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Adducts of the zwitterionic (ZW) form of merocyanine (MC) of five photochromic spirooxazines (SO) and spiropyrans (SP) have, for the first time, been prepared photochemically or thermally through a trapping reaction of metastable MC with trimethylsilyl cyanide (TMSCN) in dichloromethane. The adducts (ZWAs) form easily and are obtained in almost quantitative yields. X-ray diffraction analysis of ZWA of spirooxazine 2 shows that in the adduct the planes of the indolyl and naphthyl moieties form a ca. 29° angle. The adduct crystallizes in the monoclinic space group P21/n with a = 22.663(4) Å, b = 8.234(1) Å, c = 14.455(2) Å, β = 104.72(1)°, V = 2637.4(7) Å3, and D calcd = 1.1476 Mg/m3 for Z = 4. Least-squares refinement of the model based on 1986 reflections (F > 4σ(F)) converged to a final R = 0.066 and R w = 0.068. ZWAs form through an unprecedented, clean, and regioselective Michael-type 1,6-addition of TMSCN to either a strongly polarized quinoidal MC form or its charge-separated zwitterionic limit structure. No evidence for the more common 1,2- and/or 1,4-addition has been found. The observed high reactivity of SOs and SPs with nucleophiles may contribute to the reported limited durability of these classes of photochromes.

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The non-intrusive density measurement of the thin plasma produced by a mini-helicon space thruster (HPH.com project) is a challenge, due to the broad density range (between 10(16) m(-3) and 10(19) m(-3)) and the small size of the plasma source (2 cm of diameter). A microwave interferometer has been developed for this purpose. Due to the small size of plasma, the probing beam wavelength must be small (λ = 4 mm), thus a very high sensitivity interferometer is required in order to observe the lower density values. A low noise digital phase detector with a phase noise of 0.02° has been used, corresponding to a density of 0.5 × 10(16) m(-3).