The reaction of spiro[indoline‐naphthopyrans] with nitrogen oxides. Not a case of biradical double trapping

The reactions of six differently substituted photochromic spiro[indoline‐naphtopyrans] with ·NO or ·NO₂ under normal daylight conditions have been investigated by means of EPR spectroscopy along with those of three structurally related spiro[indoline‐benzopyrans]. The spectra due to cyclic oxynitroxides originating from double trapping of biradicals by ·NO were observed with the three latter derivatives, this finding being in agreement with previous results. Similar signals were also observed with the six former compounds, but in this case they were responsible for just a minor component of the spectra, the main spectral signals being due to hitherto unreported paramagnetic species that on the basis of their spectral parameters are identified as iminoxy radicals. DFT calculations at the B3LYP/6‐31G* level carried out on a variety of radicals support this assignment. Copyright © 2009 John Wiley & Sons, Ltd.