Quantum-classical calculations of X-ray photoelectron spectra of polymers—Polymethyl methacrylate revisited

Abstract

In this work, we apply quantum mechanics/molecular mechanics (QM/MM) approach to predict core-electron binding energies and chemical shifts of polymers, obtainable via X-ray photoelectron spectroscopy(XPS), using polymethyl methacrylate as a demonstration example. The results indicate that standard parametrizations of the quantum part (basis sets, level of correlation) and the molecular mechanics parts (decomposed charges, polarizabilities, and capping technique) are sufficient for the QM/MM model to be predictive for XPS of polymers. It is found that the polymer environment produces contributions to the XPS binding energies that are close to monotonous with the number of monomer units, totally amounting to approximately an eV decrease in binding energies. In most of the cases, the order of the shifts is maintained, and even the relative size of the differential shifts is largely preserved. The coupling of the internal core-hole relaxation to the polymer environment is found to be weak in each case, amounting only to one or two tenths of an eV. The main polymeric effect is actually well estimated already at the frozen orbital level of theory, which in turn implies a substantial computational simplification. These conclusions are best represented by the cases where the ionized monomer and its immediate surrounding are treated quantum mechanically. If the QM region includes only a single monomer, a couple of anomalies are spotted, which are referred to the QM/MM interface itself and to the neglect of a possible charge transfer.

A QM/MM Study on the X-ray Spectra of Organic Proton Transfer Crystals of Isonicotinamides

“…The usage of point charges at relatively distant atoms provides a more accurate description of the electrostatic effect than the supermolecule-continuum model. Applications in XPS/XAS spectral simulations cover a wide range of systems, including organic crystals, metallic oxides, metalloenzymes, , surface adsorbates, gas phase polymers, and solutions. − For example, Scanlon et al simulated the Ti L 3 edge XPS spectra of TiO 2 by combining DFT with a polarizable force field, which helped understand the charge carrier separation between the rutile and anatase phases. Neese and co-workers used time-dependent DFT (TDDFT)/MM to study the Fe K-edge XAS spectra of a metalloenzyme, which helped understand its catalytic reaction.…”

“…Neese and co-workers 28 used time-dependent DFT (TDDFT)/MM to study the Fe Kedge XAS spectra of a metalloenzyme, which helped understand its catalytic reaction. Ågren and co-workers applied DFT/MM to calculate the K-edge XPS or BE of polymers 31 and ethanol 33 and glycine 32 in water solutions, and they also applied the so-called quantum mechanics capacitance molecular mechanics (QMCMM) method for a monolayer of small organic molecules absorbed on the Ag(111) surface. 30 Proton transfer crystals have more structural complexities than molecular crystals with pure neutral residues.…”

Decomposition of molecular properties

“…16 These localized properties have indeed been of much use as granulated force fields in QM/MM calculations of properties, ranging from NMR, EPR over to optical properties and X-ray spectroscopy. 1,5,[21][22][23][24] In a previous work 25 we showed how a LoProp polarizable force field can be derived quantum mechanically for extended environments by using analytical response theory. The theory was recently extended to construct the localized polarizable force fields to be frequency dependent.…”

Peak Broadening in Photoelectron Spectroscopy of Amorphous Polymers: The Leading Role of the Electrostatic Landscape

“…In this scheme, the MM region is described with a classically polarizable model of atomistic resolution. Various works report BEs or ionization potentials calculated in a QM/MM framework with ΔSCF − or GW . − However, only few of these works focus on amorphous solids, , and although some attempts have been made on polymer chains, , only the work by Ehlert et al reports calculated binding energies of a full amorphous model of a polymer, although calculated only for core levels using ΔSCF.…”