Photochemical cycloaddition reactions of cyanoacetylene and dicyanoacetylene.

Photolysis of cyanoacetylene with 185- or 206-nm light yields 1,3,5-tricyanobenzene while 254-nm radiation yields a mixture of tetracyanocyclooctatetraenes, 1,2,4- and 1,3,5-tricyanobenzene. A polymer of cyanoacetylene is the major photoproduct. 1,3,5-Tricarbomethoxybenzene was the only photoproduct identified from the irradiation of methyl propiolate at 254 nm. Mono-, di-, and tricyanobenzenes are formed by irradiation of mixtures of acetylene and cyanoacetylene at 185, 206, and 254 nm along with trace amounts of cyclooctatetraenes. No photoadducts were detected on photolysis of mixtures of cyanoacetylene and CO or HCN. The tetracyanocyclooctatetraene structures were established by UV, MS, and NMR analyses. The 1H NMR of the product mixture exhibited a singlet at delta 7.028 consistent with either 1 or 2 and two singlets at delta 6.85 and 6.91 assigned to 3. Photolysis of mixtures of dicyanoacetylene and acetylene with either 185- or 206-nm light yielded 1,2-dicyanobenzene and (E,Z)-1-buten-3-yne-1,4-dicarbonitrile. These products were also obtained using 254-nm light along with a mixture of tetracyanocyclooctatetraenes. The same three singlets were observed in this product mixture as were observed in the tetracyanocyclooctatetraenes obtained from cyanoacetylene. From this observation it was concluded that the delta 7.02 signal is due to 2 and not 1. The photolysis of cyanoacetylene and dicyanoacetylene in the presence of ethylene with 185-nm light yields 1-cyanocylobutene and 1,2-dicyanocyclobutene, respectively. 2-Cyanobutadiene and 2,3-dicyanobutadiene are the photoproducts with 254-nm light. Reaction pathways are proposed to explain these findings.     

A quantum chemical study of the generation of a potential prebiotic compound, cyanoacetaldehyde, and related sulfur containing species

Cyanoacetaldehyde (NCCH 2CHO), which may have played a role in the prebiotic formation of the pyrimidine bases cytosine and uracil, is formed in water solutions by addition of water to cyanoacetylene (HCC-CN), a compound that exists in interstellar space, in comets, and planetary atmospheres. A gas-phase model of the uncatalyzed addition of water to cyanoacetylene is explored by ab initio calculations at the MP2/6-311++G** level of theory. A reaction path consisting of several steps was found in these calculations, but the activation energy of the first step is relatively high, which makes it unlikely that cyanoacetaldehyde is formed in an uncatalyzed reaction. Similar calculations were also performed for the uncatalyzed reaction of water to protonated cyanoacetylene (HCCCNH (+)), a component of the interstellar medium, forming protonated cyanoacetaldehyde (HNCCH 2CHO (+)), but a high activation energy was found for this reaction as well. Moreover, the corresponding addition reactions of hydrogen sulfide (H 2S) to HCCCN, as well as to HCCCNH (+), have been explored with similar results.     

Phenylborylene: direct spectroscopic characterization in inert gas matrices

The photolysis of bisazidophenylborane isolated in cryogenic matrices results in phenylborylene, a subvalent boron(I) species with a singlet ground state. Broad band irradiation of phenylborylene causes formation of benzoborirene by insertion into an ortho-CH bond.     

Photolysis of water/dicyanoacetylene complexes: an infrared matrix isolation and theoretical study

The structure and energy of the 1:1 complexes formed between dicyanoacetylene and water (D₂O) in argon matrix are investigated using FTIR spectroscopy and ab initio calculations at the B3LYP/6-31g** level of theory. Two types of 1:1 complexes are observed. The first one corresponds to the NH structure characterized by a hydrogen bond between H₂O and one of the nitrogen of dicyanoacetylene. The second corresponds to the CO form which involves a van der Waals interaction between the Cβ of dicyanoacetylene and the oxygen of water.

These complexes were irradiated with an Hg–Xe lamp at 10 K. Two products were observed by FTIR spectroscopy. The first one corresponds to the Isonitrile:H₂O complexes formation. The second one, resulting from the reaction of water on the photolyzed dicyanoacetylene, corresponds to the cyanoketene:HCN complex formation.     

