Water Diffusion and Transport in Oil Paints as Studied by Unilateral NMR and 1 H High-Resolution MAS-NMR Spectroscopy.

Heavy metal carboxylate degradation severely affects thousands of oil paintings. Relative humidity has been reported to accelerate the rate of the reactions. To evaluate its role further, water diffusion and molecular mobility of protons in linseed oil-based lead white paints were studied by unilateral NMR and 1 H HRMAS spectroscopy. The results indicate that exposure to high %RH for relatively long times affects the dynamics of the oil paint's mobile fraction and that the effect is more pronounced as the thickness of the film increases. It was found that the paint can absorb appreciable amounts of water and has a porosity of approximately 6 % available for the diffusion of water, for which a regime of restricted diffusion was observed. Furthermore, the presence of bound and free-moving water, due to the possible formation of hydrated ionic-group clusters, supports the hypothesis of a polymeric/ionomeric network, as well as regions of essentially water free to move as in the bulk. The findings allow a better understanding of the role of water as a factor activating the degradation process in linseed oil-based lead white paints.

High-resolution 13 C and 43 Ca solid-state NMR and computational studies of the ethylene glycol solvate of atorvastatin calcium.

We report 43 Ca and 13 C solid-state nuclear magnetic resonance (NMR) spectroscopic studies of the ethylene glycol solvate of atorvastatin calcium. The 13 C and 43 Ca chemical shift and 43 Ca quadrupolar coupling tensor parameters are reported. The results are interpreted in terms of the reported X-ray diffraction crystal structure of the solvate and are compared with the NMR parameters of atorvastatin calcium trihydrate, the active pharmaceutical ingredient in Lipitor®. Hartree-Fock and density functional theory calculations of the NMR parameters based on a cluster model derived from the optimized X-ray diffraction crystal structure of the ethylene glycol solvate of atorvastatin calcium are in reasonable agreement with the experimental 43 Ca and 13 C NMR measurables.

Review of the use of NMR spectroscopy to investigate structure, reactivity, and dynamics of lead soap formation in paintings.

Heavy metal carboxylate or soap formation is a widespread deterioration problem affecting oil paintings and other works of art bearing oil-based media. Lead soaps are prevalent in traditional oil paintings because lead white was the white pigment most frequently chosen by old masters for the paints and in some cases for the ground preparations, until the development of other white pigments from approximately the middle of the 18th century on, and because of the wide use of lead-tin yellow. In the latter part of the 19th century, lead white began to be replaced by zinc white. The factors that influence soap formation have been the focus of intense study starting in the late 1990s. Since 2014, nuclear magnetic resonance (NMR) studies have contributed a unique perspective on the issue by providing chemical, structural, and dynamic information about the species involved in the process, as well as the effects of environmental conditions such as relative humidity and temperature on the kinetics of the reaction(s). In this review, we explore recent insights into soap formation gained through solid-state NMR and single-sided NMR techniques.

A new NMR crystallographic approach to reveal the calcium local structure of atorvastatin calcium.

We combine experimental and computational determination of 43Ca solid-state NMR parameters (chemical shift tensors, quadrupolar coupling tensors, and Euler angles) to constrain the structure of the local calcium-ligand coordination environment. A new 43Ca NMR crystallographic approach which includes an extensive survey of the Cambridge Structural Database and a new symmetry benchmark is developed to enhance the selectivity of structural screening. The application of this method to quadrupolar NMR crystallographic investigations is demonstrated by unearthing the calcium local structure of the active pharmaceutical ingredient atorvastatin calcium trihydrate, the active ingredient in Lipitor®, in the absence of diffraction data. This method has been tested by applying it to calcium acetate monohydrate which has a known structure.

Spin-orbit effects on the 125Te magnetic-shielding tensor: A cluster-based ZORA/DFT investigation.

