Intermolecular Hydrogen Atom Research Articles

ChemInform Abstract: PHOTOCHEMICAL DEUTERATION OF ORTHO‐SUBSTITUTED N‐(DIPHENYLMETHYLENE)ACETAMIDES

AbstractEs findet eine Deuterierung der o‐Alkylgruppe (Methyl und Benzyl als o‐Substituent) bei Bestrahlung in Deuteriomethanol statt.

Photochemical Deuteration of ortho-Substituted N-(Diphenylmethylene)acetamides

Abstract The alkyl hydrogens of N-(2-alkyldiphenylmethylene)acetamides were found to be readily replaced with deuterium on irradiation in deuteriomethanol (CH3OD). It is proposed that the deuteration proceeds via an enamide isomer produced by intramolecular hydrogen transfer from the o-alkyl group to the excited C=N double bond. An intermolecular hydrogen atom abstraction pathway was excluded because neither N-(4-methyldiphenylmethylene)acetamide in deuteriomethanol(CH3OD) nor diphenylmethane in deuteriomethanol(CH3OD) in the presence of N-(diphenylmethylene)acetamide was deuterated. Intramolecular hydrogen abstraction is essential for the deuteration, because no deuteration occurs on irradiation of 2-methylbenzophenone semicarbazone which undergoes no photoreduction. The difference in reactivities of the ortho-substituted imines between in solution and in a rigid matrix is discussed from the viewpoint of syn-anti isomerization.

Amine oxidation. Part VIII. Evidence for intramolecular hydrogen-atom transfer in amine radical cations

Reaction of the N-oxide of R2BuN (R = Me or Bu) with iron(II) ion gives the hydroxy-derivatives R2N·[CH2]2·CH(OH)Me and R2N·[CH2]3·CH2·OH, as well as the parent amine and a secondary amine. Evidence is presented that the hydroxy-compounds are formed from the amine radical cation R2BuN+˙via intramolecular hydrogen-atom transfer which occurs specifically at the terminal carbon of the butyl group; the resulting carbon radical yields the primary alcohol by way of one-electron oxidation, and the secondary alcohol partly by way of one-electron oxidation and intramolecular rearrangement and partly via intermolecular hydrogen-atom abstraction.

Photochemical Reactions of p-Benzoquinone Complexes with Aromatic Molecules

Abstract Electronic absorption spectra and ESR spectra have been measured for the photolyzed charge transfer complexes between p-benzoquinone and aromatic donors in low temperature matrices. When the p-benzoquinone complex with toluene or p-xylene was irradiated at 77°K, an electronic absorption spectrum attributable to the semiquinone radical was obtained together with that of the benzyl-type radical. The spectral shape and position of the former are somewhat different from those of the semiquinone radicals obtained by the photolysis of p-benzoquinone or hydroquinone. These differences indicate the presence of interaction between semiquinone and benzyl-type radicals. The ESR spectra obtained from these systems show that these radicals form pairs in the triplet (S=1) state. It is concluded that the radical pairs are produced as a result of intermolecular hydrogen atom transfer from the aromatic donor molecule to p-benzoquinone in the excited states of the CT complexes formed between them. The irradiations of various p-benzoquinone complexes were carried out in the wavelength regions corresponding to the lowest CT absorption bands at 77°K, and the reactivities were discussed in relation to the electronic structures and geometrical configurations of the CT complexes in the excited states. From the analysis of ESR spectra, the fine structure constant was evaluated for each of the radical pairs, from which the average distance between interacting unpaired electrons was calculated.

Intermolecular Hydrogen Transfer Reactions in the Excited States of EDA-Complexes of Benzophenone with Aromatic Amines at Low Temperature

Abstract Electronic absorption and emission spectra of the mixtures of benzophenone and aromatic amines have been studied in rigid solutions. From absorption spectral measurements, it has been concluded that benzophenone as an acceptor forms the molecular complexes with various aromatic amines as donors. Emission spectra different from those of the component molecules were observed at longer wavelengths in the same systems. The maxima of the anomalous emission spectra of these systems were found to shift slightly to lower frequencies with the decrease in the ionization potentials of donor molecules. On account of their decay times, the emissions may be ascribed to the phosphorescence of benzophenone-aromatic amine complexes. Photolysis of benzophenone-aromatic amine complexes in rigid matrices always resulted in the appearance of the visible absorption spectrum of benzoketyl radical, together with ESR spectra, the analysis of which indicated that there is an interacting pair of radicals which is in the combined triplet (S=1) state. From the evaluation of the zero field splitting constants, the average distances between the interacting radicals were estimated. These results suggest that intermolecular hydrogen atom transfer from aromatic amines to benzophenone occurs, yielding the ketyl and aromatic amine radicals which lie close to each other. The reaction was found to proceed by way of the lowest excited triplet state of benzophenone.

Photochemistry of biological molecules. 3. Mechanism of photodamage of alanine peptides in the solid state.

Abstract— –The structures of the primary radicals formed by 2537 Å photolysis at —196° of alanine peptides of varying size have been determined. The conversion of the primary to stable secondary radicals on heat treatment has been followed in detail by ESR techniques and the chemical change shown to be intermolecular hydrogen atom transfer. The primary photochemical event is shown to be decarboxylation, but photodamage cannot be prevented by conversion of the carboxyl group to an acid derivative.

Methyl group rotation and the low temperature transition in hexamethylbenzene. A neutron diffraction study

Neutron diffraction studies of single crystals of hexamethylbenzene at 298 K and at 130 K indicate that the molecule in phase II has approximate D3d symmetry. The amplitudes of libration of the methyl group and of rigid body motions of the molecule are consistent with earlier data, except that the barrier to methyl group rotation appears to be lower by about 0.5 kcal/mol (2100 J/mol).Consideration of intra- and inter-molecular hydrogen atom contact distances and calculated potential energy curves using a 6-exp potential function suggest that intermolecular forces are important in determining the barrier to rotation of the methyl groups and that substantial changes in the intermolecular packing must be responsible for the lambda-point transition at 116 K and the consequent profound change in the potential barrier to internal rotation which has been previously observed.