Insertion of noble gas atoms into cyanoacetylene: an ab initio and matrix isolation study

A computational and experimental matrix isolation study of insertion of noble gas atoms into cyanoacetylene (HCCCN) is presented. Twelve novel noble gas insertion compounds are found to be kinetically stable at the MP2 level of theory, including four molecules with argon. The first group of the computationally studied molecules belongs to noble gas hydrides (HNgCCCN and HNgCCNC), and we found their stability for Ng = Ar, Kr, and Xe. The HNgCCCN compounds with Kr and Xe have similar stability to that of previously reported HKrCN and HXeCN. The HArCCCN molecule seems to have a weaker H-Ar bond than in the previously identified HArF molecule. The HNgCCNC molecules are less stable than the HNgCCCN isomers for all noble gas atoms. The second group of the computational insertion compounds, HCCNgCN and HCCNgNC, are of a different type, and they also are kinetically stable for Ng = Ar, Kr, and Xe. Our photolysis and annealing experiments with low-temperature cyanoacetylene/Ng (Ng = Ar, Kr, and Xe) matrixes evidence the formation of two noble gas hydrides for Ng = Kr and Xe, with the strongest IR absorption bands at 1492.1 and 1624.5 cm(-1), and two additional absorption modes for each species are found. The computational spectra of HKrCCCN and HXeCCCN fit most closely the experimental data, which is the basis for our assignment. The obtained species absorb at quite similar frequencies as the known HKrCN and HXeCN molecules, which is in agreement with the theoretical predictions. No strong candidates for an Ar compound are observed in the IR absorption spectra. As an important side product of this work, the data obtained in long-term decay of KrHKr+ cations suggest a tentative assignment for the CCCN radical.     

2-pyrazinylnitrene and 4-pyrimidylnitrene. Ring expansion to 1,3,5-triazacyclohepta-1,2,4,6-tetraene and ring opening to (2-Isocyanovinyl)carbodiimide

Tetrazolo[1,5-a]pyrazine/2-azidopyrazine 9T/9A undergo photolysis in Ar matrix at cryogenic temperatures to yield 1,3,5-triazacyclohepta-1,2,4,6-tetraene 21 as the first observable intermediate, and 1-cyanoimidazole 11 and (2-isocyanovinyl)carbodiimide 22 as the final products. The latter tautomerizes to 2-(isocyanovinyl)cyanamide 23 on warming to 40 K. The same intermediate 21 and the same final products are obtained on matrix photolysis of the isomeric tetrazolo[1,5-c]pyrimidine/4-azidopyrimidine 24T/24A. These photolysis results as well as those of the previously reported thermal ring contraction of (15)N-labeled 2-pyrazinyl- and 4-pyrimidylnitrenes to 1-cyanoimidazoles can all be rationalized in terms of selective ring opening of 21 or nitrine 10 to a nitrile ylide zwitterion 28 prior to formation of the final products, 11 and 22. The results are supported by high-level ab initio and DFT calculations (CASPT2-CASSCF(6,6), G3(MP2), and B3LYP/6-31+G) of the energies and IR spectra of the intermediates and products.     

Conformational properties and photochemistry of tetrazolylpyridines in low temperature matrices. Spectroscopic evidence for the photochemical carbon-to-nitrogen rearrangement

The most stable conformers of 2-(tetrazol-1-yl)-, 3-(tetrazol-1-yl)- and 2-(tetrazol-5-yl)pyridines undergo photolysis in Ar matrices at cryogenic temperatures to yield pyridin-2-ylcarbodiimide or pyridin-3-ylcarbodiimide. Spectroscopic evidence of carbon-to-nitrogen rearrangement in the case of the 2-(tetrazol-5-yl)pyridine molecule is provided. For the latter molecule a second pathway leads to the 1-cyclopenta-2,4-dienylketenimine formation. The experimental findings are supported by extensive B3LYP/6-311++G(2d,2p) calculations.     

