posted on 2018-05-04, 00:00authored byBrandi West, Bethany Lowe, Paul M. Mayer
1-Methylpyrene radical
cations undergo the loss of a hydrogen atom
at internal energies above the first dissociation threshold. Imaging
photoelectron photoion coincidence spectroscopy was employed in combination
with RRKM modeling to determine a 0 K activation energy of 2.78 ±
0.25 eV and an entropy of activation of 6 ± 19 J K–1 mol–1 for this H-loss reaction. The ionization
energy of 1-methylpyrene was measured by mass-selected threshold photoelectron
spectroscopy to be 7.27 ± 0.01 eV. These values were found to
be consistent with calculations at the CCSD/6-31G(d)//B3-LYP/6-31G(d)
level of theory showing that the formation of the 1-methylenepyrene
cation (resulting from H loss from the methyl group) is kinetically
more favorable than the formation of a tropylium-containing product
ion that is structurally analogous to the formation of the tropylium
cation in H loss from ionized toluene. The shift away from a tropylium-containing
structure was found to be due to the increased ring strain imposed
on the C7 moiety when it is bound to three fused benzene rings. The
RRKM results allow for the derivation of the ΔfH0o (1-methylenepyrene cation) of
945 ± 31 kJ mol–1.