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Equilibrium Structures of Three‑, Four‑, Five‑, Six-, and Seven-Membered Unsaturated N‑Containing Heterocycles
journal contribution
posted on 2015-03-05, 00:00 authored by Attila G. Császár, Jean Demaison, Heinz Dieter RudolphUp
to six different techniques are utilized to estimate the equilibrium
structures (re) of a series of mostly
unsaturated, N-containing heterocycles. Accurate Born–Oppenheimer
(reBO) and, if allowed, semiexperimental (reSE), as well as
empirical (rm-type) estimates of the equilibrium
structures of three-membered (1H- and 2H-azirine, aziridine), four-membered (azete), five-membered (pyrrole,
pyrazole, imidazole), six-membered (pyridine, pyrimidine, uracil),
and seven-membered (1H-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 (rm)
method also delivers surprisingly accurate structures for this class
of compounds.