Vibronic coupling in ionized organic molecules: structural distortions and chemical reactions

Spectroscopic and computational studies on the rearrangement of ionized [1.1.1]propellane and some of its valence isomers: the key role of vibronic coupling

The [1.1.1]propellane radical cation 1(•+), generated by radiolytic oxidation of the parent compound in argon and Freon matrices at low temperatures, undergoes a spontaneous rearrangement to form the distonic 1,1-dimethyleneallene (or 2-vinylideneallyl) radical cation 3(•+) consisting of an allyl radical substituted at the 2-position by a vinyl cation. In similar matrix studies, it is found that the isomeric dimethylenecyclopropane radical cation 2(•+) also rearranges to 3(•+). The unusual molecular and electronic structure of 3(•+) has been established by the results of ESR, UV-vis, and IR spectroscopic measurements in conjunction with detailed theoretical calculations. Also of particular interest is an NIR photoinduced reaction by which 3(•+) is cleanly converted to the vinylidenecyclopropane radical cation 4(•+), a process that can be represented in terms of a single electron transfer from the allyl radical to the vinyl cation followed by allyl cation cyclization. The specificity of this photochemical reaction provides additional strong chemical evidence for the structure of 3(•+). Theoretical calculations reveal the decisive role of vibronic coupling in shaping the potential energy surfaces on which the observed ring-opening reactions take place. Thus vibronic interaction in 1(•+) mixes the (2)A(1)' ground state, characterized by its "non-bonding" 3a(1)' SOMO, with the (2)E'' first excited state resulting in the destabilization of a lateral C-C bond and the initial formation of the methylenebicyclobutyl radical cation 5(•+). The further rearrangement of 5(•+) to 3(•+) occurs via 2(•+) and proceeds through two additional lateral C-C bond cleavages characterized by transition states of extremely low energy, thereby explaining the absence of identifiable intermediates along the reaction pathway. In these consecutive ring-opening rearrangements, the "non-bonding" bridgehead C-C bond in 1(•+) is conserved and ultimately transformed into a normal bond characterized by a shorter C-C bond length. This work provides strong support for the Heilbronner-Wiberg interpretation of the vibrational structure in the photoelectron spectrum of 1 in terms of vibronic coupling.

Magneto-structural relationships for radical cation and neutral pyridinophane structures with intrabridgehead nitrogen atoms. An integrated experimental and quantum mechanical study

An integrated experimental and computational approach was used to compare the properties of representative molecules containing intrabridgehead nitrogen atoms with those of the corresponding radical cations issuing from one-electron oxidation with the aim of unraveling the characteristics of the three-electron sigma-bonds formed in the open-shell species. From a quantitative point of view, last-generation density functional methods coupled with proper basis sets and, when needed, continuum models for describing bulk solvent effects confirm their reliability for the computation of structures and magnetic properties of organic free radicals. From an interpretative point of view, different hybridizations of nitrogen atoms tuned by their chemical environment lead to markedly different magnetic properties that represent reliable and sensitive probes of structural and electronic characteristics.

Structure and Reactivity of Radical Ions: New Twists on Old Concepts

Electron transfer is the simplest reaction possible, yet it has a profound impact on the structure and reactivity of organic compounds. These changes allow a new look at some of the fundamental concepts that are used to explain organic chemistry, such as symmetry, aromaticity, and bonding. The results from high-level electronic structure calculations are used to analyze the mechanistic differences in the pericyclic reactions of simple hydrocarbons and their radical cation counterparts. The importance of state symmetry correlation, Jahn-Teller distortions, delocalization, and fractional bonding for the reaction pathways of hydrocarbon radical cations is discussed.

Radical Cations from Dicyclopropylidenemethane and Its Octamethyl Derivative

The radical cations of dicyclopropylidenemethane (2) and its octamethyl derivative (2-Me8) are prone to rearrangements into those of (2-methylallylidene)cyclopropane (2a) and its octamethyl derivative (2a-Me8), respectively, by opening one three-membered ring. In contrast to the radical cations of bicyclopropylidene (1) and its octamethyl derivative (1-Me8), 2*+ and 2-Me8*+ are stable to opening of the second ring, because in this case the resulting species would be a non-Kekulé hydrocarbon with a quartet ground state. Similarly to 1, octamethyl substitution in 2 promotes the tendency to rearrangement. Thus, ESR and ENDOR studies indicate that the primary radical cation 2*+, which is formed upon gamma-irradiation of 2 in a CFCl3 matrix at 77 K, does not rearrange up to 150 K. On the other hand, when 2-Me8 is treated in the same way, only the rearranged radical cation 2a-Me8*+ can be observed and characterized by its ESR and ENDOR spectra. Nevertheless, the existence of the two "missing" species, 2a*+ and 2-Me8*+, is revealed by other methods. According to UV and IR studies, X irradiation of 2 in an Ar matrix leads directly to the ring-opened radical cation 2a*+. Moreover, magnetic field effects on the decay of fluorescence, which appears upon recombination of the radical anion of p-terphenyl with a radical cation generated from 2-Me8 in liquid octane, strongly suggest that 2-Me8*+ (and not 2a-Me8*+) is formed initially. From the temperature dependence of the decay, the activation energy of the ring-opening process 2-Me8*+ --> 2a-Me8*+ is estimated. The radical cations 2a*+ and 2a-Me8*+ are formally distonic with the spin residing in the allylic moiety and the charge accommodated on the central carbon atom of the allene pi-system. The intact cyclopropylidenemethylidene moiety assumes a "bisected" conformation, thus favoring an optimal interaction with the positively charged center on the pi-system.

"Electromers" of the tetramethyleneethane radical cation and their nonexistence in the octamethyl derivative: interplay of experiment and theory

Bicyclopropylidene 1a and its octamethyl derivative 1b are subjected to ionization by X-irradiation in solid argon. In accord with previous experiments, this treatment leads to the spontaneous opening of both cyclopropylidene rings, as does ionization of 1b by gamma-irradiation in CFCl(3) at 77 K. The resulting tetramethyleneethane (or bisallyl) radical cations 2a+* and 2b+* are distinguished by a broad band in the NIR. In the case of 2a+*, wavelength-selective photolyses reveal the presence of two interconvertible species with very similar yet distinct spectra. Based on DFT and CASSCF/CASPT2 calculations, these spectra are assigned to two "electromeric" forms of 2a+* which differ in the nature of the singly occupied MO. The NIR bands correspond to charge-resonance transitions between states with fully delocalized spin and charge. Calculations predict that similar electromers should also exist in 2b+* which shows a much weaker NIR band, but no corresponding experimental evidence could be found. On the other hand, the ESR spectrum of 2b+* indicates that, in contrast to 2a+*, the spin is largely localized in one of the two allylic moieties in 2b+*. Although no theoretical method is presently available that would permit an accurate modeling of the opposing factors favoring localized or delocalized structures in molecules such as 2a+* or 2b+*, the observed trends can be satisfactorily rationalized on the basis of semiquantitative considerations. In particular, the important role of vibronic coupling in shaping the potential surfaces for such systems is emphasized.