Sergey Molchanov

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).

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).

Carbon‐13 and mercury‐199 nuclear spin relaxation study on solution dynamics of the bis(phenylethynyl)mercury molecule and shielding anisotropy of its acetyleniccarbon and mercury nuclei

Carbon‐13 longitudinal relaxation times and nuclear Overhauser effect enhancement factors at two magnetic fields and three temperatures were measured for a DMSO solution of bis(phenylethynyl)mercury and interpreted in terms of molecular mobility of the solute molecule. The problem of the effective C— H bond length, crucial for the quantitative treatment of the relaxation data, was especially addressed. Then, using relaxation data for acetylenic carbons and additionally measured longitudinal relaxation times of mercury‐199, the shielding anisotropy parameters for these nuclei were calculated. Copyright

Solvation of Uracil and Its Derivatives by DMSO: A DFT-Supported 1H NMR and 13C NMR Study

1H NMR and 13C NMR spectra of uracil, thymine, 5-hydroxymethyluracil, 5,6-dihydrouracil, and 5,6-dihydrothymine in DMSO-d6 solutions have been measured. Additionally, molecular structures as well as NMR parameters of these compounds and their various solvates have been calculated using DFT B3LYP/6-311++G(2d,p) PCM(DMSO) method. The analysis of the chemical shift data for these compounds has shown that, indeed, in DMSO solutions they occur as equilibrium mixtures of free molecules and solvates in which solute and solvent molecules are joined by NH···O or OH···O hydrogen bonds. The populations of particular species present in the solutions have been estimated. Moreover, it has been found that 5,6-dihydrothymine exists in DMSO solution preferentially in conformation with the methyl group occupying the pseudoequatorial position. This finding is based on the molecular energy calculations and remains in full agreement with the interpretation of NMR data and theoretical calculations of NMR parameters.

Scalar relaxation of the second kind - a potential source of information on the dynamics of molecular movements. 2. Magnetic dipole moments and magnetic shielding of bromine nuclei.

In this paper, we continue the exploration of possibilities, limitations, and methodological problems of the studies based on measurements of the nuclear spin relaxation rates running via the scalar relaxation of the second kind (SC2) mechanism. The attention has been focused on the (13)C-(79)Br and (13)C-(81)Br systems in organic bromo compounds, which are characterized by exceptionally small differences of Larmor frequencies, ΔωCBr, of the coupled nuclei. This unique property enables experimental observation of longitudinal SC2 relaxation of (13)C nuclei, which makes investigation of the SC2 relaxation rates an attractive experimental method of determination of spin-spin coupling constants and relaxation rates of quadrupole bromine nuclei, both types of parameters being hardly accessible by direct measurements. A careful examination of the methodology used in SC2 relaxation studies of carbon-bromine systems reveals, however, some disturbing facts that could burden the results with systematic inaccuracies. Namely, the way of calculating the Larmor frequency differences between (13)C and bromine isotopes, ΔωCBr, may cause some reservations. In this work, the values of (79)Br and (81)Br magnetogyric ratios have been rechecked using bromine NMR data for the KBr·Kryptofix 222 complex in acetonitrile solution and the results of the advanced calculations of the magnetic shielding of the bromine nucleus in the Br(-) anion. Moreover, it has been pointed out that in the case of (13)C-(79)Br, the magnetic shielding of the bromine nucleus in the investigated molecule must not be neglected during the calculation of the ΔωCBr parameter. Some recommendations concerning the exploitation of available theoretical methods to calculate bromine shielding constants for bromo compounds have also been formulated, keeping in mind relativistic effects.

Nuclear spin relaxation and magnetic shielding tensors of atoms constituting cyanogen bromide molecule

The temperature dependence of the correlation time describing reorientation kinetics of cyanogen bromide in CDCl3 solution has been determined on the basis of the linewidths of the 14N NMR signal. It has been found that the longitudinal spin relaxation of the 15N nucleus occurs by shielding anisotropy and spin-rotation mechanisms, whereas for the 13C nucleus these mechanisms are of lesser importance. In the latter case the scalar relaxation of the second kind due to carbon-bromine coupling is the predominant relaxation mechanism. The parameter values: 1 J(13C—79Br) = 349 ± 10 Hz, T 1 (79Br, 303 K) = 2.31 ± 0.22 × 10−7 s, Δσ(15N) = 565 ± 16 ppm and Δσ(13C) = 276 ± 120 ppm have been determined from the relaxation data analysis. The shielding anisotropy parameters Δσ(15N) = 580 ± 50 ppm and Δσ(13C) = 274 ± 9 ppm have been independently determined using 13C and 15N NMR in liquid crystalline solvent. The experimentally determined shielding tensors for sp-hybridized atoms in the investigated compound and in a series of bromoacetylenes have been compared with the results of quantum mechanical calculations [GIAO, DFT B3LYP/6-311 + +G(2d,p)]. The ‘heavy atom effect’ shielding bromine-bonded carbons is of the order of — 25 ppm and concerns mainly the σ⊥ component.

