Four-Component Relativistic Calculations of NMR Shielding Constants of the Transition Metal Complexes-Part 2: Nitrogen-Coordinated Complexes of Cobalt.

Both four-component relativistic and nonrelativistic computations within the GIAO-DFT(PBE0) formalism have been carried out for N and Co NMR shielding constants and chemical shifts of a number of the nitrogen-coordinated complexes of cobalt. It was found that the total values of the calculated nitrogen chemical shifts of considered cobalt complexes span over a range of more than 580 ppm, varying from -452 to +136 ppm. At that, the relativistic corrections to nitrogen shielding constants and chemical shifts were demonstrated to be substantial, changing accordingly from ca.

Four-component relativistic calculations of NMR shielding constants of the transition metal complexes. Part 1: Pentaammines of cobalt, rhodium, and iridium.

The nonrelativistic and four-component fully relativistic calculations of H, N, Co, Rh, and Ir shielding constants of pentaammineaquacomplexes of cobalt(III), rhodium(III), and iridium(III) were carried out at the density functional theory (DFT) level of theory. The noticeable deshielding relativistic corrections were observed for nitrogen shielding constants (chemical shifts), whereas those corrections were found to be negligible for protons. For the transition metals cobalt, rhodium, and iridium, relativistic corrections to their nuclear magnetic resonance (NMR) shielding constants were found to be rather small for cobalt and rhodium (some 5-10%), whereas they are essentially larger for iridium (up to 70%).

The H and C NMR chemical shifts of Strychnos alkaloids revisited at the DFT level.

The density functional theory calculation of H and C NMR chemical shifts in a series of ten 10 classically known Strychnos alkaloids with a strychnine skeleton was performed at the PBE0/pcSseg-2//pcseg-2 level. It was found that calculated H and C NMR chemical shifts provided a markedly good correlation with experiment characterized by a mean absolute error of 0.08 ppm in the range of 7 ppm for protons and 1.

Calculation of N NMR Chemical Shifts in a Diversity of Nitrogen-Containing Compounds Using Composite Method Approximation at the DFT, MP2, and CCSD Levels.

Computations of N NMR chemical shifts in 93 diverse nitrogen-containing compounds representing almost all known classes are performed at the density functional theory (DFT), second-order Møller-Plesset perturbation theory (MP2), and coupled cluster singles and doubles (CCSD) levels using the composite method approximation (CMA) in comparison with experimental results. It is shown that the CMA-DFT and CMA-CCSD methods provided the best performance characterized by a normalized mean absolute error of 1.1-1.

Computational Multinuclear NMR of Platinum Complexes: A Relativistic Four-Component Study.

The structures of 16 derivatives of cisplatin and transplatin were optimized at the DFT/dyall.ae2z levels, and their H, N, and Pt NMR chemical shifts were evaluated within the nonrelativistic and four-component relativistic approaches. Reliable correlations of calculated NMR chemical shifts with available experimental data were achieved by taking into account relativistic effects, solvent effects, and vibrational corrections.

DFT computational schemes for N NMR chemical shifts of the condensed nitrogen-containing heterocycles.

A systematic density functional theory (DFT) study of the accuracy factors (functionals, basis sets, and solvent effects) for the computation of N NMR chemical shifts has been performed in the series of condensed nitrogen-containing heterocycles. The behavior of the most representative functionals was examined based on the benchmark calculations of N NMR chemical shifts in the reference set of compounds. It was found that the best agreement with experiment was achieved with OLYP functional in combination with aug-pcS-3(N)//pc-2 locally dense basis set scheme providing mean absolute error of 5.

Calculation of N and P NMR Chemical Shifts of Azoles, Phospholes, and Phosphazoles: A Gateway to Higher Accuracy at Less Computational Cost.

A number of computational schemes for the calculation of N and P NMR chemical shifts and shielding constants in a series of azoles, phospholes, and phosphazoles was examined. A very good correlation between calculated at the CCSD(T) level and experimental N and P NMR chemical shifts was observed. It was found that basically solvent, vibrational, and relativistic corrections are of the same order of magnitude and alternate in sign, being, on average, of about 2-3 ppm in absolute value but, being much larger (up to 14 ppm) in the case of solvent molecules explicitly introduced into computational space.

Substitution effects in the N NMR chemical shifts of heterocyclic azines evaluated at the GIAO-DFT level.

A systematic study of the accuracy factors for the computation of N NMR chemical shifts in comparison with available experiment in the series of 72 diverse heterocyclic azines substituted with a classical series of substituents (CH , F, Cl, Br, NH , OCH , SCH , COCH , CONH , COOH, and CN) providing marked electronic σ- and π-electronic effects and strongly affecting N NMR chemical shifts is performed. The best computational scheme for heterocyclic azines at the DFT level was found to be KT3/pcS-3//pc-2 (IEF-PCM). A vast amount of unknown N NMR chemical shifts was predicted using the best computational protocol for substituted heterocyclic azines, especially for trizine, tetrazine, and pentazine where experimental N NMR chemical shifts are almost totally unknown throughout the series.

GIAO-DFT calculation of N NMR chemical shifts of Schiff bases: Accuracy factors and protonation effects.

