Observed 13C Chemical Shifts Research Articles

Characterization of Chemisorbed Species and Active Adsorption Sites in Mg-Al Mixed Metal Oxides for High-Temperature CO2 Capture.

Mg–Al mixed metal oxides (MMOs), derived from the decomposition of layered double hydroxides (LDHs), have been purposed as adsorbents for CO2 capture of industrial plant emissions. To aid in the design and optimization of these materials for CO2 capture at 200 °C, we have used a combination of solid-state nuclear magnetic resonance (ssNMR) and density functional theory (DFT) to characterize the CO2 gas sorption products and determine the various sorption sites in Mg–Al MMOs. A comparison of the DFT cluster calculations with the observed 13C chemical shifts of the chemisorbed products indicates that mono- and bidentate carbonates are formed at the Mg–O sites with adjacent Al substitution of an Mg atom, while the bicarbonates are formed at Mg–OH sites without adjacent Al substitution. Quantitative 13C NMR shows an increase in the relative amount of strongly basic sites, where the monodentate carbonate product is formed, with increasing Al/Mg molar ratios in the MMOs. This detailed understanding of the various basic Mg–O sites presented in MMOs and the formation of the carbonate, bidentate carbonate, and bicarbonate chemisorbed species yields new insights into the mechanism of CO2 adsorption at 200 °C, which can further aid in the design and capture capacity optimization of the materials.

Perfect and Defective 13C-Furan-Derived Nanothreads from Modest-Pressure Synthesis Analyzed by 13C NMR.

The molecular structure of nanothreads produced by the slow compression of 13C4-furan was studied by advanced solid-state NMR. Spectral editing showed that >95% of carbon atoms were bonded to one hydrogen (C-H) and that there were 2-4% CH2, 0.6% C═O, and <0.3% CH3 groups. Alkenes accounted for 18% of the CH moieties, while trapped, unreacted furan made up 7%. Two-dimensional (2D) 13C-13C and 1H-13C NMR identified 12% of all carbon in asymmetric O-CH═CH-CH-CH- and 24% in symmetric O-CH-CH═CH-CH- rings. While the former represented defects or chain ends, some of the latter appeared to form repeating thread segments. Around 10% of carbon atoms were found in highly ordered, fully saturated nanothread segments. Unusually slow 13C spin-exchange with sites outside the perfect thread segments documented a length of at least 14 bonds; the small width of the perfect-thread signals also implied a fairly long, regular structure. Carbons in the perfect threads underwent relatively slow spin-lattice relaxation, indicating slow spin exchange with other threads and smaller amplitude motions. Through partial inversion recovery, the signals of the perfect threads were observed and analyzed selectively. Previously considered syn-threads with four different C-H bond orientations were ruled out by centerband-only detection of exchange NMR, which was, on the contrary, consistent with anti-threads. The observed 13C chemical shifts were matched well by quantum-chemical calculations for anti-threads but not for more complex structures like syn/anti-threads. These observations represent the first direct determination of the atomic-level structure of fully saturated nanothreads.

Real-time prediction of 1H and 13C chemical shifts with DFT accuracy using a 3D graph neural network.

Nuclear magnetic resonance (NMR) is one of the primary techniques used to elucidate the chemical structure, bonding, stereochemistry, and conformation of organic compounds. The distinct chemical shifts in an NMR spectrum depend upon each atom's local chemical environment and are influenced by both through-bond and through-space interactions with other atoms and functional groups. The in silico prediction of NMR chemical shifts using quantum mechanical (QM) calculations is now commonplace in aiding organic structural assignment since spectra can be computed for several candidate structures and then compared with experimental values to find the best possible match. However, the computational demands of calculating multiple structural- and stereo-isomers, each of which may typically exist as an ensemble of rapidly-interconverting conformations, are expensive. Additionally, the QM predictions themselves may lack sufficient accuracy to identify a correct structure. In this work, we address both of these shortcomings by developing a rapid machine learning (ML) protocol to predict 1H and 13C chemical shifts through an efficient graph neural network (GNN) using 3D structures as input. Transfer learning with experimental data is used to improve the final prediction accuracy of a model trained using QM calculations. When tested on the CHESHIRE dataset, the proposed model predicts observed 13C chemical shifts with comparable accuracy to the best-performing DFT functionals (1.5 ppm) in around 1/6000 of the CPU time. An automated prediction webserver and graphical interface are accessible online at http://nova.chem.colostate.edu/cascade/. We further demonstrate the model in three applications: first, we use the model to decide the correct organic structure from candidates through experimental spectra, including complex stereoisomers; second, we automatically detect and revise incorrect chemical shift assignments in a popular NMR database, the NMRShiftDB; and third, we use NMR chemical shifts as descriptors for determination of the sites of electrophilic aromatic substitution.

