Hydrogen Rearrangement Research Articles

Electrospray ionization tandem mass spectrometry of 4-aryl-3,4-dihydrocoumarins.

We have investigated the gas-phase fragmentation reactions of 11 synthetic 4-aryl-3,4-dihydrocoumarins by electrospray ionization tandem mass spectrometry (ESI-MS/MS) on a quadrupole-time-of flight (Q-TOF) hybrid mass spectrometer. We have also estimated thermochemical data for the protonated coumarins (precursor ion A) and product ion structures by computational chemistry at a B3LYP level of theory to establish the ion structures and to rationalize the fragmentation pathways. The most abundant ions in the product ion spectra of coumarins 1-11 resulted from C8H8O2, CO2, C4H4O3, C8H10O3, C8H8O2, and CH3OH eliminations through retro-Diels-Alder (RDA) reactions, remote hydrogen rearrangements (β-eliminations), and β-lactone ring contraction. Although the investigated coumarins shared most of the fragmentation pathways, formation of a benzylic product ion and its corresponding tropylium ion was diagnostic of the substituents at ring C. The thermochemical data revealed that the nature and position of the substituents at ring C played a key role in the formation of this product ion and determined its relative intensity in the product ion spectrum. The results of this study contribute to knowledge of the gas-phase ion chemistry of this important class of organic compounds.

Hydrogen and halogen rearrangements in imidazoles: hydrogen, fluorine, chlorine and bromine shifts

Hydrogen and halogen rearrangements in imidazoles: hydrogen, fluorine, chlorine and bromine shifts

Fragmentation Pathway of Organophosphorus Flame Retardants by Liquid Chromatography–Orbitrap-Based High-Resolution Mass Spectrometry

Organophosphorus flame retardants (OPFRs) have been widely used in polymeric materials owing to their flame retardant and plasticizing effects. Investigating the fragmentation pathway of OPFRs is of great necessity for further discovering and identifying novel pollutants using orbitrap-based high-resolution mass spectrometry (HRMS). A total of 25 OPFRs, including alkyl, halogenated, and aromatic types, were analyzed in this study. The fragmentation pathways of the OPFRs were investigated using orbitrap-based HRMS with high-energy collision dissociation (HCD) in positive mode. The major fragmentation pathways for the three types of OPFRs are greatly affected by the substituents. In detail, the alkyl and halogenated OPFRs underwent three McLafferty hydrogen rearrangements, wherein the substituents were gradually cleaved to form the structurally stable [H4PO4]+ (m/z = 98.9845) ions. In contrast, the aromatic OPFRs would cleave not only the C-O bond but also the P-O bond, depending on the substituents, to form fragment ions such as [C6H7O]+ (m/z = 95.0495) or [C7H7]+ (m/z = 91.0530), among others. Using HRMS improved the accuracy of fragment ion identification, and the pathway became more evident. These fragmentation laws can provide identification information in pollutant screening work and theoretical references for the structural characterization of compounds with diverse substituent structures.

Fragmentation dynamics of the doubly charged aniline: the source of kinetically excited CnH3+ ions.

The goals of this work are to attempt to decipher if an aniline dication can isomerize to a picoline dication in a given astrochemical environment and if the dissociation of such dications could be a source of kinetically hot fragment ions, some of which could be of significance in the interstellar medium. Toward this purpose, the VUV-induced dication dissociation was investigated experimentally using ion-ion coincidence and computationally by optimizing various pathways. Contrary to previous reports, we show here that the dication of aniline is structurally too weak to retain its ring structure while following the dissociation pathways. A fragile open ring structure could lead to all the experimentally observed pathways of noticeable intensity. The significance of this, especially in terms of molecular dynamics, can be assessed by the fact that all the transformations were facilitated by specific hydrogen migration. A clear selectivity is seen where the dication of aniline was found to prefer a rearrangement of hydrogen within the ring rather than transferring from nitrogen to the ring, which is conventionally expected and has to do with the charge state and charge localization.

