Calculations Of Nuclear Shielding Constants Research Articles

Implementation and First Evaluation of Strong-Correlation-Corrected Local Hybrid Functionals for the Calculation of NMR Shieldings and Shifts.

Local hybrid functionals containing strong-correlation factors (scLHs) and range-separated local hybrids (RSLHs) have been integrated into an efficient coupled-perturbed Kohn-Sham implementation for the calculation of nuclear shielding constants. Several scLHs and the ωLH22t RSLH have then been evaluated for the first time for the extended NS372 benchmark set of main-group shieldings and shifts and the TM70 benchmark of 3d transition-metal shifts. The effects of the strong-correlation corrections have been analyzed with respect to the spatial distribution of the sc-factors, which locally diminish exact-exchange admixture at certain regions in a molecule. The scLH22t, scLH23t-mBR, and scLH23t-mBR-P functionals, which contain a "damped" strong-correlation factor to retain the excellent performance of the underlying LH20t functional for weakly correlated situations, tend to make smaller corrections to shieldings and shifts than the "undamped" scLH22ta functional. While the latter functional can also deteriorate agreement with the reference data in certain weakly correlated cases, it provides overall better performance, in particular for systems where static correlation is appreciable. This pertains only to a minority of systems in the NS372 main-group test set but to many more systems in the TM70 transition-metal test set, in particular for high-oxidation-state complexes, e.g., Cr(+VI) complexes and other systems with stretched bonds. Another undamped scLH, the simpler LDA-based scLH21ct-SVWN-m, also tends to provide significant improvements in many cases. The differences between the functionals and species can be rationalized on the basis of one-dimensional plots of the strong-correlation factors, augmented by isosurface plots of the fractional orbital density (FOD). Position-dependent exact-exchange admixture is thus shown to provide substantial flexibility in treating response properties like NMR shifts for both weakly and strongly correlated systems.

Synthesis and Tautomerism of Substituted Pyrazolo[4,3‐c]pyrazoles

AbstractFused five‐membered nitrogen heterocycles comprise a very important group of compounds frequently utilized in pharmaceutical applications. In this study, we report the first systematic synthesis of substituted pyrazolo[4,3‐c]pyrazoles and three regioisomers of their N‐methyl derivatives. All compounds were fully characterized by NMR spectroscopy in solution and selected compounds also were studied by X‐ray diffraction in the solid state. 1H, 13C, and 15N NMR spectroscopic data for all isomers were interpreted by DFT calculations of nuclear shielding constants and indirect spin–spin coupling constants. The N‐methyl isomers were used in the following steps as model compounds to investigate a potential N1‐H/N4‐H, N2‐H/N4‐H, and N1‐H/N5‐H tautomerism of 3,6‐substituted pyrazolo[4,3‐c]pyrazoles by using low‐temperature NMR spectroscopy. All bases were shown to occur predominantly in the N1‐H/N4‐H tautomeric form and the structure of minor form was governed by the substituents at positions 3 and 6. Stabilities of individual tautomeric forms are calculated by DFT methods and discussed. A relationship between the tautomeric populations and the ratios among N‐methyl isomers obtained upon methylation of selected bases in solution are investigated.

Scalar Relativistic Computations of Nuclear Magnetic Shielding and g-Shifts with the Zeroth-Order Regular Approximation and Range-Separated Hybrid Density Functionals.

Density functional theory (DFT) calculations of NMR chemical shifts and molecular g tensors with Gaussian-type orbitals are implemented via second-order energy derivatives within the scalar relativistic zeroth order regular approximation (ZORA) framework. Nonhybrid functionals, standard (global) hybrids, and range-separated (Coulomb-attenuated, long-range corrected) hybrid functionals are tested. Origin invariance of the results is ensured by use of gauge-including atomic orbital (GIAO) basis functions. The new implementation in the NWChem quantum chemistry package is verified by calculations of nuclear shielding constants for the heavy atoms in HX (X = F, Cl, Br, I, At) and H2X (X = O, S, Se, Te, Po) and (125)Te chemical shifts in a number of tellurium compounds. The basis set and functional dependence of g-shifts is investigated for 14 radicals with light and heavy atoms. The problem of accurately predicting (19)F NMR shielding in UF6-nCln, n = 1-6, is revisited. The results are sensitive to approximations in the density functionals, indicating a delicate balance of DFT self-interaction vs correlation. For the uranium halides, the range-separated functionals are not clearly superior to global hybrids.

