Quantum-chemical calculation of NMR chemical shifts of organic molecules: III. Intramolecular coordination effects on the 31P NMR chemical shifts of phosphorylated N-vinylazoles

Theoretical and experimental studies on magnetic shielding of the phosphorus nucleus in trichloro-[2-(1H-pyrazol-1-yl)ethenyl]phosphonium hexachlorophosphate(V) and 1,1,1,1-tetrachloro-1H-1λ⁶-pyrazolo-[1,2-a][1,2,3]diazaphosphol-8-ium-1-ide showed that intramolecular coordination of the phosphorus atom in the chlorophosphonium group to the nitrogen atom in the pyrazole ring leads to upfield shift of the phosphorus signal (to δ_P 170 ppm) and that the contribution of the spin-orbital contribution to the ³¹P chemical shift reaches 15%. Relativistic effects and effects of the medium are determining in the theoretical calculation of ³¹P NMR chemical shifts.     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: XIII. Accuracy of the calculation of 15N NMR chemical shifts of azines with account taken of solvation effects

Effects of solvation on the accuracy of the calculation of ¹⁵N chemical shifts in the azine series have been analyzed at the DFT/GIAO level of theory. The best results are obtained with the use of the Keal-Tozer KT2 functional in combination with the Dunning aug-cc-pVTZ basis set with inclusion of solvent effects according to the Tomasi polarizable continuum model (PCM). If specific solvation is strong, additional consideration of solvent effects in the supermolecule approximation is necessary with explicit inclusion of solvent molecules.     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: VIII. Solvation effects on 15N NMR chemical shifts of nitrogen-containing heterocycles

Effect of solvation on the accuracy of DFT quantum-chemical calculations of ¹⁵N NMR chemical shifts of pyrrole, N-methylpyrrole, and pyridine was studied. The use of continuum model is sufficient to obtain consistent theoretical σ_N values for weakly polar aprotic solvents, whereas solvation effects in strongly polar and protic solvents should be taken into account in the explicit form.     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: XII. Calculation of the 13C NMR chemical shifts of fluoromethanes at the DFT level

Calculation was carried out of chemical shifts in ¹³C NMR spectra for a series of fluoromethanes CH _n F_4?n (n = 0–4) by the methods of the electron density functional theory GIAO-DFT taking in consideration the solvent effect in the framework of the polarizable continuum model Tomasi IEF-PCM. The best results were obtained at the use of Keal-Tozer KT3 functional combined with Pople standard basis sets 6-311G(d,p) and 6-311++G(d,p), and also with Jensen special set pcS-2 containing tight p-functions. The optimum reference in the calculation of chemical shifts in ¹³C NMR spectra for the fluoromethanes series is TMS.     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: XIV. Solvation effects in calculations of chemical shifts in 13C NMR spectra of chlorine-containing compounds

Analysis of precision factors in calculations of ¹³C NMR chemical shifts in the series of saturated and unsaturated organochlorine compounds was performed in the framework of the method of electron density functional theory GIAO-DFT-KT3/pcS-2 in the gas phase and with accounting for solvent effect by the polarized continuum model IEF-PCM. The accounting for solvation effects in calculations of ¹³C NMR chemical shifts within the framework of the IEF-PCM model is not fundamental for organochlorine compounds, yet it considerably improves the precision of calculations up to 2.5 ppm.     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: VI. Accuracy of DFT calculations of 29Si chemical shifts of four-coordinate silicon compounds

Systematic analysis of factors affecting the accuracy of DFT calculations of ²⁹Si NMR chemical shifts in four-coordinate silicon compounds showed that the best agreement with the experimental values is attained using B1PW91 and PBE0 functionals in combination with the TZP basis set. In calculations of ²⁹Si chemical shifts by quantum-chemical methods particular attention should be given to the contribution of relativistic spin-orbit interaction and conformational equilibrium.     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: XV. Relativistic calculations of <Superscript>29</Superscript>Si NMR chemical shifts of silanes

Calculations of ²⁹Si NMR chemical shifts of 68 silanes possessing various substituents, in particular, with heavy halogens attached to silicon atom, were carried out applying an efficient calculation scheme of locally dense basis set in the framework of the electron density functional theory utilizing the Keal–Tozer functional combined with relativistic Dyall basis sets on a four-component relativistic level. The main factors of calculation accuracy of silicon chemical shifts were analyzed including the relativistic effects, environmental impact, and vibrational corrections. The mean absolute calculation error for the studied compounds series accounting for all mentioned factors was 14.0 ppm for the nonrelativistic calculation and 6.7 ppm for the four-component relativistic calculation at the range of silicon chemical shifts variation of ~250 ppm.     