Magnetic shielding tensors of carbon nuclei in 3,5-dichlorophenylacetylene, a theoretical ab initio and experimental study

Ab initio GIAO-CHF and DFT(B3LYP) molecular geometry optimization and magnetic shielding tensor calculations of carbon nuclei of 3,5-dichlorophenylacetylene have been performed using 6–311G** and 6–311+G(2d,p) basis sets. The isotropic 13C chemical shifts, needed for comparison, have been measured in C6D12 solution. The principal elements of the shielding tensor of the carbon nuclei in the investigated molecule in the solid state have been determined from an intensity analysis of the spinning sidebands in 1H-13C CP/MAS NMR spectra. Shielding anisotropy parameters of the acetylenic carbons have been independently determined using the method based on proton-coupled 13C nematic phase spectra as well as from the interpretation of the 13C longitudinal relaxation rates. The latter data have been analysed assuming the molecular reorientation to be the rotational diffusion of an asymmetrical top, which has provided, apart from the diffusion coefficients, an additional check on the reliability of the theoretical calculation of the shielding tensors. In general, satisfactory agreement between the theoretical and experimental results has been achieved.

Novel non-covalent stable supramolecular ternary system comprising of cyclodextrin and branched polyethylenimine

The synthesis of a novel supramolecular system comprising of branched polyethylenimine and cyclodextrin, is presented. The synthesis route is based on the self-assembly phenomena with the inclusion of solvent molecules. The systems are formed by a hydrogen-bonding network and host–guest type interactions between the building blocks. It was found that the native cyclodextrin and polyethylenimine are able to form stable systems when the reaction medium constitutes a polar solvent forming host–guest type complexes with cyclodextrin. A special consideration was paid on the detailed spectroscopic analyses of the obtained water-soluble constructs, including ROESY and diffusion-ordered (DOSY) NMR spectroscopy studies. The versatility and significance of DOSY technique for the analysis of the cyclodextrin complexes and its non-covalent systems with branched polymers, were presented. It was also found that the guest molecules that were incorporated in the complexes exhibited enhanced thermal stability. The morphological details in the solid state were obtained by scanning electron microscope.Graphical Abstract

Shielding and indirect spin-spin coupling tensors in the presence of a heavy atom: an experimental and theoretical study of bis(phenylethynyl)mercury.

Magnetic shielding and indirect spin-spin coupling phenomena are tensorial properties and both their isotropic and anisotropic parts do affect NMR spectra. The involved interaction tensors, σ and J, can nowadays be theoretically calculated, although the reliability of such methods in the case of anisotropic parameters, Δσ and ΔJ, in systems involving heavy nuclei, yet demands testing. In this communication the results of the experimental and theoretical investigations of bis(phenylethynyl)mercury (I) labeled with (13)C isotope at positions neighboring Hg are reported. The theoretical calculations of molecular geometry and values of NMR parameters for I have been performed by the ZORA/DFT method, including the relativistic scalar and spin-orbit coupling contributions, using the PBE0 functional and TZP (or jcpl) basis set. These values have been confronted with the experimentally measured ones. The isotropic parameters have been measured by the standard (13)C and (199)Hg NMR spectra. The shielding anisotropies for the atoms in the central part of molecule I have been determined in a liquid sample using magnetic relaxation measurements. The relaxation data have been interpreted within the rotational diffusion theory, assuming the symmetrical top reorientation model. The anisotropies of one-bond (13)C-(199)Hg and two-bond (13)C-Hg-(13)C spin-spin couplings have been determined exploiting the temperature-dependent (13)C NMR spectra of I in the ZLI1167 liquid-crystal phase. We have found that our theoretical calculations reproduce experimental values of both isotropic and anisotropic NMR parameters very well.

Solvation of Amides in DMSO and CDCl3: An Attempt at Quantitative DFT-Based Interpretation of 1H and 13C NMR Chemical Shifts

The study concerns N-methyl-2-pyrrolidinone, N,N-dimethylformamide, 2-pyrrolidinone, N-methylformamide, and formamide in DMSO-d6 and CDCl3 solutions. It has been shown that the results of DFT calculations [B3LYP and/or PBE0 6-311++G(2d,p), PCM] of molecular geometries and magnetic shielding are able to reproduce very well the amide 1H NMR and 13C NMR chemical shifts measured in these solvents provided that the specific solvation of the solute molecules and their association are taken into account and also that comparison of the experimental and theoretical data is carefully done. Analysis of the chemical shift data points out that in CDCl3 solutions primary and secondary amides are partially associated and that their carbonyl oxygen lone electron pairs are specifically solvated by solvent molecules. At the same time, association of the amides seems to be of minor importance in DMSO, while their N-H hydrogens form strong hydrogen bonds with solvent molecules.