Transient ions, reactions

We have already mentioned that Dorfman and collaborators have developed a versatile technique to observe ort-lived carbenium ions in solution generated by dissociative pulse radiolysis. This novel approach to the characterisation of transient species has also allowed this schod to measure the rate constants of many electrophilic reactions between carbenium ions (the benzylium ion in particular) and various nucleophiles. In the first paper of the series Jones and Dorfman reported the rate constants of the benzylium ion reaction with methanol, ethanol, the bromide and the iodide ions in ethylene chloride at 24 C. Values of about 5 x 10 sec were obtained for the halide ions and of around 10 sec for the alcohols. Later studies confirmed that the reaction of halide ions vrith benzylium, diphenyl-methylium and triphenylmethylium ions is at the limit of diffusion control. Reaction rate constants of these three carbenium ions with amines and alcdiols were also reported in the same paper. More recently, these studies have been extended to include cyclopropylphenylmetiiylium ion as electrophile, ammonia as nucleophile and methylene chloride and trichloroethane as solvents These results are extremely... 

C atom reaction which is the rate determining step. The dynamics of transient ion pairs and radical pairs in anthracene derivative/ tetranitromethane systems have been examined as a... 

The manual interpretation of a mass spectrum - nature and origin of the fragmentation peaks - is a difficult but interesting exercise. Organic chemists are usually familiar with these methods of interpretation since they retrieve different types of transient ions considered to explain reaction mechanisms in condensed phases. The difference here is that these ions move in a vacuum and do not collide. The very short period of time between ion formation and ion detection (a few pis), allows to observe the existence of very unstable species that are unstable under normal conditions. 

With this terminology we may not only describe a fully concerted, one-step Sn2 reaction (AnDn), and a stepwise SnI reaction involving intermediate ions that diffuse apart (Dn -H An), but we may also concisely represent a stepwise reaction involving a transient ion pair (E An). The lUPAC nomenclature system can also be used to describe other substitution reactions. Among them are the SnI (substitution nucleophilic unimolecular with rearrangement) reaction, equation 1.8, which is denoted by lUPAC as an (1/Dn -I- 3/An) reaction. The numbers before the slash symbols indicate atoms involved in the dissociation and association steps. Thus, 1/Dn means that the nucleofuge dissociates from one atom (1), while the 3/An term means that the nucleophile associates at an allylic position (3). 

Kinetic studies on Eq. 12.28 suggest that the reaction goes by nucleophilic attack of the hydride on CO2 to give the transient ion pair shown this then collapses to give the final product. [Rh(dpe)2H] forms the stable salt [Rh(dpe)2][HC02]. ... 

---