Nitrene electronic states

The nitrogen analogs of carbenes are called nitrenes. As with carbenes, both singlet and triplet electronic states are possible. 

As mentioned in Section II.A, several possible electronic states can arise from the presence of two diradical sites (carbenes and/or nitrenes) in a molecule. When... 

Figure 3 Valence-bond depictions of electronic states arising from the coupling of two reactive centers (carbene, nitrene) g via the 1,4-phenylene linker. 5 ...

Figure 11 shows some of the geometrical parameters (computed at the MCSCF level of theory) for the five electronic states of 33. The local geometries of the carbene and nitrene subunits (bond angle at the carbene center, C—bond length of the carbene and the bond lengths between the diradical centers and the benzene ring) of the quintet state ( A ) are very similar to that of triplet phenylcar-bene and phenynitrene computed at the same level of theory (Fig. 12). The situation is less clear for the other two A states ( A and A ), but the geometry of...

The nitrogen analogs of carbenes are called nitrenes. As with carbenes, both singlet and triplet electronic states are possible. The triplet state is usually the ground state for simple structures, but either species can be involved in reactions. The most common... 

Electronic States and Structure of Triplet Nitrenes 2.1. Calculations... 

The smaller the value of D, the smaller the interaction between the unpaired electrons, or in other words the greater is their separation. So, from the zero field splitting parameter of phenylnitrene (D =0.99 cm-1) one can deduce that one unpaired electron is delocalized into the aromatic system. Although ESR shows that these nitrenes have triplet ground states it tells nothing about the electronic state of the nitrenes at the moment of generation. 

One of the problems that has received some attention concerns the nature of the electronic state of the nitrene produced on photolysis. Reiser and co-workers approached this question by comparing the photolysis of 2-azidobiphenyl (356) in a solid matrix and in solution. It was found that photolysis gave the nitrene (357) as identified by its electronic absorption spectrum, and that the reaction could be followed by ultraviolet spectroscopy, monitoring azide disappearance, nitrene appearance and disappearance, and finally, the... 

The energy separations between ground and excited states of nitrenes are relatively small in comparison with the energy separations between these states in closed-shell molecules. This difference dramatically increases the level of complexity and the difficulty in assigning the chemistry and spectroscopy of a transient nitrene to a particular electronic state. In this regard, the proper use of theoretical methods has been invaluable. 

NH in its lowest singlet state (a A) inserts readily into paraffin CH bonds. In this electronic state the nitrene also abstracts hydrogen atoms from hydrocarbons and undergoes quenching to the ground triplet state. For example, the ratio of these channels is 0.6 0.1 03 in the case of the reaction of NH with ethane ... 

Negative hyperconjugation, 274, 275 Neighboring group effect, 82 Net Charge SHMO, 91-92 Mulliken, 236 Nickel tetracarbonyl, 252 Nitrene (FIN) electronic states, 116-117 S—T gap, 116... 

Nitrenes, 116-118, see also Nitrene (FIN) aminonitrenes, 118 from azide photolysis, 116 phenylnitrene, 118 substituents on, 117 Nitrenimn ion (NF12+) electronic states, 118-119 Nitreniun ions, 118-120, see also Nitrenimn ion (NF12+)... 

In the case of transient species with unpaired electrons such as free radicals, and the triplet states of carbenes or nitrenes, electron spin resonance (ESR) spectroscopy can provide unique evidence about the structure of the intermediate. Useful information about intermediates in reactions involving radical pair coupling can also be obtained by a technique known as chemically-induced dynamic nuclear polarization (CIDNP). However, detailed discussions of ESR and CIDNP are outside the scope of this book and for further information suitable text books on physical organic chemistry or the references given in the Further Reading section should be consulted. 

Smith, B.A. and Cramer, C.J. (1996) How do different fluorine substitution patterns affect the electronic state energies of phenyl nitrene . Am. Chem. Soc., 118, 5490. 

These results clearly showed that biradical 83 was not formed from ground state triplet nitrene 82, either classically or via tunneling. The anthors specnlated that the biradical might have been prodnced from a higher energy electronically excited nitrene than was available from irradiation of 82. Alternatively, it is possible that the biradical might have formed directly from excited azide. 

Aminonitrene (isodiazene) (145) is the least stable of all isomers of the elemental composition N2H2 and was isolated in an argon matrix for the first time in 1984.195 Theoreticians also gave much attention to aminonitrene (145).196 This molecule is exceptional in so far, that it is one of the few nitrenes with a singlet electronic ground state and is best described by the dipolar structure 145. ... 

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