Core-dependent and ligand-dependent relativistic corrections to the nuclear magnetic shieldings in MH4−n Y n (n = 0–4; M = Si, Ge, Sn, and Y = H, F, Cl, Br, I) model compounds

2014

J. Phys. Chem. A

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The reference values for NMR magnetic shieldings, σ ref , are of highest importance when theoretical analysis of chemical shifts are envisaged. The fact that the non relativistically valid relationship among spin-rotation constants and magnetic shieldings is not any longer valid for heavy atoms makes that the search for σ ref for such atoms needs new strategies to follow.We present here results of σ ref that were obtained applying an own simple procedure which mix accurate experimental chemical shifts (δ) and theoretical magnetic shieldings (σ). We calculated σ(Sn) and σ(Pb) in a family of heavy-halogen containing molecules.We found out that σ ref (Sn; Sn(CH 3 ) 4 ) in gas phase should be close to 3864.11 ± We studied tin and lead shieldings of the XY 4−n Z n (X = Sn, Pb; Y , Z = H, F, Cl, Br, I) and PbH 4−n I n (n = 0, 1, 2, 3, 4) family of compounds with four-component functionals as implemented in the DIRAC code. For these systems results of calculations with RelPPA-RPA are more reliable than the DFT ones. We argue on why those DFT functionals must be modified in order to obtain more accurate results of NMR magnetic shieldings within the relativistic regime: first, there is a dependence among both, electron correlation and relativistic effects that should be introduced in some way in the functionals; and second, the DIRAC code uses standard non-relativistic functionals and the functionals B3LYP and PBE0 were parameterized only with data taken from light elements. It can explain why they are not able to properly introduce relativistic effects on nuclear magnetic shieldings.We finally show that in the analysis of magnetic shieldings for the family of compounds mentioned above, one must consider the newest and so called heavy-atom effect on vicinal heavy atoms, HAVHA. Such effects are among the most important relativistic effects in these kind of compounds.

Section: 62mentioning

confidence: 91%

Relativistic and Electron-Correlation Effects on the Nuclear Magnetic Resonance Shieldings of Molecules Containing Tin and Lead Atoms

Maldonado

2014

J. Phys. Chem. A

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“…It can be seen in Table that the convergence of calculations using different basis sets is obtained with the MMA‐lresc basis set, which is an smaller version of aug‐cc‐pVTZ‐lresc basis set and was named aug‐Jun1 basis set in earlier works . The differences in the values of the total shielding, obtained as the addition of the NR shielding constant with all relativistic corrections, is smaller than 1 ppm compared with the values obtained using the larger basis set (aug‐cc‐pVTZ‐lresc).…”

Section: Applications Of the Lresc Modelmentioning

confidence: 95%

“…We also aim to have an insight on each correcting term, and finally on 2014 we proposed an overview to treat relativistic corrections grouped in two terms. We named those terms as core‐ and ligand‐dependent . We present in this section a brief account of LRESC achievements in describing heavy atom containing molecular systems.…”

Section: Applications Of the Lresc Modelmentioning

confidence: 99%

Foundations of the LRESC model for response properties and some applications

Melo

Int J of Quantum Chemistry

et al. 2017

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Accurate calculations of some response properties, like the NMR spectroscopic parameters, are quite exigent for the theoretical quantum chemistry models together with the computational codes that are written from them. They need to include a very good description of the electronic density in regions close to the nuclei. When heavy-atom containing systems are studied, those requirements become even higher. Given that relativistic effects must be included in one way or another on the calculation of response properties of heavy-atoms and heavy-atom containing molecules, different schemes were developed during the past decades to include them in as good as possible way. There are some four-component models, which include relativistic effects in a very compact way, although calculations have large time-consumption; one also needs to deal with new and unusual four-component operators. There are also two-component models, which in general may be less accurate, although their application to property calculations on medium-size and large-size molecules are feasible, and they maintain the application of usual operators. In this review, we give the fundamentals of the two-component linear response elimination of small component formalism, LRESC, together with some applications to few selected response properties.New physical insights do appear when the LRESC model is used to analyze the effect of the environment on magnetic shieldings, and when one search for the relativistic extension of well-known nonrelativistic relationships like Flygare's relation among the NMR magnetic shielding and the nuclear spin-rotation constant. A similar relationship is found for the g-tensor and the susceptibility tensor. K E Y W O R D Sg-tensor, NMR, response properties, spin-rotation tensor, two-component methods | I N TR ODU C TI ONThe strong influence of relativistic effects on atomic and molecular response properties of heavy-atom containing molecules was first shown few decades ago.[1] Pyykk€ o included relativistic effects in the calculations of NMR spectroscopic parameters by applying a relativistic model that resemble Ramsey's theory [2] and use relativistic molecular wave function of the relativistically parameterized extended H€ uckel method, REX.[3] Other more elaborated semi-empirical methods and codes were later on used to improve those first results. [4][5][6] The importance of including relativistic effects on the calculation of response properties compelled the theoretical chemists to develop new specific relativistic theories and models. Several formalisms and models appeared in the literature from that time, whose implementations gave more accurate results than that obtained using previous schemes. [7][8][9][10][11][12] We can split them into two broad groups: four-component methods and two-component methods. [12][13][14][15] Even though accurate calculations of response properties of medium-size molecules, meaning molecules containing more than 10 heavy atoms (belonging to the fourth row or below of the Periodic Ta...

“…They were studied since 30 years ago for many research groups in a vast variety of molecular systems. However, few years ago a third heavy atom effect was proposed in our research group, termed the HAVHA (heavy‐atom effects on vicinal‐heavy‐atoms) effect . New studies were then needed to find enough evidence for this effect.…”

Section: Few Recent Applications Of Relativistic Polarization Propagamentioning

confidence: 99%

“…The electronic mechanisms that underlie these two terms are thus important in the HAVHA effect. We showed that the HAVHA effect is due to all three ligand‐dependent relativistic effects . The percentages of the SO correction for molecular systems containing tin as the central atom goes down.…”

Section: Few Recent Applications Of Relativistic Polarization Propagamentioning

confidence: 99%

Theoretical developments and applications of polarization propagators

Int J of Quantum Chemistry

Maldonado

Montero

et al. 2018

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The Argentinian studies of response properties by applying polarization propagators started more than 35 years ago. It began when the research group led by Professor Ruben Contreras in Buenos Aires started to apply it to the study of NMR spectroscopic parameters on top of semiempirical methods. Novel theoretical developments and its successful early applications make that this group quickly grew up with students from different regions of Argentina. In this review, we shall expose some Argentinian developments of that formalism, including its extension to the relativistic regime and also its latest formal development, which shows how can it be derived from the more fundamental path integral formalism. The interpretative power and the analysis of different electronic effects, that influence the NMR spectroscopic parameters, will be shown with few selected examples. They include molecules with light and heavy atoms, and also hydrogen‐bonded systems. Novel results and analysis of earlier and latest developments will also be given. One of the main advantages of the formalism of polarization propagators is its flexibility for being applied in both regimes, relativistic and nonrelativistic in the same manner. One can go from one to the other framework, by only scaling the velocity of light.