New stationary phase based on β-cyclodextrin for normal-phase HPLC group-separation of organic nitrates

The synthesis of a β-cyclodextrin-silica normal-phase (NP)-liquid chromatography (LC) stationary phase is reported. Silica gel was modified with (3-bromopropyl)trimethoxysilane to a 3-bromopropyl-silica that was reacted with β-cyclodextrin, resulting in a β-cyclodextrin-silica. To prove its usefulness in group-separation of organic nitrates among others, a mixture of three groups of organic nitrates was separated. The results are compared with those obtained on a nitrated polyol-silica that has recently been reported. The alkyl dinitrates exhibt higher retention relatively to alkyl mononitrates on the new phase. This allows to cut the LC fractions in a way that the alkyl mononitrates and phenylalkyl nitrates appear in one fraction and the dinitrates in a second one without any overlap of the two fractions. Received: 3 August 1999 / Revised: 27 September 1999 / Accepted: 30 September 1999     

Multifunctional organic nitrates as constituents in European and US urban photo-smog

Air samples of the atmospheric ground layer in the cities of Ulm in Germany, Las Vegas, Nevada, and Salt Lake City, Utah, in the US were analyzed for organic nitrates. The air samples were taken around noon in summer at sunny weather conditions. 43 (C6-C13) alkyl mononitrates, 24 (C3-C6) alkyl dinitrates, and 19 (C2-C6) hydroxy alkyl nitrates have been identified. The analytical protocol included high-volume-sampling, NP-HPLC group separation, high resolution capillary gas chromatography, and detection by the highly selective mass spectrometer detector (SIM mode, m/z = 46). The levels of the sum of 15 (C6-C10) alkyl mononitrates ranged from 2.9 to 11.0 parts per trillion (ppt(v)). The levels of the sum of 21 (C3-C6) alkyl dinitrates ranged from 2.3 to 10.5 ppt(v), and the levels of the sum of 7 (C2-C4) hydroxy alkyl nitrates ranged from 7.3 to 28 ppt(v), respectively, in the urban air samples. These results emphazise the contribution of the alkyl dinitrates and hydroxy alkyl nitrates besides the alkyl mononitrates to the budget of NO(Y) compounds. No major differences in levels and pattern of the organic nitrates are present in air of the German and the US cities.     

Multifunctional organic nitrates as constituents in European and US urban photo-smog

Air samples of the atmospheric ground layer in the cities of Ulm in Germany, Las Vegas, Nevada, and Salt Lake City, Utah, in the US were analyzed for organic nitrates. The air samples were taken around noon in summer at sunny weather conditions. 43 (C₆–C₁₃) alkyl mononitrates, 24 (C₃–C₆) alkyl dinitrates, and 19 (C₂–C₆) hydroxy alkyl nitrates have been identified. The analytical protocol included high-volume-sampling, NP-HPLC group separation, high resolution capillary gas chromatography, and detection by the highly selective mass spectrometer detector (SIM mode, m/z = 46). The levels of the sum of 15 (C₆–C₁₀) alkyl mononitrates ranged from 2.9 to 11.0 parts per trillion (ppt(v)). The levels of the sum of 21 (C₃–C₆) alkyl dinitrates ranged from 2.3 to 10.5 ppt(v), and the levels of the sum of 7 (C₂–C₄) hydroxy alkyl nitrates ranged from 7.3 to 28 ppt(v), respectively, in the urban air samples. These results emphazise the contribution of the alkyl dinitrates and hydroxy alkyl nitrates besides the alkyl mononitrates to the budget of NO_Y compounds. No major differences in levels and pattern of the organic nitrates are present in air of the German and the US cities.     

Bifunctional alkyl nitrates – trace constituents of the atmosphere

Mono- and multifunctional esters of nitric acid (alkyl nitrates or organonitrates) form very complex mixtures of organic trace constituents in air. An analytical method was developed which combines selectivity in separation and detection in order to simplify this complexity in analytical terms. Mononitrates, dinitrates, keto nitrates, hydroxy nitrates of alkanes and alkenes, respecitvely, and bifunctional terpene nitrates were synthesized as reference substances. A specially developed new HPLC stationary phase (organonitrate phase) allows a group separation of mono-, di-, and hydroxy nitrates. After the HPLC preseparation the single components were finally separated by capillary HRGC-ECD and HRGC-MSD on polar and non-polar stationary phases. Mass spectrometric detection in the selected-ion-mode using the highly selective NO₂ ⁺ fragment (m/z = 46 amu) led to very good selectivities for the nitric acid ester moiety. The analysis of a 100 m³ ambient air sample using this new analytical protocol allowed the identification of seven hydroxy nitrates and 24 dinitrates ranging from C2 to C7, 22 of them for the first time ever.     

