Abstract:
A large set of lowest and medium energy conformers
of aliphatic tetramine trien was used to uncover
structural-topological preferences of poliamines. Numerous
common structural features among HL and H2L tautomers
were identified, e.g., H-atoms of protonated functional groups
are always involved in intramolecular NH•••N interactions
and they result in as large and as many as possible rings in
lowest energy conformers. Largest, 11-membered, molecular
rings stabilize a molecule most and they appeared to be strain
free whereas 5-memebred intramolecular rings were most
strained (all formed due to NH•••N interactions). The
CH•••HC interactions with QTAIM-defined atomic interaction
lines were also found but, surprisingly, mainly in the
lowest energy conformers of HL tautomers. According to
the non-covalent interaction-based (NCI) analysis, 5-
memebered rings formed by CH•••HC interactions are not
strained and, in general, 3D NCI isosurfaces mimic those obtained
for weaker NH•••N interactions. Also, 3D NCI
isosurfaces found for NH•••N and CH•••HC interactions, regardless
whether linked or not by an atomic interaction line,
appeared to be indistinguishable. Using lowest energy conformers,
theoretically predicted mixture of primary (HLp)
and secondary (HLs) forms of trien was found to be in accord
with the literature reports; using linear conformers resulted in predicting HLs as the only tautomer formed. In contrast to HF,
the overall performance of B3LYP was found satisfactory for
the purpose of the study.
