Conformations and CH/π Interactions in Aliphatic Alkynes and Alkenes

The carbon 1s photoelectron
spectra of a series of aliphatic alkynes
and alkenes that have the possibility of possessing two or more conformers
have been recorded with high resolution. The two conformers of 2-hexyne
and 4-methyl-1-pentyne, anti and gauche, have been identified and
quantified from an analysis of their carbon 1s photoelectron spectra,
yielding 30 ± 5% and 70 ± 6% anti conformers, respectively.
In the case of 1-hexyne, the photoelectron spectrum is shown to provide
partial information on the distribution of conformers. Central to
these analyses is a pronounced ability of the C1s photoemission process
to distinguish between conformers that display weak γ-CH/π
hydrogen bonding and those that do not. For the corresponding alkene
analogs, similar analyses of their C1s photoelectron spectra do not
lead to conclusive information on the conformational equilibria, mainly
because of significantly smaller chemical shifts and higher number
of conformers compared with the alkynes.