Cooperativity and cluster growth patterns in acetonitrile: a DFT study.

Cooperativity in intermolecular interactions and cluster growth patterns of acetonitrile has been studied using M06L density functional theory. Cyclic, ladder-type, stacked, cross-stacked, and mixed patterns are studied. Total interaction energy (E(int)) and interaction energy per monomer (E(m)) show maximum stability and cooperativity in stacked clusters followed by cross-stacked ones. As cluster size increased, magnitude of E(m) showed significant increase. Compared to E(m) of dimer (-2.97 kcal/mol), the increase is 2.6-fold for 27mer. Higher stabilization in larger clusters is attributed to strong cooperativity in intermolecular C-H···N and dipolar interactions. Enhanced cooperativity in stacked structures is supported by atoms-in-molecule electron density (ρ) data. Sum of ρ at intermolecular bond critical points is the highest for stacked clusters. Further, area of negative-valued molecular electrostatic potential is linearly related with E(int) and showed the lowest value in stacked followed by cross-stacked clusters, indicating maximum utilization of lone pair density and maximum cooperativity in such growth patterns. A red shift in the average C-N stretching frequencies with increase in the number of monomers and its direct correlation with E(int) in stacked clusters also supported their stability. Further, two known crystal patterns of acetonitrile (α and β) with 16 monomers are optimized and compared with the most stable hexadecamer pattern and are found to show lower values for E(int) and E(m) compared to the latter. Based on this result, we predict the existence of a third crystal pattern for acetonitrile which will be more ordered and more stable than α and β forms.