Equilibrium Structures of Three‑, Four‑, Five‑, Six-, and Seven-Membered Unsaturated N‑Containing Heterocycles

Up to six different techniques are utilized to estimate the equilibrium structures (<i>r</i><sub>e</sub>) of a series of mostly unsaturated, N-containing heterocycles. Accurate Born–Oppenheimer (<i>r</i><sub>e</sub><sup>BO</sup>) and, if allowed, semiexperimental (<i>r</i><sub>e</sub><sup>SE</sup>), as well as empirical (<i>r</i><sub>m</sub>-type) estimates of the equilibrium structures of three-membered (1<i>H</i>- and 2<i>H</i>-azirine, aziridine), four-membered (azete), five-membered (pyrrole, pyrazole, imidazole), six-membered (pyridine, pyrimidine, uracil), and seven-membered (1<i>H</i>-azepine) rings, containing usually one but in some cases two N atoms, are determined. The agreement among the structural results of the different techniques is very satisfactory. It is shown that it is possible to use the CCSD­(T) electronic structure method with the relatively small wCVTZ basis set, with all electrons correlated, and the effect of further basis set enlargement, wCVTZ → wCVQZ, computed at the MP2 level, to obtain reliable equilibrium structures for the semirigid molecules investigated. Extension to larger basis sets does not significantly improve the accuracy of the computed results. Although all molecules investigated are oblate, and their principal axis system is subject to large rotations upon isotopic substitution, the semiexperimental method, when applicable, provides accurate results, though in the difficult cases it must be augmented with the mixed regression method. Finally, it is noteworthy that the empirical mass-dependent (<i>r</i><sub>m</sub>) method also delivers surprisingly accurate structures for this class of compounds.