Michal Straka | Academy of Sciences of the Czech Republic (original) (raw)

Papers by Michal Straka

The importance of relativistic effects on the NMR parameters in heavy-atom (HA) compounds, partic... more The importance of relativistic effects on the NMR parameters in heavy-atom (HA) compounds, particularly the SO-HALA (Spin−Orbit Heavy Atom on the Light Atom) effect on NMR chemical shifts, has been known for about 40 years. Yet, a general correlation between the electronic structure and SO-HALA effect has been missing. By analyzing 1 H NMR chemical shifts of the sixth-period hydrides (Cs−At), we discovered general electronic-structure principles and mechanisms that dictate the size and sign of the SO-HALA NMR chemical shifts. In brief, partially occupied HA valence shells induce relativistic shielding at the light atom (LA) nuclei, while empty HA valence shells induce relativistic deshielding. In particular, the LA nucleus is relativistically shielded in 5d 2 −5d 8 and 6p 4 HA hydrides and deshielded in 4f 0 , 5d 0 , 6s 0 , and 6p 0 HA hydrides. This general and intuitive concept explains periodic trends in the 1 H NMR chemical shifts along the sixth-period hydrides (Cs−At) studied in this work. We present substantial evidence that the introduced principles have a general validity across the periodic table and can be extended to nonhydride LAs. The decades-old question of why compounds with occupied frontier π molecular orbitals (MOs) cause SO-HALA shielding at the LA nuclei, while the frontier σ MOs cause deshielding is answered. We further derive connection between the SO-HALA NMR chemical shifts and Spin−Orbit-induced Electron Deformation Density (SO-EDD), a property that can be obtained easily from differential electron densities and can be represented graphically. SO-EDD provides an intuitive understanding of the SO-HALA effect in terms of the depletion/ concentration of the electron density at LA nuclei caused by spin−orbit coupling due to HA in the presence of a magnetic field. Using an analogy between the SO-EDD concept and arguments from classic NMR theory, the complex question of the SO-HALA NMR chemical shifts becomes easily understandable for a wide chemical audience.

Resveratrol is antioxidant naturally occurring in wine grapes. It is thought to have a preventive... more Resveratrol is antioxidant naturally occurring in wine grapes. It is thought to have a preventive biological activity against number of diseases. However, it has been recently shown that in the presence of metal ions, such as Cu 2+ , resveratrol forms oxidative radicals. Cu 2+ is usually present in wine due to former usage of bluestone in vineyards. Fungicide tebuconazole has substituted bluestone and is presently one of the most widely used agrochemicals in wine industry; wine thus may contain traces of tebuconazole. Here, we study the ternary system of resveratrol, Cu 2+ , and tebuconazole experimentally and theoretically (using mass spectrometry, an-tioxidant capacity assay and quantum-chemical calculations) to model the redox behaviour of resveratrol in wine. We show that tebuconazole prevents formation of oxidative resveratrol radicals (induced by Cu 2+ reaction with resveratrol) via preferential Cu 2+ capture and protection of the binding sites of resveratrol. This positive effect of tebuconazole has not been observed before.

We report the F2@C60 system as the first example of an endohedral fullerene in which C 60 acts as... more We report the F2@C60 system as the first example of an endohedral fullerene in which C 60 acts as a cation C60+ interacting with endohedral anion, F2-.

The stability of H 2 molecules trapped inside a 720 carbon atoms fullerene (C 720) was studied us... more The stability of H 2 molecules trapped inside a 720 carbon atoms fullerene (C 720) was studied using the path integral Monte Carlo formalism. The change in free energy with respect to the free molecules as a function of H 2 molecules inside the fullerene was computed using alchemy methods coupled to replica exchange algorithms. The most stable number of molecules and the maximum number of molecules that could be trapped inside C 720 were obtained at T = 300 K and 200 K. Modifications in the interparticle potential were considered to increase the trapping capacity of C 720. Four clearly defined concentric layers of H 2 in the molecular fluid phase were observed inside the C 720 at loading that are acceptable for efficient storages.