Nuclear shielding constants from localized local hybrid exchange-correlation potentials

Localized local hybrid (LLH) potentials derived from corresponding local hybrid functionals with position-dependent exact-exchange admixture governed by appropriate local mixing functions (LMFs) have been tested for the first time in calculations of nuclear shielding constants for main-group molecules. Two different types of LMFs have been evaluated, based either on the kinetic energy density or on the dimensionless density gradient. LLH potentials provide comparable quality shielding constants as the localized hybrid potentials from global hybrid functionals. However, this is achieved (a) without generalized gradient corrections, and (b) with LMFs providing also excellent thermochemical performance and built-in correct long-range asymptotic behavior.

OPBE: A promising density functional for the calculation of nuclear shielding constants

We investigate the performance for nuclear magnetic constant calculations for a selective set of density functional methods (B3LYP, PBE0, BLYP, PBEPBE, OLYP and OPBE). The testing set includes the 13C, 15N, 17O and 19F magnetic shieldings and chemical shifts of 23 molecules with 64 comparisons altogether. The results are compared and contrasted to the experimental gas phase data. We find that the OPBE exchange-correlation functional performs remarkably well for the whole set of the testing system, rendering OPBE the best GGA (generalized gradient approximation) functional, determined from energy criteria, for the prediction of nuclear magnetic constants.

Localized hybrid exchange‐correlation potentials for Kohn–Sham DFT calculations of NMR and EPR parameters

AbstractIn contrast to the conventional, nonlocal, and nonmultiplicative implementation of hybrid exchange‐correlation functionals in self‐consistent calculations, the exact‐exchange contribution to the functional has been implemented as a proper local and multiplicative Kohn–Sham potential, using the localized Hartree–Fock approximation to the optimized effective potential. The resulting localized hybrid potentials provide improved performance over nonlocal implementations in density functional theory (DFT) calculations of nuclear shielding constants of main‐group molecules and electronic g‐tensors of 3d transition–metal complexes. Optimum performance of the localized potentials is found at very similar amounts of exact‐exchange admixture for both properties and is almost independent of the “pure‐DFT” exchange and correlation functionals combined with exact exchange. Based on local kinetic energy density, a measure of exact‐exchange admixture is constructed that helps rationalize the relatively large optimal exact‐exchange contributions found. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Construction of local hybrid exchange-correlation potentials and their evaluation for nuclear shielding constants

The optimized effective potential (OEP) method has been applied to convert ‘traditional’ non-local and non-multiplicative hybrid exchange-correlation potentials into local and multiplicative Kohn–Sham potentials. The latter have been evaluated in calculations of nuclear shielding constants for 22 small main-group molecules. The results obtained with local potentials are in much better agreement with experiment than those from their non-local counterparts. Exact-exchange admixture between 0.4 and 0.7 provides the best agreement with the experimental shielding constants. This should be contrasted to much lower exact-exchange admixture in a recent multiplicative Kohn–Sham procedure. With local hybrid potentials no coupling terms due to exact-exchange admixture have to be evaluated in the perturbational treatment.

Calculation of nuclear shielding constants and magnetizabilities of the hydrogen fluoride molecule

Nuclear shielding constants and magnetizabilities of the hydrogen fluoride molecule have been calculated adopting the multiconfigurational self-consistent field method. Effects of vibrational averaging are calculated by adopting a Gaussian function with variable width and position to describe the vibrational part of the wave function. The rovibrationally averaged 19F shielding constant is calculated to 407.4 ppm and its anisotropy to 111.2 ppm, both in good agreement with experimental results. Also the isotope effect, 〈σF(DF)〉−〈σF(HF)〉, which is calculated to 3.0 ppm agrees well with the available NMR experiment. The effective geometry, dipole moment, vibrational frequencies, and rotational constants are also calculated and discussed.

The density functional calculation of nuclear shielding constants using London atomic orbitals

The theory for the calculation of nuclear shielding constants with London atomic orbitals using density functional theory is presented. The theory includes the use of a local exchange–correlation functional which depends on both the electron density ρ(r) and the paramagnetic current density jp(r). The resulting coupled-perturbed Kohn–Sham equations are presented, together with the working expression for the nuclear shielding tensor. The entire theory has been programmed and exhaustively checked, using standard Gaussian basis sets. A variety of ρ(r) dependent exchange–correlation functionals have been used, together with a current dependence suggested by Vignale, Rasolt, and Geldart. Certain numerical difficulties arose with this form which necessitated a cutoff in its evaluation for low densities. Calculations have been performed on HF, N2, CO, F2, H2O, and CH4. Including the current dependence is seen here to have a slightly deshielding effect. The major deficiency in the reported calculations appears to arise from the use of local functionals which give poor values for the Kohn–Sham eigenvalues.