Nitric Oxide Adsorption in MIL-100(Al) MOF Studied by Solid-State NMR

Adsorption of nitric oxide (NO) in the metal–organic framework (MOF) MIL-100­(Al) is studied by solid-state NMR. Owing to a modified synthesis, no extraframework benzenetricarboxylate is present on the cost of a small amount of extraframework Al­(OH)₃ as evident from ²⁷Al, ¹H, as well as heteronuclear correlation spectra. Five-coordinated aluminum sites represent about 50% of the aluminum in a dehydrated sample, which remain open for adsorption. With increasing NO loading, a decrease of five-coordinated aluminum with a subsequent increase of six-coordinated aluminum site intensity is found. Additionally, ¹H spin–lattice relaxation time <i>T</i>₁ is decreasing with an increasing amount of NO, which also supports the NO interaction with the MOF because of the paramagnetism of NO. Fourier-transform infrared spectroscopy data further hint at Al–NO interactions

Effects of Aromatic Substitution on the Photodimerization Kinetics of β-<i>trans</i> Cinnamic Acid Derivatives Studied with ¹³C Solid-State NMR

In our efforts to study photodimerizations in the solid state, we present data on the influence of the position of aromatic substitution by bromine on the photodimerization rate in cinnamic acid derivatives. Results were obtained by ¹³C CPMAS NMR spectroscopy together with chemical shift tensor analysis, DFT calculations using the NMR-CASTEP program, and crystal structure data. Reaction rates are highest for <i>para</i> bromo substitution, whose parent crystal structure was solved in this work. To explain the differences in photoreaction rate, several factors such as distance between double bonds, best π-orbital overlap of the reacting CC double bonds, and CSA tensor analysis (using 2D PASS) were taken into account. Calculations of ¹³C chemical shifts and chemical shift anisotropy tensor parameters show very good agreement with experimental data, including the carboxylic carbon that is hydrogen bonded to the neighboring cinnamic acid molecule. For the cinnamic acid photodimerization, the best angle between reacting double bonds and the smallest degree of molecular reorientation favor faster photoreaction

Effects of Aromatic Substitution on the Photodimerization Kinetics of β-<i>trans</i> Cinnamic Acid Derivatives Studied with ¹³C Solid-State NMR

In our efforts to study photodimerizations in the solid state, we present data on the influence of the position of aromatic substitution by bromine on the photodimerization rate in cinnamic acid derivatives. Results were obtained by ¹³C CPMAS NMR spectroscopy together with chemical shift tensor analysis, DFT calculations using the NMR-CASTEP program, and crystal structure data. Reaction rates are highest for <i>para</i> bromo substitution, whose parent crystal structure was solved in this work. To explain the differences in photoreaction rate, several factors such as distance between double bonds, best π-orbital overlap of the reacting CC double bonds, and CSA tensor analysis (using 2D PASS) were taken into account. Calculations of ¹³C chemical shifts and chemical shift anisotropy tensor parameters show very good agreement with experimental data, including the carboxylic carbon that is hydrogen bonded to the neighboring cinnamic acid molecule. For the cinnamic acid photodimerization, the best angle between reacting double bonds and the smallest degree of molecular reorientation favor faster photoreaction

Adsorption of Small Molecules on Cu₃(btc)₂ and Cu_3–<i>x</i>Zn_<i>x</i>(btc)₂ Metal–Organic Frameworks (MOF) As Studied by Solid-State NMR

Static and MAS ¹³C NMR techniques are used to investigate the interaction of CO and CO₂ molecules with the host structure of the MOFs Cu₃(btc)₂ and Cu_2.97Zn_0.03(btc)₂. A defined amount of ¹³C-enriched molecules per copper atom was adsorbed. The ¹³C chemical shift anisotropy and isotropic chemical shift were studied over a temperature range from 10 to 353 K. Already above 30 K an isotropic line for CO is found superimposed to the solidlike spectra belonging to the majority of adsorbed CO molecules. For adsorbed CO₂ an isotropic line can be detected above 70 K. This observation reflects differences in the local motion of both molecules. At high temperatures it is found that CO is desorbed more easily from the MOF framework in comparison to CO₂. This is in agreement with conclusions derived from desorption measurements on Cu₃(btc)₂. From the temperature dependence of the chemical shift for adsorbed CO₂ molecules (measured by means of ¹³C MAS NMR between 213 and 353 K) and from the deconvolution of the overlapping ¹³C NMR lines for adsorbed CO molecules (between 180 and 323 K), the activation energy for the local motion of the adsorbed molecules was determined as 3.3 and 6.1 kJ/mol, respectively. Additionally, the motion is accompanied by a partial desorption of the adsorbed species

Aging effects on WaMB transition temperature (A), Lorentzian line fraction (B) and on the amount of mobilisable water (C), expressed by the difference of the respective parameters after and before aging.

<p>Positive values indicate an increase, and negative values indicate a decrease in the parameter, respectively. Dotted line along zero shown in (C) distinguishes the positive and negative effects.</p

Hydrides of Alkaline Earth–Tetrel (AeTt) Zintl Phases: Covalent Tt–H Bonds from Silicon to Tin

