Concomitant Polymorphism in an Organic Solid: Molecular and Crystal Structure and Intra‐ and Intermolecular Potential Contributions to tert‐Butyl and Methyl Group Rotation

Abstract

We investigate the relationship between structure (crystal and molecular) and tert-butyl and methyl group dynamics in 2-(tertbutyl)-9-(4-(tert-butyl)phenyl)anthracene. Powder and singlecrystal X-ray diffraction, taken together, show that different polycrystalline samples recrystallized from different solvents have different amounts of at least four polymorphs (crystallites having different crystal structures), of which we have identified three by single crystal X-ray diffraction. The molecules in the asymmetric units of the different crystal structures differ by the dihedral angle the tert-butylphenyl group makes with the anthracene moiety. Ab initio electronic structure calculations on the isolated molecule show that very little intramolecular energy is required to change this angle over a range of about 60°which is probably the origin of the concomitant polymorphism (crystals of more than one polymorph in a polycrystalline sample). Solid state 1 H nuclear magnetic resonance (NMR) spin-lattice relaxation experiments support the powder and single-crystal X-ray results and provide average NMR activation energies (closely related to rotational barriers) for the rotation of the tert-butyl groups and their constituent methyl groups. These barriers have both an intramolecular and an intermolecular component. The latter is sensitive to the crystal structure. The intramolecular components of the rotational barriers of the two tert-butyl groups in the isolated molecule are investigated with ab initio electronic structure calculations.[a] Prof. Figure 1. The molecule 2-(tert-butyl)-9-(4-(tert-butyl)phenyl)anthracene. C atoms are black and green, H atoms in methyl groups in the plane of the neighboring aromatic ring are purple, H atoms in the out-of-plane methyl groups are cyan, and aromatic ring H-atoms are red. Of interest is the dihedral angle the 4-tert-butylphenyl group makes with the anthracene moiety defined using the four C atoms in green. For the isolated molecule in the ground state this dihedral angle is 68°(shown) or 112°(see Figure 2) but in the six independent molecules in three of the polymorphs found in the polycrystalline samples, this angle varies between 66 and 106°. See Table 2. The orientation of the two tert-butyl groups shown defines rotation angles of 0°for these groups.

Molecular Design of Pyrene and Its Derivatives to Make Microcrystalline Films with Silver Luster Effect Using Drop-Casting Method

“…Single-crystal X-ray crystallographic data of 1 and 3 have already been reported (Table ). , To make a comparison with these data, we ground the crystal films of 1 and 3 and performed powder XRD measurements. The powder XRD patterns of 1 and 3 with the ground crystal films and the pattern calculated from single-crystal X-ray crystallographic data showed similar patterns (Supporting Information, Figure S7a,c).…”

Single-Molecule Localization to Demonstrate the Optical Diffraction of Materials with 2D or Helical Structures

“…Although vibrational and rotational spectroscopy has also been used to identify the structure of molecules, those methods do not provide direct data about the structure of molecules but only provide information on specific parts of molecules, such as chemical functional groups. Hence, the technique of X-ray diffraction has provided a unique and helpful tool for chemists as well as biophysicists. ,,, Utilizing single-molecule localization with optical diffraction in our demonstration yields extremely accurate measurements of diffraction patterns.…”

“…Hence, the technique of X-ray diffraction has provided a unique and helpful tool for chemists as well as biophysicists. 3,4,6,7 Utilizing single-molecule localization with optical diffraction in our demonstration yields extremely accurate measurements of diffraction patterns.…”

“…In this demonstration, we introduce undergraduate and graduate students to this important concept using optical diffraction. X-ray diffraction crystallography is one of the best scientific tools to analyze the structures of biological molecules including DNA, RNA, proteins, and associated complexes, − as well as of chemicals, including inorganic supramolecules and most organic molecules. − Compared to spectroscopic tools, it offers a much more direct method to investigate the structure of molecules. Although vibrational and rotational spectroscopy has also been used to identify the structure of molecules, those methods do not provide direct data about the structure of molecules but only provide information on specific parts of molecules, such as chemical functional groups.…”

Modulation of Rotational Dynamics in Halogen-Bonded Cocrystalline Solids

“…Although noncovalent interactions, typically hydrogen bonds and sometimes halogen bonds, are often used as the anchor points of supramolecular rotors in, e.g., MOFs and other porous systems, the possible indirect or unanticipated steric and electronic effects of these noncovalent bonds on rotational energy barriers are less understood. The factors which may influence rotational energy barriers have been often discussed, − and while there are certain classes of systems whose rotational energy barriers may be well described by, e.g., a steric model, the role of noncovalent interactions as tools to expressly modulate rotational energy barriers is an open question. In a series of papers, Baudry − showed computationally that the rotational barriers of methyl groups may either increase or, somewhat counterintuitively, decrease depending on the microenvironment.…”