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.