Survival of the weakest: The existence of a new class of centrosymmetric radical cations in which H2 bridges two identical main group elements was recently proposed in this journal by Uggerud and co-workers. By growing complexes inside helium nanodroplets at subkelvin temperatures, we obtained experimental evidence for the existence of the most weakly bound member of this class, He-H-H-He+ (see picture), although in a metastable, electronically excited state.
In a recent report, Uggerud and co-workers (A. Krapp et al., Chem. Eur. J.2008, 14, 4028) proposed the existence of a new class of radical cations in which a dihydrogen bridges two identical main group elements. Upon electron impact ionization of helium nanodroplets doped with one or more H2 molecules we observe various HexHy+ cluster ions, including He2H2+, which would belong to the proposed class of radical cations. Mass-analyzed kinetic energy scans reveal that the ion is metastable; it dissociates in the field-free region of the mass spectrometer. One reaction is into HeH2+ + He with a low kinetic energy release of 15±4 meV. Surprisingly, another unimolecular reaction is observed, into HeH+ + HeH (or He + H). The probability of this reaction is an order of magnitude higher, and the average kinetic energy release is four times larger. These findings suggest the presence of a metastable electronically excited state; they are consistent with the proposed linear, centrosymmetric ion structure of He-H-H-He+.