Showing papers on "Hydrogen atom abstraction published in 1974"

Infrared Studies of Water in Crystalline Hydrates: Location of Hydrogen Atoms and Evidence for a Bifurcated Hydrogen Bond in K2SnCl4•H2O

Abstract: Infrared spectra of polycrystalline K2SnCl4•H2O at different degrees of deuteration were recorded between 4000 and 300 cm−1 at temperatures between 30 and −160 °C. The spectra are consistent with the crystal structure proposed by Kamenar and Grdenic. The water molecules are all equivalent and asymmetric. They are sufficiently well separated from one another for the dynamic coupling of vibrations to be negligible. Both hydrogens of the water molecule participate in hydrogen bonding; one of the hydrogen bonds is weak and bifurcated. Positional parameters of the hydrogen atoms have been calculated using supplementary information from available n.m.r. studies. Infrared spectral characteristics of water molecules engaged in bifurcated hydrogen bonding are discussed.

Aminyle, 7. Über Diarylaminyle

Abstract: Die Diarylaminyle 1b–22b und die heterocyclischen Aminyle 23b und 26b wurden durch thermische oder photolytische Dissoziation der entsprechenden Hydrazine oder durch Wasserstoffabstraktion aus den entsprechenden Aminen erzeugt und wie die entsprechenden Nitroxide 1c–26c ESR-spektroskopisch untersucht. Der Einflus der Substitution auf die Eigenschaften des Diphenylaminyl-Systems und auf das Dissoziationsverhalten der Tetraarylhydrazine wird diskutiert. Aminyls, 7. Diarylaminyls The diarylaminyls 1b–22b and the heterocyclic aminyls 23b and 26b were generated by thermal or photolytic dissociation of the corresponding hydrazines or by hydrogen abstraction of the corresponding amines and studied by e.s.r. as were the corresponding nitroxides 1c–26c. The influence of substitution on the properties of the diphenylaminyl system and on the dissociation of the tetraarylhydrazines is discussed.

Intramolecular catalysis in the mass spectrometer: Mechanisms for loss of methanol from methyl esters upon electron‐impact

Abstract: A review of the literature indicates compelling evidence that: (1) loss of ROH from esters requires protonation of the alkoxy oxygen; (2) the (symmetry forbidden) [1,3] hydrogen migration from protonated carbonyl to alkoxy oxygen does not occur in the mass spectra of esters; (3) hydrogen abstraction in esters occurs almost exclusively to the carbonyl oxygen. Mechanisms are proposed which account for all examples of ROH loss from esters. Alkanol loss from molecular ions in esters requires the presence of a second functional group to act as an intramolecular catalyst, either as a general acid in transferring a proton to the alkoxy oxygen, or as a general base in assisting the [1,3] carbonyl oxygen to alkoxy oxygen proton transfer. Loss of ROH from fragment ions requires proton transfer from an atom α to the positive charge to the alkoxy oxygen. These mechanisms are generalized to include a wide class of bifunctional esters and a selection of natural products.

Further studies on the crystal-violet-sensitized photooxidation of cysteine to cysteic acid.

Abstract: —Crystal violet sensitizes the selective photooxidation of cysteine to cysteic acid; hydrogen peroxide is also formed as an end product. The participation of singlet oxygen in the photoreaction has been ruled out, since exposure of cysteine to this reagent, generated by chemical or photochemical processes, gives only cystine as a product. The photoreaction is inhibited by radical scavengers such as hydroquinone and allylic alcohol. A mechanism is proposed involving hydrogen abstraction by the triplet dye from the thiol group of cysteine.

Photolysis of Aqueous Solutions of Hydrogen Peroxide Containing β-Ammonio Alcohols

Abstract: During the photolysis of aqueous H2O2 added β-ammonio alcohols suffer oxidative attack initiated through hydrogen abstraction by the •OH radical. At pH 7.0 the initially formed radical, undergoes fragmentation to yield and CH3COR′ exclusively. At pH 1.0 fragmentation is less important and direct oxidation to dominates. A radical deprotonation step is proposed to account for the pH dependence.

