Role of hyperconjugation in the 1,2-shift reactivity of bicyclo[2.1.0]pentane and cyclopropane radical cations: a computational study.

Hyperconjugation and its relationship with the 1,2-shift rearrangement reactivity in bicyclo[2.1.0]pentane and cyclopropane radical cations have been studied with density functional theory (PBE0/6-311G**). Hyperconjugation has been evaluated by calculating the (1)H hyperfine coupling constants, atomic spin densities, and dihedral angles of ß hydrogens with respect to the axes of the nearest p-orbitals bearing the main part of the localized spin density. The calculated hyperfine couplings are in good agreement with the experimental values, and the calculated couplings and angles satisfy the Heller-McConnell relationship, which validates our approach to measure hyperconjugation. Significantly, it is the endo ß-hydrogen on the single methylene bridge of the housanes 1a, 1b, and 1d that has the largest hyperconjugative interaction, and this is also the migrating hydrogen in the 1,2-shift reaction leading to the rearrangement of these housanes to cyclopentene radical cations. As a result of this stereoelectronic preference, the migrating entity from the methylene bridge is the endo rather than the exo bond, irrespective of the nature of the substituent. Accordingly, for the 1a-1d housanes, the key role of hyperconjugation lowers the endo C-H or C-Me bond strength selectively, and thereby assists the preferred sigmatropic migration of the endo substituent to the bridgehead carbon. By comparison, the extent of hyperconjugation is found to be much reduced in the cyclopropane radical cations 2a-2d, and the latter species do not undergo the corresponding 1,2-shift rearrangement reaction. This absence of reactivity in 2a-2d is therefore attributed to the weaker hyperconjugative interaction as well as to the less favorable energetics for the overall reaction.

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.

Structures of the hexafluorocyclopropane, octafluorocyclobutane, and decafluorocyclopentane radical anions probed by experimental and computational studies of anisotropic electron spin resonance (ESR) spectra.

Anisotropic electron spin resonance (ESR) spectra are reported for the radical anions of hexafluorocyclopropane (c-C(3)F(6)(-)), octafluorocyclobutane (c-C(4)F(8)(-)), and decafluorocyclopentane (c-C(5)F(10)(-)) generated via gamma-irradiation in plastically crystalline tetramethylsilane (TMS) and rigid 2-methyltetrahydrofuran (MTHF) matrices. By combining the analysis of these experimental ESR spectra involving anisotropic hyperfine (hf) couplings with a series of quantum chemical computations, the geometrical and electronic structure of these unusual perfluorocycloalkane radical anions have been characterized more fully than in previous studies that considered only the isotropic couplings. Unrestricted Hartree-Fock (UHF) computations with the 6-311+G(d,p) basis set predict planar ring structures for all three radical anions, the ground electronic states being (2)A(2)(") for c-C(3)F(6)(-) (D(3h) symmetry), (2)A(2u) for c-C(4)F(8)(-) (D(4h)), and (2)A(2)(") for c-C(5)F(10)(-) (D(5h)), in which the respective six, eight, and ten 19F-atoms are equivalent by symmetry. A successful test of the theoretical computation is indicated by the fact that the isotropic 19F hf couplings computed by the B3LYP method with the 6-311+G(2df,p) basis set for the optimized geometries are in almost perfect agreement with the experimental values: viz., 19.8 mT (exp) vs 19.78 mT (calc) for c-C(3)F(6)(-); 14.85 mT (exp) vs 14.84 mT (calc) for c-C(4)F(8)(-); 11.6 mT (exp) vs 11.65 mT (calc) for c-C(5)F(10)(-). Consequently, the same computation method has been applied to calculate the almost axially symmetric anisotropic 19F hf couplings for the magnetically equivalent 19F atoms: (-4.90 mT, -4.84 mT, 9.75 mT) for c-C(3)F(6), (-3.54 mT, -3.48 mT, 7.02 mT) for c-C(4)F(8)(-), and (-2.62 mT, -2.56 mT, 5.18 mT) for c-C(5)F(10)(-). ESR spectral simulations performed using the computed principal values of the hf couplings and the spatial orientations of the 19F nuclei as input parameters reveal an excellent fit to the experimental anisotropic ESR spectra of c-C(3)F(6)(-), c-C(4)F(8)(-), and c-C(5)F(10)(-), thereby providing a convincing proof of the highly symmetric D(nh) structures that are predicted for these negative ions. Furthermore, using the computed 19F principal values and their orientations, the effective 19F anisotropic hf couplings along the molecular symmetry axes were evaluated for c-C(3)F(6)(-) and c-C(4)F(8)(-) and successfully correlated with the positions of the characteristic outermost features in both the experimental and calculated anisotropic spectra. In addition, the electronic excitation energies and oscillator strengths for the c-C(3)F(6)(-) , c-C(4)F(8)(-), and c-C(5)F(10)(-) radical anions were computed for the first time using time-dependent density functional theory (TD-DFT) methods.

