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.