Taye B. Demissie

The Absolute Shielding Constants of Heavy Nuclei: Resolving the Enigma of the (119)Sn Absolute Shielding.

We demonstrate that the apparent disagreement between experimental determinations and four-component relativistic calculations of the absolute shielding constants of heavy nuclei is due to the breakdown of the commonly assumed relation between the electronic contribution to the nuclear spin-rotation constants and the paramagnetic contribution to the NMR shielding constants. We demonstrate that this breakdown has significant consequences for the absolute shielding constant of (119)Sn, leading to errors of about 1000 ppm. As a consequence, we expect that many absolute shielding constants of heavy nuclei will be in need of revision.

A Combined Atomic Force Microscopy and Computational Approach for the Structural Elucidation of Breitfussin A and B: Highly Modified Halogenated Dipeptides from Thuiaria breitfussi

The use of atomic-force microscopy (AFM) with atomic resolution shows great potential for the structural characterization of planar, proton-poor compounds, as these compounds are prone to structural corrections. [1,2] Currently, AFM has limited ability to identify element type and consequently functional groups. Additional computational techniques, such as computer-aided structure elucidation (CASE) and the calculation of 13 C NMR shifts using electronic structure calculations (DFT) may assist in this respect. Herein we show the combined use of spectroscopic methods, AFM, CASE, and DFT to solve the structures of breitfussins A and B, which could not be solved using either method alone. The subject of this study was the Arctic hydrozoan Thuiaria breitfussi (Family Sertulariidae). The few publications on the chemistry of this family show the presence of sterols, [3] polyhalogenated monoterpenes, [4] and anthracenone derivatives. [5] Arctic marine environments support highly diverse and dense populations of marine invertebrates. [6,7] A

Ab initio and relativistic DFT study of spin–rotation and NMR shielding constants in XF6 molecules, X = S, Se, Te, Mo, and W

We present an analysis of the spin-rotation and absolute shielding constants of XF6 molecules (X = S, Se, Te, Mo, W) based on ab initio coupled cluster and four-component relativistic density-functional theory (DFT) calculations. The results show that the relativistic contributions to the spin-rotation and shielding constants are large both for the heavy elements as well as for the fluorine nuclei. In most cases, incorporating the computed relativistic corrections significantly improves the agreement between our results and the well-established experimental values for the isotropic spin-rotation constants and their anisotropic components. This suggests that also for the other molecules, for which accurate and reliable experimental data are not available, reliable values of spin-rotation and absolute shielding constants were determined combining ab initio and relativistic DFT calculations. For the heavy nuclei, the breakdown of the relationship between the spin-rotation constant and the paramagnetic contribution to the shielding constant, due to relativistic effects, causes a significant error in the total absolute shielding constants.

Structure and NMR Spectra of Some (2.2)Paracyclophanes. The Dilemma of (2.2)Paracyclophane Symmetry

Density functional theory (DFT) quantum chemical calculations of the structure and NMR parameters for highly strained hydrocarbon [2.2]paracyclophane 1 and its three derivatives are presented. The calculated NMR parameters are compared with the experimental ones. By least-squares fitting of the (1)H spectra, almost all J(HH) coupling constants could be obtained with high accuracy. Theoretical vicinal J(HH) couplings in the aliphatic bridges, calculated using different basis sets (6-311G(d,p), and Huz-IV) reproduce the experimental values with essentially the same root-mean-square (rms) error of about 1.3 Hz, regardless of the basis set used. These discrepancies could be in part due to a considerable impact of rovibrational effects on the observed J(HH) couplings, since the latter show a measurable dependence on temperature. Because of the lasting literature controversies concerning the symmetry of parent compound 1, D(2h) versus D(2), a critical analysis of the relevant literature data is carried out. The symmetry issue is prone to confusion because, according to some literature claims, the two hypothetical enantiomeric D(2) structures of 1 could be separated by a very low energy barrier that would explain the occurrence of rovibrational effects on the observed vicinal J(HH) couplings. However, the D(2h) symmetry of 1 with a flat energy minimum could also account for these effects.

Four-Component Relativistic Density-Functional Theory Calculations of Nuclear Spin–Rotation Constants: Relativistic Effects in p-Block Hydrides

We present an implementation of the nuclear spin-rotation (SR) constants based on the relativistic four-component Dirac-Coulomb Hamiltonian. This formalism has been implemented in the framework of the Hartree-Fock and Kohn-Sham theory, allowing assessment of both pure and hybrid exchange-correlation functionals. In the density-functional theory (DFT) implementation of the response equations, a noncollinear generalized gradient approximation (GGA) has been used. The present approach enforces a restricted kinetic balance condition for the small-component basis at the integral level, leading to very efficient calculations of the property. We apply the methodology to study relativistic effects on the spin-rotation constants by performing calculations on XHn (n = 1-4) for all elements X in the p-block of the periodic table and comparing the effects of relativity on the nuclear SR tensors to that observed for the nuclear magnetic shielding tensors. Correlation effects as described by the density-functional theory are shown to be significant for the spin-rotation constants, whereas the differences between the use of GGA and hybrid density functionals are much smaller. Our calculated relativistic spin-rotation constants at the DFT level of theory are only in fair agreement with available experimental data. It is shown that the scaling of the relativistic effects for the spin-rotation constants (varying between Z(3.8) and Z(4.5)) is as strong as for the chemical shieldings but with a much smaller prefactor.

