Singlet state nitrenium ions

Calculations of the singlet-triplet energy gaps of a series of nitrenium ions [X(H)N ]+ in the gas phase and in solution indicate that the gap decreases in the order X = H > CN > F as a result of stabilization of the singlet state by r-donation from the substituent. For strong tt-donors (Cl, F), the singlet state is more stable than the triplet. ... 

Calculations on the isoelectronic series Me(Ph)B , Me(Ph)C , and [Me(Ph)N ]+ show that the singlet-state geometries are different, reflecting differences in the orbital interactions between the hypovalent atom and the 7r-system. The high calculated barrier (21.5 kcal moP ) for [1,2]-H shift in the nitrenium ion is the result of migration using the orbital which is conjugated with the tt-system. 

The nitrenium ion +NH2 has been the subject of a detailed, comprehensive calculation. Calculations on (48) with 15 different X substituents reveal a large substituent sensitivity, and also that aqueous solvation preferentially stabilizes the singlet state. This substiment sensitivity agrees with the results of a time-resolved IR study of the diphenylnitrenium ion (49), which shows that resonance contributors such as (50) and (51) are very important to the overall structure. Substituted 4-biphenyl nitrenium ions... 

Nitrenium ions (or imidonium ions in the contemporaneous nomenclature) were described in a 1964 review of nitrene chemistry by Abramovitch and Davis. A later review by Lansbury in 1970 focused primarily on vinylidine nitrenium ions. Gassmann s ° 1970 review was particularly influential in that it described the application of detailed mechanistic methods to the question of the formation of nitrenium ions as discrete intermediates. McClelland" reviewed kinetic and lifetime properties of nitrenium ions, with a particular emphasis on those studied by laser flash photolysis (LFP). The role of singlet and triplet states in the reactions of nitrenium ions was reviewed in 1999. Photochemical routes to nitrenium ions were discussed in a 2000 review. Finally, a noteworthy review of arylnitrenium ion chemistry by Novak and Rajagopal " has recently appeared. 

It is important to note that for methyl nitrenium ion the singlet state is not predicted at higher levels of theory to show a potential energy minimum. Rather this species is a transition structure that eliminates H2 without a barrier (Fig. 13.15). It seems likely that the difficulties encountered in attempting to study alkyl nitrenium ions might be traced to their propensity to rearrange or eliminate. 

Gonzales and co-workers extended their work on simple nitrenium ions, examining the reactions of several small, monosubstituted nitrenium ions (XNH+, where X = H, F, Cl, CN, and Me) with water. These computational studies (QCISD(T)/6-311-1—1- G ) treated only the singlet states of the nitrenium ion. According to these calculations, each singlet nitrenium ion adds to water to form an O-protonated hydroxylamine intermediate (R2N—OHj). No activation barrier was detected for... 

The parent nitrenium ion (NH2) is firmly established as a ground-state triplet both extensive ab initio calculations as well as PES experiments all agree that the singlet-triplet energy gap is 30 kcal/mol. There have been several investigations on its behavior in solution. Takeuchi et al. " showed that this species could be generated by photolysis of l-amino-(2,4,6-triphenylpyridinium) ion. These photolyses were carried out in the presence of various aromatic compounds. It was found that the triplet state abstracted hydrogen atoms from traps such as toluene... 

The DFT calculations (BPW91/cc-pVDZ) on the same singlet nitrenium ions give excellent agreement with these measurements. For calculated and experimental C=C stretches, deviation between experiment and calculation is generally <5 cm On the other hand, similar calculations carried out for the triplet states showed significantly greater deviation from the measured values. 

By the late 1980s it was clear that a significant number of thermal and photochemical reactions of arylhydroxylamines and their derivatives, N-chloroanilines, aryl azides, anthranilium salts, and other compounds could be explained in terms of nitrenium ions or transition states that resembled nitrenium ions. Since no monoarylnitrenium ion had been directly observed, and data on the lifetimes and quantitative reactivity/selectivity of these species were not available, it was not possible to assess whether the reactions that had been observed were due to free ions, or ion pairs, or preassociation processes. In many cases Sn2 reactions could not be ruled out because appropriate kinetics experiments had not been performed. Most authors had attributed the presence of reduction products in thermal and photochemical reactions to triplet ions, but calculations suggested that the triplet species may not be accessible in thermal processes. It was clear that singlet ions could be reduced under certain conditions, so the presence of the... 

Nitrenes ([NH]) are the neutral nitrogen analogs of carbenes, while nitrenium ions ([NH2]+) are isoelectronic to carbenes. Many of the reactions which are observed for carbenes have parallels in nitrene and nitrenium ion chemistry. Like carbenes, nitrenes and nitrenium ions can exist in both singlet and triplet states. There are some interesting divergences in chemical properties and in the effects of substituents, however, which are readily understood on the basis of orbital interaction diagrams. 

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