EPR studies of amine radical cations. Part 2. Thermal and photo-induced rearrangements of propargylamine and allylamine radical cations in low-temperature freon matrices.

Matrix EPR studies and quantum chemical calculations have been used to characterize the consecutive H-atom shifts undergone by the nitrogen-centered parent radical cations of propargylamine (1b*+) and allylamine (5*+) on thermal or photoinduced activation. The radical cation rearrangements of these unsaturated parent amines occur initially by a 1,2 H-atom shift from C1 to C2 with pi-bond formation at the positively charged nitrogen; this is followed by a consecutive reaction involving a second H-atom shift from C2 to C3. Thus, exposure to red light (lambda > 650 nm) converts 1b*+ to the vinyl-type distonic radical cation 2*+ which in turn is transformed on further photolysis with blue-green light (lambda approximately 400-600 nm) to the allene-type heteroallylic radical cation 3*+. Calculations show that the energy ordering is 1b*+ > 2*+ > 3*+, so that the consecutive H-atom shifts are driven by the formation of more stable isomers. Similarly, the parent radical cation of allylamine 5*+ undergoes a spontaneous 1,2-hydrogen atom shift from C1 to C2 at 77 K with a t1/2 of approximately 1 h to yield the distonic alkyl-type iminopropyl radical cation 6*+; this thermal reaction is attributed largely to quantum tunneling, and the rate is enhanced on concomitant photobleaching with visible light. Subsequent exposure to UV light (lambda approximately 350-400 nm) converts 6*+ by a 2,3 H-shift to the 1-aminopropene radical cation 7*+, which is confirmed to be the lowest-energy isomer derived from the ionization of either allylamine or cyclopropylamine. Although the parent radical cations of N, N-dimethylallylamine (9*+) and N-methylallylamine (11*+) are both stabilized by the electron-donating character of the methyl group(s), the photobleaching of 9*+ leads to the remarkable formation of the cyclic 1-methylpyrrolidine radical cation 10*+. The first step of this transformation now involves the migration of a hydrogen atom to C2 of the allyl group from one of the methyl groups (rather than from C1); the reaction is then completed by the cyclization of the generated MeN + (=CH2) CH2CH2CH2* distonic radical cation, possibly in a concerted overall process. In contrast to the ubiquitous H-atom transfer from carbon to nitrogen that occurs in the parent radical cations of saturated amines, the alternate rearrangements of either 1b*+ or 5*+ to an ammonium-type radical cation by a hypothetical H-atom shift from C1 to the ionized NH2 group are not observed. This is in line with calculations showing that the thermal barrier for this transformation is much higher (approximately 120 kJ mol-1) than those for the conversion of 1b*+ --> 2*+ and 5*+--> 6*+ (approximately 40-60 kJ mol-1).

Structures of tetrafluorocyclopropene, hexafluorocyclobutene, octafluorocyclopentene and related perfluoroalkene radical anions revealed by electron spin resonance spectroscopic and computational studies.

Isotropic and anisotropic ESR spectra were observed for the radical anions of hexafluorocyclobutene (c-C(4)F(6)(-)), octafluorocyclopentene (c-C(5)F(8)(-)) and perfluoro-2-butene (CF(3)CF=CFCF(3)(-)) in gamma-irradiated plastically crystalline neopentane, tetramethylsilane (TMS) and TMS-d(12) matrices, or the rigid 2-methyltetrahydrofuran (MTHF) matrix. The isotropic spectra of c-C(4)F(6)(-) and c-C(5)F(8)(-) are characterized by three different sets of pairs of (19)F nuclei with the isotropic hyperfine (hf) splittings of 15.2 (2F), 6.5 (2F), 1.1 (2F) mT for c-C(4)F(6)(-) and 14.7 (2F), 7.4 (2F), 1.0 (2F) mT for c-C(5)F(8)(-). By comparison with the results of ab initio quantum chemical computations, the large triplet (19)F hf splittings of ca. 15 mT are assigned to the two fluorines attached to the C=C bond. The UHF, B3LYP and MP2 computations predict that the geometrical structures of the perfluoroalkenes are strongly distorted by one-electron reduction to form their radical anions; c-C(3)F(4)(-): C(2) symmetry ((2)A state) <-- C(2)(v) ((1)A(1)), c-C(4)F(6)(-): C(1) ((2)A) <-- C(2)(v) ((1)A(1)) and c-C(5)F(8)(-): C(1) ((2)A) <-- C(s) ((1)A'). The structural distortion arises from a mixing of the pi* and higher-lying sigma* orbitals at the C=C carbons similar to that previously found for CF(2)=CF(2)(-) with a C(2)(h) distortion. The isotropic (19)F hf splittings computed with the B3LYP method with 6-311+G(2df,p) basis set for the geometry optimized by the UHF and/or MP2 methods are within 6% error of the experimental values. The experimental anisotropic spectra of c-C(4)F(6)(-), c-C(5)F(8)(-) and CF(2)=CF(2)(-) were satisfactorily reproduced by the ESR spectral simulation method using the computed hf principal values and orientation of (19)F nuclei. In addition, the electronic excitation energies and oscillator strengths for the CF(2)=CF(2)(-), c-C(3)F(4)(-), c-C(4)F(6)(-) and c-C(5)F(8)(-) radical anions were computed for the first time by TD-DFT methods.

