Abstract:
Investigation of cooperative effect exhibited by purely C-H—O hydrogen bonded (H-bonded) networks in linear and cyclic clusters of (1,3-cyclohexanedione)n (n = 2 to 6) has been carried out using density functional theoretical calculations. Linear clusters were found to show anti-cooperative behavior, while the cyclic clusters showed positive cooperativity. H-bond strengths and binding energies per bimolecular interaction were found to decrease with increasing cluster size for the linear clusters whereas their cyclic counterparts showed opposite trends. 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.
Introduction:
Hydrogen bond (H-bond) is one of the most extensively studied among various non-covalent interactions. The interest in H-bond is mainly owing to its importance in influencing the structures and activities of various chemical and biological systems, such as proteins, DNA, enzymes etc1,2,3,4,5,6. One of the more interesting attributes of H-bond is its role in formation and stabilization of molecular clusters at ambient condition by virtue of a special non-additive property, which is absent in covalent interactions, known as cooperative effect. It plays pivotal roles in various fields like structural stabilization of biological macromolecules, facilitating enzyme reactions and formation of stable supramolecules7,8,9,10,11,12,13,14. Depending on the collaborative or antagonistic nature of their mutual behavior, cooperativity could be classified as positive or negative, respectively. Cooperativity between H-bonds was first postulated qualitatively by Frank and Wen15 as early as in 1957. However, it took almost another 3-4 decades for quantitative estimation employing advanced spectroscopic16,17,18,and computational methods19,20,21,22. Since then, many research groups have extensively investigated the cooperative nature of H-bonds by studying several H-bonded molecular clusters like of water18,23,24,25,26ammonia27,28,291,3-diones30 carboxylic acids31amides32,33,34. It is worth mentioning here that a large majority of the existing works on cooperativity is focused on the behavior of classical H-bonds. Over the last couple of decades, however, various classes of weak H-bonding interactions have been found to play important roles in dictating structures, and consequently functions, of biological macromolecules35,36,37,Cooperative effects imparted by neighboring H-bonds, either classical or weak, on these bonds, and also in the reverse direction, inside the macromolecules are expected to provide appreciable impetus in their extra stabilization. This has prompted several studies on cooperativity involving weak H-bonds, a few of them dealing with negative cooperativity as well38,39,40,41,42,43,44. Nevertheless, most of these studies, that include both experimental45,17,46,47,48and theoretical49,50,51,52,53works, have dealt with how a weak H-bond cooperatively stabilizes (or destabilizes) a classical H-bond in either inter- or intra molecular fashion. On the other hand, not too many studies are available in existing literature on how weak H-bonds cooperatively stabilize themselves in molecular clusters, investigations on C-H—N interactions in HCN54and cyanoacetylene55clusters and C-H—π in ethyne clusters56 and S-H—S H-bond in H2S clusters57 being a few of them. However, to the best of our knowledge, there exists no work on the negative cooperativity exhibited by one weak H-bond on another.
Among various weak H-bonds known to us, C-H—O type has been studied very extensively over the past couple of decades, in an earlier work it was shown that C-H—O H-bond could dramatically facilitate the diketo to keto-enol tautomerization process in cyclic 1,3-diketones, the barrier for tautomerization being highly sensitive to both H-bond strength as well as size of the C-H—O H-bonded diketone cluster. 1,3-cyclohexanedione (CHD) has been widely considered to be a prototype for studies focused on cyclic 1,3-diketones58. When the diketo form of CHD forms homomeric clusters, then it is bound by only C-H—O type H-bonds. Due to the above reasons, molecular clusters of CHD could prove to be a very suitable system for studying the cooperative nature of C-H—O H-bonds. Besides, CHD provides multiple donors (via eight C-H bonds) and acceptor sites (via two carbonyl groups) which makes possible to monitor chelate effects on cooperative nature. As the change in cluster size would inevitably result in a change in relative orientation between adjacent monomeric moieties, it is expected that interactions between these multiple donor and acceptor sites would undergo mutual alteration.
In this work, we have investigated C-H—O H-bonded molecular clusters (CHD)n (n = 2 to 6) employing density functional theoretical methods. The evolution of geometric and energetic parameters with cluster size has been studied to assess the extent of cooperativity in those clusters. The geometric and energetic trends shown by C-H—O H-bonds have been corroborated with hyper conjugative charge-transfer energies calculated using natural bond orbital (NBO) analysis and electron density values at bond critical points provided by atoms in molecule (AIM) calculations.