EPR Studies of Amine Radical Cations,Part 1:Thermal and Photoinduced Rearrangements of n-Alkylamine Radical Cations to their Distonic Forms in Low-Temperature Freon Matrices

摘要

The thermal and photochemical transformations of primary amine radical cations (n-propyl l~+,n-butyl 5~+) generated radiolytically in freon matrices have been investigated by using low-temperature EPR spec-troscopy.Assignment of the spectra was facilitated by parallel studies on the corresponding N,N-dideuterio-amines.The identifications were supported by quantum chemical calculations on the geometry,electronic structure,hyperfine splitting constants and energy levels of the observed transient radical species.The rapid generation of the primary species by a short exposure (1-2 min) to electron-beam irradiation at 77 K allowed the thermal rearrangement of l~+ to be monitored kinetically as a first-order reaction at 125-140 K by the growth in the well-resolved EPR signal of the distonic radical cation -CH_2CH_2CH_2NH_3~+.By comparison,the formation of the corresponding -CH_2CH_2CH_2CH_2NH_3~+ species from 5~+ is considerably more facile and already occurs within the short irradiation time.These results directly verify the intramolecular hydrogen-atom migration from carbon to nitrogen in these ionised amines,a reaction previously proposed to account for the fragmentation patterns observed in the mass spectrometry of these amines.The greater ease of the thermal rearrangement of 5~+ is in accordance with calculations on the barrier heights for these intramolecular 1,5- and 1,4-hy-drogen shifts,the lower barrier for the former being associated with minimisation of the ring strain in a six-mem-bered transition state.For l~+,the 1,4-hydrogen shift is also brought about directly at 77 K by exposure to -350 nm light,although there is also evidence for the 1,3-hydrogen shift requiring a higher energy.A more surprising result is the photochemical formation of the H_2C=N radical as a minor product under hard-matrix conditions in which diffusion is minimal.It is suggestedthat this occurs as a consequence ofthe beta-fragmentation of l~+ to the ethylradical and the CH_2=NH_2~+ ion,followed by consecutive cage reactions ofdeprotonation and hydrogen transferfrom the iminonium group.Additionally,secondary ion-molecule reactionswere studied in CFC1_2CF_2C1 undermatrix conditions that allow diffusion.The propane-1-iminyl radicalCH_3CH_2CH=N was detected at highconcentrations of the n-propylaminesubstrate.Its formation is attributed toa modified reaction sequence in whichl~+ first undergoes a proton transferwithin a cluster of amine molecules toyield the aminyl radicalCH_3CH_2CH_2NH.A subsequent dispro-portionation of these radicals can then yield the propane-1-imine precursor CH_3CH_2CH=NH,which is known to easily undergo hydrogen abstraction from the nitrogen atom.The corresponding butane-1-iminyl radical was also observed.