Nonbonding interaction analyses on PVDF/[BMIM][BF₄] complex system in gas and solution phase

The present study provides a detailed quantum chemical description of the physicochemical interactions between poly-vinylidene fluoride (PVDF) and 1-butyl-3-methyl-imidazolium tetrafluoro borate ([BMIM][BF₄]) ionic liquid (IL). Geometry optimization and frequency calculations are carried out for four monomer units of α- and β-PVDF, [BMIM][BF₄], and PVDF/[BMIM][BF₄] using dispersion corrected density functional theory. The effects of solvation on the systems under study are demonstrated for three polar aprotic solvents, namely tetra-hydrofuran (THF), acetone, and n,n-dimethyl formamide (DMF) using the integral equation formalism polarizable continuum model (IEFPCM). Calculated negative solvation free energy values suggest solution phase stability of the systems under study. Binding and interaction energies for β-PVDF/IL are found higher in magnitude than those for α-PVDF/IL. The nonbonding interaction phenomenon of β-PVDF/[BMIM][BF₄] is elucidated on the basis of natural bond orbital (NBO), Bader's quantum theory of atoms in molecules (QTAIM), delocalization indices, Hirshfeld surface, and reduced density gradient (RDG) analyses. Both anions and cations of ionic liquids are found to show weak van der Waals interaction with PVDF molecule but the anion ([BF₄]^-)/PVDF interaction is found to be stronger than cation ([BMIM]⁺)/PVDF interaction. Inter-unit C-H⋯F type hydrogen bonds are found to show improper (causing blue shifts in vibrational frequencies) nature. Frontier molecular orbital analysis is carried out, and different chemical parameters like electronegativity, chemical potential, chemical hardness and softness, and electrophilicity index are calculated using Koopmans' theorem. Thermochemical calculations are also performed, and the variation in different standard thermodynamic parameters with temperature is formulated. Graphical abstract (a) Hirshfeld surface mapped onto electron density and (b) NCI isosurfaces showing inter-unit interactions of β-PVDF/[BMIM][BF₄].