Cluster-based calculations of 125Te magnetic-shielding tensors demonstrate that inclusion of spin-orbit effects is necessary to obtain the best agreement of theoretical predictions with experiment. The spin-orbit contribution to shielding depends on the oxidation state and stereochemistry of the 125Te site. Comparison of the performance of various density functionals indicates that GGA functionals behave similarly to each other in predicting NMR magnetic shielding. The use of hybrid functionals improves the predictive ability on average for a large set of 125Te-containing materials. The amount of Hartree-Fock exchange affects the predicted parameters. Inclusion of larger Hartree-Fock exchange contributions in hybrid functionals results in larger slopes of the correlation between calculated magnetic-shielding and experimental chemical-shift principal components, by 10-15% from the ideal value.

Molecular dynamics of palmitic acid and lead palmitate in cross-linked linseed oil films: Implications from deuterium magnetic resonance for lead soap formation in traditional oil paintings.

Many oil paintings, dating from the 15th century to the present, are affected by the formation of heavy-metal carboxylates (soaps) that alter the structural integrity and appearance of the works. Through transport phenomena not yet understood, free fatty acids formed from oils used as binders migrate through the paint film and react with heavy-metal ions that are constituents of pigments and/or driers, forming metal carboxylates. The local molecular dynamics of fatty acids and metal carboxylates are factors influencing material transport in these systems. We report temperature-dependent 2H NMR spectra of palmitic acid and lead palmitate as pure materials, in cross-linked linseed oil films, and in a lead white linseed oil paint film as part of our broader research into metal soap formation. Local dynamics at the α carbon, at the terminal methyl group, and at the middle of the fatty acid chain were observed in specifically deuterated materials. Changes in the dynamic behavior with temperature were observed by the appearance of two species, a solid-like material and a liquid-like material. The relative amounts of the two phases and their deuterium NMR parameters indicate that the amount of liquid-like material and the local dynamics at that site increase with temperature. At the three locations along the chain and at all temperatures, there is a larger percentage of acyl chains of both palmitic acid and lead palmitate that are "mobile" or liquid-like in linseed oil films than there are in the pure materials. However, the percentage of liquid-like species is decreased in a lead white paint film, as compared to a linseed oil matrix. In addition, these experiments indicate that there is a larger percentage of liquid-like acyl chains of palmitic acid than of lead palmitate under identical conditions in these model paint systems.

Calculations of solid-state 43 Ca NMR parameters: A comparison of periodic and cluster approaches and an evaluation of DFT functionals.

We present a computational study of magnetic-shielding and quadrupolar-coupling tensors of 43 Ca sites in crystalline solids. A comparison between periodic and cluster-based approaches for modeling solid-state interactions demonstrates that cluster-based approaches are suitable for predicting 43 Ca NMR parameters. Several model chemistries, including Hartree-Fock theory and 17 DFT approximations (SVWN, CA-PZ, PBE, PBE0, PW91, B3PW91, rPBE, PBEsol, WC, PKZB, BMK, M06-L, M06, M06-2X, M06-HF, TPSS, and TPSSh), are evaluated for the prediction of 43 Ca NMR parameters. Convergence of NMR parameters with respect to basis sets of the form cc-pVXZ (X = D, T, Q) is also evaluated. All DFT methods lead to substantial, and frequently systematic, overestimations of experimental chemical shifts. Hartree-Fock calculations outperform all DFT methods for the prediction of 43 Ca chemical-shift tensors. © 2017 Wiley Periodicals, Inc.

Semi-empirical refinements of crystal structures using 17O quadrupolar-coupling tensors.

We demonstrate a modification of Grimme's two-parameter empirical dispersion force field (referred to as the PW91-D2* method), in which the damping function has been optimized to yield geometries that result in predictions of the principal values of 17O quadrupolar-coupling tensors that are systematically in close agreement with experiment. The predictions of 17O quadrupolar-coupling tensors using PW91-D2*-refined structures yield a root-mean-square deviation (RMSD) (0.28 MHz) for twenty-two crystalline systems that is smaller than the RMSD for predictions based on X-ray diffraction structures (0.58 MHz) or on structures refined with PW91 (0.53 MHz). In addition, 13C, 15N, and 17O chemical-shift tensors and 35Cl quadrupolar-coupling tensors determined with PW91-D2*-refined structures are compared to the experiment. Errors in the prediction of chemical-shift tensors and quadrupolar-coupling tensors are, in these cases, substantially lowered, as compared to predictions based on PW91-refined structures. With this PW91-D2*-based method, analysis of 42 17O chemical-shift-tensor principal components gives a RMSD of only 18.3 ppm, whereas calculations on unrefined X-ray structures give a RMSD of 39.6 ppm and calculations of PW91-refined structures give an RMSD of 24.3 ppm. A similar analysis of 35Cl quadrupolar-coupling tensor principal components gives a RMSD of 1.45 MHz for the unrefined X-ray structures, 1.62 MHz for PW91-refined structures, and 0.59 MHz for the PW91-D2*-refined structures.

