Nanosecond transient absorption methods were used to determine accurate oxidation potentials (Eox) in acetonitrile for benzene and a number of its alkyl-substituted derivatives. Eox values were obtained from a combination of equilibrium electron-transfer measurements and electron-transfer kinetics of radical cations produced from pairs of benzene and biphenyl derivatives, with one member of the pair acting as a reference. Using a redox-ladder approach, thermodynamic oxidation potentials were determined for 21 benzene and biphenyl derivatives. Of particular interest, Eox values of 2.48 ± 0.03 and 2.26 ± 0.02 V vs SCE were obtained for benzene and toluene, respectively. Because of a significant increase in solvent stabilization of the radical cations with decreasing alkyl substitution, the difference between ionization and oxidation potentials of benzene is ∼0.5 eV larger than that of hexamethylbenzene. Oxidation potentials of the biphenyl derivatives show an excellent correlation with substituent σ+ values...

Accurate Oxidation Potentials of Benzene and Biphenyl Derivatives via Electron-Transfer Equilibria and Transient Kinetics

Photoelectron spectroscopy (PES) provides valuable information on the ionization energies of atoms and molecules. The ionization energy (IE) is given by the relation. hv = IE + T where hv is t h e energy of the radiation and T i s the kinetic energy of the electron. The IEs are directly related to the orbital energies (Koopmans' theorem). By employing UV radiation (HeI. 21.2 eV. or HeII. 40.8 eV). extensive data on the ionization of valence electrons in organic molecules have been obtained in recent years. These studies of UV photoelectron spectroscopy. originated by Turner, have provided a direct probe into the energy levels of organic molecules. Molecular orbital calculations of various degrees of sophistication are generally employed to make assignments of the PES bands. Analysis of the vibrational structure of PES bands has not only provided structural information on the molecular ions, but has also been of value in band assignments. Dewar and co-workers [1, 2) presented summaries of availabl...

Systematic Organic UV Photoelectron Spectroscopy

Applications of laser ionization/fourier-transform mass spectrometry to the study of metal ions and their clusters in the gas phase

Nitrogen hyperfine coupling constants (hfccs) of organic radicals have been calculated by density functional theory (DFT) methodology. The capability of the B3LYP functional, combined with 6-31G*, TZVP and EPR-III basis sets, to reproduce experimental nitrogen coupling constant data has been analyzed for 109 neutral, cationic and anionic radicals, all of them containing at least one nitrogen atom. The results indicate that the selection of the basis set plays an important role in the accuracy of DFT calculations of hfccs, mainly in relation with the composition of the primitive functions and the quantum number of those functions. The main conclusion obtained is the high reliability of the scheme B3LYP/6-31G* for the prediction of nitrogen hfccs with very low computational cost.

Density functional theory study of 14N isotropic hyperfine coupling constants of organic radicals.

Substituent Effects on 13C Chemical Shifts in Aliphatic Molecular Systems. Dependence on Constitution and Stereochemistry

Single-electron oxidation equilibriums of tetraalkylhydrazines. Comparison of solution E0 values and vapor-phase ionization potentials

Photoelectron spectra of 1,2-dialkyl- 1,2-diazetidines show that the 1,2-dimethyl compound (1) exists overwhelming- ly in the "diequatorial" trans conformation, while a significant amount of the "diaxial" trans conformation is detected for the diisopropyl compound (2), and the tricyclic structure holds the diazetidine ring in nearly eclipsed geometry for 3,4-dimethyl- 3,4-diazatricyclo(4.2.1 .02~5)non-7-ene (4) and its saturated analogue (3). Although N-dimethylaminoazetidine (5), N-dipro- pylaminoazetidine (6), and biazetidine (8) are predominantly in gauche conformations, the anti conformation predominates for N-piperidinylazetidine (7). The rates of double nitrogen inversion for 3 and 4 are nearly identical and increase slightly with increasing solvent polarity, and AG*298 is about 3.8 kcal/mol lower than for 1. The ESR splitting constants for the cations from 1,2, and 8 are reported, and compared to those for other tetraalkylhydrazine radical cations. Although many techniques have been used to study the conformations of tetraalkylhydrazines,2 two of the most useful are NMR and photoelectron (PE) spectroscopy. NMR allows accurate quantitation of the relative amounts of conformations if their interconversion is slow on the NMR time scale, and also determination of the rates for conformational equilibration. NMR does not directly yield information on the dihedral angle between the nitrogen long pairs (e), but the cis or trans dis- position of the N-alkyl groups attached to a ring may be es- tablished. This allows inferring 0 to within several degrees from a knowledge of the CN,NC endocyclic torsional angle allowed by the ring system in bicyclic233 and monocyclic The PE technique measures the vertical ionization potentials for the lone pair combination orbitals. Their separation is rather sensitive to 0, experimentally varying from a maximum of 2.3 eV near 0 = 180 and 0°5,6 to a minimum of about 0.5 eV near 0 = 90O.' The PE time scale is exceedingly short, so even if conformational interconversiuri ;S iuo rapid to be "frozen out" by NMR spectroscopy, the PE spectrum observed is the su- perposition of those for the conformations present. Although the relative amounts of different 0 value conformations thus detected cannot be nearly as accurately quantitated as in NMR experiments, the qualitative predominance of conformations in six-ring hydrazines determined by PE experiments has al- ways agreed with that determined by 13C NMR in solu- tion." Conformational information on tetraalkylhydrazine radical cations has come almost exclusively from ESR studies,* which have established that the barrier for double nitrogen inversion is quite low, and that for some cases in which the planar form is strained, the equilibrium geometry is not planar at nitro- gen.

V. E. Peacock

Papers

Single-electron oxidation equilibriums of tetraalkylhydrazines. Comparison of solution E0 values and vapor-phase ionization potentials

Conformations of four-membered ring hydrazines and hydrazine radical cations

Carbon-13 NMR studies of 9-methyl-9-azabicyclo[3.3.1]nonane and related compounds

Conformations of Four-Membered Ring Hydrazines and Hydrazine Radical Cations

Carbon-13 nmr studies of 9-methyl-9-azabicyclo(3.3.1)nonane and related compounds