Thermal Stability of Schottky Contacts and Rearrangement of Defects in β‐Ga2O3 Crystals

AbstractThe thermal stability of different Schottky contacts (Au, Pt, and Ni) on (100) β‐Ga2O3 single crystals grown by the Czochralski method is investigated. Besides the examination of the Schottky barrier parameters, contact‐dependent defect levels are investigated by deep‐level transient spectroscopy (DLTS) in a 100–650 K (ramp‐up) and 650–100 K (ramp‐down) temperature cycle. Several defect levels are detected below the conduction band minimum at 0.41, 0.60, 0.77, 0.96, and 1.17 eV. In the temperature ramp‐down DLTS, the 1.17 eV level disappears, and the 0.60 eV level appears for all Schottky contacts. DFT calculations suggest that rearrangement and dissociation of a single hydrogen from a doubly‐hydrogenated Ga─O divacancy complex occurs during the temperature sweep under bias. The trap level at 0.96 eV only appears after the thermal load for the Ni contact, in contrast to Au and Pt, where it is present without a thermal budget. Temperature‐dependent leakage current (at −4 V) measurements indicate oxidation of Ni, and further thermodynamic analysis suggests alloying of Au‐Ga atoms at the Au/β‐Ga2O3 interface. These studies provide insight into the behavior induced by these common Schottky contacts and the alteration associated with temperature cycling.

The Second Derivative of Fullerene C60 (SD-C60) and Biomolecular Machinery of Hydrogen Bonds: Water-Based Nanomedicine.

The human body contains 60-70% water, depending on age. As a body fluid, it is not only a medium in which physical and chemical processes take place, but it is also one of the active mediators. Water is the richest substance with non-covalent hydrogen bonds. Water molecules, by themselves (in vacuum), are diamagnetic but when organized into clusters, they become diamagnetic or paramagnetic. Also, biomolecules (DNA, collagen, clathrin, and other proteins) have non-covalent hydrogen bonds in their structure. The interaction, as well as signal transmission, between water and biomolecules is achieved through the vibrations of covalent and non-covalent hydrogen bonds, which determine the state and dynamics of conformational changes in biomolecules. Disruptive conformational changes in biomolecules, cells, and tissues lead to their dysfunctionality, so they are a frequent cause of many disorders and diseases. For example, the rearrangement of hydrogen bonding due to mitochondrial disease mutation in cytochrome bc1 disturbs heme bH redox potential and spin state. In order to prevent and repair the dysfunctional conformational changes, a liquid substance was developed based on the second derivative of the C60 molecule (SD-C60), which has classical and quantum properties. The characterization of SD-C60 by UV-VIS-NIR, FTIR, TEM, and AFM/MFM was performed and it is shown that SD-C60 water layers generate vibrations with near-zero phase dispersion which are transmitted through Fibonacci's water chains to biomolecules. In comparison with previously published SD-C60 derivate (3HFWC, size until 10 nm, and 1-5 water layers), the improved formulation (3HFWC-W, size 10-25 nm, and 6-9 water layers) showed multiplied cytotoxic activity against melanoma cell lines of different aggressiveness. Apart from this, the mode of action was preserved and based on an induction of senescence rather than cell death. Importantly, high selectivity towards malignant phenotypes was detected. Observed effects can be ascribed to a machinery of hydrogen bonds, which are generated in SD-C60 and transmitted through water to biomolecules. This approach may open a new field in science and healthcare-a "water-based nanomedicine".