Analysis of the bond-valence method for calculating (29) Si and (31) P magnetic shielding in covalent network solids.

(29) Si and (31) P magnetic-shielding tensors in covalent network solids have been evaluated using periodic and cluster-based calculations. The cluster-based computational methodology employs pseudoatoms to reduce the net charge (resulting from missing co-ordination on the terminal atoms) through valence modification of terminal atoms using bond-valence theory (VMTA/BV). The magnetic-shielding tensors computed with the VMTA/BV method are compared to magnetic-shielding tensors determined with the periodic GIPAW approach. The cluster-based all-electron calculations agree with experiment better than the GIPAW calculations, particularly for predicting absolute magnetic shielding and for predicting chemical shifts. The performance of the DFT functionals CA-PZ, PW91, PBE, rPBE, PBEsol, WC, and PBE0 are assessed for the prediction of (29) Si and (31) P magnetic-shielding constants. Calculations using the hybrid functional PBE0, in combination with the VMTA/BV approach, result in excellent agreement with experiment. © 2016 Wiley Periodicals, Inc.

Spin-orbit effects on the (119)Sn magnetic-shielding tensor in solids: a ZORA/DFT investigation.

Periodic-boundary and cluster calculations of the magnetic-shielding tensors of (119)Sn sites in various co-ordination and stereochemical environments are reported. The results indicate a significant difference between the predicted NMR chemical shifts for tin(ii) sites that exhibit stereochemically-active lone pairs and tin(iv) sites that do not have stereochemically-active lone pairs. The predicted magnetic shieldings determined either with the cluster model treated with the ZORA/Scalar Hamiltonian or with the GIPAW formalism are dependent on the oxidation state and the co-ordination geometry of the tin atom. The inclusion of relativistic effects at the spin-orbit level removes systematic differences in computed magnetic-shielding parameters between tin sites of differing stereochemistries, and brings computed NMR shielding parameters into significant agreement with experimentally-determined chemical-shift principal values. Slight improvement in agreement with experiment is noted in calculations using hybrid exchange-correlation functionals.

Carbon-13 chemical-shift tensors in indigo: A two-dimensional NMR-ROCSA and DFT Study.

The principal components of the (13)C NMR chemical-shift tensors for the eight unique carbon sites of crystalline indigo have been measured using the ROCSA pulse sequence. The chemical shifts have been assigned unambiguously to their respective nuclear sites through comparison of the experimental data to the results of density-functional calculations employing a refined X-ray diffraction structure. These measurements expand the database of measured aromatic (13)C chemical-shift tensors to the indole ring. Magnetic shielding calculations for hypoxanthine and adenosine are also reported. Comparisons of calculations that include the effect of the crystalline lattice with calculations that model indigo as an isolated molecule give an estimate of the intermolecular contribution to the magnetic shielding.

207Pb and 119Sn solid-state NMR and relativistic density functional theory studies of the historic pigment lead-tin yellow type I and its reactivity in oil paintings.

Lead soaps (lead carboxylates) have been detected in traditional oil paintings in layers containing the pigment lead-tin yellow type I (LTY-I). LTY-I has been used by artists from at least the second quarter of the 15th century until the first half of the 18th century. Soap formation can lead to protrusions in paint layers and increased transparency, causing the paint support to become visible. We have characterized LTY-I by (119)Sn and (207)Pb solid-state NMR (ssNMR) spectroscopy. Using a combination of NMR techniques and DFT molecular cluster calculations, we identify the individual species in LTY-I and determine their (119)Sn and (207)Pb chemical-shift tensors. The presence of starting materials from the synthesis, minium, and tin(IV) oxide was also verified. Knowledge of the chemical-shift tensor components and the impurities in LTY-I is important for examining the chemistry of degradation processes and soap formation. We demonstrate that ssNMR can be used to detect reaction between Pb2SnO4 and added palmitic acid in a model paint sample containing LTY-I.

