The vibrational spectra of germane and silane derivatives—XII. A reassignment of the dimethylhalosilanes

The i.r. and Raman spectra of the dimethylhalosilanes, Me₂SiHZ and Me₂SiDX (X = F, Cl, Br, I) have been recorded and the fundamental vibrations assigned. Analysis of the spectra of the deuterated compounds indicate that existing assignments for Me₂SiHX are in error.     

The measurement of derivative i.r. spectra—II. Experimental measurements

The performances of the Butler and Hopkins, and Savitzky and Golay methods, used in conjunction with the Trott and Beynon smoothing technique, for generating derivative i.r. spectra have been assessed experimentally, primarily in the context of peak finding. The test bands used were tert.-butyl benzene (761 cm-⁻¹), 1,2,3-trimethylbenzene (764.5 cm⁻¹) and m-xylene (766.5 cm⁻¹), together with doublets from two component mixtures of these hydrocarbons and a computer-generated doublet from the 1,2,3-trimethylbenzene band. Measurements were made on Perkin-Elmer 577 and 580 spectrometers, with off-line data processing; very satisfactory second and fourth derivative spectra were obtained. Conditions for the optimum compromise between signal to noise ratio and resolution, in terms of the derivatization and smoothing parameters, were established and prove to be similar to those previously deduced from a study of synthetic Lorentzian peak systems. Of the two derivatization techniques the Butler and Hopkins method gives the somewhat superior performance.     

Polymerization of spiro-orthoesters—2. A kinetic approach

The polymerization behaviour of a typical spiro-orthoester, 2-Phenoxymethyl-1,4,6-trioxaspiro(4,4)-nonane, was investigated. The polymerization rate decreases abnormally with conversion; the effect is explained by a mechanism involving deactivation of the active centres by the polymer.     

The i.r. spectroscopy of some highly conjugated systems—I. Rationale of the investigation

The solvent shift method of BELLAMY is re-examined, particularly as a tool for the identification of vibrations that include and are coupled to carbonyl, all of which show solvent sensitivity. For α,β-unsaturated ketones, it is shown that s-cis and s-trans conformers are readily distinguished by plots of νCC vs. νCO. The nature of this coupling is discussed and it is concluded that, in so far as coupling occurs, νCO and νCC take on the character of out-of-phase and in-phase modes respectively. In either conformer, the in-phase (νCC) mode gains in relative intensity as the frequencies converge. While s-cis conformers couple more, conjugation is unaffected by this; the true index of conjugation is neither νCO nor νCC but mean frequency ν_m. The whole of the solvent effect appears to be relayed by carbonyl: even a highly polar carbon double bond is insensitive to solvent unless so coupled.     

The secondary thioamide function—I. The vibrational analysis of NN′ dimethyl-dithiooxamide

The i.r., far i.r. and Raman spectra of CH₃HNCSCSNHCH₃ and the N and C deuterated derivatives are given. The assignments and a complete vibrational analysis has been proposed for the four isotopes.     

The reaction of perfluoroalkanesulfinates——Ⅶ.Fenton reagent-initiated addition of sodium perfluoroalkanesulfinates to alkenes

The addition of sodium perfluoroalkanesulfinates (R_FSO_2Na) to various olefins (CH_2=CHR)initiated by Fenton reagent (Fe~(2+)-H_2O_2) in the presence of sodium azide gave the adduets R_FCH_2-CHN_3R in good yield.A radical mechanism was proposed based on the EPR and other evidences.The adducts were readily reduced through catalytic hydrogenation to give the corresponding aminesR_FCH_2CH(NH_2)R in high yield.The reaction represents a convenient and effective route to theseuseful organofluorine compounds.     

