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.

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The elusive protonated ozone ion (O(3)H(+)) has been long postulated as a reactive intermediate but never experimentally observed. This ion has been detected here in mass spectrometric experiments with the use of Fourier transform ion cyclotron resonance. In these experiments, ozone (O(3)) was protonated by strong acids-for example, H(3)(+), KrH(+), XeH(+), and CH(5)(+). The hitherto experimentally unknown proton affinity of O(3) was evaluated by a "bracketing" technique and determined to be 148 -/+ 3 kilocalories mole(-1) at 298 kelvin, in excellent agreement with a value determined in a recent theoretical study of the O(3)/O(3)H(+) system, which was 148 kilocalories mole(-1) at zero temperature ( approximately 149.5 kilocalories mole(-1) at 298 kelvin).

A direct experimental approach based on the evaluation of the ligand exchange equilibrium H20-N02+ + CH30H CH3(H)O-N02+ + H 2 0 has been exploited for the determination of APA = PA(HN03) -PA(CH30N02) at 298 K. The result, APA = 4.0 f 1.2 kcal mol-', is sufficiently accurate to provide firm experimental support to the counterintuitive prediction, based on high-level ab initio calculations, that the PA of HN03 exceeds that of CH3-ON02 by 5.6 f 5 kcal mol-'. Combination of the experimental APA with the known PA of CH30N02 gives PA(HNO3) = 182.0 f 2.3 kcal mol-', in excellent agreement with the theoretically computed value, 182.5 f 3 kcal mol-'. An explanation of the considerably lower PA(HN03) value derived from earlier ICR bracketing experiments is offered, based on the R(H)O-N02+ vs RON02H+ isomerism. whose role in determining the observed PA trend along the RON02 series (R = H, CH3, C2H5) is discussed.