The cyclodimerisation of 3-methyl-2-butenenitrile

3-Methyl-2-butenenitrile (1) cyclodimerised on treatment with lithium diisopropylamide in dimethoxyethane at temperatures between ?78°C and 0°C to 3-amino-4-cyano-1,5,5-trimethyl-1,3-cyclohexadiene (2) the structure of which was established by acid hydrolysis to the known 4-cyano-1,5,5-trimethyl-1-cyclohexene-3-one (3).     

Conical parametric scattering of a single extraordinary beam in BaTiO3 crystal

In this paper, the parametric scattering of a single extraordinary polarized beam of laser in BaTiO₃ photorefractive crystal has been investigated experimentally and theoretically. The resulting pattern consists of beam fanning, isotropic ring, and anisotropic one. Among all parts of scattering pattern, isotropic ring has not been studied as much as beam fanning and anisotropic ring, and there still are some differences in reports about it. Therefore, the study has mainly focused on this part. In this experimental configuration, isotropic ring is just visible in positive angles although the other parts of parametric scattering pattern can be visible from behind and in front of the crystal. In addition to steady state pattern in forward and backward directions, its transient behavior with the rotation of crystal has been studied. The results of experiments have been analyzed carefully, and their theoretical explanations have been presented based on the standard theory of parametric scattering in photorefractive crystals. It has been shown that this configuration corresponds to the so called parametric B-process scattering.     

Experimental implementation of ultrashort laser pulses in the von Neumann picture

The characterization and interpretation of ultrashort laser pulses is most intuitive in the joint time–frequency domain, where structures like multiple pulses become immediately apparent. For practical reasons, ultrafast femtosecond laser pulse shaping, however, is commonly carried out in the frequency domain. Here we implement pulse shaping of optical fields defined in the von Neumann representation, which is a joint time–frequency distribution with complex-valued Gaussian basis functions. We discuss the feasibility as well as the principal limitations of this technique, show illustrative examples, and propose possible applications in coherent control.     

Self-chemical ionization of diethylzinc

The self-chemical ionization of diethylzinc is examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and semiempirical molecular orbital calculations. Electron impact of diethylzinc neutral produces the radical cation, C(4)H(15)Zn(+) (m/z x 122), which reacts further with the neutral (C(2)H(5))(2)Zn to give the following product ions: Zn(+) (m/z x 64), C(2)H(5)Zn(+) (m/z x 93), C(4)H(9)Zn(+) (m/z x 121), C(4)H(11)Zn(2)(+) (m/z x 187), and C(6)H(15)Zn(2)(+) (m/z x 215). To determine the structure and pathways for production of these ions, monoisotopic (12)C(4)H(15)(64)Zn(+), (64)Zn(+) and (12)C(2)H(5)(64)Zn(+) were individually isolated and reacted with the neutral background. We also performed semiempirical molecular orbital calculations (ZINDO/1). The molecular orbital calculations and experimental data are consistent in predicting that the ethyl group on the diethylzinc cation carries the positive charge. Copyright 1999 John Wiley & Sons, Ltd.     

Direct determination of methionine sulfoxide in milk proteins by enzyme hydrolysis/high-performance liquid chromatography

A direct and simultaneous HPLC/UV determination of methionine and methionine sulfoxide in enzyme-hydrolyzed milk proteins is described. Protein hydrolysis is accomplished by a three-enzyme (pronase, leucine aminopeptidase, prolidase) 20-h/37 degrees C digestion. A gradient elution reversed-phase HPLC system with UV detection at 214 nm and 280 nm is then used to determine the quantitative releases of methionine sulfoxide, methionine, tyrosine, and tryptophan. The ease of methionine oxidation by a wide variety of oxidants, coupled with the quantitative release of both methionine and its sulfoxide by the three-enzyme hydrolysis, renders the approach valuable for identifying oxidized milk proteins. The relatively simple method proved accurate and precise in its application to commercial milk products, finding methionine sulfoxide levels as high as 74% of the total methionine.     

怀孕期头发中一些元素含量的变化趋势

用同步辐射激发X射线荧光分析,对11个产妇怀孕期间头发内一些元素含量的变化趋势进行了测量,在各自对比的基础上,得到了一些结果。     

Generation of polarization-shaped ultraviolet femtosecond pulses

We experimentally demonstrate the generation and characterization of polarization-shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 400 nm. Near-infrared laser pulses are first polarization shaped and then frequency doubled in an interferometrically stable setup that employs two perpendicularly oriented nonlinear crystals. A new pulse shaper design involving volume phase holographic gratings reduces losses and hence leads to an increase in pulse energy.     

Product accumulation for ultrasensitive femtochemistry

We present a novel experimental method for studying photochemical reactions that involve permanent products. The accumulation of photoproducts facilitates the measurement of extremely small product yields. A calibration of the setup accounts for diffusion effects, and the experimental results can be expressed in terms of single-pulse photochemical efficiencies. A demonstration experiment on indocyanine green (ICG) is presented. The general method is suited both for femtosecond spectroscopy and quantum control experiments.