Rotational equilibria in 1,2,6-trisubstituted pyridinium cations and reactions of 2-Isopropylpyrylium cations

2-Isopropyl-6-phenyl- and 2,6-diisopropyl-pyridiniums with bulky 1-substituents show temperature-variable NMR spectra which are interpreted in terms of restricted rotation. 2-Isopropyl-4,6-diphenylpyrylium can be deprotonated at the isopropyl group to give an anhydro base which forms new pyryliums with electrophiles.     

Collisionally activated dissociation of some pyridinium cations: Novel fragmentation pathways

Three novel collisionally induced dissociation pathways, additional to the usual formation of pyridine or pyridinium cation, are described for laser-desorbed N-substituted pyridinium cations. Particularly prevalent is the formation of an ion of m/z 94, corresponding to [PyCH₃]⁺. Doubly charged pyridiniums tend to lose H⁺, and one example of the apparent formation of the neutral radical C₅H₆N˙ is reported.     

The peculiarities of the spectral luminescence properties of N-anthryl-substituted pyridinium cations

A structureless fluorescence band λ^max_f ≈ 530 nm) in liquid solution and a fluorescence band with vibrational structure (λ = 376–480 nm) in rigid media (ethanol, 77 K and poly(methyl methacrylate) (PMMA), 296 K) were observed for N-anthryl-substituted pyridinium cations. In liquid solution the long-wave fluorescence is attributed to the biradical structure resulting from the photoinduced electron transfer (PET) from anthracene to the pyridinium ring, and the long-wave, low intensity absorption (and fluorescence excitation) band (λ=465 nm) is attributed to this interfragment PET which is weakly allowed by the torsional vibrations. In rigid media the PET is strongly forbidden (in the strict orthogonal pyridinium-anthracene structure) and radiative deactivation of the anthracene fragment is observed.     

Kinetics and mechanisms of the pyridinolysis of phenyl and 4-nitrophenyl chlorothionoformates. Formation and hydrolysis of 1-(aryloxythiocarbonyl)pyridinium cations

The title reactions are subjected to a kinetic study in water, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). By following the reactions spectrophotometrically two consecutive reactions are observed: the first is formation of the corresponding thionocarbamates (1-(aryloxythiocarbonyl)pyridinium cations) and the second is their decomposition to the corresponding phenol and pyridine, and COS. Pseudo-first-order rate coefficients (k(obsd1) and k(obsd2), respectively) are found under excess amine. Plots of k(obsd1) vs free pyridine concentration at constant pH are linear, with the slope (k(N)) independent of pH. The Br?nsted-type plots (log k(N) vs pK(a) of the conjugate acids of the pyridines) are linear with slopes beta = 0.07 and 0.11 for the reactions of phenyl and 4-nitrophenyl chlorothionoformates, respectively. These Br?nsted slopes are in agreement with those found in other stepwise reactions of the same pyridines in water, where the formation of a tetrahedral intermediate is the rate-determining step. In contrast to the stepwise mechanism of the title reactions that for the reactions of the same substrates with phenols is concerted, which means that substitution of a pyridino moiety in a tetrahedral intermediate by a phenoxy group destabilizes the intermediate. The second reaction corresponds to the pyridine-catalyzed hydrolysis of the corresponding 1-(aryloxythiocarbonyl)pyridinium cation. Plots of k(obsd2) vs free pyridine concentration at constant pH are linear, with the slope (k(H)) independent of pH. The Br?nsted plots for k(H) are linear with slopes beta = 0.19 and 0.26 for the reactions of the phenyl and 4-nitrophenyl derivatives, respectively. These low values are explained by the fact that as pK(a) increases the effect of a better pyridine catalyst is compensated by a worse leaving pyridine from the corresponding thionocarbamate     

Origin of attraction, magnitude, and directionality of interactions in benzene complexes with pyridinium cations

Geometries and interaction energies of benzene complexes with pyridine, pyridinium, N-methylpyridinium were studied by ab initio molecular orbital calculations. Estimated CCSD(T) interaction energies of the complexes at the basis set limit were -3.04, -14.77, and -9.36 kcal/mol, respectively. The interactions in the pyridinium and N-methylpyridinium complexes should be categorized into a cation/pi interaction, because the electrostatic and induction interactions greatly contribute to the attraction. On the other hand, the interaction in the pyridine complex is a pi/pi interaction. The dispersion interaction is mainly responsible for the attraction in the benzene-pyridine complex. Short-range interactions including charge-transfer interactions are not important for the attraction in the three complexes. The most stable pyridinium complex has a T-shaped structure, in which the N-H bond points toward the benzene, while the N-methylpyridinium complex prefers a slipped-parallel structure. The benzene-pyridine complex has two nearly isoenergetic (Slipped-parallel and T-shaped) structures.     