Identification of alkyl dinitrates in ambient air of Central Europe

Bifunctional organic nitrates – organic esters of nitric acid – are part of the pool of components of odd nitrogen (NO_Y) in the atmosphere. In this paper, we report the identification of dinitrates in ambient air of Central Europe (Ulm, Germany). The organic nitrates were sampled by adsorptive high volume sampling of 100 m³ of air with 100 g precleaned silica. Sample preparation and group separation was performed by NP-HPLC. Separation and detection was done by HRGC/EI-MSD. We have extended the identification of dinitrates in ambient air to a number of 30, of which only five have been reported by other authors before. To our knowledge, this is the first time that 13 non-vicinal dinitrates and two branched dinitrates have been identified in ambient air. Four organic nitrates have never been reported before: the non-vicinal 1,3-dinitrooxy-pentane (1,3C5), the two branched 2-methyl-1,2-dinitrooxy-propane (2M1,2C3) and 2-methyl-2,3-dinitrooxy-butane (2M2,3C4), and one diastereomer of 3,4-dinitrooxy-hexane (RR/SS-3,4C6). We are also assigning the configuration of the two diastereomers of 2,4-dinitrooxy-pentane (2,4C5) found in air before using reference standards obtained by enantioselective synthesis. Received: 8 June 1998 / Revised: 7 September 1998 / Accepted: 14 September 1998     

Separation, detection and occurrence of (C2–C8)-alkyl- and phenyl-alkyl nitrates as trace compounds in clean and polluted air

Nitric acid, HNO₃, and nitrous acid, HNO₂, are forming stable esters with alcohols, the alkyl nitrates and alkyl nitrites. Both groups of compounds are used as fuel additives, explosives and pharmaceuticals. Alkyl nitrates are also formed as complex mixtures during incomplete combustion and the abiotic transformation of alkanes, alkenes and aldehydes in air. Organic nitrates can be assigned to anthropogenic and natural sources alike. Here the synthesis of reference mixtures of alkyl nitrates is described starting with alcohols, alkyl bromides, alkyl iodides or alkanes, respectively, sampling techniques in air analysis, and the separation of alkyl nitrates and alkyl dinitrates by high resolution capillary gas chromatography using various stationary phases and electron capture (HRGC/ECD) as well as mass spectrometric detection (HRGC/MSD). A highly selective detection mode for alkyl nitrates and alkyl di- and trinitrates — in general in the presence of other organic trace compounds — is the single ion monitoring of 46 amu in GC/MS. The separation and occurrence of alkyl- and phenyl-alkyl nitrates in polluted air is reported using capillary gas chromatography with electron capture and ion trap MS detection. The reaction of alkanes with nitric acid at room temperature simulates in a good approximation the pattern of alkyl nitrates formed in air chemistry.     

Reaction of oleic acid particles with NO3 radicals: Products, mechanism, and implications for radical-initiated organic aerosol oxidation

The heterogeneous reaction of liquid oleic acid aerosol particles with NO3 radicals in the presence of NO2, N2O5, and O2 was investigated in an environmental chamber using a combination of on-line and off-line mass spectrometric techniques. The results indicate that the major reaction products, which are all carboxylic acids, consist of hydroxy nitrates, carbonyl nitrates, dinitrates, hydroxydinitrates, and possibly more highly nitrated products. The key intermediate in the reaction is the nitrooxyalkylperoxy radical, which is formed by the addition of NO3 to the carbon-carbon double bond and subsequent addition of O2. The nitrooxyalkylperoxy radicals undergo self-reactions to form hydroxy nitrates and carbonyl nitrates, and may also react with NO2 to form nitrooxy peroxynitrates. The latter compounds are unstable and decompose to carbonyl nitrates and dinitrates. It is noteworthy that in this reaction nitrooxyalkoxy radicals appear not to be formed, as indicated by the absence of the expected products of decomposition or isomerization of these species. This is different from gas-phase alkene-NO3 reactions, in which a large fraction of the products are formed through these pathways. The results may indicate that, for liquid organic aerosol particles in low NOx environments, the major products of the radical-initiated oxidation (including by OH radicals) of unsaturated and saturated organic compounds will be substituted forms of the parent compound rather than smaller decomposition products. These compounds will remain in the particle and can potentially enhance particle hygroscopicity and the ability of particles to act as cloud condensation nuclei.     

