Th. J. de Boer

C-nitroso compounds—VI : Acyl-alkyl-nitroxides from acyl radicals and nitroso compounds as studied by ESR☆

Abstract Acryl radicals combine with nitroso compounds to give acyl-alkyl-nitroxides, which are identified by their ESR spectra. The acyl radicals are generated by oxidation of aldehydes with suitable hydrogen-acceptors such as t-butoxy radicals or by intermediates obtained int he photochemical and thermal decomposition of nitroso compounds. During photolysis of alkyl nitrites in hydrocarbon solvents the acyl-alkyl-nitroxides appear also and this is explained on the basis of known reactions of alkyl nitrites and their decomposition products.Abstract Acryl radicals combine with nitroso compounds to give acyl-alkyl-nitroxides, which are identified by their ESR spectra. The acyl radicals are generated by oxidation of aldehydes with suitable hydrogen-acceptors such as t-butoxy radicals or by intermediates obtained int he photochemical and thermal decomposition of nitroso compounds. During photolysis of alkyl nitrites in hydrocarbon solvents the acyl-alkyl-nitroxides appear also and this is explained on the basis of known reactions of alkyl nitrites and their decomposition products.

C-nitroso compounds—II : On the photochemical and thermal formation of nitroxides from nitroso compounds, as studied by E.S.R.

Abstract It is demonstrated that nitroxides can be generated by photochemical or thermal decomposition of C-nitroso compounds. The nitroxide-formation takes place in both cases via the nitroso monomer. The influence of the wavelength on the photochemical reaction is discussed.

On the question of one-electron transfer in the mechanism of reduction by nadh-models

Abstract The fluorescence of 1-benzyl-1,4-dihydronicotinamide (BNAH) is quenched by a variety of electron acceptors. The dependence of the rate constant of the quenching process on the electrochemical reduction potentials of the quenchers corresponds with that expected for quenching by an electron transfer mechanism in which BNAH acts as an electron donor with a one electron oxidation potential of 0.76 ± 0.02 V (in acetonitrile relative to the saturated calomel electrode). From this oxidation potential, and the reduction potentials of a number of substrates reported to be reduced by BNAH, the rates of thermal one-electron transfer from BNAH to these substrates were estimated via the Rehm-Weller relation for outersphere one-electron transfer. These calculated rates are many orders of magnitude lower than experimental rates reported for the overall reduction processes. This seems to exclude outersphere one-electron transfer as an intermediate step in such reductions.

Fluorescence of intramolecular electron donor—acceptor systems; the importance of through-bond interaction

Abstract Intramolecular electron donor—acceptor (EDA) systems have been studied in which D and A are separated by three carboncarbon sigma bonds. With D = (di)methoxyphenyl and A = 1,1-dicyanomethylene, such systems are found to show intramolecular charge-transfer (CT) type emission from a polar excited state even when the interconnecting sigma bonds are part of a rigid framework which prevents close approach between D and A. Through-bond interaction is proposed to provide the coupling between the ground state and the polar excited state in these systems.

Studies of inter- and intra-molecular donor-acceptor interactions—II : Intermolecular charge transfer involving substituted pyridinium ions

Abstract The intermolecular charge transfer (CT) interaction of various substituted aromatics and the iodide ion as donors, and substituted pyridinium ions as acceptors was studied. Relatively strong acceptor properties are displayed by the N-methyl-4-cyanopyridinium ion and with this ion a linear relationship is shown to exist between the ionization potential of the donor and the energy of the CT-transition as observed in the absorption spectrum. The complexes between N-methyl-4-cyanopyridinium ion and neutral aromatic organic donors are very weak as concluded from their equilibrium constants; the solvent dependence of the corresponding CT-bands cannot be correlated with solvent polarity parameters. It is shown that the double CT-band pattern of many pyridinium iodides is probably caused by the presence of two closely located vacant molecular orbitais in the pyridinium ion.

Mass spectrometry of aralkyl compounds with a function group—IV : Specific exchange between the α- and Ortho-hydrogen atoms in the molecular ion of γ-phenylpropylbromide☆

Abstract The mass spectrum of γ-phenylpropylbromide has been compared with that of analogues, specifically deuterated in the aliphatic chain and in the Ph ring. It is shown that in the molecular ion a mutual exchange takes place between the H atoms from the α-methylene group and in the ortho -positions of the aromatic ring. Moreover, it is demonstrated that the Br atom as well as an α-H atom are transferred to one of the ortho -positions of the Ph nucleus via a McLafferty rearrangement.

Studies of inter- and intra-molecular donor-acceptor interactions—IV: Intramolecular charge transfer phenomena in substituted N-aralkyl-pyridinium ions

Abstract The absorption spectra of compounds with the general structure D-(CH2)n-A, in which D is a substituted Ph group and A a 4-cyanopyridinium group show long-wavelength absorption not attributable to transitions within the separate chromophores. This absorption at long-wavelength results from an intramolecular Charge Transfer (CT) interaction between D and A. The CT nature of the absorption is proved by the linear relationship between the ionization potential of D and the energy of the CT transition. Furthermore the intramolecular CT is comparable with the intermolecular CT interaction between analogous D and A systems, both in transition energy and solvent dependence. The appearance of an intramolecular CT band in the spectra of compounds with n = 1 can qualitatively be predicted from the character of the highest and one but highest filled molecular orbital in the D part of the molecule. Simple MO calculations indicate that the 4-cyanopyridinium ion has stronger acceptor properties than the 2- and 3-cyano isomers, which agrees with our previous findings,3 for intermolecular CT complexes.

Photophysics of intramolecular electron donor—acceptor systems; N-carbazolyl(CH2)ntetrachlorophthalimide

Abstract In systems N-carbazolyl—(CH 2 ) n —tetrachlorophthalimide ( n = 2, 3, 4, 7) virtually complete quenching of the carbazole fluorescence via an intramolecular electron-transfer mechanism occurs, with concomitant formation of an emissive polar excited state. The rate of formation of this polar excited state is found to exceed considerably those reported for formation of intramolecular exciplex states in other non-conjugated bichromophoric systems with a less pronounced electron donor—acceptor character.

Mass spectrometry of aralkyl compounds with a functional group—III : Specific exchange between some hydrogen atoms in the molecular ion of γ-phenylpropanol☆

Abstract The mass spectrum of γ-phenylpropanol has been compared with that of analogues, specifically deuterated in the aliphatic chain and in the Ph nucleus. It is shown, that in the molecular ion a muual exchange takes place between the H atoms from the OH group, the γ-methylene group and in the ortho -positions of the aromatic ring.

The chemistry of small ring compounds—XIV : Acidity of cyclopropanes with electron-withdrawing substituents

Abstract The hydrogen isotope exchange rates of α-deuterio- and α-tritiocyclopropanes with electronwithdrawing substituents and the corresponding open 2-substituted propanes have been measured in solutions of sodium methoxide in methanol. Substituents fall in two classes: CF3, CN and ∅SO2 make the α-hydrogen in cyclopropanes more acidic than in the open chain analogues. On the other hand acyl groups (t-C4H9CO, iso-C3H7CO, C6H5CO) make the cyclopropanes less acidic than the corresponding 2-substituted propanes. An explanation based on inductive and mesomeric stabilisation of carbanions is given. A large isotope effect has been found for bulky isopropyl t-butyl ketone, suggesting rate limiting hydrogen abstraction by base. In most other cases (Table 2) isotope effects are small, presumably because anions remain strongly associated with methanol by hydrogen bonding (e.g. R− … DOCH3) and consequently solvent exchange (with MeOH) becomes rate limiting for D/H (T/H) exchange.