Terahertz Spectroscopy and Quantum Mechanical Simulations of Crystalline Copper-Containing Historical Pigments

Abstract

Terahertz spectroscopy, a noninvasive and nondestructive analytical technique used in art conservation and restoration, can provide compelling data concerning the composition and condition of culturally valuable and historical objects. Terahertz spectral databases of modern and ancient artists' pigments exist but lack explanations for the origins of the unique spectral features. Solid-state density functional theory simulations can provide insight into the molecular and intermolecular forces that dominate the observed absorption features as well as reveal deviations from simple harmonic vibrational behavior that can complicate these spectra. The characteristic terahertz spectra of solid azurite, malachite, and verdigris are presented here, along with simulations of their crystalline structures and sub-3.0 THz lattice vibrations. The powerful combination of theory and experiment enables unambiguous spectral assignment of these complex materials and highlights the challenges that anharmonic peak broadening in organic-containing materials may present in the construction of reference pigment databases.

True Polymorphic Phase Transition or Dynamic Crystal Disorder? An Investigation into the Unusual Phase Behavior of Barbituric Acid Dihydrate

“…This is typical in such calculations, as the lattice dimensions used for normal mode calculations are contracted from the actual experimental dimensions due to neglect of thermal expansion in the DFT-optimized unit cells and contributions from basis set superposition error. , The result of constricting the unit cell dimensions is the general blue-shifting of vibrational frequencies as extensively shown in previous reports. In addition, vibrational anharmonicity of phonon modes contributes to overestimation of normal modes calculated in a harmonic limit. ,, Frequencies were scaled by 0.80 in the spectral overlays provided in Figure to aid in visual comparison. Lorentzian line shapes were convolved into calculated modes using the fwhm of the 33.6 cm –1 spectral feature to emphasize the large differences in line widths observed in the THz spectrum.…”

“…In addition, vibrational anharmonicity of phonon modes contributes to overestimation of normal modes calculated in a harmonic limit. 11,48,49 Frequencies were scaled by 0.80 in the spectral overlays provided in Figure 5 to aid in visual comparison. Lorentzian line shapes were convolved into calculated modes using the fwhm of the 33.6 cm −1 spectral feature to emphasize the large differences in line widths 2A).…”

Quantitative Analysis of Minium and Vermilion Mixtures Using Low-Frequency Vibrational Spectroscopy

“…These types of calculations explicitly include effects of the solid-state environment on the pigment structure and dynamics, and provide insight into the rotational, torsional, and other phonon modes that dominate the low-frequency spectral region and enable unambiguous spectral assignments. , Simulating THz-TDS and LFRS spectra with ss-DFT highlights the connections between chemical identity, crystalline structure, and the intermolecular forces that collectively contribute to the characteristic vibrational spectra of these materials. This approach has been successfully demonstrated on both organic and inorganic pigments. , …”

Terahertz Spectroscopic Analysis of the Vermilion Pigment in Free-Standing and Polyethylene-Mixed Forms

“…This fingerprinting capability of terahertz spectroscopy is highly advantageous in efficient identification of ancient and modern pigments. Consequently, a number of representative pigments, such as azurite, − malachite, − , verdigris, , and magenta, , have been extensively studied in the terahertz range, and a few spectral databases have been established at various institutes around the world . However, in these data sets, most often, only transmission spectra are given, which include rather distorted spectral features of individual absorption lines.…”

“…Furthermore, most existing terahertz databases of pigments contain only room-temperature spectra where a substantial broadening of all spectral lines is inevitable. This is especially relevant to our efforts to connect the experimental data to density functional theory (DFT) calculations frequently performed on crystal structures stabilized at absolute zero without consideration of thermal broadening. ,,, Consequently, closely spaced or nearly overlapping absorption lines are not amenable to accurate mode assignments based on DFT. Additionally, the anharmonic nature of some lattice vibrational modes predicted by DFT needs to be unveiled experimentally in the absence of thermal broadening at elevated temperatures.…”