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Tan, Melissa

Publications and source records attributed to Tan, Melissa.

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Suppression of Disorder in Benzamide and Thiobenzamide Crystals by Fluorine Substitution

Disorder is a common feature of molecular crystals that complicates determination of structures and can potentially affect electric and mechanical properties. Suppression of disorder is observed in otherwise severely disordered benzamide and thiobenzamide crystals by substituting hydrogen with fluorine in the ortho-position of the phenyl ring. Fluorine occupancies of 20–30% are sufficient to suppress disorder without changing the packing motif. Crystal structure prediction calculations reveal a much denser lattice energy landscape for benzamide compared to 2-fluorobenzamide, suggesting that fluorine substitution makes disorder less likely.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Competing Singlet Fission and Excimer Formation in Solid Fluorinated 1,3-Diphenylisobenzofurans

Singlet fission (kSF) and excimer formation (kEXC) rate constants along with other photophysical properties of thin solid layers of 1,3-diphenylisobenzofuran and 11 of its fluorinated derivatives have been determined. The molecular properties of these compounds are similar, but their crystal packing varies widely. Most of them undergo singlet fission whereas excitation in others is trapped in excimers. The trend in rate constants kSF agrees qualitatively with results of calculations by a simplified version of the frontier orbital model for a molecular pair. The main shortcoming of the model is discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Melt Crystallization for Paracetamol Polymorphism

Trimorphic paracetamol, one of the most commonly used analgesic and antipyretic drugs, has been a model system for studying transformations among phases of molecular crystalline materials. During crystallization from the melt and the glass above 0 °C, three new polymorphs of paracetamol (N-acetyl- para -aminophenol or acetaminophen) were discovered, doubling the number of known ambient forms. The crystal structure of one new form was solved using a combination of powder X-ray diffraction and computational techniques. Growth kinetics became anomalous near the glass transition: as temperature decreased, growth rate increased; this rare and poorly understood phenomenon is commonly identified as the glass-to-crystal (GC) growth mode. In addition, two polymorphs displayed optical evidence of helicoidal morphologies, a characteristic of at least 25% of molecular crystals, that has been resistant to a universal explanation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

ROY Revisited, Again: The Eighth Solved Structure

X-ray powder diffraction and crystal structure prediction (CSP) algorithms were used in synergy to establish the crystal structure of the eighth polymorph of 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (ROY), form R05. R05 crystallizes in the monoclinic space group P2_1 with lattice parameters a = 11.479(4) Å, b = 11.030(1) Å, c = 10.840(6) Å, β = 118.23(1)°. This is both the first acentric ROY polymorph, and the first with Z' > 1. The torsion angles defined by the S-C-N-C atom sequence of each molecule in the asymmetric unit are -34.0° and 44.9°. These angles fall between those previously determined for the red and orange forms of ROY. Hirshfeld surface analysis was employed to understand the crystal packing and intermolecular interactions in R05 and an updated energy stability ranking was determined using computational methods. Finally, although the application of CSP was critical to the structure solution of R05, energy stability rankings determined using a series of DFT vdW-inclusive models substantially deviate from experiment, indicating that ROY polymorphism continues to be a challenge for CSP.

36 MATERIALS SCIENCE↗
DOE OSTI2017

The Third Ambient Aspirin Polymorph

Polymorphism in aspirin (acetylsalicylic acid), one of the most widely consumed medications, was equivocal until the structure of a second polymorph II, similar in structure to the original form I, was reported in 2005. Here, the third ambient polymorph of aspirin is described. Lastly, it was crystallized from the melt and its structure was determined using a combination of X-ray powder diffraction analysis and crystal structure prediction algorithms.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