Adam Gryff-Keller

Remarks on GIAO-DFT predictions of 13C chemical shifts†

Predicting 13C chemical shifts by GIAO‐DFT calculations appears to be more accurate than frequently expected provided that: (a) the comparison between experimental and theoretical data is performed using the linear regression method, (b) a sufficiently high level of theory [e.g. B3LYP/6‐311 + + G(2d,p)//B3LYP/6‐311 + + G(2d,p) or PBE1PBE/6‐311 + G(2df,p)//B3LYP/6‐311 + + G(2d,p)] is used, (c) the experimental data originate from the measurements performed in one solvent whose influence is taken into account at the molecular geometry optimization step and, first of all, during the shielding calculation, (d) the experimental data are free of heavy atom effects or such effects are appropriately treated in calculations, and finally (e) the conformational compositions of the investigated objects are known. Copyright

The Influence of a Presence of a Heavy Atom on (13)C Shielding Constants in Organomercury Compounds and Halogen Derivatives.

(13)C nuclear magnetic resonance shielding constants have been calculated by means of density functional theory (DFT) for several organomercury compounds and halogen derivatives of aliphatic and aromatic compounds. Relativistic effects have been included through the four-component Dirac-Kohn-Sham (DKS) method, two-component Zeroth Order Regular Approximation (ZORA) DFT, and DFT with scalar effective core potentials (ECPs). The relative shieldings have been analyzed in terms of the position of carbon atoms with respect to the heavy atom and their hybridization. The results have been compared with the experimental values, some newly measured and some found in the literature. The main aim of the calculations has been to evaluate the magnitude of heavy atom effects on the (13)C shielding constants and to check what are the relative contributions of scalar relativistic effects and spin-orbit coupling. Another object has been to compare the DKS and ZORA results and to check how the approximate method of accounting for the heavy-atom-on-light-atom (HALA) relativistic effect by means of scalar effective core potentials on heavy atoms performs in comparison with the more rigorous two- and four-component treatment.

Determination of the absolute configuration of 2-hydroxyglutaric acid and 5-oxoproline in urine samples by high-resolution NMR spectroscopy in the presence of chiral lanthanide complexes

Determination of the absolute configuration of some metabolites in body fluids is important for the diagnosis of some inborn errors of metabolism. Presently available methods of such determinations are tedious and usually require highly specialized instrumentation. In this work, an alternative method, based on high-resolution nuclear magnetic resonance spectroscopy in the presence of the chiral lanthanide shift reagent as an auxiliary additive, has been proposed (NMR/LSR). The method involves the lineshape analysis of a chosen multiplet of the one-dimensional 1H NMR spectrum or application of the two-dimensional 1H-13C correlation spectroscopy (HSQC). In order to confirm the resonance assignments and to boost the signal-noise ratio, the addition of an amount of racemic analyte to the urine sample is recommended. The entire procedure is simple in application and demands minimal or no preprocessing of urine samples. The effectiveness of the method has been confirmed by finding the expected forms of 2-hydroxyglutaric acid and 5-oxoproline in the urine samples of an independently diagnosed patient with 2-D-hydroxyglutaric aciduria and 5-L-oxoprolinuria, respectively.

13C NMR spectroscopy: a convenient tool for detection of argininosuccinic aciduria.

Proton decoupled high resolution 13C NMR spectra of argininosuccinic acid have been measured in a series of dilute water solutions of various acidity. These data have provided a basis for unequivocal determination of the presence of this metabolite in the investigated sample. The method additionally enables simultaneous rough estimation of the metabolite concentration. In order to check the practicability of the usage of this spectroscopy for diagnostic purposes, the spectra of several unprocessed urine samples have been recorded including three from patients with argininosuccinic aciduria. It has been concluded that 13C NMR spectroscopy can be a convenient method of recognising the above syndrome and probably many other inborn metabolic errors which manifest themselves with the excretion of the marker metabolite in amounts comparable to (or larger than) creatinine.

Systematic discrepancy of theoretical predictions of NMR chemical shifts for chlorinated aromatic carbons using the GIAO DFT method

Theoretical calculations of carbon-13 NMR chemical shifts using the gauge including atomic orbitals (GIAO) DFT approach with a moderately large set of basis functions usually yield quite satisfactory results. In the case of chlorinated aromatic carbons, however, abnormally large differences between experimental and calculated values have been noticed. This discrepancy has been proven not to be caused by improper referencing, or the basis set effect, and probably not by neglect of vibrational corrections. One of the possible sources of the chlorine effect could be the impact of relativistic phenomena on electrons moving about the chlorine nucleus. The second, probably more important factor is the influence of electron correlations, ignored in Hartree–Fock SCF and only partially included in DFT calculations. Surprisingly, however, the observed divergence has been significantly larger for DFT than for Hartree–Fock results. In the latter case the observed divergence between theoretical and experimental 13C NMR chemical shifts of chlorine-bonded carbons is systematic but rather small (3.4–4.4ppm).

