Tetranitrogen (N4), which has been the subject of great theoretical interest, has been prepared from the N4+ cation and positively detected as a gaseous metastable molecule with a lifetime exceeding 1 microsecond in experiments based on neutralization-reionization mass spectrometry. An examination of the geometry of N4+ and the fragmentation pattern of the (14)N2(15)N2 neutral molecule has revealed that the latter is characterized by an open-chain geometry with two distinct, closely bound N2 units joined by a longer weaker bond.

Hydrogen trioxide (HO3) has long been postulated as a key intermediate in important atmospheric processes but has proved difficult to detect. The molecule was unequivocally detected in experiments based on neutralization-reionization and neutralization-reionization/collisionally activated dissociation mass spectrometry, using protonated ozone (HO3+) as the charged precursor. Hydrogen trioxide is a relatively stable species and has a H-O-O-O connectivity and a lifetime exceeding 10(-6) seconds at ambient temperature.

A precise jab to methane: The SO(2)(*+) radical cation (see figure) effectively activates CH(4) at room temperature through a [H(3)C(*)...HOSO(+)] methyl intermediate isolated in the gas phase by mass spectrometry. Methanol and ionized methyl hydrogen sulfoxylate, CH(3)OSOH(*+), are formed by selective, direct attack of the incipient methyl radical at the O atom of the intermediate. The reaction shows radical and charge effects in the activation of methane by metal-free radical cations.

The search for tetraoxygen, which dates back to a study by Lewis in 1924, [1] is being actively pursued, owing to its fundamental interest, its potential role as a material of high energy density, [2] and, in particular, the current upsurge of interest in excited states of O 2 with relevance to atmospheric chemistry and to terrestrial and Venusian nightglow. [3] Numerous calculations at different levels of theory predict the existence of metastable O 4 molecules with a D 2d cyclic geometry [4] or a D 3h ªpinwheelº geometry. [5,6] Positive experimental detection of such species has not been reported, whereas weakly bonded (O 2 ) 2 van der Waals dimers, with dissociation enthalpies of less than 1 kcal mol À1 , have long been known and thoroughly characterized. [7] Indirect evidence for metastable tetraoxygen was derived from experiments involving electron transfer from Cs to O 4 , followed by the measurement of the kinetic energy spectrum of the O 2 fragments. The structure of the kinetic energy distribution was consistent with the intervention of some O 4 species that were not observable under the conditions of the experiment. [8] Strong evidence for metastable O 4 was derived from studies involving the photoionization of O 2 excited by a DC discharge. [9,10] Interestingly, the results of both experimental studies were interpreted as suggestive of the existence of a third metastable O 4 species in addition to the theoretically predicted molecules, namely, a relatively long lived complex between a ground-state O 2 molecule and an O 2 molecule in the excited c 1 S À u electronic state. [10] To obtain conclusive proof, we have carried out an experiment aimed at detecting intact O 4 . Our approach is also based, like previous studies, [8] on the neutralization of O 4 cations in the gas phase, but uses for the identification of the intact O 4 molecule neutralization reionization (NR) mass spectrometry [11] on a highly sensitive instrument that allowed the detection of other elusive atmospheric species, such as hydrogen trioxide [12] and the [H 2 O ´O 2 À ] charge-transfer complex. [13] The O 4 ions were generated in a chemical ionization (CI) source by association of O 2 molecules with O 2 primary ions, formed both in the X 2 P g ground state and in electronically excited states by electron impact. [14] The ions were then accelerated through 4 ± 8 keV, magnetically mass resolved, and probed by collisionally activated dissociation (CAD) mass spectrometry. Whereas the CAD spectrum of 16 O 4 from the ionization of 16 O 2 shows 16 O 2 as the charged fragment, 16 O 2 18 O 2 ions from 16 O 2 / 18 O 2 mixtures give 16 O 2 and 18 O 2 without the 16 O 18 O ions [15] that would suggest isotopic scrambling ( Figure 1). This shows that the O 4 ions probed contain two discrete O 2 units, each of which retains its original identity.In the NR experiments the O 4 ions were also accelerated to 4 ± 8 keV and mass-selected before undergoing two consecutive collision events in separate cells aligned along the beam path. In the first c...

The acid-catalyzed racemization and regioisomerization of (S)-trans-4-hexen-3-ol (1S) has been investigated in gaseous CH4 and C3H8 at 720 Torr and in the 40−120 °C temperature range. The contribution to the racemization and isomerization products by free 1-methyl-3-ethylallyl cations, arising from unimolecular fragmentation of excited O-protonated (S)-trans-4-hexen-3-ol (IS), was evaluated by generating them from protonation of isomeric 2,4-hexadienes and by investigating their behavior toward H2 18O under the same experimental conditions. The rate constant of the gas phase racemization of IS (1.4−21.3 × 106 s-1) was found to exceed that of its isomerization (1.0−9.9 × 106 s-1) over the entire temperature range. The experimental results, combined with ab initio theoretical calculations on the model [C3H5 +/H2O] system, are consistent with a gas phase intramolecular IS racemization and isomerization involving the intermediacy of structured ion−molecule complexes, wherein the H2O molecule is coplanarly coordinated to the hydrogen atoms of the 1-methyl-3-ethylallyl moiety. The rate of formation of these structured complexes, their relative stability, and the dynamics of their evolution to the racemized and isomerized products depend, in the gas phase, on the specific conformation of IS. The relevant activation parameters point to transition structures wherein a substantial fraction of the positive charge is located on the allyl moiety. The results obtained in the present gas phase investigation confirm previous indications about the occurrence and the role of intimate ion−molecule pairs in acid-catalyzed racemization and isomerization of optically active alcohols, including allylic alcohols, in solution.