posted on 2023-11-28, 13:35authored byEdgar White Buenger, Paul M. Mayer
The atmospheric chemistries
of isoprene and prenol have been studied
extensively; however, much of that research has focused on neutral
or radical chemistry. Recent studies have demonstrated that under
acidic conditions, isoprene and prenol can become protonated in the
atmosphere, and we have explored the unimolecular chemistry of protonated
isoprene and prenol with tandem mass spectrometry (using a triple-quadrupole
mass spectrometer) and density functional theory. The collision-induced
dissociation of protonated isoprene revealed two product ion channels:
the neutral losses of C2H4 and H2, the former dominating over the latter. Protonated prenol dissociates
by four product ion channels: the neutral losses of water, formaldehyde,
methanol, and propene, with the former two being minor channels and
the latter two being major channels. Density functional theory supplemented
with CBS-QB3 single-point calculations revealed the underlying mechanisms
to explain the breakdown behavior. The two competing channels from
protonated isoprene could easily be rationalized due to the relative
energy difference between key transition states along the reaction
coordinates. However, in the case of protonated prenol, it was revealed
that the minor products observed in the breakdown of protonated prenol
had significantly lower reaction barriers when compared to the major
products, an apparent contradiction. This could be rationalized if
the initial ion population entering the collision cell is comproed
of several isomeric species on the minimum energy reaction pathway,
species populated by collisional excitation in the ion source region.