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The rigidity of poly- l-proline is an important contributor to the stability of many protein secondary structures, where it has been shown to strongly influence bulk flexibility. The experimental Young's moduli of two known poly- l-proline helical forms, right-handed all- cis (Form I) and left-handed all- trans (Form II), were determined in the crystalline state by using an approach that combines terahertz time-domain spectroscopy, X-ray diffraction, and solid-state density functional theory. Contrary to expectations, the helices were found to be considerably less rigid than many other natural and synthetic polymers, as well as differing greatly from each other, with Young's moduli of 4.9 and 9.6 GPa for Forms I and II, respectively. [ABSTRACT FROM AUTHOR]

Terahertz vibrational spectroscopy has evolved into a powerful tool for the detection and characterization of transition metal sulfate compounds, specifically for its ability to differentiate between various hydrated forms with high specificity. Copper(II) sulfate is one such system where multiple crystalline hydrates have had their terahertz spectra fully assigned, and the unique spectral fingerprints of the forms allows for characterization of multicomponent systems with relative ease. Yet the most commonly occurring form, copper(II) sulfate pentahydrate (CuSO4·5H2O), has proven elusive due to the presence of a broad absorption across much of the terahertz region, making the unambiguous identification of its spectral signature difficult. Here, it is shown that the sub-100 cm-1 spectrum of CuSO4·5H2O is obscured by absorption from adsorbed water and that controlled drying reveals sharp underlying features. The crystalline composition of the samples was monitored in parallel by X-ray diffraction as a function of drying time, supporting the spectroscopic results. Finally, the terahertz spectrum of CuSO4·5H2O was fully assigned using solid-state density functional theory simulations, helping attribute the additional absorptions that appear after excessive drying to formation of CuSO4·3H2O. [ABSTRACT FROM AUTHOR]

Terahertz spectroscopy probes thelow-frequency vibrations that are sensitive to both the intermolecularand intramolecular interactions of molecules in the solid state. Thus,terahertz spectroscopy can be a useful tool in the investigation ofcrystalline pharmaceutical compounds, where slight changes in thepacking arrangement can modify the overall effectiveness of a drugformulation. This is especially true for cases of polymorphic systems,hydrates/solvates, and cocrystals. In this work, the cocrystal offlufenamic acid with nicotinamide was investigated using terahertzspectroscopy and solid-state density functional theory. The solid-statesimulations enable understanding of the low-frequency vibrations seenin the terahertz spectra, while also providing insight into the energeticsinvolved in the formation of the cocrystal. The comparison of thecocrystal to the pure forms of the molecular components reveals thatthe cocrystal has better overall binding energy, driven by increasedintermolecular hydrogen bond strength and greater London dispersionforces and that the trifluoromethyl torsional potential is significantlydifferent between the studied solids. [ABSTRACT FROM AUTHOR]

Solid-statedensity functional theory is a powerful computationalmethod used to provide insight into the low-frequency vibrations ofcrystalline solids. A known limitation of this method is its generalunderestimation of weak intermolecular forces. Semiempirical Londonforce corrections have been developed to augment density functionaltheory calculations with the ultimate goal being corrections thatare applicable to a range of compounds. In this study, two structurallysimilar amino acids, asparagine and glutamine, were chosen to gaugethe proximity of the widely used DFT-D2 approach to this goal. Despitetheir chemical similarities, it was determined that the two molecularsolids required considerably different semiempirical correction magnitudes,with asparagine requiring a 42% greater London force correction factorwhen compared to glutamine. To validate these findings, terahertzspectroscopy was used to investigate the intermolecular vibrationsof both amino acids in the low-frequency, sub-100 cm–1region. The excellent correlation between the experimental and thetheoretical spectra demonstrates that the noncovalent interactionsare well represented by the applied model despite the correction disparity.These results have highlighted a practical shortcoming of a commonsemiempirical method for the modeling of weak forces and emphasizesthat care must be exercised for effective use of such correctionsin crystalline solids. [ABSTRACT FROM AUTHOR]