Zintl phases form hydrides either by incorporating hydride anions (interstitial hydrides) or by covalent bonding of H to the polyanion (polyanionic hydrides), which yields a variety of different compositions and bonding situations. Hydrides (deuterides) of SrGe, BaSi, and BaSn were prepared by hydrogenation (deuteration) of the CrB-type Zintl phases AeTt and characterized by laboratory X-ray, synchrotron, and neutron diffraction, NMR spectroscopy, and quantum-chemical calculations. SrGeD_{4/3–<i>x</i>} and BaSnD_{4/3–<i>x</i>} show condensed boatlike six-membered rings of Tt atoms, formed by joining three of the zigzag chains contained in the Zintl phase. These new polyanionic motifs are terminated by covalently bound H atoms with <i>d</i>(Ge–D) = 1.521(9) Å and <i>d</i>(Sn–D) = 1.858(8) Å. Additional hydride anions are located in Ae₄ tetrahedra; thus, the features of both interstitial hydrides and polyanionic hydrides are represented. BaSiD_2–<i>x</i> retains the zigzag Si chain as in the parent Zintl phase, but in the hydride (deuteride), it is terminated by H (D) atoms, thus forming a linear (SiD) chain with <i>d</i>(Si–D) = 1.641(5) Å

Restructuring of a Peat in Interaction with Multivalent Cations: Effect of Cation Type and Aging Time

<div><p>It is assumed to be common knowledge that multivalent cations cross-link soil organic matter (SOM) molecules via cation bridges (CaB). The concept has not been explicitly demonstrated in solid SOM by targeted experiments, yet. Therefore, the requirements for and characteristics of CaB remain unidentified. In this study, a combined experimental and molecular modeling approach was adopted to investigate the interaction of cations on a peat OM from physicochemical perspective. Before treatment with salt solutions of Al³⁺, Ca²⁺ or Na⁺, respectively, the original exchangeable cations were removed using cation exchange resin. Cation treatment was conducted at two different values of pH prior to adjusting pH to 4.1. Cation sorption is slower (>>2 h) than deprotonation of functional groups (<2 h) and was described by a Langmuir model. The maximum uptake increased with pH of cation addition and decreased with increasing cation valency. Sorption coefficients were similar for all cations and at both pH. This contradicts the general expectations for electrostatic interactions, suggesting that not only the interaction chemistry but also spatial distribution of functional groups in OM determines binding of cations in this peat. The reaction of contact angle, matrix rigidity due to water molecule bridges (WaMB) and molecular mobility of water (NMR analysis) suggested that cross-linking via CaB has low relevance in this peat. This unexpected finding is probably due to the low cation exchange capacity, resulting in low abundance of charged functionalities. Molecular modeling demonstrates that large average distances between functionalities (∼3 nm in this peat) cannot be bridged by CaB-WaMB associations. However, aging strongly increased matrix rigidity, suggesting successive increase of WaMB size to connect functionalities and thus increasing degree of cross-linking by CaB-WaMB associations. Results thus demonstrated that the physicochemical structure of OM is decisive for CaB and aging-induced structural reorganisation can enhance cross-link formation.</p></div

Hydrides of Alkaline Earth–Tetrel (AeTt) Zintl Phases: Covalent Tt–H Bonds from Silicon to Tin

Zintl phases form hydrides either by incorporating hydride anions (interstitial hydrides) or by covalent bonding of H to the polyanion (polyanionic hydrides), which yields a variety of different compositions and bonding situations. Hydrides (deuterides) of SrGe, BaSi, and BaSn were prepared by hydrogenation (deuteration) of the CrB-type Zintl phases AeTt and characterized by laboratory X-ray, synchrotron, and neutron diffraction, NMR spectroscopy, and quantum-chemical calculations. SrGeD_{4/3–<i>x</i>} and BaSnD_{4/3–<i>x</i>} show condensed boatlike six-membered rings of Tt atoms, formed by joining three of the zigzag chains contained in the Zintl phase. These new polyanionic motifs are terminated by covalently bound H atoms with <i>d</i>(Ge–D) = 1.521(9) Å and <i>d</i>(Sn–D) = 1.858(8) Å. Additional hydride anions are located in Ae₄ tetrahedra; thus, the features of both interstitial hydrides and polyanionic hydrides are represented. BaSiD_2–<i>x</i> retains the zigzag Si chain as in the parent Zintl phase, but in the hydride (deuteride), it is terminated by H (D) atoms, thus forming a linear (SiD) chain with <i>d</i>(Si–D) = 1.641(5) Å

Structural–Spectrochemical Correlations of Variable Dimensionality Crystalline Metal–Organic Framework Materials in Hydrothermal Reactivity Patterns of Binary–Ternary Systems of Pb(II) with (a)Cyclic (Poly)carboxylate and Aromatic Chelator Ligands

Efforts to comprehend the structural–spectrochemical correlations of crystalline metal–organic framework materials of Pb­(II) with (a)­cyclic and aromatic chelators linked to photoluminescent applications led to the hydrothermal pH-specific synthesis of crystalline materials [Pb­{H₂BTC}­(phen)­(H₂O)]_<i>n</i>·2<i>n</i>H₂O­(<b>1</b>), [Pb₂{CBTC}]_<i>n</i>(<b>2</b>), [Pb₄(phen)₈{CBTC}₂(H₂O)₄]₃·70.3H₂O­(<b>3</b>), and [Pb­{HCTA}­(H₂O)₂]_<i>n</i>·<i>n</i>H₂O­(<b>4</b>). X-ray studies showed that <b>1</b>–<b>4</b> exhibit unique architectures linked to 2D–3D coordination polymers formulated by Z-type units composed of Pb₂O₂ cores, unusually high number of lattice–water molecules, and π–π and H-bond interactions. The contribution of the nature–structure properties of the aliphatic-(a)­cyclic organic (poly)­carboxylic/aromatic chelators-ligands to binary-ternary Pb­(II) reactivity weaves into the assembly of supramolecular networks, thereby providing clear structural–spectroscopic inter-relationships exemplifying the observed photoluminescent activity in a distinct MOF-linked fashion

Langmuir fit parameters of sorption curves.

<p>Langmuir fit parameters of sorption curves.</p