19 research outputs found
Direct Comparison of Solution and Gas-Phase Reactions of the Three Distonic Isomers of the Pyridine Radical Cation with Methanol
To directly compare the reactivity of positively charged
carbon-centered
aromatic σ-radicals toward methanol in solution and in the gas
phase, the 2-, 3-, and 4-dehydropyridinium cations (distonic isomers
of the pyridine radical cation) were generated by ultraviolet photolysis
of the corresponding iodo precursors in a mixture of water and methanol
at varying pH. The reaction mixtures were analyzed by using liquid
chromatography/mass spectrometry. Hydrogen atom abstraction was the
only reaction observed for the 3- and 4-dehydropyridinium cations
(and pyridines) in solution. This also was the major reaction observed
earlier in the gas phase. Depending on the pH, the hydrogen atom can
be abstracted from different molecules (i.e., methanol or water) and
from different sites (in methanol) by the 3- and 4-dehydropyridinium
cations/pyridines in solution. In the pH range 1–4, the methyl
group of methanol is the main hydrogen atom donor site for both 3-
and 4-dehydropyridinium cations (just like in the gas phase). At higher
pH, the hydroxyl groups of water and methanol also act as hydrogen
atom donors. This finding is rationalized by a greater abundance of
the unprotonated radicals that preferentially abstract hydrogen atoms
from the polar hydroxyl groups. The percentage yield of hydrogen atom
abstraction by these radicals was found to increase with lowering
the pH in the pH range 1.0–3.2. This pH effect is rationalized
by polar effects: the lower the pH, the greater the fraction of protonated
(more polar) radicals in the solution. This finding is consistent
with previous results obtained in the gas phase and suggests that
gas-phase studies can be used to predict solution reactivity, but
only as long as the same reactive species is studied in both experiments.
This was found not to be the case for the 2-iodopyridinium cation.
Photolysis of this precursor in solution resulted in the formation
of two major addition products, 2-hydroxy- and 2-methoxypyridinium
cations, in addition to the hydrogen atom abstraction product. These
addition products were not observed in the earlier gas-phase studies
on 2-dehydropyridinium cation. Their observation in solution is explained
by the formation of another reactive intermediate, the 2-pyridyl cation,
upon photolysis of 2-iodopyridinium cation (and 2-iodopyridine). The
same intermediate was observed in the gas phase but it was removed
before examining the reactions of the desired radical, 2-dehydropyridinium
cation (which cannot be done in solution)
Characterization of Asphaltene Deposits by Using Mass Spectrometry and Raman Spectroscopy
Crude oil deposition in oil transfer
pipelines and bore wells afflicts
many oil reservoirs. Asphaltenes play a major role in this process
because of their tendency to precipitate in pipelines upon changes
in temperature and/or pressure. Asphaltenes are defined by their lack
of solubility in <i>n</i>-alkane solvents, which means that
they likely contain many compounds that do not actively contribute
to the deposition of crude oil in pipelines. The preponderance of
studies in the literature have focused on asphaltenes derived from
crude oil, whereas far fewer investigations have focused on asphaltenes
derived from oil deposits. In this study, structural parameters of
oil-deposit asphaltenes were examined using Raman spectroscopy and
tandem mass spectrometry and compared to results reported previously
for petroleum asphaltenes. On the basis of D1 and G band intensities
in the Raman spectrum of oil-deposit asphaltenes, the average aromatic
sheet size of these molecules was 21.0 Ã…, slightly larger than
earlier values reported for petroleum asphaltenes (15.2–18.8
Ã…). Mass spectrometric experiments of oil-deposit asphaltenes
ionized via atmospheric pressure chemical ionization (APCI) using
CS<sub>2</sub> solvent were used to measure the molecular weight distribution
(MWD), saturated carbon content, and the number of fused aromatic
rings in the cores of the asphaltene molecules. The MWD was found
to be 150–1050 Da with an average molecular weight (average <i>M</i><sub>W</sub>) of 497 Da, which are significantly lower
than those reported previously for petroleum asphaltenes (200–1500
Da and 570–700 Da, respectively). Aromatic core sizes were
estimated to contain 8 fused rings on average for the most abundant
species in oil-deposit asphaltenes, with 5–15 carbons in their
alkyl side chains, as compared to averages of 3–7 aromatic
rings and 17–41 alkyl carbons for petroleum asphaltenes
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
Separation of Asphaltenes by Reversed-Phase Liquid Chromatography with Fraction Characterization
The use of a 4.6 × 250 mm, 5 μm cyanopropyl
column is
effective for the liquid chromatography (LC) separation of asphaltenes
with sequential ultraviolet (UV) and florescence detection. The mobile-phase
composition is an optimized gradient from acetonitrile (MeCN) and
water to <i>N</i>-methyl-2-pyrrolidone (NMP) and tetrahydrofuran
(THF). A low flow rate of 0.5 mL min<sup>–1</sup> is used to
maintain lower operating pressure to minimize aggregate formation.