NMR chemical shifts of urea loaded copper benzoate. A joint solid-state NMR and DFT study

We report solid-state 13C NMR spectra of urea-loaded copper benzoate, Cu2(C6H5CO2)4·2(urea), a simplified model for copper paddlewheel-based metal-organic frameworks (MOFs), along with first-principles density functional theory (DFT) computation of the paramagnetic NMR (pNMR) chemical shifts. Assuming a Boltzmann distribution between a diamagnetic open-shell singlet ground state (in a broken-symmetry Kohn-Sham DFT description) and an excited triplet state, the observed δ(13C) values are reproduced reasonably well at the PBE0-⅓/IGLO-II//PBE0-D3/AE1 level. Using the proposed assignments of the signals, the mean absolute deviation between computed and observed 13C chemical shifts is below 30 ppm over a range of more than 1100 ppm.

Surface Interactions and Confinement of Methane: A High Pressure Magic Angle Spinning NMR and Computational Chemistry Study.

Characterization and modeling of the molecular-level behavior of simple hydrocarbon gases, such as methane, in the presence of both nonporous and nanoporous mineral matrices allows for predictive understanding of important processes in engineered and natural systems. In this study, changes in local electromagnetic environments of the carbon atoms in methane under conditions of high pressure (up to 130 bar) and moderate temperature (up to 346 K) were observed with 13C magic-angle spinning (MAS) NMR spectroscopy while the methane gas was mixed with two model solid substrates: a fumed nonporous, 12 nm particle size silica and a mesoporous silica with 200 nm particle size and 4 nm average pore diameter. Examination of the interactions between methane and the silica systems over temperatures and pressures that include the supercritical regime was allowed by a novel high pressure MAS sample containment system, which provided high resolution spectra collected under in situ conditions. For pure methane, no significant thermal effects were found for the observed 13C chemical shifts at all pressures studied here (28.2, 32.6, 56.4, 65.1, 112.7, and 130.3 bar). However, the 13C chemical shifts of resonances arising from confined methane changed slightly with changes in temperature in mixtures with mesoporous silica. The chemical shift values of 13C nuclides in methane change measurably as a function of pressure both in the pure state and in mixtures with both silica matrices, with a more pronounced shift when meso-porous silica is present. Molecular-level simulations utilizing GCMC, MD, and DFT confirm qualitatively that the experimentally measured changes are attributed to interactions of methane with the hydroxylated silica surfaces as well as densification of methane within nanopores and on pore surfaces.

Characterization of Red/Green Cyanobacteriochrome NpR6012g4 by Solution Nuclear Magnetic Resonance Spectroscopy: A Protonated Bilin Ring System in Both Photostates.

Cyanobacteriochromes (CBCRs) are cyanobacterial photoreceptors distantly related to phytochromes. Both CBCRs and phytochromes use photoisomerization of a linear tetrapyrrole (bilin) chromophore to photoconvert between two states with distinct spectral and biochemical properties, the dark state and the photoproduct. The isolated CBCR domain NpR6012g4 from Nostoc punctiforme is a well-characterized member of the canonical red/green CBCR subfamily, photosensory domains that can function as sensors for light color or intensity to regulate phototactic responses of filamentous cyanobacteria. Such red/green CBCRs utilize conserved Phe residues to tune the photoproduct for green light absorption, but conflicting interpretations of the photoproduct chromophore structure have been proposed. In the hydration model, the proposed photoproduct state is extensively solvated, with a loosely bound, conformationally flexible chromophore. In the trapped-twist model, the photoproduct chromophore is sterically constrained by hydrophobic amino acids, including the known Phe residues. Here, we have characterized chromophore structure in NpR6012g4 using solution nuclear magnetic resonance spectroscopy and a series of labeled chromophores. Four NH resonances are assigned for both the red-absorbing dark state and the green-absorbing photoproduct. Moreover, observed (13)C chemical shifts are in good agreement with those obtained for protonated rather than deprotonated bilins in ab initio calculations. Our results demonstrate that NpR6012g4 has a protonated, cationic bilin π system in both photostates, consistent with a photoproduct structure in which the chromophore is not extensively hydrated.

