Experimental Detection of Achiral and Chiral Naturally Abundant ¹³C–²H Isotopomers by 2D-NMR in Liquids and Chiral Oriented Solvents

The recent technological and methodological developments in NMR spectroscopy have dramatically decreased its detection limit, hence expanding its application areas. Herein, we report the first experimental detection of 2H–13C isotopomeric isomers (isotopomers) in natural abundance (1.7 × 10–4%) both in liquids and oriented systems by combining cryoprobe, high magnetic field, and carefully chosen 2D heteronuclear correlation techniques. These experimental results require the detection of one molecule among 600 000 and demonstrate the sensivity and the selectivity of the employed NMR techniques. Besides, we show that 2H–13C enantio-isotopomers can be distinguished using 2D NMR in chiral polypeptide alignment media, thus providing a new tool for the chiral analysis. This method opens fruitful propects in various analytical fields such as molecular source authentication, metabolism studies, or the fight against counterfeiting

Enantiodiscrimination of Flexible Cyclic Solutes Using NMR Spectroscopy in Polypeptide Chiral Mesophases: Investigation of <i>cis</i>-Decalin and THF

The conformational dynamics and orientational behavior of two model cyclic molecules, cis-decalin (cis-dec) and tetrahydrofurane (THF), dissolved in weakly ordering, polypeptidic chiral liquid crystals (CLCs) are theoretically discussed and experimentally investigated using deuterium and carbon-13 NMR spectroscopies. The analysis of enantiomeric and enantiotopic discriminations in these compounds is shown to depend on the rate of conformational exchange regime, slow or fast. The slow exchange regime is illustrated through the case of cis-dec at low temperature (243 K). We show that the deuterium NMR spectra in this regime can be qualitatively and quantitatively interpreted by restricting the conformational pathway of cis-dec to two enantiomeric conformers of C2-symmetry. The orientational order parameters of these interconverting enantiomers are calculated by matching the 2H quadrupolar splittings with calculated conformer structures. The fast exchange regime is investigated through the examples of cis-dec at high temperature (356 K) and THF at room temperature (300 K). The 2H NMR spectra above the coalescence temperature are analyzed by introducing the concept of “average molecular structure”. This fictitious structure allows easily identifying NMR equivalences of solutes dissolved in CLC. However, it cannot be applied to determine consistent orientational order parameters. This study emphasizes that enantiotopic discriminations observed for flexible molecules in the fast exchange regime can be quantitatively interpreted only by considering the orientational order of each conformer

Primostrato Solid-State NMR Enhanced by Dynamic Nuclear Polarization: Pentacoordinated Al³⁺ Ions Are Only Located at the Surface of Hydrated γ‑Alumina

Aluminas (Al2O3) are ubiquitous functional materials. In particular, the γ-alumina form is extensively used in research and industry as a catalyst and catalyst support. Nevertheless, a full structural description, which would aid in comprehension of its properties, is lacking and under large debate. Solid-state NMR has been used previously to study γ-alumina but is limited for certain applications, such as surface studies, due to intrinsic low sensitivity. Here, we detail the implementation of low temperature (∼100 K) magic angle spinning combined with dynamic nuclear polarization (MAS-DNP) to significantly enhance the sensitivity of solid-state NMR experiments and gain structural insights into this important material. Notably, we analyze hydrophilic and hydrophobic sample preparation protocols and their implications on the sample and resulting NMR parameters. We show that the choice of preparation does not perturb the spectrum, but it does have a large effect on NMR coherence lifetimes, as does the corresponding required (hyper)­polarizing agent. We use this preliminary study to optimize the absolute sensitivity of the following experiments. We then show that there are no detectable hydroxyl groups in the bulk of the material and that DNP-enhanced 1H → 27Al cross-polarization experiments are selective to only the first surface layer, enabling a very specific study. This primostrato NMR is integrated with multiple-quantum magic angle spinning (MQMAS) and it is demonstrated, interestingly, that pentacoordinated Al3+ ions are only observed in this first surface layer. To highlight that there is no evidence of subsurface pentacoordinated Al3+, a new bulk-filtered experiment is described that can eliminate surface signals

