Colour formation in poly(ethylene terephthalate) during melt processing

The discolouration, that occurs in virgin poly(ethylene terephthalate) - PET during melt processing, was studied using various bulk and surface analytical techniques. Proton nuclear magnetic resonance (¹H NMR) was used to study the bulk chemical changes occurring in the polymer during thermo-oxidative degradation. Chemical derivatisation with trifluoroacetic anhydride (TFAA) was used to label the hydroxyl groups introduced on the polymer surface by thermal oxidation.From the surface analysis studies using photoacoustic Fourier transform infrared spectroscopy (PA/FT-IR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) and X-ray photoelectron spectroscopy (XPS) it was evident that colour formation starts initially with the hydroxylation of the terephthalic ring. Further, the formation of additional carbonyl functionalities and conjugated chromophoric systems complete the colour formation process.     

Facile Self‐Assembly of Metallo‐Supramolecular Ring‐in‐Ring and Spiderweb Structures Using Multivalent Terpyridine Ligands

A series of metallo‐supramolecular ring‐in‐ring structures was generated by assembling Cd^II ions and the multivalent terpyridine ligands ( L^1‐3 ) composed of one 60°‐bent and two 120°‐bent bis(terpyridine)s with varying alkyl linker lengths. The mechanistic study for the self‐assembly process excluded an entropically templated pathway and showed that the intramolecularly complexed species is the key intermediate leading to ring‐in‐ring formation. The next‐generation superstructure, a spiderweb, was produced in quantitative yield using the elongated decakis(terpyridine) ligand ( L⁵ ).     

Spectroscopic evidence for mobilization of amide position protons during CID of model peptide ions

Infrared multiple photon dissociation (IRMPD) spectroscopy was used to study formation of b ₂⁺ from nicotinyl-glycine-glycine-methyl ester (NicGGOMe). IRMPD shows that NicGGOMe is protonated at the pyridine ring of the nicotinyl group, and more importantly, that b ₂⁺ from NicGGOMe is not protonated at the oxazolone ring, as would be expected if the species were generated on the conventional b _n ⁺ /y _n ⁺ oxazolone pathway, but at the pyridine ring instead. IRMPD data support a hypothesis that formation of b ₂⁺ from NicGGOMe involves mobilization and transfer of an amide position proton during the fragmentation reaction.     

Ring-chain isomerism in conjugated guanylhydrazones: Experimental and theoretical study

Treatment of 1,3-diaryl-propene-2-one with aminoguanidine under acidic conditions for a short reaction period (1?h) delivers - in accordance with a literature report - the corresponding guanylhydrazones. However, when the reaction time was increased to 12?h, formation of the ring annulated product 4,5-dihydro-1H-pyrazole-1-carboximidamide was observed. This is the first case of ring-chain isomerism in conjugated guanylhydrazones. The acyclic conjugated guanylhydrazone and the corresponding annulated product (4,5-dihydro-1H-pyrazole-1-carboximidamide) could be clearly distinguished by means of UV and NMR spectroscopy. The formation of the ring isomer was further confirmed by single crystal XRD analysis. The time-dependent ¹H NMR study indicated the gradual transformation of the open-chain compound into the cyclic one. The mechanistic insights into the formation of these two products were explored using quantum chemical methods which revealed that the ring isomer is thermodynamically more stable than the open-chain isomer by 6–11?kcal/mol and the barrier for cyclization was found to be 31.37?kcal/mol.     

