Poly-p-phenylenes (PPs) are prototype systems for understanding the charge transport in π-conjugated polymers. In a combined computational and experimental study, we demonstrate that the smooth evolution of redox and optoelectronic properties of PP cation radicals toward the polymeric limit can be significantly altered by electron-donating iso-alkyl and iso-alkoxy end-capping groups. A multiparabolic model (MPM) developed and validated here rationalizes this unexpected effect by interplay of the two modes of hole stabilization: due to the framework of equivalent p-phenylene units and due to the electron-donating end-capping groups. A symmetric, bell-shaped hole in unsubstituted PPs becomes either slightly skewed and shifted toward an end of the molecule in iso-alkyl-capped PPs or highly deformed and concentrated on a terminal unit in PPs with strongly electron-donating iso-alkoxy capping groups. The MPM shows that the observed linear 1/n evolution of the PP cation radical properties toward the polymer limit originates from the hole stabilization due to the growing chain of p-phenylene units, while shifting of the hole toward electron-donating end-capping groups leads to early breakdown of these 1/n dependencies. These insights, along with the readily applicable and flexible multistate parabolic model, can guide studies of complex donor–spacer–acceptor systems and doped molecular wires to aid the design of the next generation materials for long-range charge transport and photovoltaic applications.

Magic blue (MB+• SbCl6− salt), i.e. tris-4-bromophenylamminium cation radical, is a routinely employed one-electron oxidant that slowly decomposes in solid state upon storage to form so called ‘blues brothers’, which often complicate the quantitative analyses of the oxidation processes. Herein, we disclose the identity of main ‘blues brother’ as the cation radical and dication of tetrakis-(4-bromophenyl)benzidine (TAB) by a combined DFT and experimental approach, including isolation of TAB+• SbCl6− and its X-ray crystallography characterization. The formation of TAB in aged magic blue samples occurs by a Scholl-type coupling of a pair of MB followed by a loss of molecular bromine. This recognition led us to rational design and synthesis of tris(2-bromo-4-tert-butylphenyl)amine, referred to as ‘blues cousin’, (BC: Eox1 = 0.78 V vs Fc/Fc+, λmax(BC+•) = 805 nm, εmax = 9930 cm−1 M−1), whose oxidative dimerization is significantly hampered by positioning the sterically demanding tert-butyl groups at the para-positions of aryl rings. A ready two-step synthesis of BC from triphenylamine and the high stability of its cation radical (BC+•) promises that BC will serve as a ready replacement for MB and oxidant of choice for mechanistic investigations of one-electron transfer processes in organic, inorganic, and organometallic transformations.

We performed a detailed study of the NH + O(2) potential energy surface by means of a number of multireference (CASSCF, MC-QDPT2, MR-AQCC, MR-CISD(18;13)+Q with 6-311+G(d,p), and aug-cc-pVTZ basis sets) and composite (G3B3, G3MP2B3, CBS-QB3, W1U) methods. Parent nitroso oxide, HNOO, was found to be the key intermediate of this process. In its ground state, (1)A', HNOO exists in two conformations, where the cis form is 8.1-10.9 kJ x mol(-1) more stable than the trans-nitroso oxide. The mechanism of nitrene oxidation by dioxygen may be represented as a set of various transformations of vibrationally excited HNOO, namely, decomposition into NO and OH radical pair, O-O dissociation reaction, and a number of thermal deactivation processes. We localized all stationary points of these transformations on both the singlet and the triplet reaction PES. The energies of reactants, products, and transition states were calculated at the RI-MR-CISD(18;13)+Q/aug-cc-pVTZ level of theory; the vibrational analysis of these species was done by means of CASSCF(18;13)/6-311+G(d,p). Apparent rate constants of the NH + O(2) reaction were calculated using RRKM theory. The total rate constant k(total) corresponds well to available experimental data. The temperature dependence of k(total) is rather nontrivial and consists of three quasi-linear intervals. At low temperatures (up to room temperature) the slope of log(k(total)) vs 1/T is negative due to prevailing stabilization of HNOO. The rate-determining channel of the "NH + O(2)" reaction in the medium-temperature interval (up to approximately 1000 K) was found to be formation of the NO + OH radical pair via H transfer to the terminal oxygen atom. This reaction is accelerated by a factor of 4.2 (214 K) and 1.2 (2500 K) due to tunnel effect. The distinctive feature of the NH + O(2) high-temperature chemistry is the increase of the effective activation energy due to prevailing dissociation of the HNOO peroxide bond.

There is currently great interest in S-nitrosothiols (RSNOs) because formation of protein-based RSNOs-protein S-nitrosation-has been recently recognized as a major pathway of the biological function of nitric oxide, NO. Despite the growing number of S-nitrosated proteins identified in vivo, enzymatic processes that control reactions of biological RSNOs are still not well understood. In this article, we use a range of models to computationally demonstrate that specific interactions of RSNOs with charged and polar residues in proteins can result in dramatic modification of RSNO structure, stability, and reactivity. This unprecedented sensitivity of the -SNO group toward interactions with charged species is related to their unusual electronic structure that can be elegantly expressed in terms of antagonistic resonance structures. We propose a 'ligand effect map' (LEM) approach as an efficient way to estimate the environment effects on the -SNO groups in proteins without performing electronic structure calculations. Furthermore, the calculated (15)N NMR signatures of these specific interactions suggest that (15)N NMR spectroscopy can be an effective technique to identify and study these interactions experimentally. Overall, the results of this study suggest that RSNO reactions in vivo should be tightly controlled by the protein environment via modulation of the RSNO electronic structure.