Persistent Organic High‐Spin Trinitrenes

The septet ground state trinitrenes 1,3,5‐trichloro‐2,4,6‐trinitrenobenzene and 1,3,5‐tribromo‐2,4,6‐trinitrenobenzene were isolated in inert (Ar, Ne, and Xe) as well as reactive matrices (H₂, O₂, and H₂O) at cryogenic temperatures. These trinitrenes were obtained in high yields by UV photolysis of the corresponding triazides and characterized by IR and UV/Vis spectroscopy. The trinitrenes, despite bearing six unpaired electrons, are remarkably unreactive towards molecular oxygen and hydrogen and are persistent in water ice up to 160 K where the water matrix starts to sublime off.     

Acetylenic/cyanoacetylenic complexes: simulation of the Titan’s atmosphere chemistry

The structures and energies of the 1:1 acetylene/cyanoacetylene, acetylene/dicyanoacetylene and cyanoacetylene/dicyanoacetylene complexes in solid argon matrices have been investigated using FT-IR spectroscopy and ab initio calculations, at the B3LYP/6-31G** level of theory. For the three complexes, predicted frequency shifts for the L shaped structures, characterized by a hydrogen bond between the nitrogen of the cyano group and the acetylenic proton, were found to be in good agreement with those experimental. Only in the case of acetylene/cyanoacetylene complex, we obtained a second minimum with a T shaped structure characterized by an interaction between the proton of cyanoacetylene and the Π system of acetylene. It appears clearly that HC₃N acts as an electrophile or as a nucleophile in these complexes.     

Ultraviolet Photolysis and Ortho-Phthalaldehyde-Mercaptoethanol Derivatization of Pharmaceuticals for Enhanced Fluorescence Detection in HPLC

Abstract

Several classes of nitrogenous pharmaceutical were examined for fluorescence after ultraviolet (UV) radiation induced photolysis followed by reaction with o-phthalaldehyde-2-mercaptoethanol (OPA-MERC), and after UV photolysis alone. Photolyses were examined in water, mixtures of methanol/water (1:1), and acetonitrile/water (1:1). Acetone was assessed as a photosensitizer to enhance photolysis and fluorescence response. Flow injection analysis and high-performance liquid chromatographic techniques were used for several pharmaceuticals. The analytes were subjected to UV photolysis and reaction with OPA-MERC reagent for generation of fluorophores that responded to fluorescence detection. During photolysis, solvent type as well as the presence of photosensitizers seem to play a significant role in the formation of primary amines and fluorophores. Photochemical transformation products of some of the pharmaceutical chemicals are proposed. Analytical figures of merit were determined for some analytes. This fluorescence detection approach is applicable for a number of pharmaceuticals at nanogram level.     

Absorption spectra and photolysis of methyl peroxide in liquid and frozen water

Methyl peroxide (CH(3)OOH) is commonly found in atmospheric waters and ices in significant concentrations. It is the simplest organic peroxide and an important precursor to hydroxyl radical. Many studies have examined the photochemical behavior of gaseous CH(3)OOH; however, the photochemistry of liquid and frozen water solutions is poorly understood. We present a series of experiments and theoretical calculations designed to elucidate the photochemical behavior of CH(3)OOH dissolved in liquid water and ice over a range of temperatures. The molar extinction coefficients of aqueous CH(3)OOH are different from the gas phase, and they do not change upon freezing. Between -12 and 43 °C, the quantum yield of CH(3)OOH photolysis is described by the following equation: Φ(T) = exp((-2175 ± 448)1/T) + 7.66 ± 1.56). We use on-the-fly ab initio molecular dynamics simulations to model structures and absorption spectra of a bare CH(3)OOH molecule and a CH(3)OOH molecule immersed inside 20 water molecules at 50, 200, and 220 K. The simulations predict large sensitivity in the absorption spectrum of CH(3)OOH to temperature, with the spectrum narrowing and shifting to the blue under cryogenic conditions because of constrained dihedral motion around the O-O bond. The shift in the absorption spectrum is not observed in the experiment when the CH(3)OOH solution is frozen suggesting that CH(3)OOH remains in a liquid layer between the ice grains. Using the extinction coefficients and photolysis quantum yields obtained in this work, we show that under conditions with low temperatures, in the presence of clouds with a high liquid-water content and large solar zenith angles, the loss of CH(3)OOH by aqueous photolysis is responsible for up to 20% of the total loss of CH(3)OOH due to photolysis. Gas phase photolysis of CH(3)OOH dominates under all other conditions.     

