Narrow-band seeding of an optical parametric amplifier in the femtosecond regime

Nearly transform limited femtosecond pulses tunable between 2.56 and 3.16 m have been generated by optical parametric amplification. The single stage parametric amplifier is pumped by a tunable high power femtosecond Ti:sapphire laser system at 1 kHz repetition rate and seeded by quasi-continuous wave (cw) radiation from the Q-switched Nd:YLF laser used to pump the regenerative amplifier. The 100 fs idler pulses are shorter than the pump pulses. The mechanism of the achieved pulse compression is discussed and experimental results are compared with numerical simulations.     

Radiothermoluminescence of polycrystalline high-temperature superconducting YBa2Cu3O6.94, EuBa2Cu3O6.9, and SmBa2Cu3O6.9 samples

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, p. 1934, August, 1991.     

Oxidative coupling of malononitrile with formation of 1,1,2,3,3-pentacyanopropene salts

A simple and convenient route for the synthesis of 1,1,2,3,3-pentacyanopropene salts is reported. These salts are formed by interaction of malononitrile with SeO₂ in presence of organic N-containing bases or pyridinium salts.     

New guanosine derivatives: facile O6-phosphorylation,thiophosphinylation sulfonylation and silylation of guanosine derivatives by 4-dimethylaminopyridine catalized reaction

Appropriately protected guanosine derivatives were successfully converted to the corresponding O⁶-substituted guanosine derivatives by treatment with dialkyl- or diaryl-phosphoryl halides, dialkyl- or diaryl-phosphinothioyl halides, arenesulfonyl chlorides, and trialkylsilyl chlorides.     

Isomerization of perfluorinated cyclic amines under the action of antimony pentafluoride

Conclusions When perfluorinated tertiary cyclic amines containing a substituent at the nitrogen atom are heated with antimony pentafluoride, they isomerize to the corresponding azomethines.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 646–649, March, 1989.     

Transformations of Cyclohexane under the Action of in situ Generated Aluminum Halides

Transformations of inert cyclohexane into dimethyldecalins under mild conditions in the presence of a simple catalytic system consisting of aluminum metal and halomethanes were studied.     

SPECTROCSOPIC STUDIES OF PURINES—III. PROPERTIES OF PURINES SUBSTITUTED AT THE SIXTH AND NINTH POSITIONS*

Abstract— We have investigated the effects of solvent and pH on the absorption and emission propertied of various 6- and 9-substituted purines as a means of examining the nature of the directed not only at deteriming the relative energies of (π,π*) and (π,π*) states, but in particular, at examining the nature and energies of the two lowest-lying (π,π*) states. For examplem, the attachment of a methyl or ribosyl group to the N9 of purine does not change the relative energies of the lowest-lying (π,π*) and (π,π*) states; thus, the latter is the singlet of lowest energy. However, the lowest singles in derivatives formed by substitution at C6 are(π,π*) states—i.e. there is no long-wavelength tail in the absorption spectra and florescence is comparable in intensity to phosphorescence. Further the absence of emission from neutaral adenine at room temperature is due to temperature quenching. Both lowest (π–π*) transtions. are contained under a common envelope and cannot be resolved in the absorption spectra. The shoulder observed in some of these compounds on the long-wavelength slope of absorption envelope is vibrational in nature. Consideration of the effects of solvents on absorption and the analysis of luminescence spectra make it possible to locate the relative position of these (π–π*) transitions within the common envelope. In compounds with an -H on N9 and a free (aza) N1 (e.g. adenine, anionic hupoxanthine) the weaker, solvent-sensitive (W) band has a higher energy then the more intense, solvent-insensitive (S) band. When N1 is protonated, (e.g., in hypoxanthine or cationic adenine) and/or when methyl or ribosyl is substituted at N9, the order of these bands is inverted due ot a red shift of the W band. This shift is most apparent in the 9-substituted hypoxanthines, where in non-polar solvents the W band can be readily resolved in the absoption spectra. This inversion results in a red shift of both fluorescence and phophorescence and an increase of the P/F ratio. When chlorine or iodine is attached at C6 only phosphorescence having a very short lifetime appears due to heavy atom enhancement of single—triplet transitions.     

Carbon-carbon and carbon-nitrogen coupling reactions catalyzed by palladium nanoparticles derived from a palladium substituted Keggin-type polyoxometalate

Palladium 15-20 nm particles stabilized by a Keggin-type polyoxometalate were prepared by reduction of K(5)PPdW(11)O(39) with H(2). The nanoparticles were shown to be effective catalysts for Suzuki-, Heck-, and Stille-type carbon-carbon coupling and carbon-nitrogen coupling reactions of bromoarenes in aqueous media. Chloroarenes were also reactive in reaction media without solvent. [reaction: see text]