CPRM: a new method for quantitative detection of peroxide formation during oxygen reduction using the rotating ring-disc electrode technique

A new procedure to detect peroxide formation quantitatively during oxygen reduction using the rotating ring-disc electrode (RRDE) technique is described. The new procedure is called the Cyclic Potential Ring Measurement (CPRM) method. In this technique, the ring electrode is continuously cycled between 850 mV and 1600 mV versus RHE at a sweep rate of 500 . Concurrently, the disc electrode is stepped potentiostatically in the potential region of oxygen reduction. For oxygen reduction on gold, the CPRM technique indicates 100% peroxide formation in the first wave region whereas the conventional RRDE technique indicated only 20 to 30% peroxide formation. The continuous sweeping of the ring electrode in the CPRM technique regenerates continuously a fresh, active ring electrode surface that is less susceptible to interference from low levels of impurities in the solution. Consequently, the CPRM technique provides a more stable, reproducible ring surface for peroxide detection than the conventional technique.To insure that the potential sweeping of the ring does not affect its response adversely, we measured the electrode collection coefficient with a model system (Fe(CN)³⁻₆/Fe(CN)⁴⁻₆), in which low levels of solution impurities would not interfere. The collection coefficient measured using the CPRM and conventional techniques with the model system agreed within ±2.4%.     

A Study on the Intramolecular Mitsunobu Reaction of N6‐(ω‐Hydroxyalkyl)adenines

The cyclization of N ⁶‐(ω‐hydroxyalkyl)adenines with a N⁶H‐group leads to N⁶,N1 ring closure regardless of the method of the cyclization that was used. Five‐membered to eight‐membered rings were obtained using NBS/PPh₃; however, under Mitsunobu conditions, the eight‐membered fused purine was not formed. Surprisingly, the cyclization of N ⁶‐methyl‐N ⁶‐(4‐hydroxybutyl)adenine only leads to N⁶,N7 ring closure using both methods.     

Uracil ring opening in the reaction of 5-formyl-2′-deoxyuridine with primary alkyl amines

Treatment of 5-formyl-2′-deoxyuridine (f⁵dU) with stoichiometric amounts of strongly nucleophilic, non-hindered primary alkyl amines led to fast and quantitative formation of the corresponding Schiff bases. In the presence of excess amines, novel nucleosides with ring opened pyrimidine bases were formed as a result of the Michael addition of a second amine to the pre-formed imines. In the reaction of f⁵dU with aromatic amines, the formation of Schiff base derivatives was slower and even under prolonged treatment with an excess of amine the uracil ring remained intact.     

The Synthesis of Some 9-,10-, and 11-Membered Ring Compounds Containing a Nitrogen Atom: 2,3,4,5,6,7-Hexahydro-1H-3-Benzazonines, 1,2,3,4,5,6,7,8-Octahydro-3-Benzazocines,and 2,3,4,5,6,7,8,9-Octahydro-1H-3-Benzazacy-cloundecanes

In previous papers the syntheses of seven-¹ and eight-membered ^{2a, 2b} ring compounds of the general formulas 5a, b have been described and the nine-membered^2c homolog^5c briefly mentioned. It was of interest to see whether the method of ring formation using an amino acid of type 3 was applicable to the preparation of larger ring systems. In this paper we would like to report the preparation of the bicyclic amines 5 which are the 9-, 10-, and 11-membered ring homologs of this series.     

Mechanism of the cyclization of dimethyl diethynyl silane with selenium tetrabromide: Computational and structural studies, and monitoring

The structure of 2,4-dibromo-2-dibromomethyl-3,3-dimethyl-1-selena-3-silacyclopentene-4, formed by regioselective electrophilic addition of SeBr₄ to dimethyl diethynyl silane, has been determined using X-ray analysis technique. Quantum chemistry methods were used to study elementary stages of the reaction. It was found that the first stage consisted of SeBr₄ conversion into bimolecular complex Br₂?SeBr₂, initiated by dimethyl diethynyl silane. Possible formation of five-membered and six-membered heterocycles involves different cyclization mechanisms. The formation of only five-membered heterocycle is explained by kinetically preferable ring closure through four-center transition state. The conclusions obtained by calculations were confirmed by monitoring of the reaction using ¹H NMR method.     

