3 research outputs found
Reactivity of a σ,σ,σ,σ-Tetraradical: The 2,4,6-Tridehydropyridine Radical Cation
The 2,4,6-tridehydropyridine radical cation, an analogue
of the
elusive 1,2,3,5-tetradehydrobenzene, was generated in the gas phase
and its reactivity examined. Surprisingly, the tetraradical was found
not to undergo radical reactions. This behavior is rationalized by
resonance structures hindering fast radical reactions. This makes
the cation highly electrophilic, and it rapidly reacts with many nucleophiles
by quenching the N–C <i>ortho</i>-benzyne moiety,
thereby generating a relatively unreactive <i>meta</i>-benzyne
analogue
Substituent Effects on the Nonradical Reactivity of 4-Dehydropyridinium Cation
Recent studies have shown that the reactivity of the
4-dehydropyridinium
cation significantly differs from the reactivities of its isomers
toward tetrahydrofuran. While only hydrogen atom abstraction was observed
for the 2- and 3-dehydropyridinium cations, nonradical reactions were
observed for the 4-isomer. In order to learn more about these reactions,
the gas-phase reactivities of the 4-dehydropyridinium cation and several
of its derivatives toward tetrahydrofuran were investigated in a Fourier
transform ion electron resonance mass spectrometer. Both radical and nonradical
reactions were observed for most of these positively charged radicals.
The major parameter determining whether nonradical reactions occur
was found to be the electron affinity of the radicalsî—¸only
those with relatively high electron affinities underwent nonradical
reactions. The reactivities of the monoradicals are also affected
by hydrogen bonding and steric effects
On-Line Mass Spectrometric Methods for the Determination of the Primary Products of Fast Pyrolysis of Carbohydrates and for Their Gas-Phase Manipulation
Mass
spectrometric methodology was developed for the determination
and manipulation of the primary products of fast pyrolysis of carbohydrates.
To determine the true primary pyrolysis products, a very fast heating
pyroprobe was coupled to a linear quadrupole ion trap mass spectrometer
through a custom-built adaptor. A home-built flow tube that simulates
pyrolysis reactor conditions was used to examine the secondary reactions
of the primary products. Depending on the experiment, the pyrolysis
products were either evaporated and quenched or allowed to react for
a period of time. The quenched products were ionized in an atmospheric
pressure chemical ionization (APCI) source infused with one of two
ionization reagents, chloroform or ammonium hydroxide, to aid in ionization.
During APCI in negative ion mode, chloroform produces chloride anions
that are known to readily add to carbohydrates with little bias and
little to no fragmentation. On the other hand, in positive ion mode
APCI, ammonium hydroxide forms ammonium adducts with carbohydrates
with little to no fragmentation. The latter method ionizes compounds
that are not readily ionized upon negative ion mode APCI, such as
furan derivatives. Six model compounds were studied to verify the
ability of the ionization methods to ionize known pyrolysis products:
glycolaldehyde, hydroxyacetone, furfural, 5-hydroxymethylfurfural,
levoglucosan, and cellobiosan. The method was then used to examine
fast pyrolysis of cellobiose. The primary fast pyrolysis products
were determined to consist of only a handful of compounds that quickly
polymerize to form anhydro-oligosaccharides when allowed to react
at high temperatures for an extended period of time