Hydrazine Oxidation in Aqueous Solutions I: N4H6 Decomposition

A mixture of nonlabeled (14N2H4) and 15N labeled hydrazine (15N2H4) in an aqueous solution is oxidized to 15N2, 14N2, and 14N15N molecules, indicating the intermediate existence of the 14NH2-14NH-15NH-15NH2 with subsequent hydrogen transfers and splitting of side N-N bonds. The structures, thermodynamics and electron characteristics of various N4H6 molecules in aqueous solutions are investigated using theoretical treatment at the CCSD/cc-pVTZ level of theory to explain the crucial part of the hydrazine oxidation reaction. Most N4H6 structures in aqueous solutions are decomposed during geometry optimization. Splitting the bond between central nitrogen atoms is the most frequent method, but the breakaway of the side nitrogen is energetically the most preferred one. The N-N fissions are enabled by suitable hydrogen rearrangements. Gibbs free energy data indicate the dominant abundance of NH3... N2... NH3 species. The side N atoms have very high negative charges, which should support hydrogen transfers in aqueous solutions. The only stable cyclo-(NH)4…H2 structure has a Gibbs energy that is too high and breaks the H2 molecule. The remaining initial cyclic structures are split into hydrazine and HN≡NH or H2N≡N species, and their relative abundance in aqueous solutions is vanishing.

Scott Blair Fractional-Type Viscoelastic Behavior of Thermoplastic Polyurethane.

In this paper, the experimental characterization of the viscoelastic properties of thermoplastic polyurethane (TPU) samples through creep experiments is presented. Experiments were conducted at different constant temperature levels (15, 25, and 35 ∘C), for three different tensile stress levels (0.3, 0.5, and 0.7 MPa), and at different physisorbed water contents, providing access to: (i) the temperature dependency of creep parameters and (ii) the assessment, if behavior is indeed viscoelastic. The physisorbed water content was achieved by exposing virgin samples to environments with relative humidity ranging from 0 to 80 percent until mass stability was reached. Creep tests were conducted immediately afterwards with this particular humidity level. The main results of this study are as follows. The temperature dependency of the obtained creep parameters is well described in Arrhenius plots. With regard to water content, two prototype material responses were observed in the experimental program and accurately modeled using the following fractional-type models: (i) Scott Blair-type (i.e., power-law-type) only behavior, pronounced for the combination of low water content/low temperature; (ii) combined Scott Blair plus Lomnitz (i.e., log-type) behavior for high water content/high temperature. This change in behavior associated with certain thresholds for the specified environmental conditions (temperature and relative humidity) may indicate the initiation of hydrogen bond breakage and rearrangement (carbamate H-bonds and physisorbed water H-bonds). Regarding the short-term or quasi-instantaneous behavior, the Scott Blair element seems highly appropriate and may be better suited than the standard elastic model: the Hookean spring. We associated Scott Blair behavior with the load-induced, quasi-instantaneous re-arrangement of polymer network chains. The secondary viscoelastic mechanism associated with the Lomnitz element, hydrogen bond breakage and rearrangement, comes into play for higher temperatures and/or higher physisorbed water contents. In this case, the contribution of the two constitutive elements is well separated due to the large number of the characteristic time of the Lomnitz element, much larger than the respective value for the Scott Blair element.

Gold-Catalyzed Regioselective Synthesis of Crowded Cyclopentadienes by Migratory Cycloisomerization of Vinylallenes.

We report the regioselective synthesis of silyl-substituted cyclopentadienyl esters through gold-catalyzed migratory cycloisomerization of silyl-substituted vinylallenes. This transformation is proposed to proceed through a perfectly orchestrated sequence of events including Nazarov-like cyclization and several silyl and hydrogen rearrangements. Furthermore, exploiting the multifaceted nature of the gold catalyst, we have also identified suitable conditions for the synthesis of these cyclopentadienes in a more straightforward manner through gold-catalyzed reaction of propargyl esters and alkynylsilanes.

A 300-fold conductivity increase in microbial cytochrome nanowires due to temperature-induced restructuring of hydrogen bonding networks.

Although proteins are considered as nonconductors that transfer electrons only up to 1 to 2 nanometers via tunneling, Geobacter sulfurreducens transports respiratory electrons over micrometers, to insoluble acceptors or syntrophic partner cells, via nanowires composed of polymerized cytochrome OmcS. However, the mechanism enabling this long-range conduction is unclear. Here, we demonstrate that individual nanowires exhibit theoretically predicted hopping conductance, at rate (>1010 s−1) comparable to synthetic molecular wires, with negligible carrier loss over micrometers. Unexpectedly, nanowires show a 300-fold increase in their intrinsic conductance upon cooling, which vanishes upon deuteration. Computations show that cooling causes a massive rearrangement of hydrogen bonding networks in nanowires. Cooling makes hemes more planar, as revealed by Raman spectroscopy and simulations, and lowers their reduction potential. We find that the protein surrounding the hemes acts as a temperature-sensitive switch that controls charge transport by sensing environmental perturbations. Rational engineering of heme environments could enable systematic tuning of extracellular respiration.

