4 Summary
Cooperativity exhibited by purely C-H—O H-bonded networks in
linear and cyclic clusters of (1,3-cyclohexanedione)n(n=2 to 6) has been systematically studied using density functional
theoretical calculations. As linear cluster having cis conformers tend
to undergo cyclization after tetramer, only the all-trans conformers
were chosen for this purpose. In case of their cyclic counterparts,
trans conformers were found to get puckered. Therefore, only the all-cis
conformers were considered for cyclic clusters. It was found that the
linear clusters show anti-cooperative behavior, with H-bond strengths
and binding energies per bimolecular interaction decreasing
monotonically with increasing cluster size. On the other hand, their
cyclic counterparts were found to show positive cooperativity; both
H-bond strengths and binding energies per bimolecular interaction
systematically increasing with increasing cluster size. All the studied
linear clusters were found to have two C-H donor groups H-bonded to each
carbonyl oxygen, while in the cyclic clusters, a third C-H group
participates in H-bonding in trimer and higher clusters. The extent of
cooperativity has been found to show monotonic behavior for both linear
and cyclic clusters and was found to reach an asymptotic limit with
increasing cluster size. Natural bond orbital (NBO) analysis and atoms
in molecule (AIM) calculations were found to corroborate the obtained
results.