Pyridines gas-phase

The chemical scheme for C-terminal sequencing is shown in Figure 2. The first step involves treatment of the peptide or protein sample with diisopropylethylamine in order to convert the C-terminal carboxylic acid into a carboxylate salt. Derivatization of the C-terminal amino acid to a thiohydantoin is accomplished with diphenylisothiocyanatidate (liquid phase) and pyridine (gas phase). The peptide is then extensively washed with ethyl acetate and acetonitrile to remove reaction by-products. The peptide is then treated briefly with gas phase trifluoroacetic acid, followed by water vapor in case the C-terminal residue is a proline (this treatment has no effect on residues which are not proline). The derivatized amino acid is then specifically cleaved with sodium or potassium trimethylsilanolate to generate a shortened peptide or protein which is ready for continued sequencing. In the case of a C-terminal proline which was already removed by water vapor, the silanolate treatment merely converts the C-terminal carboxylic acid group on the shortened peptide to a carboxylate. The thiohydantoin amino acid is then quantitated and identified by reverse-phase HPLC. 

Horsley, J.A. et al.. Resonances in the K-sheU excitation-spectra of benzene and pyridine — Gas-phase, sohd, and chemisorhed states, J. Chem. Phys. 83, 6099-6107, 1985. 

Horsley JA, Stohr J, Hitchcock AP, Newbury DC, Johnson AL, Sette F (1985b) Resonances in the shell excitation spectra of benzene and pyridine Gas phase, solid, and chemisoibed states. J Chem Phys 83 6099-6107... 

Treatment of pentachloropyridine with hydrogen fluoride at 180 °C under pressure gives a mixture of trichloro-2,6-difluoro- and dichIoro-2,4,6-tri-fluoro-pyridine. Gas-phase fluorination of pentachloropyridine and of 2,4, 2,4,6-trichlorop3rrimidine with hydrogen fluoride over chromium oxide-fluoride and aluminium oxide-fluoride (or A1203) has also been achieved tetrachloropyrimidine gives 5-chloro-2,4,6-trifluoro- (61%), 5,6-dichloro-2,4-difluoro- (33 %), and 4,5,6-trichloro-2-fluoro-pyrimidine (6 %). ... 

Raw Material and Energy Aspects to Pyridine Manufacture. The majority of pyridine and pyridine derivatives are based on raw materials like aldehydes or ketones. These are petroleum-derived starting materials and their manufacture entails cracking and distillation of alkanes and alkenes, and oxidation of alkanes, alkenes, or alcohols. Ammonia is usually the source of the nitrogen atom in pyridine compounds. Gas-phase synthesis of pyridines requires high temperatures (350—550°C) and is therefore somewhat energy intensive. 

Gas-phase pK values for azoles are unavailable except for imidazole and 1-methyl-imidazole. Imidazole is ca. 75kJmor more basic than ammonia, i.e. approximately the same as pyridine 1-methylimidazole is about 29kJmor more basic than imidazole (82PC40200). 

Pyridine, l-lithio-2-phenyl-l,2-dihydro-, 2, 266 Pyridine, 2-methoxy-IR spectroscopy, 2, 129 photoelectron spectroscopy, 2, 140 pulsed ion gas-phase cyclotron resonance spectroscopy, 2, 157 Pyridine, 3-methoxy-nitration, 2, 191 Pyridine, 4-methoxy-pKa,, 2. 150... 

Apply extracts of cereals or fungal cultures apply [66] 50 pi pyridine — acetic anhydride (1 -I-1) on top remove the excess reagent in a stream of cold air and chromatograph. The reagents can also be applied via gas phase... 

A mixture of water/pyridine appears to be the solvent of choice to aid carbenium ion formation [246]. In the Hofer-Moest reaction the formation of alcohols is optimized by adding alkali bicarbonates, sulfates [39] or perchlorates. In methanol solution the presence of a small amount of sodium perchlorate shifts the decarboxylation totally to the carbenium ion pathway [31]. The structure of the carboxylate can also support non-Kolbe electrolysis. By comparing the products of the electrolysis of different carboxylates with the ionization potentials of the corresponding radicals one can draw the conclusion that alkyl radicals with gas phase ionization potentials smaller than 8 e V should be oxidized to carbenium ions [8 c] in the course of Kolbe electrolysis. This gives some indication in which cases preferential carbenium ion formation or radical dimerization is to be expected. Thus a-alkyl, cycloalkyl [, ... 

Silver(II) complexes in the gas phase have been prepared with several nitrogen- and oxygen-donor ligands including pyridine, tetrahydrofurane, benzene, C02, etc. The complexes are of the type [AgLra]2+, and the compound [Ag(C02)4]2+ is particularly stable.216... 

Thermodynamics of complex formation of silver with several ligands such amines,368 hindered pyridine bases,369 nitrogen donor solvents,370 and azoles371 have been carried out. Other studies include the secondary-ion mass spectra of nonvolatile silver complexes,372 the relationship between Lewis acid-base behavior in the gas phase and the aqueous solution,373 or the rates of hydride abstraction from amines via reactions with ground-state Ag+.374... 

The addition of pyridine directly into the gas phase also led to a significant reduction of prereaction and an improvement in the structural quality of the ZnSe layers, compared with layers grown using Me2Zn and Fl2Se in the absence of pryridine.66... 

