Succinimidyl radical, phthalimidyl radical, and 1,8-naphthalimidyl radical were generated in argon matrices by cocondensation of Na atoms and corresponding imide triflates at near liquid helium temperature. Spontaneous Na atom-to-triflate electron transfer occurred, and the resulting anions spontaneously dissociated to yield imidyl radicals and triflate anions. In the case of succinimidyl triflate, the observed ESR spectrum was determined as that of β-(isocyanatocarbonyl)ethyl radicals resulting from ring-opening of the succinimidyl radical. In the cases of phthalimidyl triflate and 1,8-naphthalimidyl triflate, the ESR spectra due to the “nitrogen centered” imidyl radicals were observed. The 14N hyperfme coupling tensors determined for these radicals unambiguously revealed their π radical ground state. When the matrix containing the phthalimidyl radical was exposed to mild radiation (λ = 600 ± 50 nm), the ESR spectrum changed to that of an ortho-substituted phenyl radical consistent with its ring-opening process. 1,8-Naphthalimidyl radical did not undergo the corresponding ring-opening process when treated similarly. © 1992, American Chemical Society. All rights reserved.