Abstract: The experimental mid-infrared spectra and structures of the α1, α2, and β phases of vanadium phosphate (VOPO4) are compared to solid-state density functional theory (DFT) simulations. The effect of basis set is also explored in the orbital descriptions of oxygen, and it is shown that there are drastic changes in the results for each crystal phase. Overall, there was a good agreement in both bond lengths and angles as well as the infrared vibrational modes between the experimental and theoretically obtained values. These results indicate that solid-state DFT provides sufficiently high accuracy to be able to distinguish between energetically similar yet distinct crystalline polymorphs, providing a tool for understanding and predicting the complex polymorphism in VOPO4 and other inorganic solids. [Copyright &y& Elsevier]

Flufenamic acid is the most polymorphic rich molecular solid currently known. Here, the origins of the two most common polymorphs of flufenamic acid (Form I and Form III) were investigated using terahertz spectroscopy and solid-state density functional theory. Terahertz spectroscopy probes the low-frequency vibrations that are sensitive to both the intermolecular and intramolecular interactions of molecules in the solid-state. This ability makes terahertz spectroscopy a particularly useful tool for characterizing polymorphic systems, where slight changes in packing arrangements distinguish different polymorphs. Solid-state density functional theory provides a means to achieve better understanding of the chemical origins of the low-frequency vibrations, while also revealing the energetic details of the polymorph relative stabilities. The exploration of these flufenamic acid polymorphs revealed that molecular conformation drives the formation of these particular forms, specifically the magnitude and orientation of the molecular dipoles dictate the preferred solid-state packing arrangements. [ABSTRACT FROM AUTHOR]

Highlights: [•] The THz spectra of hydrohalide salts of 5-(4-pyridyl)tetrazole were compared. [•] The structure and vibrational spectra were modeled using solid-state DFT methods. [•] The mass and size of the halide anion has a large effect on the vibrational modes. [•] The vibrational mode shifts were purely dependent on mass of halide anion. [ABSTRACT FROM AUTHOR]

Highlights: [•] The THz spectra of isomorphous metal 5-(4-pyridyl)tetrazolato complexes were compared. [•] The structure and vibrational spectra were modeled using solid-state DFT methods. [•] The mass and size of the central metal has a large effect on the vibrational modes. [•] A mode was found that shifts to higher frequencies as the mass of the metal increases. [ABSTRACT FROM AUTHOR]

Abstract: The role of London dispersion forces in crystalline magnesium nitrate hexahydrate [Mg(NO3)2·6H2O] has been investigated using terahertz spectroscopy and solid-state density functional theory modeling. The solid-state simulations were augmented with semi-empirical London dispersion corrections and revealed that such corrections are of negligible importance in this ionic solid. The comparison of the simulated and experimental crystal structures indicated a London force correction magnitude of less than 10% of that required in organic molecular solids of similar size and complexity. While London forces are certainly present in magnesium nitrate hexahydrate, electrostatic forces clearly dominate the intermolecular interactions governing its crystalline structure and dynamics. [Copyright &y& Elsevier]

The potential applications of terahertz (THz) spectroscopy in the analysis of pharmaceutical products in their crystalline state have prompted the need for a more thorough understanding of the fundamental vibrational motions contributing to the THz spectra. The detection of variations in crystal structure and the reliable assignment of observed THz absorption features can be aided by the use of solid-state density functional theory (DFT). In this study, solid-state DFT with periodic boundary conditions was used to simulate the crystalline structure and assign the experimental THz spectra (10-90 cm) of the enantiomerically pure and racemic forms of the common pharmaceutical compound ibuprofen. The results clearly demonstrate the capabilities of DFT methodologies to accurately reproduce the THz spectra of large complicated molecular systems and provide insight into the internal and external vibrational motions that form the basis of THz spectroscopy. © 2010 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 100:1116-1129, 2011 [ABSTRACT FROM AUTHOR]