Using a 0.02 g L<sup>–1</sup> asphaltene sample for preliminary
optimization, three peaks, with two partially resolved, are evident
in the fluorescence chromatogram. The UV chromatogram revealed an
extra weakly retained peak, suggesting aggregates that quench fluorescence.
Aggregation of asphaltenes increases with time up to about 10 h and
is dependent upon the choice of sample solvent. On the basis of the
reversed-phase mobile-phase gradient, the relative polarity of the
peaks from least to most retained can be estimated over the polarity
index (<i>P</i>′) range from about 6.3–4.3
on a scale of 0.1 for hexane (least polar) to 10.6 for water (most
polar). The sample concentration is increased to 1 g L<sup>–1</sup> for separation and collection of 12 fractions. Selected fractions
are subjected to characterization using atmospheric pressure chemical
ionization mass spectrometry (APCI–MS) using a linear quadrupole
ion trap (LQIT). The variation of the molecular-weight distribution
of the asphaltenes for the 12 fractions is fairly constant, indicating
that the retention mechanism is not controlled by size exclusion but
likely a partitioning/adsorption mechanism
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
Development of a High-Throughput Laser-Induced Acoustic Desorption Probe and Raster Sampling For Laser-Induced Acoustic Desorption/Atmospheric Pressure Chemical Ionization
Laser-induced
acoustic desorption (LIAD) was recently coupled to
atmospheric pressure chemical ionization (APCI) and shown to be of
great utility for the analysis of a variety of thermally labile nonpolar
analytes that are not amenable to ionization via electrospray ionization,
such as nonvolatile hydrocarbons. Despite these advancements, LIAD
still suffered from several limitations, including only being able
to sample a small fraction of the analyte molecules deposited on a
Ti foil for desorption, poor reproducibility, as well as limited laser
power throughput to the backside of the foil. These limitations severely
hinder the analysis of especially challenging analytes, such as asphaltenes.
To address these issues, a novel high-throughput LIAD probe and an
assembly for raster sampling of a LIAD foil were designed, constructed,
and tested. The new probe design allows 98% of the initial laser power
to be realized at the backside of the foil over the 25% achieved previously,
thus improving reproducibility and allowing for the analysis of large
nonvolatile analytes, including asphaltenes. The raster assembly provided
a 5.7 fold increase in the surface area of a LIAD foil that could
be sampled and improved reproducibility and sensitivity for LIAD experiments.