Unimolecular and bimolecular transfer of N-substituents from pyridinium cations: Evidence for a clear mechanistic changeover

$\text{N}$-Substituents in 2, 4, 6-triphenylpyridiniums are transferred to piperidine, morpholine and pyridine by unimolecular and/or bimolecular processes in chlorobenzene solution. These processes are quite distinct and afford no evidence for a mechanism intermediate between S_N1 and S_N2.     

Uphill transport of acetate anion through synthetic polymer membranes with pyridinium cations as fixed carrier

Acetate anion was transported against its concentration gradient through a synthetic polymer membrane with pyridinium cation units as fixed carrier. Two synthetic membrane materials were studied, poly(1-butyl-4-vinylpyridinium iodide-co-acrylonitrile) and poly(1-methyl-4-vinylpyridinium iodide-co-acrylonitrile). Acetate anion was transported by an antiport mechanism, with halogen ion transfer as driving force.     

Fluorescence and photodissociation of rhodamine 575 cations in a quadrupole ion trap

The fluorescence and photodissociation of rhodamine 575 cations confined to a quadrupole ion trap are observed during laser irradiation at 488 nm. The kinetics of photodissociation is measured by time-dependent mass spectra and time-dependent fluorescence. The rhodamine ion signal and fluorescence decay are studied as functions of buffer gas pressure, laser fluence, and irradiation time. The decay rates of the ions in the mass spectra agree with decay rates of the fluorescence. Some of the fragment ions also fluoresce and further dissociate. The photodissociation rate is found to depend on the incident laser fluence and buffer gas pressure. The implications of rapid absorption/fluorescence cycling for photodissociation of dye-labeled biomolecular ions under continuous irradiation are discussed.     

Rapid and simultaneous determination of imidazolium and pyridinium ionic liquid cations by ion-pair chromatography using a monolithic column

A method for rapid and simultaneous determination of imidazolium and pyridinium ionic liquid cations by ion-pair chromatography with ultraviolet detection was developed.Chromatographic separations were performed on a reversed-phase silica-based monolithic column using 1-heptanesulfonic acid sodium-acetonitrile as mobile phase.The effects of ion-pair reagent and acetonitrile concentration on retention of the cations were investigated.The retention times of the cations accord with carbon number rule.The method has been successfully applied to the determination of four ionic liquids synthesized by organic chemistry laboratory.     

Amination of the pyridinium cations in betaines. Synthesis of substituted pyrimido[5,6-c]-1′-azaquinolizine and its dihydro analogs

The pyridinium cations within betaines can be aminated under mild conditions with aqueous ammonia, ultimately undergoing cyclization to a new tricyclic system: 4-substituted-3,5-dioxopyrimido[5,6-c]-1-azaquinolizine. Intermediates in the reaction are 4-substituted-3,5-dioxo-2,2-dihydropyrimido[5,6-c]-6H-1-quinolizines.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 677–680, May, 1987.     

Fluorescence anisotropy and FRET studies of G-quadruplex formation in presence of different cations