A recent study (Sci. Adv. 2017, 3, e1602833) has shown that FH···OH 2 hydrogen bond in a HF·H 2 O... more A recent study (Sci. Adv. 2017, 3, e1602833) has shown that FH···OH 2 hydrogen bond in a HF·H 2 O pair substantially shortens, and the HÀF bond elongates upon en-capsulation of the cluster in C 70 fullerene. This has been attributed to compression of the HF·H 2 O pair inside the cavity of C 70. Herein, we present theoretical evidence that the effect is not caused by a mere compression of the H 2 O·HF pair, but it is related to a strong lone-pair–p (LP–p) bonding with the fullerene cage. To support this argument , a systematic electronic structure study of selected small molecules (HF, H 2 O, and NH 3) and their pairs enclosed in fullerene cages (C 60 , C 70 , and C 90) has been performed. Bonding analysis revealed unique LP–p cage interactions with a charge-depletion character in the bonding region, unlike usual LP–p bonds. The LP–p cage interactions were found to be responsible for elongation of the HÀF bond. Thus, the HF appears to be more acidic inside the cage. The shortening of the FH···OH 2 contact in (HF·H 2 O)@C 70 originates from an increased acidity of the HF inside the fullerenes. Such trends were also observed in other studied systems.

Journal of Chemical Theory and Computation, 2017

Relativistic effects significantly affect various spectro-scopic properties of compounds containi... more Relativistic effects significantly affect various spectro-scopic properties of compounds containing heavy elements. Particularly in Nuclear Magnetic Resonance (NMR) spectroscopy, the heavy atoms strongly influence the NMR shielding constants of neighboring light atoms. In this account we analyze paramagnetic contributions to NMR shielding constants and their modulation by relativistic spin−orbit effects in a series of transition-metal complexes of Pt(II), Au(I), Au(III), and Hg(II). We show how the paramagnetic NMR shielding and spin− orbit effects relate to the character of the metal−ligand (M−L) bond. A correlation between the (back)-donation character of the M−L bond in d 10 Au(I) complexes and the propagation of the spin−orbit (SO) effects from M to L through the M−L bond influencing the ligand NMR shielding via the Fermi-contact mechanism is found and rationalized by using third-order perturbation theory. The SO effects on the ligand NMR shielding are demonstrated to be driven by both the electronic structure of M and the nature of the trans ligand, sharing the σ-bonding metal orbital with the NMR spectator atom L. The deshielding paramagnetic contribution is linked to the σ-type M−L bonding orbitals, which are notably affected by the trans ligand. The SO deshielding role of σ-type orbitals is enhanced in d 10 Hg(II) complexes with the Hg 6p atomic orbital involved in the M−L bonding. In contrast, in d 8 Pt(II) complexes, occupied π-type orbitals play a dominant role in the SO-altered magnetic couplings due to the accessibility of vacant antibonding σ-type MOs in formally open 5d-shell (d 8). This results in a significant SO shielding at the light atom. The energy-and composition-modulation of σ-vs π-type orbitals by spin−orbit coupling is rationalized and supported by visualizing the SO-induced changes in the electron density around the metal and light atoms (spin−orbit electron deformation density, SO-EDD).

Only a single thorium fullerene, Th@C 84 , has been reported to date (Akiyama, K.; et al. J. Nucl... more Only a single thorium fullerene, Th@C 84 , has been reported to date (Akiyama, K.; et al. J. Nucl. Radiochem. Sci. 2002, 3, 151−154). Although the system was characterized by UV−vis and XANES (X-ray absorption near edge structure) spectra, its structure and properties remain unknown. In this work we used the density functional calculations to identify molecular and electronic structure of the Th@C 84. Series of molecular structures satisfying the ThC 84 stoichiometric formula were studied comprising 24 IPR and 110 non-IPR Th@C 84 isomers as well as 9 ThC 2 @C 82 IPR isomers. The lowest energy structure is Th@C 84-C s (10) with the singlet ground state. Its predicted electronic absorption spectra are in agreement with the experimentally observed ones. The bonding between the cage and Th was characterized as polar covalent with Th in formal oxidation state IV. The NMR chemical shifts of Th@C 84-C s (10) were predicted to guide the future experimental efforts in identification of this compound.

The role of relativistic effects on 1H NMR chemical shifts of Sn(II) and Pb(II) hydrides is inves... more The role of relativistic effects on 1H NMR chemical shifts of Sn(II) and Pb(II) hydrides is investigated by using fully relativistic DFT calculations. The stability of possible Pb(II) hydride isomers is studied together with their 1H NMR chemical shifts, which are predicted in the high-frequency region, up to 90 ppm. These 1H signals are dictated by sizable relativistic contributions due to spin-orbit coupling at the heavy atom and can be as large as 80 ppm for a hydrogen atom bound to Pb(II). Such high-frequency 1H NMR chemical shifts of Pb(II) hydride resonances cannot be detected in the 1H NMR spectra with standard experimental setup. Extended 1H NMR spectral ranges are thus suggested for studies of Pb(II) compounds. Modulation of spin-orbit relativistic contribution to 1H NMR chemical shift is found to be important also in the experimentally known Sn(II) hydrides. Because the 1H NMR chemical shifts were found to be rather sensitive to the changes in the coordination sphere of the central metal in both Sn(II) and Pb(II) hydrides, their application for structural investigation is suggested.