Group separation of organic nitrates on a new nitric acid ester NP-LC stationary phase

Summary The synthesis of a nitric acid ester NP-LC stationary phase (organonitrate phase), by nitrating a commercial polyol phase (Serva, Polyol Si 100, 5 μm) with mixed sulfuric-and nitric acid, is reported. The nitric acid ester bond is quite stable towards hydrolysis. To examine its possible use as a stationary phase for the group separation of alkyl nitrates, several bifunctional polar alkyl nitrates (di-nitrates, hydroxy-nitrates, keto-nitrates) have been synthesized. In order to simplify their names a new abbreviating nomenclature for multifunctional organic nitrates is presented. With our new stationary phase it is possible to separate mono-, di-, and hydroxy-nitrates as groups completely, a major advantage in sample preparation in the chemistry of smog formation. However a co-elution of di- and keto-nitrates is observed. A distinction of the latter two organonitrate groups is possible by gas chromatography-mass spectrometric detection (MSD). Apart from the fragment ion NO₂ ⁺ (m/z=46 amu), which is nearly specific for alkyl nitrates, aliphatic di- and keto-nitrates show different characteristic fragment ions.     

Simple and Versatile Nitrooxylation: Noncyclic Hypervalent Iodine Nitrooxylating Reagent

Organic nitrates are broadly applied as pharmaceuticals (acting as efficient nitric oxide donor), energetic materials, building blocks in organic synthesis, etc. However, practical and direct methods to access organic nitrates efficiently are still rare, mainly due to the lack of powerful nitrooxylating reagents. Herein, we report bench-stable and highly reactive noncyclic hypervalent iodine nitrooxylating reagents, oxybis(aryl-λ³-iodanediyl) dinitrates (OAIDNs, 2 ), which are prepared just by using aryliodine diacetate and HNO₃. The reagents are used to achieve a mild and operationally simple protocol to access diverse organic nitrates. By employing of 2 , zinc-catalyzed regioselective nitrooxylation of cyclopropyl silyl ethers is realized efficiently to access the corresponding β-nitrooxy ketones with high functional-group tolerance. Moreover, a series of direct and catalyst-free nitrooxylations of enolizable C−H bonds are carried out smoothly to afford the desired organic nitrates within minutes by just mixing the substrates with 2 in dichloromethane.     

C4-C14-alkyl nitrates as organic trace compounds in air

The long chain alkyl nitrates (C _n >5) form a complex spectrum of natural and anthropogenic organic trace compounds in air. HRGC/ECD and HRGC/MSD using 56 amu as the signal reveal a standard pattern of isomeric n-alkyl nitrates in semi-rural air. This is regulated by the input of the corresponding alkanes, their rate constants for the reaction with OH, the rate constant of the alkylperoxy radicals for the reaction to alkyl nitrates, the atmospheric concentrations of NO/NO₂ and by the rate constants of the alkyl nitrates for the reaction with OH radicals as the major removal reaction. The complex pattern of signals given by the ECD in the retention index range between 700 and 2000 has been observed before but this is the first time that it has been assigned to a defined group of chemical compounds.The environmental impact of the occurrence of the different groups of alkyl nitrates has yet to be evaluated. Their general property as organic stabilizers for NO/NO₂ and therefore as precursors of NO ₃ ^- ions in rain and their biological potentials are also known. The long chain alkyl nitrates act as lipophilic carriers for nitric acid.     

Alkyl nitrates as achiral and chiral solute probes in gas chromatography. Novel properties of a beta-cyclodextrin derivative and characterization of its enantioselective forces.