The acidic and adsorptive properties of the samples in gas phase were evaluated in a microcalorimeter of Tian-Calvet type (C80, Setaram) linked to a volumetric line. For the estimation of the acidic properties, NH3 (pKa = 9.24, proton affinity in gas phase = 857.7 kJ.mol-1, kinetic diameter = 0.375 nm) and pyridine (pKa = 5.19, proton affinity in gas phase = 922.2 kJ.mol-1, kinetic diameter = 0.533 nm) were chosen as basic probe molecules. Different VOC s such as propionaldehyde, 2-butanone and acetonitrile were used in gas phase in order to check the adsorption capacities of the samples. 

The number of sites titrated by NH3 and pyridine are similar except for sample Al-SBA-15(15) which means a good accessibility of pyridine in the solid pores without any steric hindrance. On the contrary, the integral heats of adsorption are higher when using pyridine due to its higher protonic affinity in gas phase compared to NH3 and the way in which probe molecules bind on the solid surface [6, 7]. 

One of the problems encountered when dealing with the interaction of Lewis acids and bases in a quantitative way is in evaluating the role of the solvent. Bond energies in molecules are values based on the molecule in the gas phase. However, it is not possible to study the interaction of many Lewis acids and bases in the gas phase because the adducts formed are not sufficiently stable to exist at the temperature necessary to convert the reactants to gases. For example, the reaction between pyridine and phenol takes place readily in solution as a result of hydrogen bonding ... 

However, the adduct is not stable enough to exist in significant amounts at a temperature of 116 °C, the boiling point of pyridine. As a result, such interactions are studied in solutions, although it is the strength of the bond that would be produced in the gas phase that is desired. A thermochemical cycle... 

The isopyridine 179 (3<52-lH-pyridine) is the result if the oxygen atom of 180 is replaced by an NH group. Owing to the better electron-donor quality relative to that of an oxygen atom, the NH group could have the effect that the zwitterion 179-Zj is more stable than the allene structure 179, even in the gas phase. Experiments and quantum-chemical calculations support this expectation. 

The gas-phase pyrolysis of vinylogous systems of isopropylidene amino-methylenemalonates (1280,1287, and 1290), prepared from the appropriate enaminone or dienaminone and Meldrum s acid in pyridine, was studied by McNab etal. at 500°C and 10 2 torr (87CC140). Flash vacuum pyrolysis of 1280 gave l//-azepinones (1283) in —60% yields, together with a small amount of cyclopentadienone dimer (1284). They suggested that the azepi-nones (1283) were formed by electrocyclization from dipolar intermediates (1282) produced from the methyleneketenes (1281) by hydrogen transfer (Scheme 54). Cycloaddition of 1282 yielded bicyclics (1285), which col-... 

Fluorinated alkyl cyanides, such as trifluoroacetonitrile, pentafluoropropionitrile, per-fluorobutyronitrile and chlorodifluoroacetonitrile, react with butadiene in the gas phase at 350-400 °C to afford pyridines in high yields (equation 82)72. The push-pull diene 150 and electron-rich cyanides (acetonitrile or acrylonitrile) furnish pyridines (equation 83)73. 

Studies of gas-phase S"n2 reactions at sp carbon have been made by Fourier transform ion cyclotron resonance mass spectrometry (FTlCRMS) and complemented by both semiempirical and ab initio MO calculations. The particular processes of interest involved intramolecular reactions in which neutral nucleophiles displace neutral leaving groups within cationic substrates, e.g. A-(2-piperidinoethyl)-2,4,6-triphenylpyridinium cation (59), in which the piperidino moiety is the nucleophile and 2,4,6-triphenylpyridine (60) is the leaving group. No evidence has been obtained for any intermolecular gas-phase 5) 2 reaction involving a pyridine moiety as a leaving group. The quantum mechanical treatments account for the intramolecular preference. 

Chemical/Physical. The gas-phase reaction of ozone with pyridine in synthetic air at 23 °C yielded a nitrated salt having the formula [CeHsNHJ NOs (Atkinson et al., 1987). Ozonation of pyridine in aqueous solutions at 25 °C was studied with and without the addition of ferf-butyl alcohol (20 mM) as a radical scavenger. With tert-hniyX alcohol, ozonation of pyridine yielded mainly pyridine W-oxide (80% yield), which was very stable towards ozone. Without terf-butyl alcohol, the heterocyclic ring is rapidly cleaved forming ammonia, nitrate, and the amidic compound W-formyl oxamic acid (Andreozzi et al., 1991). 

More advanced semiempirical molecular orbital methods have also been used in this respect in modeling, e.g., the structure of a diphosphonium extractant in the gas phase, and then the percentage extraction of zinc ion-pair complexes was correlated with the calculated energy of association of the ion pairs [29]. Semiempirical SCF calculations, used to study structure, conformational changes and hydration of hydroxyoximes as extractants of copper, appeared helpful in interpreting their interfacial activity and the rate of extraction [30]. Similar (PM3, ZINDO) methods were also used to model the structure of some commercial extractants (pyridine dicarboxylates, pyridyloctanoates, jS-diketones, hydroxyoximes), as well as the effects of their hydration and association with modifiers (alcohols, )S-diketones) on their thermodynamic and interfacial activity [31 33]. In addition, the structure of copper complexes with these extractants was calculated [32]. 

Four diastereomers of 2-ethynyl-7a-hydroxy-3a,7-dimethyl-4,6-diphe-nylperhydropyrrolo[3,2-c]pyridine (58B) were recently isolated (92KGS-903). It was established by means of H-NOESY and C-NMR spectroscopy that the stereochemical differences in the structures of the diastereomers involve cis and transfusion of the rings and different configurations at C(2). In solution and in the gas phase, the corresponding ring-chain equilibrium 58A 58B was detected, with two open-chain epimers (different configurations at C—NH2). 

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