Results of the steady-state fluorescence, anisotropy and FRET measurements of G-quadruplex formation in the presence of selected cations (Li(+), Na(+), K(+), NEt(4)(+) and Mg(2+)) are reported. Three different fluorescent oligonucleotides with human telomeric sequence labeled with fluorescein (FAM) and tetramethylrhodamine (TAMRA) were investigated: a dual-labeled 21-mer denoted as PSO (Potassium Sensing Oligonucleotide) and two 5'- and 3'- single-labeled probes, FAM-21 and 21-TAMRA, respectively. The fluorescence signal of FAM-21 increased significantly for all systems and the fluorescence enhancement was comparable in magnitude for monovalent cations but it was more pronounced for Mg(2+) cation. This phenomenon was attributed to the protolytic equilibria of FAM affected by the variation in ionic strength. On the other hand, fluorescence of TAMRA was enhanced selectively by Na(I) cation that was explained by the dequenching of TAMRA emission originated from the peculiarity of the basket-type structure of Na(I)-quadruplex. Anisotropy of FAM-21 (but not 21-TAMRA) appeared to be sensitive to the G-quadruplex formation, showing significant increase with an increase in cation concentration and indicating some restrictions in rotational depolarization of FAM. FRET experiments revealed that all tested cations caused quenching of FAM fluorescence in PSO, but only Na(+) and K(+) ions produced sensitized emission of TAMRA acceptor. Higher FRET efficiency observed in the presence of sodium ion was attributed to the specific spectral factor and steric interactions in the basket-type Na(I)-quadruplex.     

Fluorescence spectra of fluorobenzene cations excited in a supersonic molecular beam

Fluorescence spectra of several fluorobenzene cations in the gas phase have been observed following electron impact on a supersonic beam of the neutral molecule. The very low rotational temperature of the beam is not disturbed by the ionisation process, so the different vibronic states of the cation ar produced rotationally cold; the fluorescence spectra are therefore very simple as every vibronic band has been condensed into a very few rotational com Vibrational frequencies obtained from the analyses agree excellently with values from other techniques. The effects of Jahn—Teller distortion are cle in the spectra of C₆F₆⁺ and 1,3,5-C₆F₃H₃⁺.     

High resolution NMR spectroscopy of heteroaromatic cations. I. 13C?1H coupling constants in the pyridinium cation

High-resolution ¹H? {¹⁴N} and proton-coupled natural-abundance ¹³C? {¹⁴N} double resonance spectra have been recorded for pyridinium tetrafluoroborate dissolved in CD₃CN. Iterative analysis of these spectra has provided the accurate values and relative signs of all possible long-range ¹³C—¹H coupling constants. These are compared with the respective values in pyridine and pyridine-N-oxide and discussed in terms of the relationship with the electronegativity of the N-substituent. Experimental conditions allowing the observation of well-resolved proton-coupled ¹³C NMR spectra of charged heteroaromatics are also presented.     

Fluorescence quenching of water-soluble conjugated polymer by metal cations and its application in sensor

The effects of different metal cations on the fluorescence of water-soluble conjugated polymer (CP) and their quenching mechanism have been explored. Most transition metal cations, especially noble metal cations, such as Pd2+, Ru3+, and Pt2+ possessed higher quenching efficiency to CP fluorescence than that of the main group metal cations and other transition metal cations, which have filled or half-full outmost electron layer configurations. Base on this, rapid, sensitive detection of noble metal cations can be realized and a novel quencher-tether-ligand (QTL) probe was developed to detect avidin and streptavidin.     

Luminescence-spectral characteristics and conformational transformations in the electronically excited state of the dimethylaminophenyl derivatives of pyridine and pyridinium and pyrylium cations

The electronic absorption spectra and luminescence-spectral characteristics of the 4-dimethylaminophenyl derivatives of pyridine and pyridinium and pyrylium cations in solutions and polymeric films were studied. The obtained data make it possible to suppose that conformations with the orthogonal arrangement of the donating and accepting fragments (TICT structure) both in the cations and in the neutral molecules arc formed as a result of structural relaxation in the excited S₁ state.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, 625–630, May, 1992     

Fluorescence switching of imidazo[1,5-a]pyridinium ions: pH-sensors with dual emission pathways

Imidazo[1,5-a]pyridinium ions are identified as highly emissive and water-soluble fluorophores accessed by an efficient three-component coupling reaction. Synthetic modifications of groups conjugated to the polyheterocyclic core are shown to profoundly impact the emission properties of these molecules. Notably, two structural isomers of functionalized imidazo[1,5-a]pyridinium ions were found to exhibit distinct de-excitation pathways, which are responsible for either a fluorescence turn-on or ratiometric response to pH change.