Oxygen K-edge inner-shell calculations of polymers in solutions realized by the extraction of local structures from molecular dynamics simulations

Abstract

Inner-shell quantum chemical calculations of large molecular systems, such as polymers and soft matter in solution, were performed to understand the phase transition dynamics of these systems using soft x-ray absorption spectroscopy (XAS). The molecular structures of 40-mer poly(N-isopropylacrylamide) (PNIPAM) chains in solutions were obtained using molecular dynamics simulations. The 5-mer PNIPAM chains with terminated H atoms, including the second coordination shells of the solvent methanol and water molecules, were extracted from the 40-mer PNIPAM chains in solutions. The O K-edge inner-shell spectra of the 5-mer PNIPAM chains were obtained by averaging the inner-shell spectra of 9700 extracted polymer structures. This calculation method can be used to precisely evaluate the energy shifts of the C=O π* peaks of PNIPAM caused by the structural changes of the polymer chains, the substitutions of the hydrogen bonds of the C=O groups in PNIPAM from methanol to water molecules, and the increase in the coordination numbers of solvent molecules with the C=O groups, which were observed in the O K-edge XAS experiments.