Density functional investigation of intermolecular effects on 13C NMR chemical-shielding tensors modeled with molecular clusters.

A quantum-chemical method for modeling solid-state nuclear magnetic resonance chemical-shift tensors by calculations on large symmetry-adapted clusters of molecules is demonstrated. Four hundred sixty five principal components of the (13)C chemical-shielding tensors of 24 organic materials are analyzed. The comparison of calculations on isolated molecules with molecules in clusters demonstrates that intermolecular effects can be successfully modeled using a cluster that represents a local portion of the lattice structure, without the need to use periodic-boundary conditions (PBCs). The accuracy of calculations which model the solid state using a cluster rivals the accuracy of calculations which model the solid state using PBCs, provided the cluster preserves the symmetry properties of the crystalline space group. The size and symmetry conditions that the model cluster must satisfy to obtain significant agreement with experimental chemical-shift values are discussed. The symmetry constraints described in the paper provide a systematic approach for incorporating intermolecular effects into chemical-shielding calculations performed at a level of theory that is more advanced than the generalized gradient approximation. Specifically, NMR parameters are calculated using the hybrid exchange-correlation functional B3PW91, which is not available in periodic codes. Calculations on structures of four molecules refined with density plane waves yield chemical-shielding values that are essentially in agreement with calculations on clusters where only the hydrogen sites are optimized and are used to provide insight into the inherent sensitivity of chemical shielding to lattice structure, including the role of rovibrational effects.

Multi-technique structural analysis of zinc carboxylates (soaps).

A series of medium- and long-chain zinc carboxylates (zinc octanoate, zinc nonanoate, zinc decanoate, zinc undecanoate, zinc dodecanoate, zinc pivalate, zinc stearate, zinc palmitate, zinc oleate, and zinc azelate) was analyzed by ultra-high-field 67Zn NMR spectroscopy up to 35.2 T, as well as 13C NMR and FTIR spectroscopy. We also report the single-crystal X-ray diffraction structures of zinc nonanoate, zinc decanoate, and zinc oleate-the first long-chain carboxylate single-crystals to be reported for zinc. The NMR and X-ray diffraction data suggest that the carboxylates exist in three distinct geometric groups, based on structural and spectroscopic parameters. The ssNMR results presented here present a future for dynamic nuclear polarization (DNP)-NMR-based minimally invasive methods for testing artwork for the presence of zinc carboxylates.

15N and 13C NMR chemical shifts of 6-(fluoro, chloro, bromo, and iodo)purine 2'-deoxynucleosides: measurements and calculations.

The (15)N and (13)C chemical shifts of 6-(fluoro, chloro, bromo, and iodo)purine 2'-deoxynucleoside derivatives in deuterated chloroform were measured. The (15)N chemical shifts were determined by the (1)H-(15)N HMBC method, and complete (15)N chemical-shift assignments were made with the aid of density functional theory (DFT) calculations. Inclusion of solvation effects significantly improved the precision of the calculations of (15)N chemical shifts. Halogen-substitution effects on the (15)N and (13)C chemical shifts of purine rings are discussed in the context of DFT results. The experimental coupling constants for (19)F interacting with (15)N and (13)C of the 6-fluoropurine 2-deoxynuleoside are compared with those from DFT calculations.

Nano- to microscale three-dimensional morphology relevant to transport properties in reactive porous composite paint films.

