Electronic structure of some pyridylacetylenes studied by He I photoelectron spectroscopy: A weak orbital interaction between the nonbonding electron pair on the nitrogen atom and the in-plane π orbital in the ethynyl group

Photoelectron Spectroscopy Study of Orbital Interactions. Ethynylfurans

“…S3p orbitals have a He II/He I cross-sectional ratio of 0.139 compared to 0.639 for O2p. π CC ‘ and π CC ‘‘ can both gain heteroatom character, but the σ X −π CC ‘ interactions are weak, as had been demonstrated recently for ethynylpyridines ethynylbenzene, ethynylthiophenes, and ethynylfurans (this work) supports this conclusion.…”

“…Studies of interactions between the ethyne moiety and aromatic systems have been reported, 8,9 but few investigations of ethynyl-heteroaromatic interactions exist. The electronic structures of ethynylthiophenes 4 and some isomeric ethynylpyridines 10 have been reported. We wish to extend the study to ethynylfurans which are π isoelectronic with ethynylthiophenes and ethynylpyridines.…”

Orbital Interactions in Ethynylpyridines

“…Okubo et al have suggested, on the basis of perturbation MO theory applied to 2- and 4-ethynylpyridine, that a weak intramolecular n−π‘ cc interaction takes place in the 2- but not in 4-ethynylpyridine. In 2-ethynylpyridine the n−π 2 energy difference is 0.95 eV, while in 4-ethynylpyridine it is 0.85 eV. This small (0.1 eV) difference was considered by Okubo et al as an indication of such interactions.…”

“…Okubo et al have suggested, on the basis of perturbation MO theory applied to 2- and 4-ethynylpyridine, that a weak intramolecular n−π‘ cc interaction takes place in the 2- but not in 4-ethynylpyridine. In 2-ethynylpyridine the n−π 2 energy difference is 0.95 eV, while in 4-ethynylpyridine it is 0.85 eV. This small (0.1 eV) difference was considered by Okubo et al as an indication of such interactions. We wish to present additional experimental evidence which will clarify not only the presence of n−π‘ cc interactions but also the influence of the nitrogen atom on the substituted aromatic ring as a whole.…”

“…The problem was finally settled by the work of Piancastelli et al, who demonstrated in their synchrotron radiation study that the order of ionic states in terms of increasing ionization energies is n < π 3 < π 2 . The influence of substituents on the electronic structure is still being studied. , The results from previous studies can be summarized in two main conclusions: (i) The presence of substituents alters relative ordering of n and π ionic states and can be described in the language of orbital interactions and the composite-molecule model; (ii) The MO calculations are not a reliable guide to the assignment of ionic states.…”