10.1021/jp5084168.s001
Attila G. Császár
Attila G.
Császár
Jean Demaison
Jean
Demaison
Heinz Dieter Rudolph
Heinz Dieter
Rudolph
Equilibrium Structures of Three‑, Four‑,
Five‑, Six-, and Seven-Membered Unsaturated N‑Containing
Heterocycles
American Chemical Society
2015
MP 2 level
technique
equilibrium structures
method
basis
semiexperimental
CCSD
wCVTZ
molecule
2015-03-05 00:00:00
Journal contribution
https://acs.figshare.com/articles/journal_contribution/Equilibrium_Structures_of_Three_Four_Five_Six_and_Seven_Membered_Unsaturated_N_Containing_Heterocycles/2189974
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.