Hydroxy-Substituted Polycyclic Aromatic Hydrocarbon Ions as Sources of CO and HCO in the Interstellar Medium

Tandem mass spectrometry was used to explore the trends
in the
unimolecular fragmentation of the ionized hydroxy-substituted polycyclic
aromatic hydrocarbons 1-naphthol, 9-hydroxyphenanthrene, and 1-hydroxypyrene.
The main dissociation reactions across all ring systems were CO- and
HCO-losses, with ionized 1-naphthol also losing H atoms. Both ionized
1-naphthol and 9-hydroxyphenanthrene displayed the sequential loss
of C2H2 and C4H2 from
the [M-HCO]+ ions, reminiscent of unsubstituted PAH ions.
CO-loss is slightly favored for 1-naphthol and 9-hydroxyphenanthrene,
at low collision energy, but less so for 1-hydroxypyrene. Reaction
mechanisms for HCO- and CO-losses from 1-hydroxypyrene were derived
from CCSD/6-31G­(d)//B3-LYP/6-31G­(d) calculations. The CO-loss mechanism
is dominated by two transition states: TS-A governing a 1,3-H shift
in the molecular ion and TS-C which governs a ring-closing step to
form a five-member ring in the product ion. HCO-loss occurs over a
much flatter potential energy surface with the intermediate being
the product ion bound to the carbon atom of HCO. Imaging photoelectron
photoion coincidence spectroscopy of 1-hydroxypyrene yielded threshold
photon-energy resolved breakdown curves and time-of-flight distributions
that were modeled with RRKM theory to give 0 K reaction energies for
HCO- and CO-losses of 3.92 ± 0.05 and 2.91 ± 0.05 eV, respectively.
The entropies of activation for the two channels were very different,
14 and 95 JK–1 mol–1, respectively,
a result consistent with the calculated mechanisms. The threshold
photoelectron spectrum yielded an IE value of 7.14 ± 0.01 eV
for 1-hydroxypyrene.