On the basis of four-component relativistic DFT calculations, large relativistic spin−orbit effec... more On the basis of four-component relativistic DFT calculations, large relativistic spin−orbit effects amounting to more than 200 ppm for 13C and more than 1000 ppm for 29Si are identified to be the main contributions to high-frequency 13C and 29Si NMR chemical shifts in Tl(I) and Pb(II) compounds. Origin of the large SO deshielding is traced to highly efficient 6p−6p* orbital magnetic couplings.

We propose a two-state electric field-driven room-temperature single-molecule switch based on a d... more We propose a two-state electric field-driven room-temperature single-molecule switch based on a dipolar molecule enclosed inside ellipsoidal fullerene C 70. We show that the two low-energy minima of the molecular dipole inside the C 70 cage provide distin-guishable molecular states of the system that can be switched by application of an external electric field.

Endohedral actinide fullerenes are rare and a little is known about their molecular properties. ... more Endohedral actinide fullerenes are rare and a little is known about their molecular properties.
Here we characterize U2@C80 system, which was recently detected experimentally by means
of mass spectrometry (Akiyama et al., JACS 2001, 123, 181). Theoretical calculations predict
a stable endohedral system, 7U2@C80 derived from the C80:7 IPR fullerene cage, with six
unpaired electrons. Bonding analysis reveals a double ferromagnetic (one-electron-twocenter)
U‒U bond at rU‒U distance of 3.9 Å. This bonding is realized mainly via U(5f) orbitals.
The U-U interaction inside the cage is estimated to be about -18 kcal/mol. The U‒U bonding
is further studied along the U2@Cn (n = 60, 70, 80, 84, 90) series and the U‒U bonds are also
identified in U2@C70 and U2@C84 systems at rU‒U ~4 Å. It is found that the character of U‒U
bonding depends on the U‒U distance, which is dictated by the cage type. A concept of
unwilling metal-metal bonding is suggested: Uranium atoms are strongly bound to the cage
and carry a positive charge. Pushing the U(5f) electron density into U-U bonding region
reduces electrostatic repulsion between enclosed atoms, thus forcing U‒U bonds.

Theoretical Chemistry Accounts: Theory, Computation, and Modeling (Theoretica Chimica Acta), 2003

Relativistic small-core pseudopotential B3LYP and CCSD(T) calculations and frozen-core PW91-PW91 ... more Relativistic small-core pseudopotential B3LYP and CCSD(T) calculations and frozen-core PW91-PW91 studies are reported for the series UF 4 X 2 (X =H, F, Cl, CN, NC, NCO, OCN, NCS and SCN). The bonding in UF 6 is analyzed and found to have some multiple-bond character, approaching at a theoretical limit a bond order of 1.5. In addition to these r and p orbital interactions, the electrostatic attraction is important. Evidence for p bonding in the other systems studied was also found. The triatomic pseudohalides as well as fluorine and chlorine are in this sense better ligands than cyanide. The -CN group is a r donor and p acceptor, as uranium itself, and hence is unfit to bond to U(VI). The r-bonded UH 6 is octahedral.

Chemical Communications, 2002

Theoretical Chemistry Accounts, 2003

Relativistic small-core pseudopotential B3LYP and CCSD(T) calculations and frozen-core PW91–PW91 ... more Relativistic small-core pseudopotential B3LYP and CCSD(T) calculations and frozen-core PW91–PW91 studies are reported for the series UF 4X 2 ( X=H, F, Cl, CN, NC, NCO, OCN, NCS and SCN). The bonding in UF 6 is analyzed and found to have some multiple-bond character, approaching at a theoretical limit a bond order of 1.5. In addition to these s and

