The structures of σ-radical cations formed by ionization of adamantane, twistane, noradamantane, cubane, 2,4-dehydroadamantane, and protoadamantane were optimized at the B3LYP, B3LYP-D, M06-2X, B3PW91, and MP2 levels of theory using 6-31G(d), 6-311+G(d,p), 6-311+G(3df,2p), cc-PVDZ, and cc-PVTZ basis sets. On the whole, single-configuration approximations consistently describe the structure and transformations of the examined σ-radical cations. The best correlations (r = 0.97-0.98) between the calculated adiabatic ionization potentials and experimental oxidation (anodic) potentials of hydrocarbons were obtained in terms of B3PW91 approximation.Radical cations are formed as intermediates in a number of important natural processes. They are responsible for repair of damaged DNA [1], antioxidant properties of carotenoids [2], photosynthesis [3,4], and visual process [5]; radical cations are also formed in chemical reactions occurring in atmosphere [6]. Radical cations are generated during important preparative oxidative transformations, and their study is necessary for understanding fundamental reaction mechanisms [7,8]. Detection and structural analysis of hydrocarbon radical cations are limited by the existing physical methods. Recording of their electronic spectra is possible when characteristic absorption bands are intrinsic to radical cations [9]. Magnetic resonance methods are more informative, but their application is limited by short lifetime of radical cation intermediates [10]. Unfortunately, most available methods are almost inapplicable to extremely unstable hydrocarbon σ-radical cations, and only a few examples of such studies have been reported [11].On the other hand, modern computational procedures make it possible to predict the structure and behavior of hydrocarbon σ-radical cations, though the results of theoretical calculations sometimes do not agree with the experimental data because of dynamic effects. For example, DFT quantum-chemical calculations predicted an energy minimum for methane radical cation having C 2v symmetry [12], whereas analysis of the ESR spectrum recorded in neon matrix at 4 K indicated equivalence of all four protons [13].We previously studied the structure and transformations of radical cations derived from adamantane and its alkyl derivatives [14], cubane [15], protoadamantane [16], homoadamantane [17], propellanes [18], and rotanes [19] in terms of the density functional theory (DFT) with the use of popular B3LYP functional. Although in some cases good correlations between the calculated (B3LYP) and experimental data were observed, in particular for diamondoids [20], the problem related to applicability of various theoretical approaches to hydrocarbon σ-radical cations and improvement of their quality remains important. Among other factors, the reason is that standard DFT approximations tend to localize charge and spin density in radical cations [21].Furthermore, during the past 5 years, appropriateness of DFT methods (especially of B3LYP) for the calculation of even simp...