Cyanoacetylene adsorption on amorphous and crystalline water ice films: investigation through matrix isolation and quantum study.

The structure and energy properties of the 1:1 complexes formed between cyanoacetylene and H2O (D2O) are investigated using FT-IR matrix isolation spectroscopy and ab initio calculations at the MP2/ 6-31G(d,p) level. Cyanoacetylene adsorption and desorption on amorphous ice film are monitored by FT-IR using the temperature-programmed desorption method. In an argon matrix, two types of 1:1 complexes are observed. The first one corresponds to the NH structure, which involves a hydrogen bond with the terminal nitrogen of cyanoacetylene. The second corresponds to the HO form, which involves a hydrogen bond from the cyanoacetylene to the oxygen of water. This last complex is the more stable (DeltaE = -8.1 kJ/mol.). As obtained in argon matrixes, two kinds of adsorption site are observed between HC3N and ice. The first one, stable between 25 and 45 K is characterized by a nu(OH) shift similar to the one observed in matrix for the NH complex. The second, stable at higher temperatures (between 45 and 110 K), corresponds to an interaction with the dangling oxygen site of ice and is similar to the HO complex observed in matrix. From theoretical calculations (DFT method combined with a plane wave basis set and ultrasoft pseudopotentials), it is shown that, for this adsorption site, the HC3N moiety is flattened on the ice surface and stabilized by a long-distance interaction ( approximately 3 A) between one dangling OH and the pi system of the C triple bond C triple bond. The HC3N desorption occurs between 110 and 140 K, and the associated desorption energy is 39 kJ/mol. This value is in good agreement with the first principle calculation based on density functional theory and ultrasoft pseudopotentials (34 kJ/mol). These calculations confirm the electrostatic nature of the interaction forces. A small amount of cyanoacetylene is incorporated into the bulk and desorbs at the onset of the ice crystallization near 145 K. In these two kinds of experiments, HC3N acts as both an electrophilic and a nucleophilic molecule.     

Photolysis of polycyclic aromatic hydrocarbons on water and ice surfaces

Laser-induced fluorescence detection was used to measure photolysis rates of anthracene and naphthalene at the air-ice interface, and the kinetics were compared to those observed in water solution and at the air-water interface. Direct photolysis proceeds much more quickly at the air-ice interface than at the air-water interface, whereas indirect photolysis due to the presence of nitrate or hydrogen peroxide appears to be suppressed at the ice surface with respect to the liquid water surface. Both naphthalene and anthracene self-associate readily on the ice surface, but not on the water surface. The increase in photolysis rates observed on ice surfaces is not due to this self-association, however. The wavelength dependence of the photolysis indicates that it is due to absorption by the PAH. No dependence of the rate on temperature is seen, either at the liquid water surface or at the ice surface. Molecular oxygen appears to play a complex role in the photolytic loss mechanism, increasing or decreasing the photolysis rate depending on its concentration.     

Photochemical synthesis of the cyanodiacetylene HC(5)N: a cryogenic matrix experiment

The reactivity of intermolecular complexes cyanoacetylene:acetylene and dicyanoacetylene:acetylene, trapped in solid argon matrixes at 10 K and irradiated with vacuum UV, has been studied. FTIR measurements, together with (2)H, (15)N, (13)C labeling experiments and with density functional theory (DFT) calculations (B3LYP/aug-cc-pVTZ), pointed to the formation of cyanodiacetylene HC(5)N (cyanobutadiyne). This synthetic route is potentially important for chemical models of the Titan's atmosphere.     

Efficient and Practical Route to α‐Aminocarbonylketene and α‐Cyanoketene Dithioacetals

An efficient and practical route to α‐aminocarbonylketene dithioacetals 5 and α‐cyanoketene dithioacetals 6 was developed. With readily available α‐acetyl‐α‐aminocarbonyl ketene dithioacetals 4 as the starting materials, α‐aminocarbonylketene dithioacetals 5 were prepared in high yield via base‐catalyzed (sodium hydroxide) deacetylation. In the presence of POCl₃ and with DMF as the solvent, α‐cyanoketene dithioacetals 6 were obtained via dehydration of 5 in excellent yield.     