Computational and ESR studies of electron attachment to decafluorocyclopentane, octafluorocyclobutane, and hexafluorocyclopropane: electron affinities of the molecules and the structures of their stable negative ions as determined from 13C and 19F hyperfine coupling constants.

High-resolution ESR spectra of the ground-state negative ions of hexafluorocyclopropane (c-C3F6*-), octafluorocyclobutane (c-C4F8*-), and decafluorocyclopentane (c-C5F10*-) are reported and their isotropic 19F hyperfine coupling constants (hfcc) of 198.6 +/- 0.4 G, 147.6 +/- 0.4 G, and 117.9 +/- 0.4 G, respectively, are in inverse ratio to the total number of fluorine atoms per anion. Together with the small value of 5.2 +/- 0.4 G determined for the isotropic 13C hfcc of c-C4F8*-, these results indicate that in each case the singly occupied molecular orbital (SOMO) is delocalized over the equivalent fluorines and possesses a nodal plane through the carbon atoms of a time-averaged D(nh) structure. A series of quantum chemical computations were carried out to further characterize these anions and their neutral counterparts. Both the B3LYP density functional and second-order Møller-Plesset perturbation theory (MP2) indicate that c-C3F6*- adopts a D(3h) geometry and a (2)A2'' ground electronic state, that c-C4F8*- adopts a D(4h) geometry and a (2)A2u ground electronic state, and that c-C5F10*- adopts a C(s) structure and a (2)A' electronic state. Moreover, the 19F hyperfine coupling constants computed with the MP2 method and a high quality triple-zeta basis set are within 1% of the experimental values. Also, the values computed for the 13C hfcc of c-C4F8*- are consistent with the experimental value of 5.2 G. Therefore, in keeping with the ESR results, these negative ions derived from first-row elements can be characterized as pi* species. In addition, the hypervalency of these perfluorocycloalkane radical anions has been clarified.

Ionized bicyclo[2.2.2]oct-2-ene: a twisted olefinic radical cation showing vibronic coupling.

The bicyclo[2.2.2]oct-2-ene radical cation (1(.+)) exhibits matrix ESR spectra that have two very different types of gamma-exo hydrogens (those hydrogens formally in a W-plan with the alkene pi bond), a(2H) about 16.9 G and a(2H) about 1.9 G, instead of the four equivalent hydrogens as would be the case in an untwisted C(2v) structure. Moreover, deuterium substitution showed that the vinyl ESR splitting is not resolved (and under about 3.5 G); this is also a result of the twist. Enantiomerization of the C(2) structures is rapid on the ESR timescale above 110 K (barrier estimated at 2.0 kcalmol(-1)). Density functional theory calculations estimate the twist angle at the double bond to be 11-12 degrees and the barrier as 1.2-2.0 kcalmol(-1). Single-configuration restricted Hartree-Fock (RHF) calculations at all levels that were tried give untwisted C(2v) structures for 1(.+), while RHF calculations that include configuration interactions (CI) demonstrate that this system undergoes twisting because of a pseudo Jahn-Teller effect (PJTE). Significantly, twisting does not occur until the sigma-orbital of the predicted symmetry is included in the CI active space. UHF calculations at all levels that include electron correlation (even semiempirical) predict twisting at the alkene pi bond because they allow the filled alpha and the beta hole of the SOMO to have different geometries. The 2,3-dimethylbicyclo[2.2.2]oct-2-ene radical cation (2(.+)) is twisted significantly less than 1(.+), but has a similar temperature for maximum line broadening. Neither the 2,3-dioxabicyclo[2.2.2]octane radical cation (3(.+)) nor its 2,3-dimethyl-2,3-diaza analogue (5(.+)) shows any evidence of twisting. Calculations show that the orbital energy gap between the SOMO and PJTE-active orbitals for 3(.+) is too large for significant PJTE stabilization to occur.