N NMR chemical shifts in the representative series of Schiff bases together with their protonated forms have been calculated at the density functional theory level in comparison with available experiment. A number of functionals and basis sets have been tested in terms of a better agreement with experiment. Complimentary to gas phase results, 2 solvation models, namely, a classical Tomasi's polarizable continuum model (PCM) and that in combination with an explicit inclusion of one molecule of solvent into calculation space to form supermolecule 1:1 (SM + PCM), were examined.

On the accuracy factors and computational cost of the GIAO-DFT calculation of N NMR chemical shifts of amides.

The main factors affecting the accuracy and computational cost of Gauge-independent Atomic Orbital-density functional theory (GIAO-DFT) calculation of N NMR chemical shifts in the benchmark series of 16 amides are considered. Among those are the choice of the DFT functional and basis set, solvent effects, internal reference conversion factor and applicability of the locally dense basis set (LDBS) scheme. Solvent effects are treated within the polarizable continuum model (PCM) scheme as well as at supermolecular level with solvent molecules considered in explicit way.

On the long-range relativistic effects in the N NMR chemical shifts of halogenated azines.

Long-range β- and γ-relativistic effects of halogens in N NMR chemical shifts of 20 halogenated azines (pyridines, pyrimidines, pyrazines, and 1,3,5-triazines) are shown to be unessential for fluoro-, chloro-, and bromo-derivatives (1-2 ppm in average). However, for iodocontaining compounds, β- and γ-relativistic effects are important contributors to the accuracy of the N calculation. Taking into account long-range relativistic effects slightly improves the agreement of calculation with experiment.

Normal halogen dependence of C NMR chemical shifts of halogenomethanes revisited at the four-component relativistic level.

The 'Normal Halogen Dependence' of C NMR chemical shifts in the series of halogenomethanes is revisited at the four-component relativistic level. Calculations of C NMR chemical shifts of 70 halogenomethanes have been carried out at the density functional theory (DFT) and MP2 levels with taking into account relativistic effects using the four-component relativistic theory of Dirac-Coulomb within the different computational methods (4RPA, 4OPW91) and hybrid computational schemes (MP2 + 4RPA, MP2 + 4OPW91). The most efficient computational protocols are derived for practical purposes.

Theoretical and experimental (15)N NMR study of enamine-imine tautomerism of 4-trifluoromethyl[b]benzo-1,4-diazepine system.

The tautomeric structure of 4-trifluoromethyl[b]benzo-1,4-diazepine system in solution has been evaluated by means of the calculation of (15)N NMR chemical shifts of individual tautomers in comparison with the averaged experimental shifts to show that the enamine-imine equilibrium is entirely shifted toward the imine form. The adequacy of the theoretical level used for the computation of (15)N NMR chemical shifts in this case has been verified based on the benchmark calculations in the series of the push-pull and captodative enamines together with related azomethynes, which demonstrated a good to excellent agreement with experiment.

Theoretical and experimental study of 15N NMR protonation shifts.

A combined theoretical and experimental study revealed that the nature of the upfield (shielding) protonation effect in 15N NMR originates in the change of the contribution of the sp(2)-hybridized nitrogen lone pair on protonation resulting in a marked shielding of nitrogen of about 100 ppm. On the contrary, for amine-type nitrogen, protonation of the nitrogen lone pair results in the deshielding protonation effect of about 25 ppm, so that the total deshielding protonation effect of about 10 ppm is due to the interplay of the contributions of adjacent natural bond orbitals. A versatile computational scheme for the calculation of 15N NMR chemical shifts of protonated nitrogen species and their neutral precursors is proposed at the density functional theory level taking into account solvent effects within the supermolecule solvation model.

Solvent effects in the GIAO-DFT calculations of the 15N NMR chemical shifts of azoles and azines.

The calculation of (15)N NMR chemical shifts of 27 azoles and azines in 10 different solvents each has been carried out at the gauge including atomic orbitals density functional theory level in gas phase and applying the integral equation formalism polarizable continuum model (IEF-PCM) and supermolecule solvation models to account for solvent effects. In the calculation of (15)N NMR, chemical shifts of the nitrogen-containing heterocycles dissolved in nonpolar and polar aprotic solvents, taking into account solvent effect is sufficient within the IEF-PCM scheme, whereas for polar protic solvents with large dielectric constants, the use of supermolecule solvation model is recommended. A good agreement between calculated 460 values of (15)N NMR chemical shifts and experiment is found with the IEF-PCM scheme characterized by MAE of 7.

On the accuracy of the GIAO-DFT calculation of 15N NMR chemical shifts of the nitrogen-containing heterocycles--a gateway to better agreement with experiment at lower computational cost.

The main factors affecting the accuracy and computational cost of the gauge-independent atomic orbital density functional theory (GIAO-DFT) calculation of (15)N NMR chemical shifts in the representative series of key nitrogen-containing heterocycles--azoles and azines--have been systematically analyzed. In the calculation of (15)N NMR chemical shifts, the best result has been achieved with the KT3 functional used in combination with Jensen's pcS-3 basis set (GIAO-DFT-KT3/pcS-3) resulting in the value of mean absolute error as small as 5 ppm for a range exceeding 270 ppm in a benchmark series of 23 compounds with an overall number of 41 different (15)N NMR chemical shifts. Another essential finding is that basically, the application of the locally dense basis set approach is justified in the calculation of (15)N NMR chemical shifts within the 3-4 ppm error that results in a dramatic decrease in computational cost.