Abstract: Crystalline 2-pyridone has been investigated using terahertz vibrational spectroscopy in the range of 10–90cm−1 (0.3–2.7THz). Solid-state density functional theory (B3LYP, BP, and PW91 with the 6-311G(d,p) basis set) was used to simulate and assign both observed terahertz spectral features and a previously published far-infrared spectrum up to 400cm−1. The PW91 functional was found to provide the best combination of crystal structure and vibrational frequency reproduction. Observed spectral features below 150cm−1 are assigned to intermolecular movements of the 2-pyridone chains within the unit cell. The use of independent intramolecular and intermolecular frequency scalars is proposed. [Copyright &y& Elsevier]

Through a combined experimental and theoretical investigation we determine that the fundamental modes of three quinacridones fall in the terahertz spectral range (1-10 THz, ~30-300 cm-1). In each spectrum the terahertz resonances correspond to wagging, rocking, or twisting of the quinacridone rings, with the most intense absorption being an in-plane rocking vibration of the carbonyl oxygens. In spite of these spectral similarities, we demonstrate that terahertz measurements readily differentiate β-quinacridone, γ-quinacridone, and 2,9-dimethylquinacridone. The spectrum of β-quinacridone has a group of closely spaced modes at ~4 THz, whereas in contrast the spectrum of γ-quinacridone displays a widely spaced series of modes spread over the range ~1-5 THz. Both of these have the strongest mode at ~9 THz, whereas in contrast 2,9-dimethylquinacridone exhibits the strongest mode at ~7 THz. Because quinacridones are the basis of widely used synthetic pigments of relatively recent origin, our findings offer promising applications in the identification and dating of modern art. [ABSTRACT FROM AUTHOR]

Terahertz(THz) vibrational spectroscopy is a promising tool forthe nondestructive and potentially noninvasive characterization ofhistorical objects, which can provide information on the materialsused for their production as well as identify and monitor their chemicaldegradation. Copper sulfate (CuSO4) has drawn interestdue to its inclusion in the preparation of iron gall inks found inhistorical artwork and documents. Copper sulfate rapidly forms hydrateswhich contribute to the formulation of these ink species and may influencetheir corrosive nature. In this study, copper sulfate has been studiedusing a combination of THz time-domain spectroscopy, powder X-raydiffraction (PXRD), and solid-state density functional theory (DFT)in order to better understand the spectral absorbances in the THzregion. The results have revealed that the THz spectrum of commerciallyavailable “anhydrous” copper sulfate results from thepresence of not only the anhydrous form but also the monohydrate (CuSO4·H2O) and trihydrate (CuSO4·3H2O) forms. Complete assignment of the experimental spectrumwas achieved through a comparison of density functionals and extensiveinvestigation of the influence of basis set polarization functionson the bonding interactions, lattice parameters, and low-frequencymotions in these crystalline solids. [ABSTRACT FROM AUTHOR]

The relative stabilities of crystallinepolymorphs are an important aspect of the manufacturing and effectiveutilization of pharmaceuticals. These stabilities are driven by bothmolecular conformational energy within the solid-state componentsand cohesive binding energy of the crystalline arrangement. The combinedapproach of experimental vibrational terahertz spectroscopy with solid-statedensity functional theory provides a powerful tool to study such propertiesand is applied here in the analysis of conformational polymorphismin crystalline aripiprazole. The low-frequency (<95 cm–1) terahertz vibrations of several aripiprazole polymorphs were measured,revealing distinct spectral features that uniquely identify each form.Solid-state density functional theory was employed to interpret theexperimental terahertz spectra, correlating the observed spectralfeatures to specific atomic motions within the crystalline lattice.The computational analysis provides insight into the formation andstability of the polymorphs by revealing the balance between the externalbinding forces and internal molecular forces that is ultimately responsiblefor the physical characteristics of the numerous crystalline polymorphsof aripiprazole. [ABSTRACT FROM AUTHOR]

Abstract: We report here, for the first time, the terahertz absorption spectrum of triacetone triperoxide (TATP). The experimental spectra are coupled with solid-state density functional theory, and preliminary assignments are provided to gain physical insight into the experimental spectrum. The calculated absorption coefficients are in excellent agreement with experiment. [Copyright &y& Elsevier]