The isotropic 129 Xe nuclear magnetic resonance (NMR) chemical shift (CS) in Xe@C 60 dissolved in... more The isotropic 129 Xe nuclear magnetic resonance (NMR) chemical shift (CS) in Xe@C 60 dissolved in liquid benzene was calculated by piecewise approximation to faithfully simulate the experimen-tal conditions and to evaluate the role of different physical fac-tors influencing the 129 Xe NMR CS. The 129 Xe shielding constant was obtained by averaging the 129 Xe nuclear magnetic shield-ings calculated for snapshots obtained from the molecular dy-namics trajectory of the Xe@C 60 system embedded in a periodic box of benzene molecules. Relativistic corrections were added at the Breit–Pauli perturbation theory (BPPT) level, included the sol-vent, and were dynamically averaged. It is demonstrated that the contribution of internal dynamics of the Xe@C 60 system represents about 8% of the total nonrelativistic NMR CS, whereas the effects of dynamical solvent add another 8%. The dynamically averaged relativistic effects contribute by 9% to the total calculated 129 Xe NMR CS. The final theoretic...

Physical Review E, 2007

We present constant-pressure Monte Carlo simulations of nuclear magnetic resonance (NMR) spectral... more We present constant-pressure Monte Carlo simulations of nuclear magnetic resonance (NMR) spectral parameters, nuclear magnetic shielding relative to the free atom as well as nuclear quadrupole coupling, for atomic xenon dissolved in a model thermotropic liquid crystal. The solvent is described by Gay-Berne (GB) molecules with parametrization kappa=4.4 , kappa'=20.0 , and mu=nu=1 . The reduced pressure of P*=2.0 is

The importance of relativistic effects on the NMR parameters in heavy-atom (HA) compounds, partic... more The importance of relativistic effects on the NMR parameters in heavy-atom (HA) compounds, particularly the SO-HALA (Spin−Orbit Heavy Atom on the Light Atom) effect on NMR chemical shifts, has been known for about 40 years. Yet, a general correlation between the electronic structure and SO-HALA effect has been missing. By analyzing 1 H NMR chemical shifts of the sixth-period hydrides (Cs−At), we discovered general electronic-structure principles and mechanisms that dictate the size and sign of the SO-HALA NMR chemical shifts. In brief, partially occupied HA valence shells induce relativistic shielding at the light atom (LA) nuclei, while empty HA valence shells induce relativistic deshielding. In particular, the LA nucleus is relativistically shielded in 5d 2 −5d 8 and 6p 4 HA hydrides and deshielded in 4f 0 , 5d 0 , 6s 0 , and 6p 0 HA hydrides. This general and intuitive concept explains periodic trends in the 1 H NMR chemical shifts along the sixth-period hydrides (Cs−At) studied in this work. We present substantial evidence that the introduced principles have a general validity across the periodic table and can be extended to nonhydride LAs. The decades-old question of why compounds with occupied frontier π molecular orbitals (MOs) cause SO-HALA shielding at the LA nuclei, while the frontier σ MOs cause deshielding is answered. We further derive connection between the SO-HALA NMR chemical shifts and Spin−Orbit-induced Electron Deformation Density (SO-EDD), a property that can be obtained easily from differential electron densities and can be represented graphically. SO-EDD provides an intuitive understanding of the SO-HALA effect in terms of the depletion/ concentration of the electron density at LA nuclei caused by spin−orbit coupling due to HA in the presence of a magnetic field. Using an analogy between the SO-EDD concept and arguments from classic NMR theory, the complex question of the SO-HALA NMR chemical shifts becomes easily understandable for a wide chemical audience.

Resveratrol is antioxidant naturally occurring in wine grapes. It is thought to have a preventive... more Resveratrol is antioxidant naturally occurring in wine grapes. It is thought to have a preventive biological activity against number of diseases. However, it has been recently shown that in the presence of metal ions, such as Cu 2+ , resveratrol forms oxidative radicals. Cu 2+ is usually present in wine due to former usage of bluestone in vineyards. Fungicide tebuconazole has substituted bluestone and is presently one of the most widely used agrochemicals in wine industry; wine thus may contain traces of tebuconazole. Here, we study the ternary system of resveratrol, Cu 2+ , and tebuconazole experimentally and theoretically (using mass spectrometry, an-tioxidant capacity assay and quantum-chemical calculations) to model the redox behaviour of resveratrol in wine. We show that tebuconazole prevents formation of oxidative resveratrol radicals (induced by Cu 2+ reaction with resveratrol) via preferential Cu 2+ capture and protection of the binding sites of resveratrol. This positive effect of tebuconazole has not been observed before.