A Comparative 13C NMR Study of Local Ordering in a Homologous Series of Bent-core Liquid Crystals

A 13C NMR study is carried out in a number of banana-shaped molecules belonging to a homologous series. The derivatives contain chlorine or bromine substituent(s) in the center ring of 1,3-phenylene bis[4'-alkenyloxy biphenyl]-4-carboxylate (nPBBC), and different terminal chain lengths are systematically compared in terms of their local order parameters and their ability to form an aligned nematic phase in the NMR magnet. The chemical shift anisotropy tensors measured from nPBBC by fitting the 13C powder patterns are now extensively used on these molecules to interpret their observed 13C chemical shifts in the nematic phase. The bend angles are estimated as a function of temperature in two members of the nPBBC series. Conformation twists in the bent-core region inferred from the local molecular biaxial paramters are discussed.

High resolution 13C NMR studies for crystalline and liquid crystalline phases of PB-18 polyester composed of 4,4'-dihydroxybiphenyl and octadecanedioic acid

Main-chain liquid crystalline PB-18 polyester composed of 4,4'-dihydroxybiphenyl and octadecanedioic acid forms two distinct crystalline phases, K 1 and K 2, and smectic H ( S H) phase in the order of increasing temperature. The detailed structures of the three phases were studied by 13C solid-state NMR spectroscopy with variable temperature accessories. With a comparison of the observed 13C chemical shifts to the calculated 13C NMR shieldings by FPT-INDO method, the conformations of the mesogenic moiety and alkylene spacer are discussed among these three phases.

Bowl-shaped hydrocarbons related to C60

Ab initio studies at the HF/6-31G* and B3LYP/6-31G* levels are reported for two bowl-shaped hydrocarbons related to C60: C30H12 and C36H12, of C3 and C3v symmetry, respectively. The former has an approximate heat of formation of 211 kcal/mol. Bowl-to-bowl interconversion may occur through a planar (C3h) form of ca. 64 kcal/mol greater energy having one imaginary vibrational frequency. The larger C36H12 bowl has a calculated ΔH°f of 265 kcal/mol. Its HF/6-31G*, B3LYP/6-31G*, and MM3 bond lengths are in good agreement with a recent X-ray structure. Chemical shifts for both compounds calculated by the GIAO method are in good agreement with the measured NMR spectra. The observed 13C chemical shifts increase with the extent of pyramidalization. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 189–194, 1998

Molecular assemblies of diazafluorenone Schiff-base amphiphiles. III.13C NMR investigations of reversed micelles in deuteriochloroform

13C NMR investigations of a nonionic amphiphile of diazafluorenone Schiff base with an alkyl chain length of C14 (DAFSB-C14) in deuteriochloroform (CDCl3) indicate the formation of small reversed micellar aggregates. From the observed13C chemical shifts as functions of amphiphile concentration, the critical micelle concentration (cmc), aggregation numbers (n), and equilibrium constants for micelle formation (K) have been obtained. The values of chemical shifts have shown that DAFSB-C14 is aggregated using the head group. The π-π interactions between the rigid head groups, which constitute the main attractive force for aggregation, lead to the formation of relatively small aggregates. A plausible aggregation model is discussed.

NMR studies of oligosaccharides derived from hyaluronate: complete assignment of 1H and 13C NMR spectra of aqueous di- and tetra-saccharides, and comparison of chemical shifts for oligosaccharides of increasing degree of polymerisation

A series of oligosaccharides was prepared from hyaluronate by depolymerisation with bovine testicular hyaluronidase. Complete assignment of the 1H and 13C NMR spectra was obtained for the disaccharide, the tetrasaccharide, and the NaBH 4-treated tetrasaccharide, by using various 1D and 2D NMR methods. The 1H assignments for the tetrasaccharide differ from the incomplete data reported recently (ref. 11). The 13C NMR spectra of the aqueous di, tetra-, hexa-, and octa-saccharides of this series show that all resonances, apart from those subject to obvious end effects, have chemical shifts comparable to those of the corresponding resonances of hyaluronate in D 2O. The observed 13C chemical shifts suggests that cooperative intramolecular hydrogen bonds probably play a minor role in determining the conformation of hyaluronate in water.

