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

The reaction of N(2)O (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), C(6)H(5)(+), and its perdeuterated derivative C(6)D(5)(+) are produced either by electron impact or by atmospheric pressure chemical ionization of adequate neutral precursors, and product mass spectra are measured in a guided ion beam tandem mass spectrometer. The ions C(5)(H,D)(5)(+), C(6)(H,D)(5)O(+), and C(3)(H,D)(3)(+) are experimentally detected as the most relevant reaction products. In addition, the detection of the adduct (C(6)H(5)N(2)O)(+), 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 density functional theory and, for the more promising pathways, by CASPT2/CASSCF calculations. The two reacting species (1) form initially a phenoxydiazonium adduct, C(6)H(5)ON(2)(+) (2a), by involving the empty in-plane hybrid C orbital of phenylium. The alternative attack to the ring pi system to produce an epoxidic adduct 2c is ruled out on the basis of the energetics. Then, 2a loses N(2) quite easily, thus affording the phenoxyl cation 3. This is only the first of several C(6)H(5)O(+) 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 C(5)H(5)(+), 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 C(3)H(3)(+) is generally hindered and its detection depends again on the availability of some extra initial energy.