The reaction of N2O with phenylium ions C6(H,D)5+: an integrated experimental and theoretical mechanistic study

The reaction of N2O (known to be an O atom donor under several conditions) with the phenyl cation is studied by experimental and theoretical methods. Phenyl cation (or phenylium), C6H5+, and its perdeuterated derivative C6D5+ are produced either by electron impact or by APCI ionization of adequate neutral precursors, and product mass spectra are measured in a guided ion beam tandem mass spectrometer. The ions C5(H,D)5+, C6(H,D)5O+, and C3(H,D)3+ are experimentally detected as the most relevant reaction products. In addition, the detection of the adduct (C6(H,D)5 N2O)+, which is collisionally stabilized in the scattering cell of the mass spectrometer, is reported here for the first time. The reaction pathways, which could bring about the formation of the mentioned ions, are then explored extensively by DFT and, for the more promising pathways, by CASPT2/CASSCF calculations. The two reacting species (1) form initially a phenoxydiazonium adduct, C6H5ON2+ (2a), by involving the empty in-plane hybrid C orbital of phenylium. The alternative attack to the ring Π system to produce an epoxidic adduct 2b is ruled out on the basis of the energetics. Then, 2a loses N2 quite easily, thus affording the phenoxyl cation 3. This is only the first of several C6H5O+ isomers (4-6 and 8-12), which can stem from 3 upon different cleavages and formations of C-C bond and/or H shifts. As regards the formation of C5H5+, among several conceivable pathways, a direct CO extrusion from 3 is discarded, while others appear to be viable to different extents, depending on the initial energy of the system. The easiest CO loss is from 4, with formation of the cyclopentadienyl cation 7. Formation of C3H3+ is generally hindered and its detection depends again on the availability of some extra initial energy.