Unimolecular chemistry of the gaseous cyclopropylamine radical cation

The rearrangement-dissociation mechanisms of ionized cyclopropylamine have been studied with tandem mass spectrometry, photoion-photoelectron coincidence spectroscopy, and (PU)MP4SDTQ/6-311G**/MP2/6-31G*+ZPE molecular orbital calculations. The present investigation demonstrates that the cyclic structure of cyclopropylamine is not preserved after electron removal; in fact, photon or electron ionization leads to a more stable open form, [CH(2)CH(2)CHNH(2)](.+), 2, which belongs to the category of distonic ions. Dissociative ionization of cyclopropylamine leads essentially to H atom loss. Experiments demonstrate that (i) the dissociation threshold is compatible only with the [CH(2)CHCHNH(2)](+) fragment ion structure, (ii) the hydrogen expelled from ions of low internal energy comes exclusively from a carbon, and (iii) the dissociation rate exhibits a very slow rise with internal energy. Molecular orbital calculations predict that the lowest energy pathway involves isomerization of 2 into ionized 1-aminopropene, [CH(3)CHCHNH(2)](.+), 3. The degenerate 1, 2-HCNH(2) shift [CH(2)CH(2)CHNH(2)](.+), 2 reversible arrow [NH(2)CHCH(2)CH(2)](.+), 2, is found to require a negligible critical energy. A statistical (RRKM) modeling of the reaction rate accurately reproduces the experimental data, and the observed slow down effect on the dissociation rate appears to be due to the passage through the very stable structure 3. Several previously unknown thermochemical parameters, including the standard heat of formation of various neutral and ionized (C(3), H(7), N) species, are proposed; for example Delta(f)H degrees(300)[CH(2)CH(2)CHNH(2)](.+) = 840 +/- 10 kJ/mol and Delta(f)H degrees(300)[CH(3)CHCHNH(2)](.+) = 770 +/- 10 kJ/mol are deduced from a combination of experiment and theory.