No abstract

Cyclic delocalization of five and six electrons, respectively, in the plane of four N centers was explored by one-/two-electron reduction of more or less rigid, parallel bisdiazenes (1-7), with NdN/NdN distances (d ππ ) ranging from ∼2.8 to 5.0 Å, interorbital angles (p,p; ω) from ∼l75°to 90°, and with more or less kinetic protection. For the "proximate" substrates 1-5 (d ππ ≈ 2.8-3.2 Å, ω ≈ 175-156°), short contact with alkali metals (Li, Na, K, Cs) generates turquoise to deeply green radical anions (λ max (DME) ≈ 700-900 nm). The character of these radical anions as cyclically in-plane delocalized bishomoconjugated 4N/5e species with a high concentration of spin density between the NdN units is established by extensive UV/vis, electrochemical, and EPR measurements (CW, pulsed) at temperatures down to 8 K, and by DFT calculations (B3LYP/6-31G*). After longer exposure to the metals, the three most persistent 4N/5e radical anions (M + 1 •-, M + 2 •-, M + 5 •-) are further reduced to the red dianions (2M + 1 2-, 2M + 2 2-, 2M + 5 2-, λ max (THF) ≈ 360-430 nm). Of the nine ion pair combinations with Li + , Na + , and K + all except one (2K + 1 2-) are thermally highly persistent. For the dianions the l H, l3 C, and 7 Li NMR analyses assisted by DFT calculations confirm the 4N/6e σ-bishomoaromatic electronic structure. The reduction potentials determined (CV) for dialkyl-diazenes (8-10) and bisdiazenes (1, 2, and 5) allow an estimate of the thermodynamic stabilization of the respective radical anions and of the gain in electron delocalization energy in the dianions (σ-bishomoaromaticity).

The implementation of matched pulses in two-dimensional electron spin echo envelope modulation experiments results in a drastic increase in intensity of peaks representing nuclear multiple quantum coherence. For a spin system consisting of one electron spin S=12 and two nuclei with spin I=12 the nuclear coherence created by the sequence (π/2)−τ−(π/2) is calculated. The transfer amplitudes between different kinds of nuclear coherences of a microwave π pulse are given. A new matched HYSCORE pulse sequence, called SMART HYSCORE, is introduced with only one microwave pulse for the generation and detection of nuclear coherence, which does not suffer from blind spots. This sequence is applied to a single crystal and a disordered system, and it is demonstrated how the resolution of weakly coupled nuclei can be increased. Furthermore we show that information about the relative sign of hyperfine couplings and about the relative orientation of the hyperfine tensors can be derived from the ridge patterns in a HYSCORE experiment. The numerical simulations are confirmed experimentally.