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Notes: The vibrational and electronic structures of p-phenylenediamine radical cation have been studied using time-resolved resonance Raman spectroscopy and molecular orbital calculations. The Raman spectra in aqueous solution show striking variations in intensity profile when excitation is varied from 340 to 480 nm. Excitation in resonance with the 460–480 nm absorption shows enhancement of only totally symmetric vibrations. Most prominent are the δNH2 scissor mode at 1658 cm−1 and the Wilson modes ν8a (CC stretch) at 1644 cm−1, ν7a (CN stretch) at 1423 cm−1, and ν9a (CH bend) at 1184 cm−1. Also observed are ν1 (ring breathe) at 840 cm−1 and ν6a (CCC bend) at 467 cm−1. With excitation in the 340–410 nm region the ν1 band becomes relatively stronger and an additional band at 1524 cm−1 appears. This latter band, which dominates the spectrum at 360 nm, is assigned to the nontotally symmetric vibration ν8b (CC stretch) having b3g symmetry that gains intensity through vibronic coupling. Raman spectra of ring- and amine-deuterated radicals allow distinction of overtone and combination bands enhanced by Fermi resonance from fundamentals due to δNH2 and ν8a (CC stretch) modes in the 1600–1700 cm−1 region. The theoretical calculations show reasonable agreement with the observed vibrational frequencies and lead to detailed information on the bond structure and normal modes of the radical. The calculated CN bond lengths of 1.33 A(ring) are intermediate between lengths typical of single and double bonds in aromatic amines. The calculations also allow interpretation of the experimental absorption spectrum. The broad medium intensity absorption peaked at 460 nm with a shoulder at 480 nm is assigned to a 2B3u ←2B2g transition, the very weak 360 nm absorption to 2Au ← 2B2g and the very strong ∼325 nm absorption to a higher 2B3u ← 2B2g.Calculations with the simple relation Ik∝ (V'k)2/ωk, which requires evaluation only of vertical excited state gradients, are found to satisfactorily describe the dramatic changes in Raman intensity profile that are observed with different excitation wavelengths. The 460 and 480 nm Raman spectra are governed by scattering from the lowest 2B3u state. Spectra taken in the 410–340 nm range are governed mainly by preresonance scattering from the higher 2B3u state, which is vibronically coupled to the weak 2Au state via the b3g symmetry ν8b (CC stretch) vibration. The relative intensities of totally symmetric modes calculated for excitation into the two 2B3u excited states agree well with those in the two distinct patterns observed experimentally.

Notes: The spin distribution of the neutral soliton in trans-polyacetylene is controversial, as evidenced by the markedly different interpretations given by the various active experimental groups to their ENDOR measurements of proton hyperfine coupling (hfc) constants. To help resolve the discrepancies, ab initio electronic structure calculations were carried out for proton and carbon-13 hfc on the series of model polyene radicals C5H7,C9H11,...,C21H23 and the most important characteristics of the spin distribution were extrapolated to the long chain limit. Here σ spin polarization was treated through single excitation configuration interaction, both with respect to Hartree–Fock and to multiconfiguration reference spaces that include the most significant effects of π electron correlation. Particular attention was paid to basis set requirements necessary for proper treatment of hfc. It turns out that the ab initio hfc does not agree with that obtained by any of the major experimental groups. In particular, the calculated width of the bell-shaped distribution for the central and other even proton sites is much narrower than all experimental reports, and the anisotropic coupling calculated for the central proton along the fibril axis is much larger. Several assumptions commonly made in analyzing the ENDOR spectra are shown not to be quantitatively justifiable, but their improvement is unlikely to resolve the large discrepancies we have found between theory and current experimental interpretations. Thus, it appears likely that the experimental spectra are significantly influenced by environmental effects and cannot be adequately modeled by an isolated long chain polyene radical as heretofore assumed.

Notes: Spin densities that determine hyperfine splitting constants are calculated for ethyl radical from ab initio electronic wave functions. The most important direct and spin polarization contributions are obtained from single-excitation configuration interaction wave functions, in conjunction with recently developed contracted Gaussian basis sets designed specifically for spin density determination. Facile out-of-plane bending at the α-carbon center leads to a significant vibrational correction and temperature dependence for the α-carbon splitting. Coupling of torsion about the CC bond with bending at the α-carbon is found to have only a small effect on the hyperfine constants. For isotropic Fermi contact interactions, agreement with experiment is better than 10% for both carbons and for the α-hydrogens. The larger 28% error found for the β-hydrogens is attributed primarily to the missing effects of electron correlation. Anisotropic dipolar hyperfine constants are also evaluated and agree well with experiment wherever comparison is possible.