Multinuclear NMR spectroscopic studies of aryltrimethylsilanes and aryldimethylphosphaneboranes

Proton, boron-11, carbon-13, silicon-29, and phosphorus-31 NMR chemical shifts and coupling constants are reported for nine ortho- and 2,6-disubstituted aryltrimethylsilanes and five similarly substituted aryldimethylphosphaneboranes. Resonances in the natural-abundance carbon-13 NMR spectra for both sets of derivatives are assigned on the basis of additivity relationships, proton-coupled spectra, and relative magnitudes of | J( 31P 13C) | coupling constants. Carbon-13 chemical shifts and | 1 J( 13C 1H) | coupling constants indicate that the P(BH 3)(CH 3) 2 group is electron-withdrawing. The 13C chemical shifts of aryl C(5) carbons can be attributed to steric inhibition of resonance of about the same magnitude as that produced by ortho-Si(CH 3) 3. Chemical shift and coupling constant data from previous work are expanded in terms of Taft's dual substituent constants σ 1 and σ R 0. Least squares solutions of these equations for aryldimethylphosphaneborane derivatives provide values of 0.41 for σ 1 and 0.04 for σ R 0 for the P(BH 3)(CH 3) 2 group. These constants produce reasonable agreement with observed 13C chemical shifts and coupling constants in the ortho derivatives.

A carbon-13 nuclear magnetic resonance study of solid tetracyclines

Magic angle sample spinning and proton/carbon-13 cross-polarization techniques have been used to obtain high resolution solid state 13C nuclear magnetic resonance spectra of several tetracycline antibiotics. Carbon-13 resonances in the solid state are assigned by measuring the extent of dipolar broadening due to neighbouring quadrupolar nuclei and by using a pulse sequence that suppresses non-quaternary carbon resonances. The observed 13C chemical shifts are discussed in light of the known solid state structures derived from X-ray diffraction, and are compared with those in solution. It is shown that the solid state nuclear magnetic reosnance technique is capable of clearly distinguishing between the neutral and zwitterionic forms of the tetracycline free bases.

The geometry of 4-(1-ethoxyethylidene)-5-oxazolones and thiazolones: 1H and 13C studies and the crystal structure of 4-[(Z)-1-ethoxyethylidene]-2-phenyl-5-oxazolone

A series of E and Z isomers of substituted 4-(1-ethoxyethylidene)-5-oxazolones and thiazolones have been prepared and their 1H and 13C spectra recorded. The vinylic methyl 1H chemical shifts showed minimal differences between E and Z isomers whereas the vinylic OCH21H signals differed by 0.15–0.43 ppm, with the Z isomer being consistently the more deshielded. Both vinylic methyl and OCH2 groups showed different 13C resonances for each isomer, with the Z isomers being the more deshielded. The Z geometry was conclusively defined for one isomer of 4-(1-ethoxyethylidene)-2-phenyl-5-oxazolone, 5, by X-ray crystallography and this was sufficient to assign the geometry of the remaining pairs of E and Z isomers. Oxazolone 5 has the space group P21/n and cell dimensions a = 9.219(3), b = 19.899(5), c = 7.459(1) Å, β = 118.01(2)°, and has four formula units in the unit cell. Intensities were measured with use of MoKα radiation and a Nicolet P3 diffractometer. The crystal structure was determined by standard methods and refined to R1 = 0.0709, R2 = 0.0696 based on 1419 independent reflections. The molecule is essentially planar and most bond lengths and angles are normal. Exceptions are the very short C(olefin)—O(ether) bond (1.339(4) Å) and the large ether C—O—C angle (122.1(3)°) caused by extreme delocalization in the O(ether)CCCO(carbonyl) system. The planarity causes a number of strong intramolecular repulsive interactions, causing an exceptionally small external olefin angle, O(ether)CC(methyl), of 108.1(4)°. The ethoxyl side chain of 5 adopts a conformation in the solid state which places the methylene of the OCH2 group adjacent to the oxazole ring nitrogen. This conformation is proposed to persist in solution phases and is consistent with the observed 13C chemical shifts and known γ and δ substituent effects.

Organoboron compounds: XXIII. Carbon-13 studies of phenylboranes

Correlations of the ortho, para, meta and C(1) carbon resonances in phenylboranes are reported. The observed 13C chemical shifts of the para carbon atoms in phenylboranes indicate shielding decreases in the order NHR ∼ NH 2 > SR > OR > organyls > halogens. The reversed trend is observed for the C(1) 13C chemical shifts which in addition are found to be sensitive to the steric nature of the substituents on boron.