Photochemistry of o-tolualdehyde: photoenolization and photobenzoin condensation

Abstract: Photolysis of o-tolualdehyde results in the formation of dimeric hemiacetal via the photoenol, or the benzoin via intermolecular hydrogen abstraction.

An application of spin trapping to radical polymerizations

Abstract: Radical polymerizations and copolymerizations were carried out in the presence of phenyl tert-butyl nitrone(PBN) and tert-nitrosobutane (t-BuNO), and the structure of the spin adduct formed was investigated by ESR spectroscopy. PBN adducts gave the same ESR pattern, and the variation of the splitting constant was not large enough to warrant its use for the structure assignment. Therefore, the subsequent trapping experiments were performed with t-BuNO. The polymerization mixture of representative monomers showed esr patterns that are indicative of the propagating radical being trapped. These trapped radicals were not necessarily very stable and, in most cases, disappeared after long reaction periods. In the case of α-methyl substituted monomers, additional nine-line spectra were observed which were attributed to trapping of the radical species formed by hydrogen abstraction from the α-methyl group. The tert-butyl radical which was formed by decomposition of t-BuNo was probably responsible for the hydrogen abstraction. In the case of styrene, methyl acrylate, and methyl methacrylate, characteristic ESR patterns of the propagating radicals were observed with polymers which were prepared in the presence of t-BuNO and purified by reprecipitation. Simultaneous trapping of different propagating radicals was attempted in several copolymerization systems. However, this was generally unsuccessful, because of the large difference in reactivities of the propagating radical with t-BuNo.

The EPR Spectra of Hydrazyl and Some 1-Substituted Hydrazyl Radicals in Solution

Abstract: Hydrogen atom abstraction by tert-butoxy radicals from hydrazine, methyl-, benzyl-and carboethoxy-hydrazine yields the hydrazyl radical and the appropriate 1-substituted hydrazyls, The radical structures are assigned on the basis of their e.p.r. spectral parameters. It has been shown by 15N labelling that in 1-alkylhydrazyls the larger nitrogen splitting is produced by the divalent nitrogen that formally bears the unpaired electron. This result contrasts with the behavior of 2,2-dialkylhydrazyls (1).

Radicals in irradiated monocrystals of the base-pair complex 9-ethyl adenine: 1-methyl uracil.

Abstract: SummaryRadical formation in monocrystals of equimolar complexes of 9-ethyl adenine: 1-methyl uracil by irradiation with X-rays was investigated at 77 K and 300 K using e.s.r.-spectroscopy at 9·5 GHz and 35 GHz. Five different types of spectrum were identified, which corresponded to spectra of the irradiated components. A hydrogen abstraction radical (I and III) and a hydrogen addition radical (II and IV) were assigned to each of the two constituents. The fifth species, with an unidentified singlet after irradiation at 77 K, has also been found in irradiated single crystals of 9-ethyl adenine. By bleaching with U.V.-light or heating to about 150 K, it is decomposed irreversibly. After a crystal irradiated at 77 K is warmed to room temperature only pyrimidine radicals I and II are left. At 77 K the two types of radical (I and II) in 1-methyl uracil are present in addition to the singlet line; at 300 K the addition radicals (II and IV) predominate in comparable proportions, but the abstraction radicals (I an...

E.S.R.-spectroscopy of Radiation-produced Radicals in 9-ethyl Adenine

Abstract: SummaryRadical formation in monocrystals of 9-ethyl adenine by irradiation with X-rays at room temperature has been studied using e.s.r.-spectroscopy at 9·5 GHz and 35 GHz. The two types of spectra found were attributed to hydrogen abstraction and addition radicals. It was possible to separate the correlated spectra and to assign definite structures to the two species by analysing the orientational variations of the spectra. The spectra of the abstraction radical were obtained under conditions of power saturation of the addition radical at reduced temperature (77 K). The site of hydrogen dissociation and of maximum unpaired spin density was localized at the C-10-carbon atom bound to N-9 of the purine. The methyl β-protons give isotropic splittings of 24·0 G each. The abstraction radical could be bleached by illumination with U.V.-light, leaving the addition radical unchanged. Hydrogen addition was concluded on the basis of isotropic hyperfine interactions from two β-protons of 39·5 G each and anisotropic ...