Molecular recognition in molecular tweezers systems: quantum-chemical calculation of NMR chemical shifts

Quantum-chemical calculations for molecular tweezers systems are presented, where the focus is not only on the recognition process in the host-guest systems, but on the self aggregation of the tweezers host as well. Such intermolecular interactions influence the corresponding NMR spectra strongly by up to 6 ppm for proton chemical shifts, since ring-current effects are particularly important. The quantum-chemical results allow one to reliably assign the spectra and to gain information both on the structure and on the importance of intra- and intermolecular interactions. In addition, we study the accuracy of a variety of density functionals for describing the present host-guest systems, where we observe a considerable underestimation of ring-current effects on (1)H NMR chemical shifts at the density functional theory (DFT) level using smaller basis sets such as 6-31G**, so that larger bases like TZP are required. This stands in contrast to the behavior of the Hartree-Fock scheme, where small basis sets, such as 6-31G**, provide reliable (1)H NMR shieldings for molecular tweezers systems.     

Density functional theory calculation of (29)Si NMR chemical shifts of organosiloxanes

Density functional theory at the B3LYP/6-311++G(d,p) level is applied to calculate the (29)Si NMR chemical shifts of a variety of organosiloxane moieties including monomers or precursors for polymerization and representative segments of organosiloxane polymers or thin films. The calculated shifts of two linear dimethylsiloxane compounds, hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane (D4), compare well with their known values, having an average error of 3.4 ppm. The same method is applied to structures believed to occur in organosilicate glass thin films deposited using hot-filament chemical vapor deposition (HFCVD) from D3 and D4. The chemical shift at -15 ppm is identified as a cross-linking Si-Si bond between two strained D groups and has not previously been reported. Retention of the strained ringed structure in HFCVD films deposited from D3 is confirmed. The rings are bonded to the matrix through either Si-O or Si-Si bonds, with the latter only becoming prevalent when higher filament temperatures are employed. The strained ring structure is also observed in films deposited from a precursor with a larger unstrained ring structure, D4. These observations suggest that the known gas-phase conversion pathways of D4 to D3 and dimethylsilanone as well as the methyl abstraction reaction from D3 operate in the HFCVD reaction chemistry.     

MP2 calculation of 77Se NMR chemical shifts taking into account relativistic corrections

The main factors affecting the accuracy and computational cost of the Second‐order Möller‐Plesset perturbation theory (MP2) calculation of ⁷⁷Se NMR chemical shifts (methods and basis sets, relativistic corrections, and solvent effects) are addressed with a special emphasis on relativistic effects. For the latter, paramagnetic contribution (390–466 ppm) dominates over diamagnetic term (192–198 ppm) resulting in a total shielding relativistic correction of about 230–260 ppm (some 15% of the total values of selenium absolute shielding constants). Diamagnetic term is practically constant, while paramagnetic contribution spans over 70–80 ppm. In the ⁷⁷Se NMR chemical shifts scale, relativistic corrections are about 20–30 ppm (some 5% of the total values of selenium chemical shifts). Solvent effects evaluated within the polarizable continuum solvation model are of the same order of magnitude as relativistic corrections (about 5%). For the practical calculations of ⁷⁷Se NMR chemical shifts of the medium‐sized organoselenium compounds, the most efficient computational protocols employing relativistic Dyall's basis sets and taking into account relativistic and solvent corrections are suggested. The best result is characterized by a mean absolute error of 17 ppm for the span of ⁷⁷Se NMR chemical shifts reaching 2500 ppm resulting in a mean absolute percentage error of 0.7%. Copyright © 2015 John Wiley & Sons, Ltd.     

