Transient radicals and elusive species

Another theme of the research in the group concerns the chemistry of peroxides and peroxide-derived radicals, which are of relevance in various regimes and with a particular focus on atmospheric chemistry. In this context, the group members took essential part in development of a method called Neutralization and Ion Decomposition Difference (NIDD) with which the reactivity of transient neutral radicals can be investigated in a unique and highly sensitive manner (Chemical Society Reviews, 1998).

As an example from ongoing research, the neutralization-reionization (NR) and the charge-reversal (CR, red) spectra of the mass-selected prop-2-enolate ion (CH₂=CH-CH₂O-) produced by deprotonation of allyl alcohol are shown in the Figure below. In a simplified view, NR of the anion involves two single-electron transitions from the anion to the neutral and then from the neutral to the cation, whereas in CR a single two-electron transition (from anion to cation) takes place. According to the NIDD approach, the difference between NR and CR hence reflects the reactivity of the transient neutral radical involved as intermediate.

While both spectra in the above Figure are quite similar, the abundances of the C₃H_nO⁺ ions (n = 3 - 5) are significantly larger in the NR spectrum. The reason for this difference is the occurrence of a fragmentation of CH₂=CH-CH₂O• radical being formed as short-lived intermediate to yield acrolein according to equation (1).

The so-formed neutral acrolein is then ionized in the second step (therein also undergoing dissociative ionization to C₃H₃O⁺). In contrast, the CR experiment does not permit the occurrence of reaction (1) because the transition from anions to cations occurs without a neutral intermediate.

Other interests in this area deal with the preparation of unusual or exotic molecules, such as neutral P₆ as a new allotrope of the element phosphorus (Angewandte Chemie International Edition, 1999 / Chemistry - A European Journal, 2002) or the elusive meta-benzoquinone (Angewandte Chemie International Edition, 2005).

Mass spectrometric generation of the elusive neutral hexaphosphorous molecule
from the corresponding bis(pentamethylcyclopentadienyl) derivative.