Achiral and chiral interactions of alkyl nitrates (R-O-NO2) with heptakis-(3-O-acetyl-2,6-di-O-pentyl)-beta-cyclodextrin (LIPODEX-D) in the gas phase were investigated chromatographically. Two major outcomes can be summarized. First, LIPODEX-D shows very fast temperature-dependent variations of the selectivity up to changes in the order of elution for C1-C5 alkyl nitrates. These changes in selectivity reveal that LIPODEX-D possesses different shape selectivities for small alkyl nitrates at different temperatures (40-80 degrees C), i.e. with increasing isothermal separation temperature extended (chain-like) alkyl nitrates have increased retention relative to bulky alkyl nitrates. The observations are highly reproducible and might indicate conformational changes of the cyclodextrin, however, an ultimate proof would require further spectroscopic investigation. Secondly, the chiral separations of systematically varied sets of C4-C11 alkyl nitrates allowed the thermodynamic characterization of enantiodiscriminating interactions. Quantitative evidence is provided showing that the presence of an ethyl group at the asymmetric carbon atom of an n-alkyl nitrate gives a strong enthalpic contribution to the resulting enantioselectivity. The Gibbs free energy differences -deltaR,S(deltaG) decrease systematically three to six times if the ethyl group is either shortened or enlarged by only a -CH2-increment. The results are based on two separate thermodynamic approaches, i.e. the determination of thermodynamic quantities (-deltaR,S(deltaG), -deltaR,S(deltaH), -deltaR,S(deltaS), Tiso) and a theoretical concept of enthalpy-entropy compensation. The data from our laboratory experiments also indicate that van der Waals interactions are responsible for chiral discrimination.     

Induction of thermotropic bicontinuous cubic phases in liquid-crystalline ammonium and phosphonium salts

Two series of wedge-shaped onium salts, one ammonium and the other phosphonium, having 3,4,5-tris(alkyloxy)benzyl moieties, exhibit thermotropic bicontinuous "gyroid" cubic (Cub(bi)) and hexagonal columnar liquid-crystalline (LC) phases by nanosegregation between ionophilic and ionophobic parts. The alkyl chain lengths on the cationic moieties, anion species, and alkyl chain lengths on the benzyl moieties have crucial effects on their thermotropic phase behavior. For example, triethyl-[3,4,5-tris(dodecyloxy)benzyl]ammonium hexafluorophosphate forms the thermotropic Ia3d Cub(bi) LC phase, whereas an analogous compound with trifluoromethanesulfonate anion shows no LC properties. Synchrotron small-angle diffraction intensities from the Ia3d Cub(bi) LC materials provide electron density maps in the bulk state. The resulting maps show convincingly that the Ia3d Cub(bi) structure is composed of three-dimensionally interconnected ion nanochannel networks surrounded by aliphatic domains. A novel differential mapping technique has been applied successfully. The map of triethyl-[3,4,5-tris(decyloxy)benzyl]ammonium tetrafluoroborate has been subtracted from that of the analogous ammonium salt with hexafluorophosphate anion in the Ia3d Cub(bi) phases. The differential map shows that the counteranions are located in the core of the three-dimensionally interconnected nanochannel networks. Changing from trimethyl- via triethyl- to tripropylammonium cation changes the phase from columnar to Cub(bi) to no mesophase, respectively. This sensitivity to the widened shape for the narrow end of the molecule is explained successfully by the previously proposed semiquantitative geometric model based on the radial distribution of volume in wedge-shaped molecules. The LC onium salts dissolve lithium tetrafluoroborate without losing the Ia3d Cub(bi) LC phase. The Cub(bi) LC materials exhibit efficient ion-transporting behavior as a result of their 3D interconnected ion nanochannel networks. The Ia3d Cub(bi) LC material formed by triethyl-[3,4,5-tris(decyloxy)benzyl]phosphonium tetrafluoroborate shows ionic conductivities higher than the analogous Ia3d Cub(bi) material based on ammonium salts. The present study indicates great potential of Cub(bi) LC nanostructures consisting of ionic molecules for development of transportation nanochannel materials.     

