Quick reactions: The gas‐phase nucleophilic displacement reaction of fluoride ions with nitrobenzene proceeds with a rate constant close to the collision limit. The calculated energy profile indicates that the reaction proceeds by an out‐of‐plane attack, with the Meisenheimer complex being the local transition state for the reaction (see picture).

Schnell reagiert: Der nucleophile Nitrit‐Fluorid‐Austausch an Nitrobenzol in der Gasphase weist eine Geschwindigkeitskonstante nahe dem Stoßlimit auf. Aus dem berechneten Energieprofil lässt sich schließen, dass die Reaktion über einen nicht in der Ebene erfolgenden Angriff verläuft, wobei der gezeigte Meisenheimer‐Komplex der lokale Übergangszustand der Reaktion ist.

The molecular ion of allyl bromide has been characterized by ab initio molecular orbital calculations at the MP4(SDTQ) level with optimized geometries at the MP2 level in order to account for experimental data suggesting the presence of two isomers. The calculations predict the existence of an allyl bromide molecular ion with structural parameters resembling the neutral species except for a lengthening of the double bond. This structure is calculated to be more stable than a cyclic bromonium radical cation structure. Rearrangement of the molecular ion of allyl bromide to that of 1-bromopropene is shown to be possible through a transition state represented by the distonic ion, + BrHCCH 2 CH 2 • , lying just below the dissociation limit of the allyl bromide molecular ion. Studies based on ion/molecule reactivity of C 3 H 5 Br •+ ions generated from allyl bromide and 1-bromopropene with ammonia, methanol, allyl bromide, and charge transfer reactions strongly suggest that a small fraction of the molecular ions of allyl bromide isomerize to the 1-bromopropene molecular ion as predicted by the calculation. These experiments cannot establish unequivocally whether the allyl bromide molecular ions retain the structure of the parent molecules as predicted by the calculations or undergo ion/ molecule reactions mediated by a bromonium type complex. Charge transfer experiments also suggest the adiabatic ionization energy of allyl bromide to be 9.83 ( 0.07 eV.

The thermal dissociation of several substituted acetophenone molecular ions induced by infrared radiation from a hot wire has been studied in a Fourier transform ion cyclotron resonance spectrometer. The temperature dependence of the dissociation rate constants reveals that the 2′-methylacetophenone molecular ion is characterized by a much higher activation energy for dissociation than other acetophenones. This molecular ion also exhibits a very different behavior with respect to charge-transfer reactions. Unlike molecular ions obtained from other isomeric acetophenones, the 2′-methylacetophenone M +• ion does not promote charge exchange with dimethyl disulfide but does undergo relatively slow electron transfer with ferrocene (IE ) 6.74 eV). Ab initio calculations at the MP2/6-31G(d) level predict that the 2-MeC 6 H 4 COCH 3 +• ion (1) can undergo facile tautomerization to the much more stable enol ion 2, 2′-•CH 2 C 6 H 4 C + (OH)CH 3 , by a 1,4-hydrogen migration (calculated energy barrier of 20 kJ mol -1 ). The calculated recombination energy of this ion is in good agreement with the observations from the charge-exchange experiments. A full analysis of the potential energy surface suggests that, at low ionizing energies (e11.5 eV), essentially all of the long-lived molecular ions have isomerized to 2. The present example reveals the versatility and some of the advantages of the high-temperature near-blackbody-induced dissociation (hot wire emission) for probing structural problems in ion chemistry. † Part of the special issue "Jack Beauchamp Festschrift".

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