Organo–Phosphorus–Sulfur Heterocycles by Reactions of Phenylphosphine with Ketones

Abstract

The reaction of phenylphosphine (PhPH₂) with ketones showed conversion into a 5-membered heterocyclic ring of the type P₂SOC if 2–2.5 equivalents of sulfur (S₈) were used. X-Ray data and ³¹P, ¹³C, ¹H NMR spectra proved the formation of the (Z) diastereomer of 1,3,2,4-oxathiadiphospholane 1 ((PhPS)₂SOCMe₂) if acetone is used. Mechanistic studies displayed that small changes in the reaction pathway lead toward the formation and characterization of intermediates including 1,3,2,4,5-dithiatriphospholane, 2,4,5-triphenyl-2-sulfide (PhP)₃S₃ and 1,3,2,4-oxathiadiphospholane, and 5,5-dimethyl-2,4-diphenyl-2-sulfide ((PhPS)(PhP)SOCMe₂), which undergo reactions with acetone, subsequently yielding the abovementioned heterocyclic ring.     

The Driving Force for the Acylation of β-Lactam Antibiotics by L,D-Transpeptidase 2: Quantum Mechanics/Molecular Mechanics (QM/MM) Study

β-lactam antibiotics, which are used to treat infectious diseases, are currently the most widely used class of antibiotics. This study focused on the chemical reactivity of five- and six-membered ring systems attached to the β-lactam ring. The ring strain energy (RSE), force constant (FC) of amide (C−N), acylation transition states and second-order perturbation stabilization energies of 13 basic structural units of β-lactam derivatives were computed using the M06-2X and G3/B3LYP multistep method. In the ring strain calculations, an isodesmic reaction scheme was used to obtain the total energies. RSE is relatively greater in the five-(1a–2c) compared to the six-membered ring systems except for 4b, which gives a RSE that is comparable to five-membered ring lactams. These variations were also observed in the calculated inter-atomic amide bond distances (C−N), which is why the six-membered ring lactams C−N bond are more rigid than those with five-membered ring lactams. The calculated ΔG^# values from the acylation reaction of the lactams (involving the S−H group of the cysteine active residue from L,D transpeptidase 2) revealed a faster rate of C−N cleavage in the five-membered ring lactams especially in the 1–2 derivatives (17.58 kcal mol⁻¹). This observation is also reflected in the calculated amide bond force constant (1.26 mDyn/A) indicating a weaker bond strength, suggesting that electronic factors (electron delocalization) play more of a role on reactivity of the β-lactam ring, than ring strain.     

1H NMR and calorimetric study on binding ability of cyclodextrins to isomeric pyridinecarboxylic acids in aqueous solution

Complex formation of α- and β-cyclodextrins with isomeric pyridinecarboxylic acids (picolinic, nicotinic and isonicotinic acids) in water were studied by calorimetry and ¹H NMR at 298.15 K. The obtained results revealed the weak 1:1 complex formation governed by the cavity dimensions and position of the carboxylic group in the pyridine ring. It was found that selective inclusion complex formation of α- and β-cyclodextrins with nicotinic and isonicotinic acids takes place. In the case of picolinic acid, the considerable role of external interactions and formation of less stable complexes were detected. The location of the carboxylic group in the meta-position of pyridine ring is more favorable for the effective binding. The pyridinecarboxylic acids are shallow inserted into α-cyclodextrin cavity possessing the smaller diameter, while they are deeply included into β-cyclodextrin cavity. Thermodynamic parameters of complex formation (K, Δ_cG⁰, Δ_cH⁰ and Δ_cS⁰) were calculated and discussed in terms of influence of reagents structure.     

Degradation of Cephaloridine on Alkaline Hydrolysis

A kinetic study on the alkaline hydrolysis of cephaloridine ( 1 ) at pH 10.5 and 37° was carried out using ion-pair reversed-phase HPLC. The main resulting degradation products, the 7-epimer 2 of 1 , the Δ²-isomer 3 of 1 , and the 3-methylidene compound 4 were identified. The presence of a pyridinio group at C(3¹) results in a slightly increased formation constant for the 3-methylidene compound 4 and the 7-epimer 2, and introduces a new reaction: the isomerization of the double bond at C(3) in the dihydrothiazine ring to C(2).     