Practical Effects of Intramolecular Hydrogen Rearrangement in Electron Transfer Dissociation-Based Proteomics.

Ion-ion reactions are valuable tools in mass-spectrometry-based peptide and protein sequencing. To boost the generation of sequence-informative fragment ions from low charge-density precursors, supplemental activation methods, via vibrational and photoactivation, have become widely adopted. However, long-lived radical peptide cations undergo intramolecular hydrogen atom transfer from c-type ions to z•-type ions. Here we investigate the degree of hydrogen transfer for thousands of unique peptide cations where electron transfer dissociation (ETD) was performed and was followed by beam-type collisional activation (EThcD), resonant collisional activation (ETcaD), or concurrent infrared photoirradiation (AI-ETD). We report on the precursor charge density and the local amino acid environment surrounding bond cleavage to illustrate the effects of intramolecular hydrogen atom transfer for various precursor ions. Over 30% of fragments from EThcD spectra comprise distorted isotopic distributions, whereas over 20% of fragments from ETcaD have distorted distributions and less than 15% of fragments derived from ETD and AI-ETD reveal distorted isotopic distributions. Both ETcaD and EThcD give a relatively high degree of hydrogen migration, especially when D, G, N, S, and T residues were directly C-terminal to the cleavage site. Whereas all postactivation methods boost the number of c- and z•-type fragment ions detected, the collision-based approaches produce higher rates of hydrogen migration, yielding fewer spectral identifications when only c- and z•-type ions are considered. Understanding hydrogen rearrangement between c- and z•-type ions will facilitate better spectral interpretation.

Macrocyclic Aromatic Amide Foldamer: Synthesis, Twisted-to-Boxlike Conformational Switching, and Molecular Recognition.

In this study, a twisted macrocycle was synthesized via ring closure of a double-helical aromatic oligoamide foldamer with two disulfide bridges. Single-crystal X-ray structure and NMR spectroscopy demonstrate the twisted conformation of macrocycle both in the solid state and in solution. As a result of the rearrangement of hydrogen bonding preference, the twisted conformation could be transformed to boxlike through protonation of the pyridine segments of macrocycle. In addition, the NMR titration experiments revealed that flat aromatic guests (e. g., coronene and perylene) could bind to the boxlike macrocycle with a 1 : 1 binding stoichiometry. The addition of base to the host-guest complexes resulted in conformational reversal of the macrocycle from boxlike to twisted as well as the release of guest.

GC–MS and GC–IR analysis of methylenedioxyphenylalkylamine analogues of the psychoactive 25X-NBOMe drugs

Twelve secondary amines representing structural analogues of the classic NBOMe category of novel psychoactive substances were prepared and evaluated in GC-EI-MS and vapor phase GC–IR studies. These compounds all contain the methylenedioxy group fused with the aromatic ring of the phenethyl moiety in the classic NBOMe chemical framework. One subseries of analogues, the three regioisomeric phenylisopropylamines, can be viewed as the N-methoxybenzyl analogues of 3,4-methylenedioxyamphetamine (MDA). Another subseries of three regioisomers contains the bromine and two methoxy groups at positions 2, 4 and 5 of the benzyl side of the molecule. The GC properties and the correlated EI-MS fragmentation products of these amines and some trifluoroacetamide derivatives were evaluated and compared to the traditional drug of abuse 25B-NBOMe. The compounds in this study are not currently known drugs of abuse and this proactive investigation provides data for differentiation of these analogues from the current NBOMe drugs of abuse. The observed GC elution order for the regioisomeric methoxybenzyl analogues on an Rxi®-17Sil MS phase was the 2-methoxy isomer eluting before the 3-substituted isomer, and the 4-substituted isomer eluting last. A similar comparison for the position of methylenedioxy fusion shows the 2,3-substituted isomer eluted before the 3,4-substituted isomer. All these secondary amines undergo EI fragmentation yielding the iminium cation via C–C bond cleavage of the phenethyl chain and the two benzylic cationic species. The mass spectra for the trifluoroacetyl derivatives in the bromodimethoxybenzyl substituted subseries provided additional structural information based on unique fragmentation processes. These characteristic ions are formed following initial hydrogen rearrangement within the molecular radical cation. The vapor phase infrared (vpIR) spectra confirm the position of substitution of the methylenedioxy, methoxy and bromodimethoxy groups.

