Phenoxyl radical (C(6)H(5)O) was prepared photochemically in low-temperature argon matrices. The infrared absorption spectra were obtained for C(6)H(5)O and for the isotopically labeled species C(6)D(5)O and 1-(13)C(12)C(5)H(5)O. All but one IR-active fundamental vibrations were detected, most of them not previously observed. Combination of results from IR linear dichroism measurements on photooriented samples, determination of absolute IR intensities with the help of internal standards, analysis of isotopic shifts, and quantum chemical predictions (B3LYP/cc-pVTZ) led to a detailed assignment of phenoxyl radical vibrations. Significant frequency shifts are observed with respect to previously reported data based on resonance Raman studies in polar solutions. For some vibrations, these shifts reflect environment-induced structural changes, such as increase of the quinoid character of the phenoxyl radical in polar media. In particular, the frequency of the CO stretching vibration, readily observable in both IR and Raman experiments, is extremely sensitive to the environment and can thus be used to probe its polarity.

For the purpose of this review, strong hydrogen bonds have been defined on the basis of experimental data, such as OH stretching wavenumbers, νOH, and OH chemical shifts, δOH (in the latter case, after correction for ring current effects). Limits for O–H···Y systems are taken as 2800 > νOH > 1800 cm−1, and 19 ppm > δOH > 15 ppm. Recent results as well as an account of theoretical advances are presented for a series of important classes of compounds such as β-diketone enols, β-thioxoketone enols, Mannich bases, proton sponges, quinoline N-oxides and diacid anions. The O···O distance has long been used as a parameter for hydrogen bond strength in O–H···O systems. On a broad scale, a correlation between OH stretching wavenumbers and O···O distances is observed, as demonstrated experimentally as well as theoretically, but for substituted β-diketone enols this correlation is relatively weak.

The phenoxyl radical and two of its isotopomers were investigated by UV-VIS and IR polarization spectroscopy of molecular samples immobilized in cryogenic argon matrices. Analysis of the combined electronic and infrared linear dichroism data led to determination of absolute transition moment directions and symmetry assignments for four low-lying excited electronic states. The bands observed at 16 000, 25 200, 33 900, and 41 800 cm−1 were assigned to A12, B12, A12, and B12 π–π* states, respectively. A very weak transition observed in the near-infrared close to 8900 cm−1 was assigned to an optically forbidden B22 n–π* state. The electronic transitions predicted by time dependent density functional theory (TD-UB3LYP/cc-pVTZ) were in good agreement with the observed transitions.

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The algebraic form of the perimeter model for nonaromatic cyclic π-electron systems developed in parts 1−4 of this series is used to analyze the previously reported magnetic circular dichroism (MCD) of biphenylene (1) and its aza analogues, to classify its excited states, and to relate them to those of other nonaromatic cyclic π systems. The observed MCD signs are interpreted in terms of relative sizes of orbital energy differences and the resulting configuration energy ordering. These require deviations from the alternant pairing associated with the simplest classical description, which are attributed to the increased negative magnitude of the diagonal resonance integrals in the four-membered ring. The interpretation of the UV and MCD spectra of 1 is confirmed by the observed effects of aza substitution, and predictions for other types of substitution follow. The magnetic field induced state mixing deduced from the perimeter model is supported by computations by the linear combination of orthogonalized atomic orbitals (LCOAO), time-dependent density functional theory (TD DFT), and symmetry-adapted cluster configuration interaction (SAC-CI) methods.

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The Raman spectrum of phenyl radical (C6H5) isolated in low-temperature argon matrices has been obtained. The assignments of experimentally observed frequencies are based on comparison with results of quantum chemical calculations (B3LYP/cc-pVTZ) and with infrared absorption bands measured for 12C6H5 and 13C6H5 species. Analysis of the IR and Raman spectra, including IR linear dichroism, and of the isotopic shifts caused by 12C → 13C substitution allows to reconfirm most of the previous assignments and to introduce some corrections. The prominent ring breathing mode, characteristic for benzenoid compounds (991 cm-1 in benzene), occurs in phenyl radical at 998 cm-1.

A new analysis of the optical properties of the molecular rotor 1,4-diphenyl-1,3-butadiyne (diphenyl-diacetylene, DPDA) is presented, taking account of the conformational dynamics. The absorption spectra are interpreted in terms of simultaneous contributions from planar as well as non-planar rotamers, characterized by a temperature dependent equilibrium distribution. The investigation is based on IR Linear Dichroism and UV Synchrotron Radiation Linear Dichroism (SRLD) spectroscopy on oriented samples in stretched polyethylene (PE), and on variable temperature UV spectroscopy. The study is supported by the results of detailed quantum chemical Time Dependent Density Functional Theory (TD-DFT) calculations. The resulting analysis has profound implications for the understanding of the optical, photochemical, and photophysical characteristics of this and related chromophores, of importance in a variety of applications.