In an attempt to produce the 2-norbornyl cation (2NB + ) in the gas phase, protonation of norbornene was accomplished in a pulsed discharge ion source coupled with a supersonic molecular beam. The C 7 H 11 + cation was sizeselected in a time-of-flight mass spectrometer and investigated with infrared laser photodissociation spectroscopy using the method of "tagging" with argon. The resulting vibrational spectrum, containing sharp bands in the CÀH stretching and fingerprint regions, was compared to that predicted by computational chemistry. However, the measured spectrum did not match that of 2NB + , prompting a detailed computational study of other possible isomers of C 7 H 11 + . This study finds five isomers more stable than 2NB + . The spectrum obtained corresponds to the 1,3-dimethylcyclopentenyl cation, the global minimum-energy structure for C 7 H 11 + , which is produced through an unanticipated ring-opening rearrangement path.The 2-norbornyl cation (C 7 H 11 + ; 2NB + ) is the most famous and controversial carbocation. [1,2] On the basis of unusual solvolysis reaction rates and products of 2-exo-and 2-endonorbornyl derivatives, [3][4][5][6][7][8] Winstein proposed a symmetrically bridged nonclassical structure for the ion intermediate. [5] In sharp disagreement, Brown favored rapidly equilibrating classical structures. [9] This dispute continued vehemently for decades. Experiments on 2NB + under "stable ion" conditions in "superacid" media supported the non-classical structure, as did theory at ever-more sophisticated levels. [10][11][12][13][14][15][16][17][18][19][20][21] Notably, Saunders demonstrated deuterium isotope effects on the 13 C NMR spectra showing no rapid equilibration, [14] and Yannonis cryogenic (5 K) 13 C NMR spectra showed a static symmetrically bridged structure. [15] However, proponents of the classical structure argued that these results were not definitive because of the long NMR time scale. Recently, the long-sought X-ray crystal structure of 2NB + was obtained, confirming its nonclassical structure in the condensed phase. [22] Nevertheless, questions remain about the nature of this ion in the gas phase. Is the isolated ion intrinsically stable, or is it stabilized by the counter ions and solvation in the superacid media? Although aspects of its chemistry have been studied by mass spectrometry, [23][24][25][26][27] no gas-phase spectroscopy of 2NB + has been reported. Our objective was to measure the infrared spectrum of the mass-selected C 7 H 11 + ion and to determine its structure from the vibrational patterns by comparison to the predictions of theory. To our surprise, the structure of C 7 H 11 + obtained under our conditions is not that of 2NB + , but instead corresponds to a much more stable rearranged ion. This rearrangement and the unexpected C 7 H 11 + isomer are the subjects of this paper. Previous mass spectrometry of 2NB + explored its reactions and collisional dissociation behavior. [23][24][25][26][27] Fragmentation patterns of C 7 H 11 + produced by different methods ...
