Non additivity of ligand effects on 59Co NMR chemical shifts in Co(III) complex compounds

⁵⁹Co chemical shift in a number of Co(III) complexes of the type CoA_6-xB_x(x = 0–6, A and B denote corresponding ligators) containing mono- and/or bidentate ligands, was measured. In almost all cases studied it was found that chemical shift appear at lower field than expected on the basis of the Rule on Additivity of Ligand Effects on ⁵⁹Co chemical shift. The non additivity of the ligand effects on ⁵⁹Co chemical shift has been discussed in terms of ligand field theory. It was shown that negative deviations from additivity has to be expected.     

Simulation of 59Co NMR Chemical Shifts in Aqueous Solution

⁵⁹Co chemical shifts were computed at the GIAO‐B3LYP level for [Co(CN)₆]^3?, [Co(H₂O)₆]³⁺, [Co(NH₃)₆]³⁺, and [Co(CO)₄]^? in water. The aqueous solutions were modeled by Car–Parrinello molecular dynamics (CPMD) simulations, or by propagation on a hybrid quantum‐mechanical/molecular‐mechanical Born–Oppenheimer surface (QM/MM‐BOMD). Mean absolute deviations from experiment obtained with these methods are on the order of 400 and 600 ppm, respectively, over a total δ(⁵⁹Co) range of about 18 000 ppm. The effect of the solvent on δ(⁵⁹Co) is mostly indirect, resulting primarily from substantial metal–ligand bond contractions on going from the gas phase to the bulk. The simulated solvent effects on geometries and δ(⁵⁹Co) values are well reproduced by using a polarizable continuum model (PCM), based on optimization and perturbational evaluation of quantum‐mechanical zero‐point corrections.     

Computational protocols for prediction of solute NMR relative chemical shifts. a case study of L-tryptophan in aqueous solution

In this study, we have applied two different spanning protocols for obtaining the molecular conformations of L-tryptophan in aqueous solution, namely a molecular dynamics simulation and a molecular mechanics conformational search with subsequent geometry re-optimization of the stable conformers using a quantum mechanically based method. These spanning protocols represent standard ways of obtaining a set of conformations on which NMR calculations may be performed. The results stemming from the solute-solvent configurations extracted from the MD simulation at 300 K are found to be inferior to the results stemming from the conformations extracted from the MM conformational search in terms of replicating an experimental reference as well as in achieving the correct sequence of the NMR relative chemical shifts of L-tryptophan in aqueous solution. We find this to be due to missing conformations visited during the molecular dynamics run as well as inaccuracies in geometrical parameters generated from the classical molecular dynamics simulations.     

pH-induced protonation of lysine in aqueous solution causes chemical shifts in X-ray photoelectron spectroscopy

We demonstrate the applicability of X-ray photoelectron spectroscopy to obtain charge- and site-specific electronic structural information of biomolecules in aqueous solution. Changing the pH of an aqueous solution of lysine from basic to acidic results in nitrogen 1s and carbon 1s chemical shifts to higher binding energies. These shifts are associated with the sequential protonation of the two amino groups, which affects both charge state and hydrogen bonding to the surrounding water molecules. The N1s chemical shift is 2.2 eV, and for carbon atoms directly neighboring a nitrogen the shift for C1s is approximately 0.4 eV. The experimental binding energies agree reasonably with our calculated energies of lysine(aq) for different pH values.     

Calculation of the (29)si NMR chemical shifts of aqueous silicate species

A DFT methodology for calculating (29)Si NMR chemical shifts of silicate species typically present prior to nucleation in zeolite synthesis solutions, incorporating solvent effects through an implicit representation is presented. We demonstrate how our methodology can reproduce the experimentally observed spectra and, by comparison to well characterized peaks in two different experimental studies, demonstrate the transferability and robustness of the methodology. We discuss certain cases in which caution must be exercised when implicit solvent representations are used for calculating silicate cluster geometries: those cases in which intramolecular hydrogen bonding can play a significant role in the geometry. A number of reassignments of previous tentative experimental assignments are proposed, and we also make assignments for the challenging substituted four-ring species. We present all of our computed chemical shift for previously observed species together with a number of other viable silicate clusters to serve as a reference point for future experimental studies.     

NMR chemical shifts. Substituted acetylenes

The MPW1PW91/6-311+G(2d,p) and MP2/6-311+G(2d,p) GIAO nuclear shieldings for a series of monosubstituted acetylenes have been calculated using the MP2/6-311G(2d,p) geometries. Axially symmetric substituents such as fluorine may lead to large changes in the isotropic shielding but have little effect on the tensor component (zz) about the C[triple bond]C bond axis. On the other hand, substituents such as vinyl and aldehyde groups lead to essentially no difference in the isotropic shielding but are calculated to give a large zz paramagnetic shift to the terminal carbon of the acetylene group, without having much effect on the inner carbon. The tensor components of the chemical shifts for trimethylsilylacetylene, methoxyacetylene, and propiolaldehyde have been measured and are in reasonable agreement with the calculations. The downfield shift at the terminal carbon of propiolaldehyde along with a small upfield shift at the adjacent carbon has been found to result from the coupling of the in-plane pi MO of the acetylene with the pi* orbital that has a node near the central carbon. The tensor components for acetonitrile also have been measured, and the shielding of cyano and acetylenic carbons are compared.     

Oxygen-17 NMR chemical shifts of esters

The ¹⁷O NMR spectra of some esters are discussed and are correlated with ¹³C NMR chemical shift and IR carbonyl streching band data.     

