Incisive probing of intermolecular interactions in molecular crystals: core level spectroscopy combined with density functional theory.

Joanna S Stevens,Daniel A Fischer,Cherno Jaye,Sven L M Schroeder,Che R Seabourne,Andrew J Scott

doi:10.1021/jp506983s

Abstract

The α-form of crystalline para-aminobenzoic acid (PABA) has been examined as a model system for demonstrating how the core level spectroscopies X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) can be combined with CASTEP density functional theory (DFT) to provide reliable modeling of intermolecular bonding in organic molecular crystals. Through its dependence on unoccupied valence states NEXAFS is an extremely sensitive probe of variations in intermolecular bonding. Prediction of NEXAFS spectra by CASTEP, in combination with core level shifts predicted by WIEN2K, reproduced experimentally observed data very well when all significant intermolecular interactions were correctly taken into account. CASTEP-predicted NEXAFS spectra for the crystalline state were compared with those for an isolated PABA monomer to examine the impact of intermolecular interactions and local environment in the solid state. The effects of the loss of hydrogen-bonding in carboxylic acid dimers and intermolecular hydrogen bonding between amino and carboxylic acid moieties are evident, with energy shifts and intensity variations of NEXAFS features arising from the associated differences in electronic structure and bonding.

Highlights

Local molecular conformations and intermolecular bonding determine many physical properties of the organic solid state, including, for example, crystal structure, melting point, lattice energy and solubility
Often cooperatively, more complex physicochemical behavior, such as the bioavailability of drugs.[1−3] Recent work has shown that core level spectroscopies, explicitly X-ray photoelectron spectroscopy (XPS)[4] and near edge X-ray absorption fine structure (NEXAFS),[5] can provide detailed insight into the underlying local chemical interactions and the electronic structure of molecular crystals.[6−14] They are very sensitive probes of hydrogen-bonding,[6,7,11,12,15−18] which has a strong effect on physical properties
Separations in energy are revealed in the pre-edge region of the NEXAFS spectra since resonances result from electron promotion from the core levels (Figure 3), and this is evident in the energy shifts between the 1s → 1π* peaks for the different carbon (Figure 3) and oxygen (Figure 4) species

Summary

Introduction

Local molecular conformations and intermolecular bonding determine many physical properties of the organic solid state, including, for example, crystal structure, melting point, lattice energy and solubility. Interpretation of NEXAFS spectra is facilitated by computational methodologies such as modern density functional theory (DFT),[21−25] static exchange (STEX),[26] algebraic diagrammatic construction (ADC(2)),[27,28] and restricted active space self-consistent field (RASSCF).[29] The primary focus has been centered on modeling individual molecules, effectively gas phase calculations, and one or more surrounding molecules While this can often reproduce many features, for inorganic-based systems,[25,29] there are clear implications for electronic structure in not accounting for the full structure (e.g., impact on energy levels, band gaps, molecular orbital distributions30) through intermolecular and long-range interactions/effects. Recent additions to the DFT code CASTEP31 enable the prediction of core-level spectra from crystal structure information.[32−34] This development has Received: July 13, 2014

Methods

Results

Conclusion