Bandgap engineering of hydrogenated a-SiC:H thin films for photoelectrochemical water splitting applications

Bandgap engineering of a-SiC:H thin films was carried out to assess the material light absorption without compromising its photoelectrochemical water splitting capabilities. The tailoring was performed by varying the hydrogen concentration in the semiconductor and by post-deposition isochronical annealing treatments from 100 °C to 700 °C. Bandgap values were obtained by fitting the fundamental absorption region of the absorption coefficient using three different models. Differences among bandgap values extracted by these methods and its correlation with the a-SiC:H structure, demonstrate that structural features, rather than a hydrogen rearrangement or depletion, would be responsible for annealing induced optical bandgap increment. These features are taking in advantage for the bandgap engineering of a-SiC:H without changing Si-C stoichiometry. Optical bandgap values for p-doped a-SiC:H samples gradually increased from 2.59 to 2.76 eV upon performing each annealing step until 600 °C. Temperature at which an enhancement in the electric performance is observed. We believe, these results will help on the design of monolithic tandem solar cells for water splitting applications.

Fragmentation mechanisms from electron-impact of complex cyclic ethers formed in combustion

Cyclic ethers are important intermediates involved in degenerate chain-branching reactions during hydrocarbon and biofuel oxidation at temperatures below 1000 K. Depending on the molecular structure of the parent species, e.g. 1-butanol, butanone, or diethyl ether, cyclic ethers can include additional functional groups, such as an alcohol, carbonyl, or a second ether group. Electron-impact mass spectrometry (EI-MS) is a commonly used diagnostic to infer the molecular structure of cyclic ethers, and while mass spectra of simple cyclic ethers using EI are abundant in literature, measurements and related fragmentation patterns of complex cyclic ethers containing multiple functional groups are not, yet are important for determining the presence of species and inferring reaction mechanisms in combustion. The measurements in the present work provide reference EI mass spectra, which enable direct identification of cyclic ether species that are relevant to experimental chemical kinetics measurements of hydrocarbon and biofuel combustion.EI-MS measurements were conducted for 29 cyclic ethers with diverse functional groups such as alcohols, carbonyls, and alkyl substituents of varying length, 10 of which are reported for the first time: 2,3-epoxypentane, 3,4-epoxybutanol, (2-methyloxiranyl)methanol, 2,3-epoxybutanol, 1-(oxiran-2-yl)ethan-1-one, 3,4-epoxycyclohexene, 4,5-epoxycyclohexene, 2-methyloxetane, tetrahydrofuran-3-one, and 2-hydroxytetrahydrofuran. The objective of the present work is two-fold: (i) to provide reference measurements of EI mass spectra for complex combustion intermediates and (ii) to analyze fragmentation patterns of each species to assess the applicability of conventional mechanisms that are commonly employed for discerning molecular structure. The impetus for the latter objective is that, in the majority speciation experiments, EI mass spectra and fragmentation mechanisms are not reported, which raises open questions concerning the reliability of conventional mechanisms to define structural isomers of multi-functional cyclic ethers.In the present work, four conventional mechanisms were employed to analyze fragmentation patterns in the mass spectra: α-cleavage, inductive effect, hydrogen rearrangement, and transannular cleavage. The most common mechanism for the cyclic ethers was inductive cleavage, followed by α-cleavage or hydrogen rearrangement. Multi-functional cyclic ethers were more likely to undergo transannular cleavage occurring across the ring in a single step. For several species, however, abundant ion peaks in the mass spectra are not explained using conventional mechanisms. The present results highlight the importance of reference measurements of EI mass spectra for identifying complex molecules in speciation experiments, which is relevant given that ion fragments of the same mass-to-charge ratio (m/z) are common for species of the same mass.