Identification of Indigo Dyes in Painting Layers by Pyrolysis Methylation and Silylation. A Case Study: “The Dinner of Emmaus” by G. Preti

Reactive pyrolysis, under methylating or silylating conditions, in combination with gas chromatography-mass spectrometry (GC-MS) was evaluated as an analytical method for the detection of indigo dyes in painting layers. Samples with the addition of tetramethylammonium hydroxide (TMAH) or hexamethyldisilazane (HMDS), for pyrolysis/methylation and pyrolysis/silylation experiments, respectively, were pyrolysed at 600 °C by means of a heated filament pyrolyser and the evolved products were analysed on-line by GC-MS. Methyl and silylated pyrolytic markers, related to 2-aminobenzoic acid and the indoxyl moiety, were established from the analysis of synthetic indigo, neat or as a pigment of artificial painting layers containing a siccative oil. The occurrence of these markers was investigated in a real sample, taken from the painting The dinner of Emmaus by G.Preti (XVII century), in order to identify the blu pigment. Positive identification of indigo was achieved by pyrolysis/silylation and confirmed by RAMAN spectroscopy.     

The stereochemistry of glycolipids. A key for understanding membrane functions?

A Commentary on the paper ”Studies on liquid‐crystalline glycosides" by Volkmar Vill, Thomas Böcker, Joachim Thiem and Fred Fischer. First published in Liquid Crystals, 6, 349‐356 (1989).     

The effect of γ rays upon monohydrated asparagine. A DSC study

Crystalline non-irradiated and irradiated monohydrated asparagine was investigated by means of DSC. The samples were irradiated at room temperature with gamma radiations using a ¹³⁷Cs source. The exposure doses ranged between 1 and 12 kGy. All samples were scanned in non-sealed pans, from room temperature to a temperature beyond the melting point. The change of registered DSC thermograms for the irradiated samples compared to those of non-irradiated ones and the variation of the parameters associated to dehydration and melting—decomposition with the cumulative dose were analyzed. The study has put in evidence transformations related to the values of characteristic temperatures of peaks and to the thermal effects. The diffusion and desorption of water molecules is hindered by the free radicals formed by irradiation. The most stable free radical species is formed by the extraction of a hydrogen atom from the methylene group of the amino acid chain, done by another hydrogen atom, probably derived from a water molecule.     

n—π electron-donor—acceptor complexes—IV. The interaction of aliphatic amines with 1,3-dicyanobenzene in n-hexane. The importance of preferred orientation in the stability of the complexes

The stability constants (K) for the electron-donor—acceptor (EDA) complexes formed between aliphatic amines as n-donors and 1,3-dicyanobenzene (1,3-DCB) have been determined in n-hexane as solvent. By means of free energy substitutent parameters and regression analysis, the electronic and steric effects of the N-amine substituents were quantitatively separated. Thus, values of log K were correlated with Taft's polar substituents, σ*, and Hancock's corrected steric substituent constants, E^C_s. Large steric effects are inferred which lead us to propose a preferential site of interaction. This preferential orientation may also explain the greater stability of the 1,3-DCB—amine complexes with respect to the corresponding 1,4-DCB—amine complexes. The proposal is supported by the excess of atomic charge predicted by the Mulliken population analysis, obtained by MNDO calculations for the acceptors and by considering that for weak n—π complexes, the electrostatic energy may be of major importance in the energy of interaction.     

SAR—The U.S. Regulatory Perspective

Abstract

Structure-Activity Relationships (SAR) have been used for over a decade by the U.S. EPA's Office of Pollution Prevention and Toxics (OPPT) in their new chemicals program. The development and use of SAR resulted from the need to make rapid risk-based decisions on thousands of new chemicals per year while seldom receiving data on chemical properties, potential exposures, or hazards to humans or organisms in the environment. Qualitative SAR and quantitative SAR methods (QSAR) have been used to fill some of these data gaps by estimating the potential properties and hazards of such chemicals. SAR has been used to assess chemical hazards, identify testing needs, and set priorities. Validation of these SAR assessment tools is an ongoing process.     

The Reaction of β-Aminoenones with Cyanamide. A High Efficient Synthesis of 2-Aminopyrimidines

β-aminoenones react with cyanamide, molar ratio 1:2, to yield 2-aminopyrimidines in nearly quantitative yields.