Boron Adsorption Kinetics of Microcrystalline Cellulose and Polymer Resin

Tailoring boron–polysaccharide interactions is an important strategy for developing functional soft materials such as hydrogels, fire retardants, and sorbents for environmental remediation, for example, using lignocellulosic biomass. For such applications to be realized, it is paramount to understand the adsorption kinetics of borate anions on cellulose and their local structures. Here, the kinetic aspects of boron adsorption by microcrystalline cellulose, lignin, and polymeric resin are investigated and compared. Borate anions interact with the vicinal diols in the glucopyranoside moieties of cellulose to yield chemisorbed boron chelate complexes. In contrast to cellulose, technical lignin contains fewer cis-vicinal diols, and it does not have a tendency to form such chelate complexes upon treatment with the aqueous boric acid solution. The formation kinetics and stability of these chelate complexes strongly depend on nanoscale structures, as well as reaction conditions such as pH and concentration of the sorbate and sorbent. Specifically, insights into the distinct boron adsorption sites were obtained by solid-state one-dimensional (1D) 11B magic-angle spinning NMR and the local structures and intermolecular interactions in the vicinities of boron chelate complexes are elucidated by analyzing two-dimensional (2D) 1H–13C and 11B–1H heteronuclear correlation NMR spectra. The total boron adsorption capacity of cellulose is estimated to be in the 1.3–3.0 mg range per gram of sorbent, which is lower than the boron adsorption capacity of a polystyrene-based resin, ∼17.2 mg of boron per gram of Amberlite IRA 743. Our study demonstrates that the local backbone and side chain flexibility as well as the structures of polyol groups play a significant role in determining the kinetic and thermodynamic stability of chelate complexes, yielding to different boron adsorption capabilities of lignocellulosic polymers

Quantitative Analysis of the Proximities of OH Ligands and Vanadium Sites in a Polyoxovanadate Cluster Using Frequency-Selective ¹H–⁵¹V Solid-State NMR Spectroscopy

We introduce a magic-angle spinning NMR experiment to estimate specific distances in a solid material between a given site occupied by a quadrupolar nucleus and the nearby spin-1/2 nuclei. The new sequence, called DANTE-S-REDOR, consists of a frequency-selective dephasing experiment where heteronuclear dipolar couplings are reintroduced by applying a symmetry-based sequence (S-REDOR). The selectivity is achieved by applying a pulse train, such as Delays Alternating with Nutations for Tailored Excitation (DANTE), to the quadrupolar nucleus. This new method allows quantitative analysis of proximities in the 3–4 Å range of protons in OH ligands and one of the ⁵¹V sites in a complex decavanadate cluster, namely Cs₄[H₂V₁₀O₂₈]·4H₂O. The high selectivity of the DANTE-S-REDOR sequence offers the possibility to investigate a wide range of materials with different quadrupolar nuclei, including polyoxometalates, oxides, zeolites, and aluminophosphates

Advances in Structural Studies on Alkylaluminum Species in the Solid State via Challenging ²⁷Al–¹³C NMR Spectroscopy and X‑ray Diffraction

Advanced multinuclear solid state NMR experiments were developed to probe the structure of two organometallic aluminum derivatives, Li­[Al­(CH3)3CH2Si­(CH3)3] (1) and Li­[Al­(CH3)4] (2), which are relevant to olefin polymerization processes. For the first time, NMR observation of 27Al–13C covalent bonds in solids is performed with the natural abundance material 1. Unprecedented triple-resonance (1H–13C–27Al) and quadruple-resonance (1H–7Li–13C–27Al) heteronuclear correlation two-dimensional NMR experiments are also introduced to probe 27Al–13C and 13C–7Li proximities for 2. High-resolution solid-state NMR spectra thus obtained provide information on the local structure of these representative organometallic derivatives that proved to be most complementary and in full agreement with the structures obtained by X-ray diffraction

Advances in Structural Studies on Alkylaluminum Species in the Solid State via Challenging ²⁷Al–¹³C NMR Spectroscopy and X‑ray Diffraction

Advanced multinuclear solid state NMR experiments were developed to probe the structure of two organometallic aluminum derivatives, Li­[Al­(CH₃)₃CH₂Si­(CH₃)₃] (<b>1</b>) and Li­[Al­(CH₃)₄] (<b>2</b>), which are relevant to olefin polymerization processes. For the first time, NMR observation of ²⁷Al–¹³C covalent bonds in solids is performed with the natural abundance material <b>1</b>. Unprecedented triple-resonance (¹H–¹³C–²⁷Al) and quadruple-resonance (¹H–⁷Li–¹³C–²⁷Al) heteronuclear correlation two-dimensional NMR experiments are also introduced to probe ²⁷Al–¹³C and ¹³C–⁷Li proximities for <b>2</b>. High-resolution solid-state NMR spectra thus obtained provide information on the local structure of these representative organometallic derivatives that proved to be most complementary and in full agreement with the structures obtained by X-ray diffraction