Helical Organic and Inorganic Polymers

TL;DR

This paper develops an advanced ab initio Green's-function method for accurately studying infinite helical polymers, enabling detailed predictions of their electronic, structural, and vibrational properties, including complex incommensurable structures.

Contribution

It introduces a novel MBGF(2) approach with symmetry-adapted basis functions for infinite helical polymers, capable of handling incommensurable structures and providing comprehensive property predictions.

Findings

Validated methods on polyethylene, polyacetylene, and polytetrafluoroethylene.

Accurately simulated ultraviolet photoelectron spectra and vibrational spectra.

Predicted properties of hypothetical nitrogen and oxygen chains, suggesting potential metastability.

Abstract

Despite being a staple of synthetic plastics and biomolecules, helical polymers are scarcely studied with Gaussian-basis-set {\it ab initio} electron-correlated methods on an equal footing with molecules. This article introduces an {\it ab initio} second-order many-body Green's-function [MBGF(2)] method with nondiagonal, frequency-dependent Dyson self-energy for infinite helical polymers using screw-axis-symmetry-adapted Gaussian-spherical-harmonics basis functions. Together with the Gaussian-basis-set density-functional theory for energies, analytical atomic forces, translational-period force, and helical-angle force, it can compute correlated energy, quasiparticle energy bands, structures, and vibrational frequencies of an infinite helical polymer, which smoothly converge at the corresponding oligomer results. These methods can handle incommensurable structures, which have an infinite translational period and are hard to characterize by any other method, just as efficiently as commensurable structures. We apply these methods to polyethylene ($2/1$ helix), polyacetylene (Peierls' system), and polytetrafluoroethylene ($13/6$ helix) to establish the quantitative accuracy of MBGF(2)/cc-pVDZ in simulating their (angle-resolved) ultraviolet photoelectron spectra, and of B3LYP/cc-pVDZ or 6-31G** in reproducing their structures, infrared and Raman band positions, phonon dispersions, and (coherent and incoherent) inelastic neutron scattering spectra. We then predict the same properties for infinitely catenated chains of nitrogen or oxygen and discuss their possible metastable existence under ambient conditions. They include planar zigzag polyazene (N$_2$)$_x$ (Peierls' system), $11/3$-helical isotactic polyazane (NH)$_x$, $9/4$-helical isotactic polyfluoroazane (NF)$_x$, and $7/2$-helical polyoxane (O)$_x$ as potential high-energy-density materials.