The quantitative evaluation of the three-dimensional (3D) morphology of porous composite materials is important for understanding mass transport phenomena, which further impact their functionalities and durability. Reactive porous paint materials are composites in nature and widely used in arts and technological applications. In artistic oil paintings, ambient moisture and water and organic solvents used in conservation treatments are known to trigger multiple physical and chemical degradation processes; however, there is no complete physical model that can quantitatively describe their transport in the paint films. In the present study, model oil paints with lead white (2PbCO3·Pb(OH)2) and zinc white (ZnO) pigments, which are frequently found in artistic oil paintings and are associated with the widespread heavy metal soap deterioration, were studied using synchrotron X-ray nano-tomography and unilateral nuclear magnetic resonance. This study aims to establish a relationship among the paints' compositions, the 3D morphological properties and degradation. This connection is crucial for establishing reliable models that can predict transport properties of solvents used in conservation treatments and of species involved in deterioration reactions, such as soap formation.

Infrared studies of lead(II) halide-1,10-phenanthroline photosensitive materials.

Infrared spectroscopic studies of 1:1 and 1:2 complexes of lead(II) bromide and lead(II) iodide with 1,10-phenanthroline were reported. Vibrational assignments are made by comparison to reported spectra of the uncomplexed 1,10-phenanthroline molecule. Small shifts of the ligand vibrational bands are characteristic of the complexes.

19F Magic Angle Spinning NMR Spectroscopy and Density Functional Theory Calculations of Fluorosubstituted Tryptophans: Integrating Experiment and Theory for Accurate Determination of Chemical Shift Tensors.

The 19F chemical shift is a sensitive NMR probe of structure and electronic environment in organic and biological molecules. In this report, we examine chemical shift parameters of 4F-, 5F-, 6F-, and 7F-substituted crystalline tryptophan by magic angle spinning (MAS) solid-state NMR spectroscopy and density functional theory. Significant narrowing of the 19F lines was observed under fast MAS conditions, at spinning frequencies above 50 kHz. The parameters characterizing the 19F chemical shift tensor are sensitive to the position of the fluorine in the aromatic ring and, to a lesser extent, the chirality of the molecule. Accurate calculations of 19F magnetic shielding tensors require the PBE0 functional with a 50% admixture of a Hartree-Fock exchange term, as well as taking account of the local crystal symmetry. The methodology developed will be beneficial for 19F-based MAS NMR structural analysis of proteins and protein assemblies.

Solid-state 207Pb NMR studies of mixed lead halides, PbFX (X=Cl, Br, or I).

Solid-state 207Pb NMR studies have been conducted on mixed lead(II) halides of the type PbFX, where X=Cl, Br, or I. NMR data for the mixed halides are compared to the solid-state NMR data for the divalent, binary lead halides, PbX2 (X=F, Cl, Br, I). The NMR data are evaluated in the context of the structures of the compounds and the effects of the mixed halides on the electronic structure of the divalent lead. Data sets for the mixed halides are discussed and compared to those for the regular lead(II) halides.

Role of Exact Exchange and Relativistic Approximations in Calculating 19F Magnetic Shielding in Solids Using a Cluster Ansatz.

Calculations of 19F magnetic shielding in various materials are presented. In calculations on gas-phase molecules, the variation of magnetic shielding with the amount of Hartree-Fock exchange (HFX) in the functional demonstrates that excellent agreement with experiment is obtained with an admixture of 50%, here denoted PBE0 (50%). Calculations at the PBE, PBE0 (25%), and PBE0 (50%) levels on 10 crystalline organofluorines and 15 crystalline inorganic fluorides, in which a cluster ansatz is used to model the lattice environment, were performed. For fluorine-containing aromatics, increasing the admixture of HFX results in the prediction of larger magnetic-shielding spans, whereas increasing the admixture of HFX in calculations for CFCl3 decreases the span. In calculations of 19F magnetic shielding of the inorganic fluorides, the use of sufficiently large clusters of inorganic fluorides results in accuracies similar to those calculated for the organofluorines. Relativistic effects on the magnetic shielding of inorganic fluorides, modeled with ZORA at both the scalar and spin-orbit levels, are dominated by the scalar terms that increase the shielding of most 19F sites over the non-relativistic results. These effects appear to scale with the atomic number of the cation. For most elements of the sixth row (Cs, Ba, La, and Pb), the scalar relativistic contribution to the magnetic shielding is in the range of 20-77 ppm. For elements of group XII (Zn, Cd, and Hg) bonded to fluorine, the scalar relativistic contribution results in deshielding of the 19F site.