Singlet state methylnitrene

The CASSCF(12,ll)/cc-pVTZ optimized geometries for the lowest singlet state of methylnitrene, methyleneimine, md the transition state that connects these two energy minima are shown in Figure 5.6. The CASSCF(12,11) and CASPT2 energies are given in Table 5.2. 

Figure 5.6 CASSCF(12,ll)/cc-pVTZ geometries (bond lengths in angstroms, bond angles in degrees) of the stationary points along the lowest-energy pathway for the rearrangement of the lowest singlet state of methylnitrene (W MN) to methyleneimine (MI). ...

Thus, in a period of only a few years (1980 to 1987), methylnitrene in its triplet ground state had gone from the tennons category to what can be considered the best characterized of all known organic nitrenes. Nevertheless, the question of whether singlet methylnitrene 14 is an intermediate in the thermal and photochemical decomposition of 13 remained nnsolved. 

A very shallow minimum for singlet methylnitrene in the A state was found, and its rearrangement to 15 was studied using the CAS(10,8)/MP2 procedure. It was possible to localize a transition state of Cj symmetry only 1.4 kcal/mol above the minimum of singlet nitrene. 

Based on the results of these CASSCF/CASPT2 calculations, singlet methylnitrene is indeed predicted to be an energy minimum rather than a transition state the barrier to rearrangement is most likely in the range of 2.5 1 kcal/mol. Thus, at least in principle, 14 should be an observ-... 

In axial biradicals, the value of 2Kj is at its maximum relative to 2Kj, and the singlet-triplet splitting implied by the simple model is so large that the effects of electrons of the fixed core are usually not capable of inverting the order of the lowest singlet and triplet states. Many of the known ground-state triplet biradicals are of this type (e.g., methylnitrene). 

The situation with vinylnitrenes is analogous to methylnitrene and it is not clear if either of these singlet nitrenes are true reactive intermediates with finite lifetimes. Quantum chemical calculations can help to explain the properties of vinylnitrenes. All calculations reported in the literature have been concerned with only the simplest vinyl azide and vinylnitrene, CH2=CH-N3 and CH2=CH-N (29a), respectively. Early theoretical studies on the vinyl azide to azirine transformation employed semi-empirical calcula-tions, or ab initio calculations performed at relatively low levels of theory, and have focused only on the closed-shell singlet excited state ( A ) of vinylnitrene. We will discuss only the latest calculations. ... 

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