Electrospray ionization tandem mass spectrometry of monoketone curcuminoids.

Although monoketone curcuminoids (MKCs) have been largely investigated due to their biological activities, data on the gas-phase fragmentation reactions of protonated MKCs under collision-induced dissociation (CID) conditions are still scarce. Here, we combined electrospray ionization tandem mass spectrometry (ESI-MS/MS) data, multiple-stage mass spectrometry (MSn ), deuterium exchange experiments, accurate-mass data, and thermochemical data estimated by computational chemistry to elucidate and to rationalize the fragmentation pathways of eleven synthetic MKCs. The MKCs were synthesized by Claisen-Schmidt condensation under basic (1-9) or acidic (10-11) conditions. ESI-CID-MS/MS analyses and deuterium-exchange experiments were carried out on a triple quadrupole mass spectrometer. MSn analyses on an ion trap mass spectrometer helped to elucidate the fragmentation pathways. Accurate-mass data and thermochemical data, obtained at the B3LYP/6-31+G(d,p) level of theory, were used to support the ion structures. The most intense product ions were the benzyl ions ([C7 H2 R1 R2 R3 R4 R5 ]+ ) and the acylium ions ([M + H - C8 H3 R1 R2 R3 R4 R5 ]+ ), which originated directly from the precursor ion as a result of two competitive hydrogen rearrangements. Product ions [M + H - H2 O]+ and [M + H - C6 HR1 R2 R3 R4 R5 ]+ , which are formed after Nazarov cyclization, were also common to all the analyzed compounds. In addition, •Br and •Cl eliminations were diagnostic for the presence of these halogen atoms at the aromatic ring, whereas •CH3 eliminations were useful to identify the methyl and methoxy groups attached to this same ring. Nazarov cyclization in the gas phase occurred for all the investigated MKCs and did not depend on the presence of the hydroxyl group at the aromatic ring. However, the presence and the position of a hydroxyl group at the aromatic rings played a key role in the Nazarov cyclization mechanism. Our results reinforce some aspects of the fragmentation pathways previously published for 1,5-bis-(2-methoxyphenyl)-1,4-pentadien-3-one and 1,5-bis-(2-hydroxyphenyl)-1,4-pentadien-3-one. The alternative fragmentation mechanism proposed herein can explain the fragmentation of a wider diversity of monoketone curcuminoids.

Hydrogen Arrangements on Defective Quasi-Molecular BN Fragments.

Considering the ever-increasing interest in metal-free materials, some potential chemical applications of quasi-molecular boron nitride (BN) derivatives were tested. Specifically, the behavior of BN fragments was analyzed when given defects, producing local electron density changes, were introduced by using topological engineering approaches. The inserted structural faults were Schottky-like divacancy (BN-d) defects, assembled in the fragment frame by the subtraction of one pair of B and N atoms or Stone–Wales (SW) defects. This study is aimed at highlighting the role of these important classes of defects in BN materials hypothesizing their future use in H2-based processes, related to either (i) H2 activation or (ii) H2 production, from preadsorbed hydrogenated molecular species on BN sites. Here, it has been observed that BN species, embodying SW defects, are characterized by endothermic H2 adsorption and fragmentation phenomena in order to guess their potential use in processes based on the transformation or production of hydrogen. On the contrary, in the presence of BN-d defects, and for reasons strictly related to local structural changes occurring along with the hydrogen rearrangements on the defective BN fragments, a possible use can be inferred. Precautions must be however taken to decrease the material rigidity that could actually decrease the ability of the BN fragment to flatten. This conversely seems to be a necessary requirement to have strong exothermic effects, following the rearrangements of the H2 molecules.