NMR solvent shifts of adenine in aqueous solution from hybrid QM/MM molecular dynamics simulations

We present first principles calculations of the NMR solvent shift of adenine in aqueous solution. The calculations are based on snapshots sampled from a molecular dynamics simulation, which were obtained via a hybrid quantum-mechanical/mechanical modeling approach, using an all-atom force field (TIP3P). We find that the solvation via the strongly fluctuating hydrogen bond network of water leads to nontrivial changes in the NMR spectra of the solutes regarding the ordering of the resonance lines. Although there are still sizable deviations from experiment, the overall agreement is satisfactory for the 1H and 15N NMR shifts. Our work is another step toward a realistic first-principles prediction of NMR chemical shifts in complex chemical environments.     

Ab initio calculations of NMR chemical shifts

The nuclear magnetic resonance chemical shift is one of the most powerful properties available for structure determination at the molecular level. A review of advances made in the ab initio calculation of chemical shielding during the past five years is presented. Specifically, progress in the areas including the effects of an unpaired electron, electron correlation, and relativistic effects into ab initio chemical shielding calculations, the tensor nature of the chemical shift, and intramolecular and intermolecular effects on the chemical shift will be covered.     

Lanthanide-ion-induced paramagnetic shifts in NMR spectra in aqueous solutions

Theoretical and Experimental Chemistry -     

A Quadrupole-Central-Transition 59Co NMR Study of Cobalamins in Solution

We report the first observation of quadrupole-central-transition (QCT) ⁵⁹Co (I=7/2) NMR signals from three cobalamin (Cbl) compounds (CNCbl, MeCbl, and AdoCbl) dissolved in glycerol/water. Measurements were performed at four magnetic fields ranging from 11.7 to 21.1 T. We found that the ⁵⁹Co QCT signals observed for cobalamin compounds in the slow motion regime (ω₀τ_C≫1) are significantly narrower than those observed from their aqueous solutions where the molecular tumbling is near the condition of ω₀τ_C≈1. We demonstrated that an analysis of ⁵⁹Co QCT signals recorded over different temperatures and at multiple magnetic fields allowed determination of both the ⁵⁹Co quadrupole coupling constant and chemical shift anisotropy for each of the three cobalamins. We successfully applied the ⁵⁹Co QCT NMR approach to monitor in situ the transformation of CNCbl to its “base off” form in the presence of KCN. We further discovered that, to obtain the maximum QCT signal intensity with the Hahn-echo sequence, a strong B₁ field should be used for the first 90° pulse, but a weak B₁ field for the second 180° pulse. The reported ⁵⁹Co QCT NMR methodology opens up a new direction for studying structure and function of cobalamin compounds and their roles in biological processes.     

Theoretical studies on NMR chemical shifts in azacubanes

Successive substitution of CH groups of cubane (CH)(8), by isoelectronic nitrogen atoms leads to a class of energy-rich azacubanes (CH)(8-alpha)N(alpha) (with alpha=1-8). In the present work, we systematically investigate how substitution of nitrogen in a cubanoid influence deshielding of carbon and manifest in the chemical shift in NMR spectra calculated using the second order M?ller-Plesset (MP2) perturbation level of theory. PMR spectra predict a large downshift of delta(H)=7.9 ppm in heptaazacubane owing to the more number of nitrogen and the stronger C-H...N interactions. These chemical shifts are explained by the net atomic charges derived from the population analysis based on Hirshfeld partitioning scheme.     

Association of sodium dodecyl sulfate in aqueous solutions according to chemical shifts in 1H NMR spectra

Proton chemical shifts of different atomic groups in sodium dodecyl sulfate (SDS) have been studied by ¹H NMR spectroscopy as functions of surfactant concentration in aqueous solutions. Three surfactant concentration ranges of the chemical shifts have been revealed. The first range corresponds to the premicellar solutions, the second one is in the vicinity of critical micelle concentration (CMC₁), and the third range corresponds to high surfactant concentrations, at which intermicellar interactions play a significant role. The parameters of SDS association (CMC₁ and CMC₂) determined based on the concentration dependences of the chemical shifts are in satisfactory agreement with the data available from the literature. The concept of critical dimerization concentration (CDC) has been introduced for the first concentration range. The values of CDC and dimerization constant K ₂ (210 × 60 dm³/mol) have been estimated within the framework of the two-state model.     

On the relativistic theory of NMR chemical shifts

A relativistic analogue to Ramsey's theory of NMR chemical shifts is formulated. Four-component spinor one-electron wavefunctions and relativistic magnetic hamiltonians are used. In contrast to the third-order Pauli approximation theory of Nakagawa et al., the main relativistic effects are then included in the usual second-order theory.     

Density-functional computation of 53Cr NMR chemical shifts

53Cr chemical shifts of CrO4(2-), Cr2O7(2-), CrO3X-, CrO2X2(X = F, Cl), and Cr(CO)5L (L = CO, PF3, CHNH2, CMeNMe2) are computed, using geometries optimized with the gradient-corrected BP86 density functional, at the gauge-including atomic orbitals (GIAO)-, BPW91-, and B3LYP levels. For this set of compounds, substituent effects on delta(53Cr) are better described with the pure BPW91 functional than with B3LYP, in contrast to most other transition-metal chemical shifts studied so far. For selected cases, 53Cr NMR line widths can be rationalized in terms of electric field gradients (EFGs) computed with the BPW91 functional, but in general other factors such as molecular correlation times appear to be dominating. 53Cr chemical shifts and EFGs are predicted for CrO3, Cr(C6H6)2, Cr(C6H6)CO3, and, with reduced reliability, for Cr2(mu2-O2CH)4.