Spin-rotation and NMR shielding constants in HCl

The spin-rotation and nuclear magnetic shielding constants are analysed for both nuclei in the HCl molecule. Nonrelativistic ab initio calculations at the CCSD(T) level of approximation show that it is essential to include relativistic effects to obtain spin-rotation constants consistent with accurate experimental data. Our best estimates for the spin-rotation constants of (1)H(35)Cl are CCl = -53.914 kHz and C(H) = 42.672 kHz (for the lowest rovibrational level). For the chlorine shielding constant, the ab initio value computed including the relativistic corrections, σ(Cl) = 976.202 ppm, provides a new absolute shielding scale; for hydrogen we find σ(H) = 31.403 ppm (both at 300 K). Combining the theoretical results with our new gas-phase NMR experimental data allows us to improve the accuracy of the magnetic dipole moments of both chlorine isotopes. For the hydrogen shielding constant, including relativistic effects yields better agreement between experimental and computed values.

NMR shielding and spin–rotation constants in XCO (X = Ni, Pd, Pt) molecules†

Ab initio nonrelativistic and four-component relativistic DFT (density functional theory) methods are combined to study the spin–rotation and absolute nuclear magnetic resonance (NMR) shielding constants of group 10 transition metal monocarbonyls. Good agreement is obtained between the calculated and available experimental data for the spin–rotation constants and shielding spans for PdCO and PtCO. These data allow us to determine accurate absolute chemical shielding constants for all the nuclei, as well as for the unknown spin–rotation constants. We compare the four-component shielding constants with those obtained from the spin–orbit zeroth-order regular approximation, together with an assessment of the performance of different basis sets. For the first time, relativistically optimised basis sets for the heavy atoms used in the four-component calculations are shown to give converged results for both magnetic properties studied.

Spin-rotation and NMR shielding constants in XF molecules (X = B, Al, Ga, In, and Tl).

Accurate spin-rotation and absolute shielding constants in a series of XF molecules (X = (11)B, (27)Al, (69)Ga, (115)In, and (205)Tl) determined using high-level ab initio coupled-cluster and four-component relativistic density-functional theory (DFT) calculations are presented. The accuracy of the results is established by comparing the relativistically and vibrationally corrected calculated values with available experimental data; for spin-rotation and shielding constants for which no experimental data exist, we provide new and reliable values. For both properties, our results can be considered as reference values against which more approximate quantum-chemical methods can be benchmarked.

Absolute NMR shielding scales and nuclear spin–rotation constants in 175LuX and 197AuX (X = 19F, 35Cl, 79Br and 127I)

We present nuclear spin-rotation constants, absolute nuclear magnetic resonance (NMR) shielding constants, and shielding spans of all the nuclei in (175)LuX and (197)AuX (X = (19)F, (35)Cl, (79)Br, (127)I), calculated using coupled-cluster singles-and-doubles with a perturbative triples (CCSD(T)) correction theory, four-component relativistic density functional theory (relativistic DFT), and non-relativistic DFT. The total nuclear spin-rotation constants determined by adding the relativistic corrections obtained from DFT calculations to the CCSD(T) values are in general in agreement with available experimental data, indicating that the computational approach followed in this study allows us to predict reliable results for the unknown spin-rotation constants in these molecules. The total NMR absolute shielding constants are determined for all the nuclei following the same approach as that applied for the nuclear spin-rotation constants. In most of the molecules, relativistic effects significantly change the computed shielding constants, demonstrating that straightforward application of the non-relativistic formula relating the electronic contribution to the nuclear spin-rotation constants and the paramagnetic contribution to the shielding constants does not yield correct results. We also analyze the origin of the unusually large absolute shielding constant and its relativistic correction of gold in AuF compared to the other gold monohalides.

Cob(II)alamin: Relativistic DFT Analysis of the EPR Parameters.

Relativistic density functional theory (DFT) has been applied to explore electron paramagnetic resonance (EPR) parameters as well as ground-state spin properties of cob(II)alamin. Cob(II)alamin is an intermediate which participates in many reactions catalyzed by derivatives of vitamin B12 and that can be detected by EPR spectroscopy due to the presence of the paramagnetic Co(II)(d(7)) center. The full structure of cob(II)alamin and its truncated analogues were examined. Three different DFT functionals, B3LYP, BP86, and PBE, have been applied to obtain the g- and A-tensors. Both tensors are axially symmetric and can provide useful insight into specific axial ligand interactions. Of the functionals tested, the hybrid B3LYP functional, was found to overestimate the axial bond length, whereas the GGA-type functionals, BP86 and PBE, produced geometries consistent with experimental data. The reliability of nonrelativistic and approximate relativistic methods for the calculation of EPR parameters has also been tested against a fully relativistic four-component approach. Since the EPR parameters are very sensitive to the local environment surrounding Co(II), a theoretical (DFT-BP86) estimate of the dependence of the g- and A-tensors on the metal-to-axial ligand interatomic distance can be directly correlated with EPR measurements. The usefulness of such an approach has been demonstrated for the methionine synthase enzyme where the reduction of cob(II)alamin takes place during the reactivation cycle.