Molecular dynamics simulation of the squarate anion in acetonitrile solution

The structure and dynamics of a diluted acetonitrile solution of the squarate dianion, C4O42-, have been investigated by molecular dynamics (MD) simulation. Detailed comparison is provided with previous simulations of C4O42- in aqueous solution. The first solvation shell around the C4O42- ion is made of 16.7 acetonitrile molecules on average, which are much less tightly bonded to the anion than water molecules in the case of aqueous solution. Probability density maps give a detailed picture of the distribution of nearest-neighbour acetonitrile molecules around the C4O42- ion. Time correlation functions indicate that rattling and librational motions of the C4O42- species in cages formed by acetonitrile molecules is not too pronounced as observed in aqueous solution, proper to the less defined solvation shell in acetonitrile solution. The differences between the MD results on acetonitrile and aqueous solutions are fully consistent with the physical picture that emerged from previous experimental works on Raman bandshape analysis of oxocarbon ions in these solvents.