Synthesis and characterization of linear high molecular weight poly(amino-s-triazine)s

A two-phase polycondensation reaction was used to prepare high molecular weight linear poly-(amino-s-triazine)s from aminodichloro-s-triazines and diamines. These polymers contain a high concentration of melamine-type units in the polymer backbone. Molded specimens exhibited good mechanical properties and resistance to organic solvents. Dimethyl substitution at the exocyclic amino group led to a significant reduction in indentation resistance, strength, and modulus.     

Synthesis of a tetraazacyclotetradecadiene and a pentaazacyclotetradecatriene

Useful procedures for preparing a novel tetraazacyclotetradecadiene and a pentaazacyclotetradecatriene are reported. J. Heterocyclic Chem., 14, 1097 (1977)     

Assessing the efficiency of free energy calculation methods

The efficiencies of two recently developed methods for calculating free energy changes along a generalized coordinate in a system are discussed in the context of other, related approaches. One method is based on Jarzynski's identity [Phys. Rev. Lett. 78, 2690 (1997)]. The second method relies on thermodynamic integration of the average force and is called the adaptive biasing force method [Darve and Pohorille, J. Chem. Phys. 115, 9169 (2001)]. Both methods are designed such that the system evolves along the chosen coordinate(s) without experiencing free energy barriers and they require calculating the instantaneous, unconstrained force acting on this coordinate using the formula derived by Darve and Pohorille. Efficiencies are analyzed by comparing analytical estimates of statistical errors and by considering two numerical examples-internal rotation of hydrated 1,2-dichloroethane and transfer of fluoromethane across a water-hexane interface. The efficiencies of both methods are approximately equal in the first but not in the second case. During transfer of fluoromethane the system is easily driven away from equilibrium and, therefore, the performance of the method based on Jarzynski's identity is poor.     

Phospha-adamantanes as ligands for organopalladium chemistry: aminations of aryl halides

The use of Pd₂dba₃·CHCl₃ and 1,3,5,7-tetramethyl-2,4,8-trioxa-6-phenyl-6-phospha-adamantane has been shown to facilitate the effective amination of aryl halides with aromatic or aliphatic amines in high yields.     

Polyfluorocycloalkenes. Part XVIII. Aryl adducts of decafluorocyclohexene

Decafluorocyclohexene reacted slowly with aniline to give 1-phenylamino- 3-phenyliminoheptafluorocyclohex-2-ene, which was hydrolysed by hydrochloric acid to 3-phenylaminoheptafluorocyclohex-2-enone. Decafluorocyclohexene reacted stepwise with phenyl lithium, giving 1-phenylnonafluorocyclohexene and thence 1,2-diphenyloctafluorocyclohexene: the former product was attacked slowly by pentafluorophenyl lithium at ?40°C affording 1-pentafluorophenyl- 1-phenyloctafluorocyclohexene. Phenyl lithium reacted sluggishly with bis(pentafluorophenyl)octafluorocyclohexene to give 1-pentafluorophenyl-2-(2′,3′,5′,6′-tetrafluoro-1′-biphenylyl)octafluorocyclohexene and 1,2-bis(2′,3′,5′,6′-tetrafluoro-1′-biphenylyl)octafluorocyclohexene. 1,2-Diphenyloctafluorocyclohexene and 1,2-bis(pentafluorophenyl) octafluorocyclohexene were fluorinated by cobalt(III) fluoride to give the olefin, 1,2-bis(undecafluorocyclohexyl)octafluorocyclohexene.     