We report the F2@C60 system as the first example of an endohedral fullerene in which C 60 acts as... more We report the F2@C60 system as the first example of an endohedral fullerene in which C 60 acts as a cation C60+ interacting with endohedral anion, F2-.

The stability of H 2 molecules trapped inside a 720 carbon atoms fullerene (C 720) was studied us... more The stability of H 2 molecules trapped inside a 720 carbon atoms fullerene (C 720) was studied using the path integral Monte Carlo formalism. The change in free energy with respect to the free molecules as a function of H 2 molecules inside the fullerene was computed using alchemy methods coupled to replica exchange algorithms. The most stable number of molecules and the maximum number of molecules that could be trapped inside C 720 were obtained at T = 300 K and 200 K. Modifications in the interparticle potential were considered to increase the trapping capacity of C 720. Four clearly defined concentric layers of H 2 in the molecular fluid phase were observed inside the C 720 at loading that are acceptable for efficient storages.

A recent study (Sci. Adv. 2017, 3, e1602833) has shown that FH···OH 2 hydrogen bond in a HF·H 2 O... more A recent study (Sci. Adv. 2017, 3, e1602833) has shown that FH···OH 2 hydrogen bond in a HF·H 2 O pair substantially shortens, and the HÀF bond elongates upon en-capsulation of the cluster in C 70 fullerene. This has been attributed to compression of the HF·H 2 O pair inside the cavity of C 70. Herein, we present theoretical evidence that the effect is not caused by a mere compression of the H 2 O·HF pair, but it is related to a strong lone-pair–p (LP–p) bonding with the fullerene cage. To support this argument , a systematic electronic structure study of selected small molecules (HF, H 2 O, and NH 3) and their pairs enclosed in fullerene cages (C 60 , C 70 , and C 90) has been performed. Bonding analysis revealed unique LP–p cage interactions with a charge-depletion character in the bonding region, unlike usual LP–p bonds. The LP–p cage interactions were found to be responsible for elongation of the HÀF bond. Thus, the HF appears to be more acidic inside the cage. The shortening of the FH···OH 2 contact in (HF·H 2 O)@C 70 originates from an increased acidity of the HF inside the fullerenes. Such trends were also observed in other studied systems.

Journal of Chemical Theory and Computation, 2017

Relativistic effects significantly affect various spectro-scopic properties of compounds containi... more Relativistic effects significantly affect various spectro-scopic properties of compounds containing heavy elements. Particularly in Nuclear Magnetic Resonance (NMR) spectroscopy, the heavy atoms strongly influence the NMR shielding constants of neighboring light atoms. In this account we analyze paramagnetic contributions to NMR shielding constants and their modulation by relativistic spin−orbit effects in a series of transition-metal complexes of Pt(II), Au(I), Au(III), and Hg(II). We show how the paramagnetic NMR shielding and spin− orbit effects relate to the character of the metal−ligand (M−L) bond. A correlation between the (back)-donation character of the M−L bond in d 10 Au(I) complexes and the propagation of the spin−orbit (SO) effects from M to L through the M−L bond influencing the ligand NMR shielding via the Fermi-contact mechanism is found and rationalized by using third-order perturbation theory. The SO effects on the ligand NMR shielding are demonstrated to be driven by both the electronic structure of M and the nature of the trans ligand, sharing the σ-bonding metal orbital with the NMR spectator atom L. The deshielding paramagnetic contribution is linked to the σ-type M−L bonding orbitals, which are notably affected by the trans ligand. The SO deshielding role of σ-type orbitals is enhanced in d 10 Hg(II) complexes with the Hg 6p atomic orbital involved in the M−L bonding. In contrast, in d 8 Pt(II) complexes, occupied π-type orbitals play a dominant role in the SO-altered magnetic couplings due to the accessibility of vacant antibonding σ-type MOs in formally open 5d-shell (d 8). This results in a significant SO shielding at the light atom. The energy-and composition-modulation of σ-vs π-type orbitals by spin−orbit coupling is rationalized and supported by visualizing the SO-induced changes in the electron density around the metal and light atoms (spin−orbit electron deformation density, SO-EDD).