Carbon‐13 NMR spectra of solid bicyclo[3. 3. 1]nonan‐9‐one. Conformational studies in the solid state

AbstractCarbon‐13 NMR spectra of solid polycrystalline bicyclo[3.3.1]nonan‐9‐one and adamantanone have been measured at 315K. The relatively narrow 13C linewidths observed for these solids, together with measured spin‐lattice relaxation times, indicate that both these solids are orientationally disordered. Observed 13C chemical shifts of solid bicyclo[3.3.1]nonan‐9‐one indicate a twin‐chair conformation for the two cyclohexanone rings.

Nuclear magnetic resonance studies of boron compounds : XVI. Carbon-13 studies of organoboranes: Phenylboranes and boron-substituted aromatic heterocycles

13C and 11B NMR data of 29 phenylboranes and 9 boron-substituted aromatic heterocycles (thiophene, N-methylpyrrole and furan) are discussed. The observed 13C chemical shifts of the para-carbon atoms in phenylboranes and the corresponding carbon atoms in the aromatic heterocycles are consistent with mesomeric interactions of the boryl group with the aromatic system. The trend of δ( 13C( para)) in phenylboranes corresponds to that observed for isoelectronic phenylcarbocations. Low temperature 13C NMR and/or 13C { 11B, 1H} heterocuclear triple resonance experiments were employed to obtain the 13C chemical shifts of the boron-bonded carbon atoms.

The structure of α‐(1,2‐dithiole‐3‐ylidene) ketones and α‐(1,2‐dithiole‐3‐ylidene) aldehydes studied by means of n.m.r. spectroscopy

Abstract1H and 13C n.m.r. studies of a series of twelve 1,2‐dithiole‐3‐ylidene ketones and aldehydes have shown that the geometry of the carbon backbone is the same as found in 1,6,6aλ4‐trithiapentalenes. No evidence has been found which favours a bicyclic structure for the system. A linear correlation of observed 13C chemical shifts with calculated charge densities is found to be valid. The observations are in agreement with a structure which is a hybrid between a true ketonic structure and a true mesoionic structure.By using the difference in the 13C chemical shifts of ortho and meta carbon atoms in substituent phenyl groups it is possible to qualify the degree of coplanarity of the phenyl groups with the backbone of the molecule.

Carbon-13 nuclear-magnetic-resonance studies on selected amino acids, peptides, and proteins.

Natural abundance high‐resolution carbon‐13 Fourier transform nuclear magnetic resonance (13C NMR) studies at 25 MHz have been carried out on selected amino acids, linear peptides and proteins. The pH dependence of the 13C resonances has been studied in detail in several cases. For l‐histidine, the 13C chemical shifts of all six carbon atoms are sensitive to the ionization state of the titrating carboxyl, imidazole and amino groups. The data for each carbon resonance have been computer‐fitted to a sum of three theoretical curves based on a simple proton‐association equilibrium for each transition. The extent of chemical shift change upon ionization of a nearby group has been interpreted in terms of two competing effects, inductive (through‐bond) and electric field (through‐space) effects.The 13C NMR spectra of the amino‐terminal 1–13, 1–15, and 1–20 peptides of bovine pancreatic ribonuclease A have been analyzed. Detailed resonance assignments have been made based on comparisons with the results for the free amino acids, shift effects for amino acids in small peptides, internal peptide bond effects and differences in amino acid composition. There is considerable correspondence between the observed 13C chemical shifts of these peptides and those predicted from the shifts of the component amino acids.Proteins of varying molecular weight have been studied to evaluate the potential of 13C NMR investigations of structure and conformation. A comparison has been made between the 13C NMR spectra of hen egg‐white lysozyme in its native and reduced states. The spectrum of the fully reduced species appears to be well represented by the sum of the spectra of the individual amino acids, both in chemical shift and line width, whereas that for the native form exhibits consistently broader resonances. Similar results were obtained for ribonuclease A. At the current level of resolution, it is impossible to discern whether this line broadening derives from shorter relaxation time effects due to restricted motion, chemical shift non‐equivalence between the same chemical groups, or both. However, it does appear that there are separately resolved peaks corresponding to Cβ‐carbons for different threonyl residues, as well as for the C‐4 ring carbons of the several tyrosyl residues in native hen egg‐white lysozyme. Spectra of carbonic anhydrases (Mr∼ 30000), which lack disulphide bonds, showed somewhat poorer resolution than the smaller proteins.