Hydrogen concentration at a-Si:H/c-Si heterointerfaces—The impact of deposition temperature on passivation performance

We studied the effect of deposition temperature on the hydrogen distribution and the passivation performance of hydrogenated amorphous silicon (a-Si:H) coated crystalline silicon (c-Si) heterojunctions as a model of high efficiency solar cell structures. Nuclear reaction analysis (NRA) was employed to obtain hydrogen depth profiles of the heterojunctions prepared at temperatures from 80 to 180 °C. The implied open circuit voltage (i-VOC) and carrier lifetime monotonically increased with increasing deposition temperature in the as-deposited samples. NRA clarified that the hydrogen concentration (CH) at the a-Si:H/c-Si interface and in the a-Si:H layer decreased with deposition temperature. The hydrogen concentration around the interface was roughly 3 × 1021 cm-3 for the sample deposited at 180 °C. The NRA results are supplemented by optical constants obtained with spectroscopic ellipsometry (SE). At higher growth temperature, larger refractive indices and extinction coefficients were confirmed by SE analysis, suggesting that fewer hydrogen atoms are incorporated into the a-Si:H layers prepared at higher growth temperature. Furthermore, the passivation performance was enhanced by post deposition annealing (PDA) at 200 °C for 30 min. No significant change of the hydrogen distribution and optical constants was observed after PDA, suggesting that improved passivation is due to a local rearrangement of hydrogen at the molecular level that results in enhanced hydrogenation of dangling bonds.

A hysteresis loop in electrical resistance of NbHx observed above the β−λ transition temperature

We investigate the electron transport properties and structures of β-NbHx(010) epitaxial thin films on Al2O3(001) substrates with a variety of hydrogen contents. NbHx epitaxial thin films with x ≥ 0.77 exhibit a hysteresis loop in their resistance near room temperature. Notably, this hysteresis loop appears above the β–λ phase transition temperature. Detailed analysis of the temperature dependence of these structures suggests that the short-range ordering of hydrogen rearrangement in the λ-phase remains locally above the transition temperature, inducing the hysteresis in the resistance.

Isolation and Identification of Six Difunctional Ethyl Esters from Bio-oil and Their Special Mass Spectral Fragmentation Pathways

Three difunctional ethyl esters (DFEEs), including ethyl 16-hydroxyhexadecanoate, ethyl (Z)-18-hydroxyoctadec-9-enoate, and diethyl (Z)-octadec-9-ene-1,18-dioate, were isolated as nearly pure compounds from sawdust-derived bio-oil. Their structures were tentatively identified by gas chromatography/mass spectrometry (GC/MS) and high-resolution mass spectrometry (HR-MS). Combination transesterification reaction of DFEEs to difunctional methyl esters (DFMEs) and determination of the GC/MS retention time differences of DFEEs and corresponding DFMEs is a simple method to determine the ester group number of unknown DFEEs. GC/MS results indicate that the fragment ions at mass-to-charge ratios (m/z) of 88 and 98 are typical ions for these DFEEs. The former ion is formed via well-known McLafferty rearrangement, whereas the formation pathway of the ion at m/z 98 is unreported before. HR-MS results show that the fragment ion at m/z 98 should be a cyclohexanone radical ion, which is possibly formed via a novel rearrangement initiated by remote hydrogen rearrangement. Besides long-chain DFEEs, long-chain difunctional carboxylic acids (including dicarboxylic acids, ω-hydroxy acids, diacid monomethyl ester, and diacid monoethyl ester) and DFMEs (including diacid dimethyl ester and ω-hydroxy methyl ester) are also typical of the fragment ion at m/z 98. Hence, it is a common characteristic to form the rearrangement ion at m/z 98 for long-chain difunctional carboxylic acids, DFEEs, and DFMEs, and this special mass spectral rearrangement could facilitate the identification of these classes of compounds. Using this rearrangement rule of DFEEs, three ω-hydroxy ethyl esters, including ethyl 22-hydroxydocosanoate, ethyl 24-hydroxytetracosanoate, and ethyl 26-hydroxyhexacosanoate, were tentatively identified by GC/MS.