) group, the pentafluorosulfanyl (SF 5 ) group, with five fluorine atoms arranged in a square pyramid about sulfur, may impart unique properties to molecules into which it has been substituted. The electronegativity of the SF 5 group has been proposed to be slightly higher than that of the CF 3 group, 3.62 in comparison to a value of 3.36 [9]. However the Hammett s p value for SF 5 was determined to be 0.68 in contrast to s p value for CF 3 of 0.54 [10].…”