Measurement of the reactivity of OH with methyl nitrate: Implications for prediction of alkyl nitrate-OH reaction rates

The rate of the gas phase reaction of hydroxyl radical with methyl nitrate has been measured to be (3.4 ± 0.4) × 10^?14 cm³ molecule^?1 s^?1 at 298 K using flow discharge/ resonance fluorescence techniques. By means of correlation methods, this rate determination is used to predict a vertical ionization potential of 12.6 eV, a bond dissociation energy for H? CH₂ONO₂ of 101 kcal mol^?1, and a rate for O(³P) reaction with methyl nitrate of ca. 9 × 10^?17 cm³ molecule^?1 s^?1. In conjunction with previously derived relative data for reaction of alkyl nitrates with OH radical in the gas phase, a priori estimated reactivities for 1-, 2-, and 3-positionally substituted straight chain alkyl nitrates have been reexamined. Revised reactivities for OH abstraction of specific hydrogens substituted on straight chain alkyl nitrates are presented and discussed, and an atmospheric lifetime of ca. 2 yrs is estimated for methyl nitrate removal due to OH.     

Liquid crystalline behavior of a semifluorinated oligomer

Monte Carlo simulations with a coarse-grained model were performed to study the microstructure of a semifluoroalkane C20 diblock oligomer [F(CF(2))(10)(CH(2))(10)H]. The coarse-grained model adopted is based on previously reported united-atom force fields for alkanes and perfluoroalkanes and was first validated by simulating the phase behavior of a mixture of hexane and perfluorohexane. These preliminary simulations established the need of a significant correction factor in the Berthelot mixing rule between alkane and perfluoroalkane groups. Using such a force field, the semifluorinated C20 oligomer liquid was simulated using efficient Monte Carlo moves to sample different molecular arrangements and box dimensions so as to allow different layering structures to form. In qualitative agreement with experimental observations, a smectic-to-isotropic phase transition occurs as temperature is increased but the transition point and the structure of the smectic phase depend on the stiffness of the torsional potential and the model of van der Waals interactions adopted. We identify two smectic phases LC1' and LC2', whose structures do not agree with those that have been postulated before to explain x-ray diffraction data, namely, LC1 and LC2. LC1' has a layer spacing similar to LC1 but the antiparallel packing is not observed with individual chains but with groups of chains producing a checkerboard pattern. LC2' has fully microsegregated blocks such as LC2 but the alkyl tails are not fully stretched or interdigitated. Despite these inconsistencies, and considering that reported experimental data also reveal the presence of mixed phases, the simulated structures suggest other plausible ways how the semifluorinated chains could pack and microsegregate to best negotiate energetic and entropic constraints.     

Peripheral chain length-dependent complex phase behaviour in C3-symmetric hexa-alkylated liquid crystals

We report the thermal and self-assembly properties of C₃-symmetric liquid crystalline (LC) molecules consisting of a conformationally tunable triazole-based mesogen and six-fold alkyl chains. Unlike the LC compound (1) based on non-crystallisable octyl chains, 2 and 3, which have crystallisable dodecyl and tetradecyl chains, respectively, exhibit a cold crystallisation which only takes place under slow heating conditions (2°C/min). In contrast with the vertically interdigitated lamellar crystalline phase of 1, a laterally interdigitated bilayered lamellar structure driven by the crystallisation of the dodecyl or tetradecyl chains is observed in the cold crystallisation temperature range. In addition to their crystalline morphology, 2 and 3 show LC morphological behaviour distinct from that of 1, 2 and 3 exhibit a hexagonal columnar LC phase consisting of T-shaped conformers rather than the lamellar LC phase of 1. The morphological transformation from the lamellar (1) to the columnar phase (2 and 3) can be rationalised by the alleviation of the conformational energy of the longer alkyl chains. Consequently, the simple variation of alkyl chain length in the C₃-symmetric LC system results in contrasting thermal and assembly properties in the crystalline and LC phases.     

