Molecular Packing of Phenoxazine: A Combined Single-Crystal/Crystal Structure Prediction Study

Martin Kaltenegger,Patrick Brocorens,Sai Manoj Gali,Roland Resel,Roberto Lazzaroni,Hans Riegler,Yves Henri Geerts,Louis Delaive,Jie Liu,Oliver Werzer

doi:10.1021/acs.cgd.1c00691

Abstract

Phenoxazine is a heterocyclic molecule, which is used either as a parent molecule or with substituents for applications in various scientific fields: e.g., as a potential antioxidant. The purpose of this work is to present the molecular packing of phenoxazine within the crystalline state, as surprisingly no crystal structure is known so far. The crystal structure solution was performed by single-crystal X-ray investigations. Although the molecule has some potential for intermolecular hydrogen bonding, the observed structure is the classical herringbone packing typical for rodlike conjugated molecules. However, severe substitutional disorder of oxygen and nitrogen atoms is observed over their two opposite positions within the molecule. To get deeper insight into this disorder phenomenon, theoretical studies were performed, including crystal structure prediction using state of the art density functional theory techniques. The theoretical investigations confirm the experimentally observed 50% occupancy of the oxygen and nitrogen atoms.

Highlights

The compound of interest in this study is the small organic molecule phenoxazine (Figure 1), which is used in pharmaceutical research.[1,2] Often, phenoxazine works as a parent molecule and is found in different biological organisms.[1]
Hydrogen bonds are quite difficult to prove for phenoxazine in the solid state, since the molecule has a symmetrical shape and the oxygen and nitrogen atoms are difficult to distinguish by X-ray diffraction techniques
Several other organic crystals display substitutional disorder.[4−6] In this work, we present the single-crystal structure of phenoxazine, which was determined by experimental diffraction data

Summary

Introduction

The compound of interest in this study is the small organic molecule phenoxazine (Figure 1), which is used in pharmaceutical research.[1,2] Often, phenoxazine works as a parent molecule and is found in different biological organisms.[1]. Phenoxazine is a heterocyclic molecule that appears as a rigid and planar molecule. It consists of two phenyl rings connected by two covalent bridges. The connection is formed on one side by a bivalent oxygen atom and on the other side by a bivalent NH group. The presence of these two chemical species at the center of the molecule makes the molecule prone to generating intermolecular hydrogen bonds. Hydrogen bonds are quite difficult to prove for phenoxazine in the solid state, since the molecule has a symmetrical shape and the oxygen and nitrogen atoms are difficult to distinguish by X-ray diffraction techniques

Objectives

Results

Conclusion