The raster assembly can also improve throughput as foils containing
multiple analytes can be prepared and analyzed
Alkali Cation Chelation in Cold β‑O‑4 Tetralignol Complexes
We
employ cold ion spectroscopy (UV action and IR–UV double
resonance) in the gas phase to unravel the qualitative structural
elements of G-type alkali metal cationized (X = Li<sup>+</sup>, Na<sup>+</sup>, K<sup>+</sup>) tetralignol complexes connected by β-O-4
linkages. The conformation-specific spectroscopy reveals a variety
of conformers, each containing distinct infrared spectra in the OH
stretching region, building on recent studies of the neutral and alkali
metal cationized β-O-4 dimers. The alkali metal ion is discovered
to bind in penta-coordinate pockets to ether and OH groups involving
at least two of the three β-O-4 linkages. Different binding
sites are distinguished from one another by the number of M<sup>+</sup>···OH···O interactions present in the
binding pocket, leading to characteristic IR transitions appearing
below 3550 cm<sup>–1</sup>. This interaction is mitigated in
the major conformer of the K<sup>+</sup> adduct, demonstrating a clear
impact of the size of the charge center on the three-dimensional structure
of the tetramer
Mechanism of MTO-Catalyzed Deoxydehydration of Diols to Alkenes Using Sacrificial Alcohols
Catalytic deoxydehydration (DODH)
of vicinal diols is carried out
employing methyltrioxorhenium (MTO) as the catalyst and a sacrificial
alcohol as the reducing agent. The reaction kinetics feature an induction
period when MTO is added last and show zero-order in [diol] and half-order
dependence on [catalyst]. The rate-determining step involves reaction
with alcohol, as evidenced by a KIE of 1.4 and a large negative entropy
of activation (Δ<i>S</i><sup>‡</sup> = −154
± 33 J mol<sup>–1</sup> K<sup>–1</sup>). The active
form of the catalyst is methyldioxorheniumÂ(V) (MDO), which is formed
by reduction of MTO by alcohol or via a novel C–C bond cleavage
of an MTO-diolate complex. The majority of the MDO-diolate complex
is present in dinuclear form, giving rise to the [Re]<sup>1/2</sup> dependence. The MDO-diolate complex undergoes further reduction
by alcohol in the rate-determining step to give rise to a putative
rheniumÂ(III) diolate. The latter is the active species in DODH extruding
stereoselectively <i>trans</i>-stilbene from (<i>R</i>,<i>R</i>)-(+)-hydrobenzoin to regenerate MDO and complete
the catalytic cycle
Comparison of Atmospheric Pressure Chemical Ionization and Field Ionization Mass Spectrometry for the Analysis of Large Saturated Hydrocarbons
Direct
infusion atmospheric pressure chemical ionization mass spectrometry
(APCI-MS) was compared to field ionization mass spectrometry (FI-MS)
for the determination of hydrocarbon class distributions in lubricant
base oils. When positive ion mode APCI with oxygen as the ion source
gas was employed to ionize saturated hydrocarbon model compounds (M)
in hexane, only stable [M – H]<sup>+</sup> ions were produced.
Ion–molecule reaction studies performed in a linear quadrupole
ion trap suggested that fragment ions of ionized hexane can ionize
saturated hydrocarbons via hydride abstraction with minimal fragmentation.
Hence, APCI-MS shows potential as an alternative of FI-MS in lubricant
base oil analysis. Indeed, the APCI-MS method gave similar average
molecular weights and hydrocarbon class distributions as FI-MS for
three lubricant base oils. However, the reproducibility of APCI-MS
method was found to be substantially better than for FI-MS. The paraffinic
content determined using the APCI-MS and FI-MS methods for the base
oils was similar. The average number of carbons in paraffinic chains
followed the same increasing trend from low viscosity to high viscosity
base oils for the two methods
Mechanism of MTO-Catalyzed Deoxydehydration of Diols to Alkenes Using Sacrificial Alcohols
Catalytic deoxydehydration (DODH)
of vicinal diols is carried out
employing methyltrioxorhenium (MTO) as the catalyst and a sacrificial
alcohol as the reducing agent. The reaction kinetics feature an induction
period when MTO is added last and show zero-order in [diol] and half-order
dependence on [catalyst]. The rate-determining step involves reaction
with alcohol, as evidenced by a KIE of 1.4 and a large negative entropy
of activation (Δ<i>S</i><sup>‡</sup> = −154
± 33 J mol<sup>–1</sup> K<sup>–1</sup>). The active
form of the catalyst is methyldioxorheniumÂ(V) (MDO), which is formed
by reduction of MTO by alcohol or via a novel C–C bond cleavage
of an MTO-diolate complex. The majority of the MDO-diolate complex
is present in dinuclear form, giving rise to the [Re]<sup>1/2</sup> dependence. The MDO-diolate complex undergoes further reduction
by alcohol in the rate-determining step to give rise to a putative
rheniumÂ(III) diolate. The latter is the active species in DODH extruding
stereoselectively <i>trans</i>-stilbene from (<i>R</i>,<i>R</i>)-(+)-hydrobenzoin to regenerate MDO and complete
the catalytic cycle