An approximate potential energy surface for HeI2 collisions

We have calculated the interaction potential for HeI₂ in T-shaped geometries using Hartree—Fock and Møller—Plesset third-order perturbation t     

Chromatographic competitive binding immunoassays: a comparison of the sequential and simultaneous injection methods

Two approaches for performing competitive binding immunoassays by HPLC and other flow-based systems are the simultaneous and sequential injection methods. Both these techniques make use of a column with a limited amount of antibody, onto which is injected a sample and a fixed amount of a labeled analyte analog. An indirect measure of the unlabeled analyte in the sample is then obtained by looking at the amount of analog in either the nonretained or retained peaks. In the simultaneous injection mode, the sample and labeled analog are applied at the same time to the column, while in the sequential mode the sample is injected first, followed by the analog. This results in a difference in the analytical characteristics of these two approaches. This study used chromatographic theory and previous data obtained for injections of human serum albumin (HSA) onto an anti-HSA antibody column to compare the response, detection limits, range, and sensitivity of these methods. Under equivalent conditions, it was found that the sequential method always provided the best lower limit of detection and sensitivity. However, the simultaneous mode had a broader dynamic range and higher upper limit of detection. From these observations, several guidelines were developed regarding the use and selection of such assays for new applications.     

Vibrational overtone spectroscopy of jet-cooled methanol from 5000 to 14 000 cm(-1)

Spectra of jet-cooled methanol in the overtone and combination region from 5000 to 14 000 cm(-1) have been obtained by means of infrared laser-assisted photofragment spectroscopy. Many of the observed features are assigned to combination bands of the type nnu(1)+nu(6), nnu(1)+nu(8), and nnu(1)+nu(6)+nu(8) (n=1,2,3), where nu(1) is the OH stretch, nu(6) is the OH bend, and nu(8) is the CO stretch. These bands show sharp torsion-rotation structure with features as narrow as 0.1 cm(-1). We also observe CH stretch overtones that are weaker than the OH containing combination bands and lack distinct torsion-rotation structure above v(CH)=2. The extent of observed structure on these bands allows us to place limits on the intramolecular vibrational energy redistribution decay rates in the upper vibrational states. We report a global fit of the observed band centers to a simple expression involving low-order anharmonicity constants.     

Efficient dehalogenation of polyhalomethanes and production of strong acids in aqueous environments: water-catalyzed O--H-insertion and HI-elimination reactions of isodiiodomethane (CH2I-I) with water

A combined experimental and theoretical study of the ultraviolet photolysis of CH2I2 in water is reported. Ultraviolet photolysis of low concentrations of CH2I2 in water was experimentally observed to lead to almost complete conversion into CH2(OH)2 and 2HI products. Picosecond time-resolved resonance Raman spectroscopy experiments in mixed water/acetonitrile solvents (25%-75% water) showed that appreciable amounts of isodiiodomethane (CH2I-I) were formed within several picoseconds and the decay of the CH2I-I species became substantially shorter with increasing water concentration, suggesting that CH2I-I may be reacting with water. Ab initio calculations demonstrate the CH2I-I species is able to react readily with water via a water-catalyzed O--H-insertion and HI-elimination reaction followed by its CH2I(OH) product undergoing a further water-catalyzed HI-elimination reaction to make a H2C=O product. These HI-elimination reactions produce the two HI leaving groups observed experimentally and the H2C=O product further reacts with water to produce the other final CH2(OH)2 product observed in the photochemistry experiments. These results suggest that CH2I-I is the species that reacts with water to produce the CH2(OH)2 and 2HI products seen in the photochemistry experiments. The present study demonstrates that ultraviolet photolysis of CH2I2 at low concentration leads to efficient dehalogenation and release of multiple strong acid (HI) leaving groups. Some possible ramifications for the decomposition of polyhalomethanes and halomethanols in aqueous environments as well as the photochemistry of polyhalomethanes in the natural environment are briefly discussed.     

Enaminone‐Based Synthesis of Dipodazine Derivatives

A series of racemic dipodazine analogues 9 were prepared in 22–80% yield from (3Z,6RS)‐3‐[(dimethylamino)methylidene]‐6‐methyl‐1‐(phenylmethyl)piperazine‐2,5‐dione ( 7 ) (Scheme 1), which was prepared in four steps from (RS)‐alanine methyl ester hydrochloride. The preparation of nonracemic 7 from (S)‐alanine methyl ester hydrochloride failed, since the introduction of the enamino functionality at position 3 of the precursor 6 was accompanied by almost complete racemization.