Fragment density functional theory calculation of NMR chemical shifts for proteins with implicit solvation

Fragment density functional theory (DFT) calculation of NMR chemical shifts for several proteins (Trp-cage, Pin1 WW domain, the third IgG-binding domain of Protein G (GB3) and human ubiquitin) has been carried out. The present study is based on a recently developed automatic fragmentation quantum mechanics/molecular mechanics (AF-QM/MM) approach but the solvent effects are included by using the PB (Poisson-Boltzmann) model. Our calculated chemical shifts of (1)H and (13)C for these four proteins are in excellent agreement with experimentally measured values and represent clear improvement over that from the gas phase calculation. However, although the inclusion of the solvent effect also improves the computed chemical shifts of (15)N, the results do not agree with experimental values as well as (1)H and (13)C. Our study also demonstrates that AF-QM/MM calculated results accurately reproduce the separation of α-helical and β-sheet chemical shifts for (13)C(α) atoms in proteins, and using the (1)H chemical shift to discriminate the native structure of proteins from decoys is quite remarkable.     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: VII. Intramolecular coordination effect in the 29Si NMR spectra of siletanes

The effects of intramolecular N→Si coordination and electronic and conformational factors on the chemical shift of ²⁹Si nucleus in silacyclobutane (siletane) derivatives were studied by quantum-chemical methods. Intramolecular coordination induces upfield shift of the ²⁹Si resonance on the average by 50 ppm when the coordination number of the silicon atom increases by unity. The state of conformational equilibrium of siletane derivatives critically affects the accuracy of δ_Si predictions.     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: X. Dynamic equilibrium effects in the 29Si NMR spectra of silacycloalkanes

Dynamic equilibria related to change of the coordination number of the silicon atom in bis[N-(dimethylamino) imidato-N′,O]silacycloalkanes in solution were studied by theoretical calculations and experimental measurement of the ²⁹Si NMR chemical shifts. Silacyclopentane derivatives were found to exist in solution as mixtures of species with six- and five-coordinate silicon atoms, and silacyclohexane derivatives, as mixtures of five- and four-coordinate silicon compounds.     

NMR chemical shifts. Substituted acetylenes

The MPW1PW91/6-311+G(2d,p) and MP2/6-311+G(2d,p) GIAO nuclear shieldings for a series of monosubstituted acetylenes have been calculated using the MP2/6-311G(2d,p) geometries. Axially symmetric substituents such as fluorine may lead to large changes in the isotropic shielding but have little effect on the tensor component (zz) about the C[triple bond]C bond axis. On the other hand, substituents such as vinyl and aldehyde groups lead to essentially no difference in the isotropic shielding but are calculated to give a large zz paramagnetic shift to the terminal carbon of the acetylene group, without having much effect on the inner carbon. The tensor components of the chemical shifts for trimethylsilylacetylene, methoxyacetylene, and propiolaldehyde have been measured and are in reasonable agreement with the calculations. The downfield shift at the terminal carbon of propiolaldehyde along with a small upfield shift at the adjacent carbon has been found to result from the coupling of the in-plane pi MO of the acetylene with the pi* orbital that has a node near the central carbon. The tensor components for acetonitrile also have been measured, and the shielding of cyano and acetylenic carbons are compared.     

Ab initio calculations of NMR chemical shifts

The nuclear magnetic resonance chemical shift is one of the most powerful properties available for structure determination at the molecular level. A review of advances made in the ab initio calculation of chemical shielding during the past five years is presented. Specifically, progress in the areas including the effects of an unpaired electron, electron correlation, and relativistic effects into ab initio chemical shielding calculations, the tensor nature of the chemical shift, and intramolecular and intermolecular effects on the chemical shift will be covered.     

Oxygen-17 NMR chemical shifts of esters

The ¹⁷O NMR spectra of some esters are discussed and are correlated with ¹³C NMR chemical shift and IR carbonyl streching band data.     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: I. Phosphines,phosphine oxides,and phosphine sulfides

The accuracy in the calculation of ³¹P NMR chemical shifts in the series of the simplest phosphines, phosphine oxides, and phosphine sulfides was estimated in terms of the Hartree-Fock self-consistent final perturbation theory and density functional theory with different basis sets. The best agreement between the calculated and experimental data was achieved at the DFT/B3LYP/IGLO-III level of theory.