Water as a clustering agent in photolysis and photonucleation of benzaldehyde vapor

The role of water vapor in benzaldehyde photolysis and photonucleation is investigated experimentally as well as with the help of semiempirical estimation of reaction intermediates. It is shown that water molecules act as a clustering agent that brings together two benzaldehyde molecules. This mechanism may explain the experimental observation of glyoxal formation during photolysis in humid carrier gas even in the presence of oxygen, the dependence of the concentrations of glyoxal and diphenyl on water-vapor concentration, and some other unusual facts concerning minor products detected in the experiment.     

Comparison of the dehalogenation of polyhalomethanes and production of strong acids in aqueous and salt (NaCl) water environments: Ultraviolet photolysis of CH(2)I(2)

The ultraviolet photolysis of CH(2)I(2) was studied in water and salt water solutions using photochemistry and picosecond time-resolved resonance Raman spectroscopy. Photolysis in both types of environments produces mainly CH(2)(OH)(2) and HI products. However, photolysis of CH(2)I(2) in salt water leads to the formation of different products/intermediates (CH(2)ICl and Cl(2) (-)) not observed in the absence of salt in aqueous solutions. The amount of CH(2)(OH)(2) and HI products appears to decrease after photolysis of CH(2)I(2) in salt water compared to pure water. We briefly discuss possible implications of these results for photolysis of CH(2)I(2) and other polyhalomethanes in sea water and other salt aqueous environments compared to nonsalt water solvated environments.     

Direct observation of an isopolyhalomethane O-H insertion reaction with water: picosecond time-resolved resonance Raman (ps-TR3) study of the isobromoform reaction with water to produce a CHBr2OH product

Picosecond time-resolved resonance Raman (ps-TR3) spectroscopy was used to obtain the first definitive spectroscopic observation of an isopolyhalomethane O-H insertion reaction with water. The ps-TR3 spectra show that isobromoform is produced within several picoseconds after photolysis of CHBr3 and then reacts on the hundreds of picosecond time scale with water to produce a CHBr2OH reaction product. Photolysis of low concentrations of bromoform in aqueous solution resulted in noticeable formation of HBr strong acid. Ab initio calculations show that isobromoform can react with water to produce a CHBr2(OH) O-H insertion reaction product and a HBr leaving group. This is consistent with both the ps-TR3 experiments that observe the reaction of isobromoform with water to form a CHBr2(OH) product and photolysis experiments that show HBr acid formation. We briefly discuss the implications of these results for the phase dependent behavior of polyhalomethane photochemistry in the gas phase versus water solvated environments.     

Novel Bis(2‐cyanoketene‐S,S‐/S,N‐acetals): Versatile Precursors for Novel Bis(aminopyrazole) Derivatives

A synthesis of novel bis(aminopyrazoles) by the reaction of hydrazine hydrate with the appropriate bis(2‐cyanoketene‐S,N‐acetals) was reported. The latter compounds were prepared by treatment of bis(cyanoacetamides) with phenyl isothiocyanate in KOH/EtOH and subsequent alkylation with methyl iodide. The utility of bis(2‐cyanoketene‐S,S‐acetals) as building blocks for novel bis(aminopyrazoles) was also investigated.     

Postcolumn formation of fluorophores from nitrogenous pesticides by UV photolysis

The ultraviolet (UV) photolysis of several classes of nitrogenous pesticides was examined with a view to photo-induced fluorescence detection in flow-injection analysis (FIA) and liquid chromatography. The solvents evaluated as typical reversed-phase mobile phases included water, methanol, and 1:1 mixtures of methanol/water and acetonitrile/water, and methanol/acetonitrilelwater mixtures. Acetone, acetophenone, the surfactant triton X-100, and the photocatalyst titanium dioxide were assessed as photosensitizers to enhance the UV photolysis and fluorescence responses. FIA and liquid chromatographic separations of several pesticides were followed by post-column UV photolysis for the fluorescence detection. Ultraviolet photolysis produces some fluorescent products. The type of photolytic solvent seems to play a significant role. The presence of photosensitizers also affects the fluorescence response of some pesticides. The photochemical transformation products of some of the pesticides are suggested. Analytical figures of merit were evaluated for determination of several pesticides in ground water. The post-column UV photolysis approach for fluorescence detection in liquid chromatography was assessed for several nitrogenous pesticides in ground water samples at ng/g concentrations.