Intermolecular interactions in binary mixtures of formamide and acetone

Trajectories from molecular dynamics simulations of binary mixtures of formamide (FA) and acetone (AT) have been analyzed across the entire concentration range by means of various pair distributions and topological network analysis techniques to probe the structure of the intermolecular interaction networks existing in these mixtures. Radial pair distributions clearly indicated H-bonding interactions between the two molecule types through acetone oxygen and the cis- and/or trans-hydrogens of formamide. Decomposition of pair energy distributions into possible intermolecular interaction types revealed that the most stable formamide-acetone dimers existing in the binary mixtures interact via an H-bonding. Orientational pair distributions have shown that both the AT-AT and the FA-FA pairs tend to adopt an antiparallel dipole orientation, as their respective pair interaction energies get smaller. And, the FA-AT pairs interacting stronger than -6.3 kcal/mol are found to adopt an orientation such that the angle between the dipole of AT and the C-N bond of FA is larger than 130 degrees. Network neighborhood analysis evidently revealed that the formamides do not show a strong preference for either molecule types. As acetone mole fraction of the mixture is increased, the added ATs are found to be capable of substituting the formamides, to a great extent, in the region spanning their second-order neighborhood. Conversely, as formamide content of the mixture is increased, the added FAs fail to adequately substitute the ATs in the region spanning their second-order neighborhood. Guided by our molecular dynamics based observations regarding the different types of intermolecular interactions existing between formamide and acetone, five FA-AT dimeric structures are highlighted. Their interaction energies are computed with density functional theory at the mPW2PLYP/def2-TZVP level. The most stable dimer is found to have an interaction energy of -7.5 kcal/mol, to involve an O center dot center dot center dot HC hydrogen-bonding, and to have nearly coplanar formamide and acetone. In addition, the DFT structures and interaction energies of trimers of the form AT center dot center dot center dot FA center dot center dot center dot AT and FA center dot center dot center dot AT center dot center dot center dot FA are also reported.