Aziridines interaction energy

Fig. 44. Proton interaction energy trend along an approach path to the aziridine N. as obtained a) by the electrostatic approximation, b) by the Hartree approximation, c) by SCF computations (GTO wave function)...

In typical organic crystals, molecular pairs are easily sorted out and ab initio methods that work for gas-phase dimers can be applied to the analysis of molecular dimers in the crystal coordination sphere. The entire lattice energy can then be approximated as a sum of pairwise molecule-molecule interactions examples are crystals of benzene [40], alloxan [41], and of more complex aziridine molecules [42]. This obviously neglects cooperative and, in general, many-body effects, which seem less important in hard closed-shell systems. The positive side of this approach is that molecular coordination spheres in crystals can be dissected and bonding factors can be better analyzed, as examples in the next few sections will show. 

The analysis of the inversion barriers in aziridine, oxaziridine and methylene imine in terms of localized bond (lone-pair) energies and interactions [see Eq. (22)] agrees with the above description. In addition it provides further insight into the energetic origin of the barrier.The main results of this study are as follows 168> ... 

The calculations predict that fMws-diaziridine is more stable than the cis isomer by 7.8 kcal/niol (4-31G). This difference is close to a previous theoretical estimate of 7.1 kcal/mol by Bonaccorsi, Scrocco and Tomasi ) who used assumed geometries and a minimal Slater basis. The lower energy for the trans form can be attributed partly to the more favorable interaction between nitrogen lone pair dipoles. The theoretical bond separation energies (Table 1) at the 6-31G level are slightly less negative than the values for cyclopropane and aziridine. 

HOMO. Conversely, it will increase the polarisation for the LUMO and hence increase the effectiveness of the interaction of the LUMO of the dipole with the HOMO of the dipolarophile, as in Fig. 6.45b. The difference in energy for the two cases is small enough that firm prediction is not possible. In practice, dipole-HO control appears to be dominant, as shown by the formation of the adduct 6.323 from methyl acrylate, but it only needs the addition of an a-methyl group, for some dipole-LU control to become evident in the formation of some of the adduct 6.324 from methyl methacrylate, in addition to the aziridine 6.326 derived from the normal regioisomer 6.325.858 Perhaps the methyl group has raised the energy of both the HOMO and the LUMO of the dipolarophile, making the HOMO/LUMO separations still more nearly equal. 

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