Effect of the mixture composition of BmimBF4/PC on the solvation structure of C153 in as seen from molecular dynamics study

In this work we report the results of the molecular dynamics (MD) simulation of the effect of the mixture composition of 1-butyl-3-methylimidazolium terafluoroborate (BmimBF(4)) and propylene carbonate (PC) on the solvation structure of coumarin 153. For this purpose, the local structure around C153 was characterized using distance and angle descriptors of the C-H.A configuration. They are based on the calculation of the nearest neighbor radial distributions between the C or H atoms of one component of the solution (C153, PC, Bmim(+)) and the electronegative A atoms (F, O) of the other ones. We also considered the combined distribution functions that include the radial and the orientation between the ring of either the cation or the solvent and that of C153. Our results show that in neat ionic liquid, the BF4- anion and in particular its F atoms, is, as expected, located close to the hydrogen atoms of the C153, while the cation, in particular its ring hydrogen atoms, are located close to the F and O atoms of the C153 and its ring has an almost the above/below parallel orientation with respect to the C153 lactone ring. In neat PC, its O atoms are located in the direct environment of the H atoms of C153 and concomitantly, its ring has the above/below parallel orientation with respect to the C153 rings. By comparing the distance and angle descriptors values of the C-H...A configuration, with a threshold hydrogen bond distance 0.30 nm and an angle of 151 degrees, we were able to rank the C-H...A interactions as weak hydrogen bonds. Furthermore, in the excited state, noticeable changes in charge values on the H, F and O atoms of C153, occurs and as a consequence, the radial distribution of the anion, cation and the solvent around these atoms of C153 is more affected. At the mixture composition around 0.20 mol fraction of BmimBF(4), the behavior of the distance angle descriptors of the solvation of C153, indicates a cross over between the situation where solvation is dominated by the ions to that dominated by the solvent molecules. In the excited state of C153, the distance descriptor values are slightly decreasing with respect to those in the ground state while the angle descriptor values remain almost the same.

Automated summary

We investigated the solvation structure of coumarin 153 in a mixture of 1-butyl-3-methylimidazolium terafluoroborate and propylene carbonate using molecular dynamics simulation. Our results show that in the neat ionic liquid, the anion is located close to the hydrogen atoms of coumarin 153, while the cation is located close to the F and O atoms of coumarin 153. In neat propylene carbonate, the O atoms of propylene carbonate are located near the H atoms of coumarin 153. The behavior of the solvation structure changes at a mixture composition of 0.20, indicating a crossover between ion-dominated and solvent-dominated solvation.