Spin distributions and geometries of the radical cations of sesquinorbornenes and other polycyclic tetraalkylethenes

Long-range coupling in strained derivatives of the 2,3-diazabicyclo[2.2.1]hept-2-ene radical anion: An EPR, ENDOR and TRIPLE-resonance study

“…the differences in nature and numbers of equivalent nuclei and on the well-established relation that the absolute values of the γ-proton-coupling constants, a H( ) , are larger for the exo-than for the endo-positions and for the anti-than for the synpositions. [16][17][18] The signs of a H follow the results of the general-TRIPLE-resonance experiment carried out on the 1 H-ENDOR signals 19 with the justified assumption that the large a H( ) value due to the protons in the 5,6-exo-positions is positive. In the case of the 14 N-and 39 K-coupling constants, a N and a K , the positive sign was taken for granted.…”

“…30 Homohyperconjugation, like hyperconjugation itself, is strongly enhanced by a positive charge at the π-centre, so that a H( ) and a H( ) values are much larger for radical cations than for neutral radicals and radical anions with a comparable geometry and π-spin distribution. Impressive examples of large long-range couplings were reported for the radical cations of benzvalene (tricyclo[3.1.0.0 2,6 ]hex-3-ene) 23 and trans-sesquinorbornene 18 with a H( ) = ϩ2.79 and ϩ1.35 mT, respectively, as well as for the radical cation of adamantylideneadamantene 37 with a H( ) = ϩ0.605 mT. Owing to the mechanism of π,σ-spin delocalization through bonds, the sign of such a H( ) and a H( ) values is the same as that of the spin population at the π-centre, i.e.…”

Organic Radical Ions as Studied by Epr Spectroscopy

“…v H and i^, are the frequencies of the free proton and 14 (90), wherein the geometry of the radical ion can be optimized by Dewar's MNDO (91) or an "ab initio" method (92) which are themselves less adequate for a direct calculation of the coupling constants. Alternatively, the more recent Dewar's AMI (93) may be used with the calculated spin populations at hydrogen atoms converted into proton-coupling constants by means of empirically derived parameters (94,95). The best results for these allvalence-electron methods are generally obtained with rigid polycyclic radical ions (95)(96)(97).…”

“…Alternatively, the more recent Dewar's AMI (93) may be used with the calculated spin populations at hydrogen atoms converted into proton-coupling constants by means of empirically derived parameters (94,95). The best results for these allvalence-electron methods are generally obtained with rigid polycyclic radical ions (95)(96)(97). Recently, DFT calculations proved promising for a variety of radical ions (143).…”

“…Prominent representatives of such compounds are adamantylideneadamantane (15), syn-and awri-sesquiterpenes, triisopropylammonia, 9,9'-bis(9- 16), and l,6-diazabicyclo [4.4.4]tetradecane (17). 15 The radical cations of 15 (84) and of the two sesquiterpenes (95) are stable because the reactive double bond is embedded in a bulky, saturated, and polycyclic carbon framework which lowers the oxidation potential of the compounds and impedes the decay of the radical cations by the loss of an allylic proton (Bredt-rule protected).…”