Tunable self-organization in n-type liquid crystalline dibenzocoronene tetracarboxdiimides for high photoconductivity

ABSTRACT

A comprehensive study on the self-organised mesomorphism and charge-transport properties of three n-type liquid crystalline (LC) dibenzocoronene tetracarboxdiimide derivatives (Dibenzo-CDI 1, 2, 3) bearing branched alkyl chains with varied side-chain length and branching point (α or β site) has been carried out. All three Dibenzo-CDI compounds possessed high solubility in common organic solvents and low-lying lowest unoccupied molecular orbital energy levels below ?4.0 eV indicating their air-stable electron-transport capacity under ambient conditions. It is discovered that the side-chain length has a moderate influence on the molecular arrangement and thus leads to only limited variation of electron-transport properties. In contrast, the branching position plays a critical role on tuning molecular packing and orientation in the bulk mesophases, thereby resulting in dramatic differences in electron mobilities. Remarkably, Dibenzo-CDI 3 demonstrates good photoconductivity by time-of-flight method with high electron mobility up to 0.075 cm²/Vs, close to the highest value reported in certain n-type LC derivatives. All these results reveal that LC Dibenzo-CDI derivatives are potential candidates for high-performance solution-processable electronics, such as air-stable n-type organic field-effect transistors (OFETs) and solar cells.     

Effect of temperature on the mechanism of retention of fullerenes in liquid chromatography using various alkyl bonded stationary phases

Summary The temperature-dependency of the separation of fullerenes in liquid chromatography (LC) has been examined using various alkyl bonded stationary phases. It has been found that a maximum retention temperature exists with long alkyl bonded stationary phases, whereas there is no similar effect with the newly synthesized alkyl bonded phases which have two phenyl groups at the base of the bonded phase. The interpretation of the retention behavior of fullerenes in the low temperature region on alkyl bonded stationary phases is discussed using information obtained by CP-MAS solid-state NMR spectroscopy and LC.     

Synthesis and tailoring phase behavior of mesogen jacketed liquid crystalline copolymers based on 2, 5‐bis[(4‐alkoxyphenyl)oxycarbonyl]styrenes

Based on 2, 5‐bis[(4‐alkoxyphenyl)oxycarbonyl]styrenes (M‐OCm, m is the number of the carbons of alkyl tails, m = 1, 4, and 18), three series of binary copolymers with high‐molecular weights, {poly(M‐OC1‐co‐M‐OC4), poly(M‐OC1‐co‐M‐OC18), and poly(M‐OC4‐co‐M‐OC18)} have been prepared via free‐radical polymerization. The random nature of the copolymers was expected on the basis of the assumed similar reactivities because of the analogous monomers. The phase behaviors of copolymers were studied by DSC, POM, and one‐dimensional wide‐angle X‐ray diffraction. The results showed that liquid crystalline (LC) phase structures of copolymers, containing smectic phase, reentrant isotropic phase, columnar phase. and isotropic phase, were strongly depended on the composition and the alkyl length due to the competing among the steric effect, the microphase separation and the driving force of the entropy. When one of them occupied a dominant position, the LC phase structure can be presented for the copolymers. Otherwise, the LC phase structure is lost despite the pair of corresponding homopolymers forming mesogenic structure. Therefore, through copolymerization, LC behavior of the mesogen‐jacketed liquid crystalline polymers can be greatly varied. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2804–2816     

Thermal and photochemical oxidation of self-assembled monolayers on alumina particles exposed to nitrogen dioxide

Alumina is an important component of airborne dust particles as well as of building materials and soils found in the tropospheric boundary layer. While the uptake and reactions of oxides of nitrogen and their photochemistry on alumina have been reported in the past, little is known about the chemistry when organics are also present. Fourier transform infrared (FTIR) spectroscopy at ～23 °C was used to study reactions of NO(2) on γ-Al(2)O(3) particles that had been derivatized using 7-octenyltrichlorosilane to form a self-assembled monolayer (SAM). For comparison, the reactions with untreated γ-Al(2)O(3) were also studied. In both cases, the particles were exposed to water vapor prior to NO(2) to provide adsorbed water for reaction. As expected, surface-bound HONO, NO(2)(-), and NO(3)(-) were formed. Surprisingly, oxidation of the organic by surface-bound nitrogen oxides was observed in the dark, forming organo-nitrogen products identified as nitronates (R(2)C[double bond, length as m-dash]NO(2)(-)). Oxidation was more rapid under irradiation (λ > 290 nm) and formed organic nitrates and carbonyl compounds and/or peroxy nitrates in addition to the products observed in the dark. Mass spectrometry of the gas phase during irradiation revealed the production of NO, CO(2), and CO. These studies provide evidence for oxidation of organic compounds on particles and boundary layer surfaces that are exposed to air containing oxides of nitrogen, as well as new pathways for the formation of nitrogen-containing compounds on these surfaces.