Methylnitrene calculations

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-... 

Recently, Tsao et al. performed CASSCF/CASPT2 calculations of the properties of the cyclopropylnitrene (16). This study was motivated by the fact that cyclopropylcarbene can be chemically intercepted and has a lifetime of about 20 ns in solution at ambient temperature. Recall that singlet methylcarbene and singlet methylnitrene have not been detected or chemically intercepted and are predicted to isomerize to, respectively, ethylene and imine over a very small barrier of 1 to 3 kcal/mol. Calculations were performed to estimate the lifetime of 16 and its suitability for laser flash photolysis studies. ... 

The CASPT2(8,8)/6-31G(2d,p) calculations on structures along the intrinsic reaction coordinate (IRC) path at the HF/6-31G level predict that the fragmentation reaction is barrierless, but the ring-expansion reaction has a small activation barrier (=2.4 kcal/mol). Therefore, the calculations indicate that, unlike cyclopropylcarbene, singlet cyclopropylnitrene will have a very short lifetime in solution at ambient temperature, and, like methylcarbene and methylnitrene, 16 will be very difficult to detect. ... 

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. ... 

Kemnitz, C. R., Ellison, G. B., Karney, W. L., and Borden, W. X, CASSCF and CASPT2 ab initio electronic structure calculations find singlet methylnitrene is an energy minimum, /. Am. Chem. 

---