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 Å, slightly larger than earlier values reported for petroleum asphaltenes (15.2–18.8 Å). Mass spectrometric experiments of oil-deposit asphaltenes ionized via atmospheric pressure chemical ionization (APCI) using CS₂ 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>_W) 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^–1 is used to maintain lower operating pressure to minimize aggregate formation. Using a 0.02 g L^–1 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^–1 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 radicals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⁺, Na⁺, K⁺) 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⁺···OH···O interactions present in the binding pocket, leading to characteristic IR transitions appearing below 3550 cm^–1. This interaction is mitigated in the major conformer of the K⁺ 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>^‡ = −154 ± 33 J mol^–1 K^–1). The active form of the catalyst is methyldioxorhenium­(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]^1/2 dependence. The MDO-diolate complex undergoes further reduction by alcohol in the rate-determining step to give rise to a putative rhenium­(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]⁺ 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>^‡ = −154 ± 33 J mol^–1 K^–1). The active form of the catalyst is methyldioxorhenium­(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]^1/2 dependence. The MDO-diolate complex undergoes further reduction by alcohol in the rate-determining step to give rise to a putative rhenium­(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