Solution Structure of Succinylacetone, An Unsymmetrical β-Diketone, As Studied by 13C NMR and GIAO−DFT Calculations

The enolization degrees of succinylacetone, an important heme biosynthesis inhibitor, have been determined in CDCl(3) and water solutions using (1)H NMR. The solution structures of SA have been investigated using a combined NMR/theoretical [GIAO DFT PBE1PBE/6-311++G(2d, p) PCM] approach. The populations of both enolic forms undergoing enol-enol equilibriums for SA and a series of unsymmetrical beta-diketones have been established by a quantitative comparison of the experimental (13)C NMR chemical shifts and calculated shielding constants. Moreover, using the same method and considering various trial structures differing in conformation and/or hydration of neutral SA molecule as well as its monoanion and dianion the structures of the most abundant species being present in the investigated water solutions have been deduced.

Structure of neutral molecules and monoanions of selected oxopurines in aqueous solutions as studied by NMR spectroscopy and theoretical calculations.

A methodology enabling investigation of a multicomponent tautomeric and acid-base equilibria by (13)C NMR spectroscopy supported by theoretical calculations has been proposed. The effectiveness of this method has been illustrated in a study of 2-oxopurine, 6-oxopurine (hypoxanthine), 8-oxopurine, and 2,6-dioxopurine (xanthine) in neutral and alkaline aqueous solutions. For each compound a series of (13)C NMR spectra were recorded at pH ranges in which neutral molecules, monoanions and/or dianions occurred in dynamic equilibrium. The carbon chemical shifts for these three forms of the investigated compounds were retrieved from the analysis of pH-dependence of the measured, dynamically averaged values of these parameters. The structures of several stable tautomers of the neutral and monoanionic oxopurine forms were predicted from theoretical calculations and nuclear magnetic shielding constants for (13)C nuclei in these tautomers were calculated. At both calculation steps (molecular geometry optimization and calculation of NMR parameters) the PBE1PBE/6-311++G(2d,p) level of theory was used. The populations of the most stable tautomers were determined from the experimental data analysis exploiting the fact that they were population-weighted averages of the chemical shifts of particular tautomers. It has been shown that only the oxo forms of the investigated oxopurines are present in aqueous solutions and that the determined populations in most cases remain in a qualitative agreement with the calculated free energies of the appropriate tautomers. The obtained results are in general agreement with other literature reports on oxopurine tautomerism and confirm importance of the hydration phenomena for the investigated systems. The data analysis has shown that the best compliance between theory and experiment is obtained when the hydration phenomenon is modeled by discrete hydration augmented by PCM (polarizable continuum solvation model).

Hindered Rotation in New Air-Stable Ruthenium Olefin Metathesis Catalysts with Chromanylmethylidene Ligands

Three new ruthenium complexes with chromanylmethylidene ligands were synthesized and dynamic NMR analysis of these compounds was carried out. The activation energy for the C–Ru rotation was measured for the first time and found to be of ~60 kJ mol–1. The complexes may potentially serve as latent catalysts for olefin metathesis – they promote the simple ring-closing metathesis reactions very slowly at room temperature, but are much more active at slightly elevated temperatures. No asymmetric induction was observed when the chiral α-tocopherol-derived catalyst was used.

Metal-side chain interaction in tricarbonylchromium complexes of phenylacetylenes studied by 13C NMR and IR spectroscopy

Abstract 13 C NMR spectra of several η 6 -arenetricarbonylchromium complexes of phenylacetylene and phenyltrimethylsilylacetylene have been interpreted. The complexing chemical shifts (CCS) observed for α and β acetylenic carbons suggest the existence of a direct interaction between the metal atom and π-electron system of the ethynyl substituent. The overall complexing effect is of the electron-withdrawing type which was confirmed by δ( 29 Si), 2 J (CH) and IR data. Some observations concerning synthesis of the investigated complexes have been also reported.

A successful DFT calculation of carbon-13 NMR chemical shifts and carbon–fluorine spin–spin coupling constants in (η6-fluoroarene)tricarbonylchromium complexes

Carbon-13 isotropic shielding constants and carbon–fluorine spin–spin coupling constants for fluorobenzene (1), 3-dimethylaminofluorobenzene (2) and 4-dimethylaminofluorobenzene (3) as well as for their tricarbonylchromium complexes (3–6) have been calculated using DFT method with B3LYP and/or BHandH functionals and 6-311++G(2d,p) basis set. In the case of the complexes, the calculations have been performed for two expected orientations of the tricarbonylchromium tripod with respect to the benzene ring. It has been found that, when using B3LYP functional, the experimental 13C NMR chemical shifts for all the investigated compounds can be well reproduced, provided that for the complexes the populations of conformers are adjusted during the experiment/theory correlation. The population parameters determined this way follow perfectly the expectations based on the literature knowledge of the effects of the benzene ring substituents. The calculations with BHandH functional have not been able to reproduce the 13C NMR chemical shifts of carbonyl carbons in tricarbonylchromium complexes. On the other hand, BHandH is a functional of choice in calculations of carbon–fluorine spin–spin coupling constants. All such constants, including the through-space coupling constants between the carbonyl carbons and fluorine, have been reproduced very well. This last coupling constant has been calculated for various relative orientations of carbonyl and C–F bond and a strong angular dependence of this constant has been found.