Reactions of molybdenum and tungsten halides with acetylenic hydrocarbons: an approach to the structure and pathways of formation of metathesis catalysts

A number of acetylene and mixed acetylene-nitrile complexes of tungsten and molybdenum halides have been prepared and investigated. Depending on the acetylene and the reaction conditions, one or more acetylene molecules may be coordinated to the metal. In the latter case acetylenes are coordinated not as single molecules but in systems of conjugated non-aromatic double bonds. On the basis of the supposed structure of such species and on their pyrolysis products a decomposition scheme is proposed, which consider the formation of apparent metathesis products as a side reaction of the cyclotrimerization of acetylenes to aromatic hydrocarbons.     

Absolute configuration of cryptostylines I,II, and III by x-ray analysis and aromatic chirality method

The S-configuration of the alkaloids cryptostylines I, II, and III was assigned on the basis of a single crystal X-ray analysis of unnatural cryptostyline II hydrobromide (5.HBr: orthorhombic, P2₁2₁2₁a = 10·162, b = 12·352, c = 16·456 Å) and confirmed by application of the aromatic chirality method. Further evidence for the configurational assignment was provided by the CD spectra of the monophenolic derivatives of unnatural cryptostyline II (14).     

Acid deuterium exchange in benzazoles

The kinetics of acid deuterium exchange in benzazoles carrying electron-donor substituents in the 5-, 6-, or 7-positions have been studied. Mass spectrometric studies have shown that exchange in 5-methoxy-1, 2-dimethylbenzimidazole takes place exclusively at one position in the benzene ring, in 5-chloro-, 7-chloro-,5-methoxy-2-methylbenzothiazole and 6-methoxy-2-methylbenzoxazole simultaneously in two positions, and in 6-methoxy-2-methylbenzothiazole the hydrogen at all three possible positions is exchangeable. Using quantum chemical reactivity indices (CNDO/2) in dynamic and state approximations, the orientational features of the reaction have been ascertained. The lack of agreement between the reactivities of the most reactive sites to exchange in heteroaromatic bicycles of similar structure and electrophilic localization energies is explained by differences in the energy profile of the reaction.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 679–687, May, 1986.     

Analytical aspects of absorption spectroelectrochemistry at a platinum electrode-I Study of metal ions

An apparatus has been built with which the intensity of a light-beam passing at grazing incidence over a platinum electrode can be monitored. The absorption of light which occurs during the electrolysis of dilute aqueous metal ion solutions has been studied as a function of a number of parameters including wavelength, potential difference and concentration. The theories of metal deposition and processes occurring at the electrode surface and in the diffusion layer have been examined and a mechanism for the production of the absorbing species in terms of increase in pH of the catholyte is proposed. The analytical potential of the technique is discussed.     

Some properties of a microwave boosted glow discharge source using neon as the operating gas

The use of neon as the operating gas for the analysis of aluminium samples with the microwave boosted glow discharge source has been studied. A new type of anode tube allowed the gas to enter the source near the sample surface so that more material was transported into the discharge. Erosion rates have been measured under conditions optimised for high line-to-background ratios and found to be lower than with argon (9 and 21 n/s, respectively). Despite the lower erosion rate the detection limits measured for a number of elements in aluminium are in the range 0.02-1 microg/g and comparable to those obtained with argon as the operating gas.     

Monolayer metal coverage of fullerenes: the optical response

A first-principles investigation of the photoabsorption cross section of the specially stable cluster Li₁₂C₆₀ is carried out, including the icosahedral symmetry of the cluster and treating the ions via ab initio pseudopotentials. The role of the coating metal is assessed by computing the same quantity for the fullerene “seed”. It is found that the main absorption features are determined by the carbon molecule, both at low and high excitation energies, in spite of a reduction of 60% for the ionization threshold of Li₁₂C₆₀ as compared to C₆₀. Nonetheless the lithium coating is responsible for small yet clearly observable effects throughout the spectrum, in particular for a more structured and broad strength distribution at excitation energies below the ionization threshold of C₆₀, and a double peak structure in the Mie resonance at ≈20 eV.     

Prediction of chromatographic parameters for some anilines by molecular connectivity

Summary The possible relation existing between R_F values obtained by thin-layer chromatography for a group of anilines with connectivity indices proposed by Kier and Hall has been studied. Using multivariable regression the corresponding connectivity functions, selected for their respective correlation coefficients, standard deviations, Snedecor's F and Student's t were obtained. Regression analysis of the connectivity functions gives a correct prediction of the experimental elution sequence for this group of substances on silica gel stationary phases and various mobile phases of different polarity. The corresponding random and stability studies of the different prediction models selected were carried out, showing good stability and null randomness in all cases.     

Addition dipolaire-1,3 aux olefines portant une substitution captodative: reactivite comparee des α et des β amino acrylnitriles vis a vis des arylazides.

Arylazides reacted with α amino-acrylonitriles $\text{1}$ to produce 1-aryl-5-amino-triazoles, and with β amino-acrylonitriles $\text{2}$ to give 1-aryl-4-cyanotriazoles. Kinetics showed the Hammett ρ to be > o and therefore, these reactions are controlled by LUMOazide-HOMOolefin interaction. Despite the captodative substitution in $\text{1}$ and $\text{2}$ (ΔΔ E^≠～ 500 cal.M^?1) does'nt agree with a diradical intermediate.     

Detection of traces of formaldehyde in pure air by gas chromatography and helium ionization detection

The sensitive helium ionization detector (HID) was used for the direct determination of ppm to ppb levels of formaldehyde in air. Two methods to generate formaldehyde in an air stream were evaluated. The first method utilized a paraformaldehyde permeation tube and the second utilized a motor driven syringe and a dilute solution of formaldehyde. The two methods were evaluated using gas chromatography with HID and spectrophotometry. The paraformaldehyde permeation tube generates only about 60% of the theoretical value, while the motor driven syringe was accurate for levels below 2 ppm; however, at a concentration of 13 ppm or above, oligomers or other chemical forms of CH2O are formed to decrease the concentration of gaseous CH2O produced.     

Rearrangement of allyl thienyl sulfides to allylthiophenethiols

It is shown that the rearrangement of allyl 2-thienyl and allyl 3-thienyl sulfides in various solvents at 89–136°C gives the corresponding allylthiophenethiols, which can subsequently undergo transallylation with the starting sulfide and cyclization to thienodihydrothiopyrans and methyldihydrothienothiophenes. The energy of activation of the rearrangement of both isomeric sulfides is 19 kcal/mole. The pK_a values of thiophenyl and some thiols of the thiophene series were determined, and it was established that the acidities of the allylthiophenethiols do not have a decisive effect on their ability to undergo cyclization.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 186–189, February, 1978.