David E. Wood

EPR of Free Radicals in an Adamantane Matrix. I. Aliphatic Aminoalkyl Radicals

Aliphatic aminoalkyl radicals RR′Ċ–NR″R″ are found to be produced by x irradiation of aliphatic amines in adamantane. The EPR spectra are isotropic with ∼ 2‐G linewidths for radicals with less than eight heavy atoms. Spectral parameters are obtained for 24 of these radicals by computer simulation of the EPR spectra. A nontwisted anti conformation is found for the geometry of the aminoalkyl radicals by comparison of observed hyperfine splitting constants with those calculated by INDO. The radicals have a substantial stabilization from a three‐electron π bond between the trigonal carbon and the nitrogen. The spin density on carbon is about 34. The stability of the radicals with respect to carbon substitution is found to be H2ĊNH2>RHĊNH2>R2ĊNH2. This ordering, which is the reverse of that found for alkyl radicals, is discussed with respect to the electron withdrawing ability of the substituents. The effects of the substituents on the geometry of the radicals is also discussed.

EPR Study of the Hindered Motion of NO2 and ClO2 Adsorbed in Synthetic Zeolites

The temperature‐dependent EPR spectra of NO2 and ClO2 adsorbed in sodium (NaX) and calcium (CaX) cation type‐X zeolite were investigated. Motional narrowing occurs above 77°K in each case. The time‐dependent spectra of NO2 in CaX zeolite were simulated by linear combinations of stationary‐state solutions of the spin Hamiltonian. The model used to produce these linear combinations required that there be no preferred axis of rotation for NO2 in this system and that rotation greater than ∼ 35° from the equilibrium position be strongly hindered. Although no comparably detailed study has been made for the other systems, it appears that NO2 in NaX also has a high barrier to rotation but that ClO2 in both NaX and CaX zeolite has a greater degree of rotational freedom.

E.P.R. of free radicals in an adamantane matrix: II. The benzyl and azabenzyl radicals

Benzyl radicals and the isoelectronic azabenzyl radicals were prepared in an adamantane matrix and their isotropic E.P.R. spectra observed and analysed. The proton hyperfine splittings were assigned by specific deuteration and by analogy with the benzyl radical. The spin density distribution is strongly affected by non-symmetrical nitrogen substitution, but is only weakly affected by symmetrical substitution. INDO calculations on the radicals predict only small deviations from the benzyl radical spin density distribution for all of the azabenzyl radicals. However, the trends in the experimental values can be rationalized in terms of the electronegativity difference between carbon and nitrogen and its effect on the molecular orbitals of benzyl radical. Furthermore the observed trend in the g values not only lends support to this view of the structure but also allows estimation of the approximate energy of the first nπ* excited states.

An E.P.R. study of group IV B-substituted methyl radicals evidence for d-p π-bonding

Axial g tensors and α-hydrogen hyperfine tensors for frozen (CH3)3MĊH2 radicals (M = C, Si, Ge, Sn and Pb) have been obtained by computer fit to first derivative E.P.R. spectra. Rotation about the carbon-metal bond is found to be rapid at 77°k except for the first in the series, neopentyl radical, which required 140°k for coalescence of the superhyperfine structure from the γ protons. The g ‖ values are constant for the series at 2·0029 (±0·0004), while the g ⊥ vales are 2·0024, 2·0023, 2·0020, 1·9997 and 1·9938 (±0·0002), in order, from M = carbon to lead. The α-hydrogen hyperfine splittings are essentially constant at 21 oe for the series. Previously reported γ-hydrogen hyperfine splittings are reflected in the observed line widths. Their values suggest a change in the sign of the spin density on γ-H in the region of germanium. The contact hyperfine splitting of 207Pb was found to be 158 oe. This measurement makes available the hyperfine splittings of α atoms in planar π radicals over the entire length ...

Tritium Decay Synthesis of Free Radicals. An EPR Study of 2‐Naphthyl Radical in Single‐Crystal Naphthalene‐d8

The 2‐naphthyl radical was prepared by allowing naphthalene‐2‐t to undergo radioactive decay in a single‐crystal naphthalene‐d8 matrix at 77°K. The resulting, oriented 2‐naphthyl radicals were then studied by EPR at 120°K. The parameters in the spin Hamiltonian were determined to be: gT1T3T4x2.004648.136.513.4y2.002465.154.721.8z2.003554.638.918.9, where the hyperfine tensor elements are in megahertz, the x direction is ⊥ to the plane of the radical, the z direction is ∥ to the broken bond (strictly true for g tensor only), and the y direction is in the plane of the radical ⊥ to x and z. These tensor elements are very nearly the same as reported for the 2‐naphthyl radical in argon at 4°K with the exception that the g tensor elements are uniformly larger by ∼ 0.0011 in naphthalene‐d8. Examination of the g shifts from free spin indicates that the unpaired‐electron σ orbital is approximately equally distant from the highest occupied π orbital and the lowest unoccupied π orbital. An INDO calculation based on ...

E.P.R. of free radicals in an adamantane matrix

Benzyl radicals and the isoelectronic azabenzyl radicals were prepared in an adamantane matrix and their isotropic E.P.R. spectra observed and analysed. The proton hyperfine splittings were assigned by specific deuteration and by analogy with the benzyl radical. The spin density distribution is strongly affected by non-symmetrical nitrogen substitution, but is only weakly affected by symmetrical substitution. INDO calculations on the radicals predict only small deviations from the benzyl radical spin density distribution for all of the azabenzyl radicals. However, the trends in the experimental values can be rationalized in terms of the electronegativity difference between carbon and nitrogen and its effect on the molecular orbitals of benzyl radical. Furthermore the observed trend in the g values not only lends support to this view of the structure but also allows estimation of the approximate energy of the first nπ* excited states.

Second-Derivative Line-Sharpening Device for Electron Paramagnetic Resonance

A second-derivative line-sharpening device is described which is capable of reducing the apparent linewidth of an EPR signal by half. The principle of operation is field modulation at odd subharmonics of the second field modulation frequency. Accumulation of the sharpening terms in a CAT is found to greatly improve the signal-to-noise ratio of the sharpened spectra and to reduce the set-up time required.