Synthesis and characterization of novel bisphthalonitrile resins linked by different molecular weight main-chain polybenzoxazines

A series of novel type bisphthalonitriles with different molecular weight main-chain polybenzoxazines as linkages have been successfully synthesized using 4, 4′-diaminodiphenyl methane, paraformaldehyde, bisphenol A and 4-nitrophthalonitrile as initial materials. The structures were characterized by Fourier transform infrared (FT-IR) and proton nuclear magnetic resonance (¹H-NMR). The formation of benzoxazine and the existence of nitrile groups were confirmed by the absorbance at 950cm^?1 of benzene attached with oxazine ring and 2231 cm^?1 of nitrile groups. The characteristic resonance peaks observed at about 4.52 (C-CH₂-N) and 5.28 ppm (N-CH₂-O) also determined the structure of benzoxazine ring. The curing behaviors were monitored by differential scanning calorimetry (DSC) and FT-IR. Two-stage polymerization mechanisms were observed according to the ring-opening of benzoxazine and the polymerization of nitrile groups catalyzed by phenolic hydroxyl groups, which generated during the curing reaction of benzoxazine. The polymerization of these bisphthalonitriles exhibited self-promoted curing behaviors. The completion of polymerization was proved by the disappearance of the band located at 950 cm^?1 in FT-IR. Thermogravimetric analysis (TGA) was used to investigate the thermal stability, and the results showed that the cured polymers achieved extremely high char yield from 61.1% up to 74.2% at 800°C under nitrogen and exhibited increasing decomposition temperature as the contents of phthalonitrile groups increased, which indicated that the polymerization of phthalonitriles could improve the thermal stability.     

Protonation and complex formation of the cis and trans isomers of alicyclic β-amino acids

The protonation constants of β-alanine and the cis and trans isomers of aliphatic 2-amino-1-carboxylic acids containing a cyclohexane, cyclopentane or cyclohexene(4) ring were determined pH-metrically, as were the formation constants of their complexes with Ni²⁺, Cu²⁺, Hg²⁺ and Cd²⁺ ions. The analysis of about 70 formation constants revealed significant differences betweeen the behaviours of the different ligands, each of which has a fixed β-alanine conformation. It is concluded that complex formation is primarily determined by the nature of the fixed β-alanine conformation and in some cases by the type of aliphatic ring.     

Synthesis and Characterization of 5‐(1,2,4‐Triazol‐3‐yl)tetrazoles with Various Energetic Functionalities

In this contribution the synthesis and full structural as well as spectroscopic characterization of three 5‐(1,2,4‐triazol‐3‐yl)tetrazoles along with selected energetic moieties like nitro, nitrimino, and azido groups are presented. The main goal is a comparative study on the influence of those variable energetic moieties on structural and energetic properties. A complete characterization including IR and Raman as well as multinuclear NMR spectroscopy of all compounds is presented. Additionally, X‐ray crystallographic measurements were performed and reveal insights into structural characteristics as well as inter‐ and intramolecular interactions. The standard enthalpies of formation were calculated for all compounds at the CBS‐4M level of theory and reveal high positive heats of formation for all compounds. The calculated detonation parameters (using the EXPLO5.05 program) are in the range of 8000 m s^?1 (8097 m s^?1 ( 5 ), 8020 m s^?1 ( 6 ), 7874 m s^?1 ( 7 )). As expected, the measured impact and friction sensitivities as well as decomposition temperatures strongly depend on the energetic moiety at the triazole ring. The C? C connection of a triazole ring with its opportunity to introduce a large variety of energetic moieties and a tetrazole ring, implying a large energy content, leads to the selective synthesis of primary and secondary explosives.     

Lactone ring formation in poly(α‐hydroxyacrylic acid)

Lactone ring formation in poly(α‐hydroxyacrylic acid) (PHA) was investigated by means of ¹H and ¹³C NMR. The stereoregularity of PHA was estimated as syndio‐rich (ca. 86%) on the basis of methylene proton peak of the sodium salt, PHANa, in aqueous solution. On the other hand, analyses of the ¹³C NMR spectra obtained as a function of the degree of dissocation (α) suggested that the lactone ring is formed at every other residue at half dissociation (α = 0.5). This specific way of lactone formation supposed for PHA is discussed in relation to the critical change in the dissociation behavior of the polyacid which has been found at α = 0.5. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1400–1405, 2002     

Steric effects on controlling of photoinduced electron transfer action of anthracene modified benzo-15-crown-5 by complexation with Mg and Ca

Benzo-crown ether based chemosensors linked by an amide bond at the 1-, 2- or 9-positions of anthracene rings were synthesized. Their complexation behavior with alkaline earth metal ions in acetonitrile was investigated using fluorescence, UV, and ¹H NMR spectroscopy. In the absence of a metal ion, all compounds showed only very slight fluorescence emissions (fluorescence ‘off’ state) because of intramolecular charge/electron transfer process. After the complex formation with Mg²⁺ and Ca²⁺, however, only the 2-position analogue gave a fluorescence ‘on’ response by inhibiting the photoinduced electron transfer. Because 2-positioned anthracene was free from steric hindrance of the crown ether ring, a strongly bent complex structure was formed with Mg²⁺ and Ca²⁺, which induced a breakdown of π-conjugation between the amide moiety and the benzene ring.     

Nitrogen-15 studies of the mechanisms of acetolyses of hexamethylenetetramine and 3,7-diacetyl-1,3,5,7-tetraazabicyclo[3.3.1]nonane (DAPT)

[¹⁵N₄]-Hexamethylenetetramine (Hexamine), and [¹⁵N₄]-3,7-diacetyl-1,3,5,7-tetraazabicyclo[3.3.1]nonane(DAPT) have been prepared starting from ¹⁵NH₃. Synthetic acetolysis reactions were performed using mixtures of pure [¹⁵N₄]- and [¹⁴N₄]-compounds and the destination of the nitrogen isotopes in the products was determined mass spectrometrically. The results show that relatively little isotopic mixing occurs in the acetolysis of hexamine to DAPT though the formation of some products with isotopic composition [¹⁴N₃¹⁵N₁] and [¹⁴N₁¹⁵N₃] indicates limited ring cleavage. However the more severe conditions used in the formation of 1,3,5-triacetyl-1,3,5-triazacyclohexane (TRAT) give rise to considerable isotopic scrambling. The acetolysis of DAPT to give 1,3,5,7-tetraacetyl-1,3,5,7-tetraazacyclooctane occurs by selective cleavage of the methylene bridge.     

Polysiloxane‐Backbone Block Copolymers in a One‐Pot Synthesis: A Silicone Platform for Facile Functionalization

Block copolymers consisting exclusively of a silicon–oxygen backbone are synthesized by sequential anionic ring‐opening polymerization of different cyclic siloxane monomers. After formation of a poly(dimethylsiloxane) (PDMS) block by butyllithium‐initiated polymerization of D3, a functional second block is generated by subsequent addition of tetramethyl tetravinyl cyclotetrasiloxane (D4^V), resulting in diblock copolymers comprised a simple PDMS block and a functional poly(methylvinylsiloxane) (PMVS) block. Polymers of varying block length ratios were obtained and characterized. The vinyl groups of the second block can be easily modified with a variety of side chains using hydrosilylation chemistry to attach compounds with Si—H bond. Conversion of the hydrosilylation used for polymer modification was investigated.