Only a single thorium fullerene, Th@C 84 , has been reported to date (Akiyama, K.; et al. J. Nucl... more Only a single thorium fullerene, Th@C 84 , has been reported to date (Akiyama, K.; et al. J. Nucl. Radiochem. Sci. 2002, 3, 151−154). Although the system was characterized by UV−vis and XANES (X-ray absorption near edge structure) spectra, its structure and properties remain unknown. In this work we used the density functional calculations to identify molecular and electronic structure of the Th@C 84. Series of molecular structures satisfying the ThC 84 stoichiometric formula were studied comprising 24 IPR and 110 non-IPR Th@C 84 isomers as well as 9 ThC 2 @C 82 IPR isomers. The lowest energy structure is Th@C 84-C s (10) with the singlet ground state. Its predicted electronic absorption spectra are in agreement with the experimentally observed ones. The bonding between the cage and Th was characterized as polar covalent with Th in formal oxidation state IV. The NMR chemical shifts of Th@C 84-C s (10) were predicted to guide the future experimental efforts in identification of this compound.

The role of relativistic effects on 1H NMR chemical shifts of Sn(II) and Pb(II) hydrides is inves... more The role of relativistic effects on 1H NMR chemical shifts of Sn(II) and Pb(II) hydrides is investigated by using fully relativistic DFT calculations. The stability of possible Pb(II) hydride isomers is studied together with their 1H NMR chemical shifts, which are predicted in the high-frequency region, up to 90 ppm. These 1H signals are dictated by sizable relativistic contributions due to spin-orbit coupling at the heavy atom and can be as large as 80 ppm for a hydrogen atom bound to Pb(II). Such high-frequency 1H NMR chemical shifts of Pb(II) hydride resonances cannot be detected in the 1H NMR spectra with standard experimental setup. Extended 1H NMR spectral ranges are thus suggested for studies of Pb(II) compounds. Modulation of spin-orbit relativistic contribution to 1H NMR chemical shift is found to be important also in the experimentally known Sn(II) hydrides. Because the 1H NMR chemical shifts were found to be rather sensitive to the changes in the coordination sphere of the central metal in both Sn(II) and Pb(II) hydrides, their application for structural investigation is suggested.

On the basis of four-component relativistic DFT calculations, large relativistic spin−orbit effec... more On the basis of four-component relativistic DFT calculations, large relativistic spin−orbit effects amounting to more than 200 ppm for 13C and more than 1000 ppm for 29Si are identified to be the main contributions to high-frequency 13C and 29Si NMR chemical shifts in Tl(I) and Pb(II) compounds. Origin of the large SO deshielding is traced to highly efficient 6p−6p* orbital magnetic couplings.

We propose a two-state electric field-driven room-temperature single-molecule switch based on a d... more We propose a two-state electric field-driven room-temperature single-molecule switch based on a dipolar molecule enclosed inside ellipsoidal fullerene C 70. We show that the two low-energy minima of the molecular dipole inside the C 70 cage provide distin-guishable molecular states of the system that can be switched by application of an external electric field.

Endohedral actinide fullerenes are rare and a little is known about their molecular properties. ... more Endohedral actinide fullerenes are rare and a little is known about their molecular properties.
Here we characterize U2@C80 system, which was recently detected experimentally by means
of mass spectrometry (Akiyama et al., JACS 2001, 123, 181). Theoretical calculations predict
a stable endohedral system, 7U2@C80 derived from the C80:7 IPR fullerene cage, with six
unpaired electrons. Bonding analysis reveals a double ferromagnetic (one-electron-twocenter)
U‒U bond at rU‒U distance of 3.9 Å. This bonding is realized mainly via U(5f) orbitals.
The U-U interaction inside the cage is estimated to be about -18 kcal/mol. The U‒U bonding
is further studied along the U2@Cn (n = 60, 70, 80, 84, 90) series and the U‒U bonds are also
identified in U2@C70 and U2@C84 systems at rU‒U ~4 Å. It is found that the character of U‒U
bonding depends on the U‒U distance, which is dictated by the cage type. A concept of
unwilling metal-metal bonding is suggested: Uranium atoms are strongly bound to the cage
and carry a positive charge. Pushing the U(5f) electron density into U-U bonding region
reduces electrostatic repulsion between enclosed atoms, thus forcing U‒U bonds.

Theoretical Chemistry Accounts: Theory, Computation, and Modeling (Theoretica Chimica Acta), 2003

Relativistic small-core pseudopotential B3LYP and CCSD(T) calculations and frozen-core PW91-PW91 ... more Relativistic small-core pseudopotential B3LYP and CCSD(T) calculations and frozen-core PW91-PW91 studies are reported for the series UF 4 X 2 (X =H, F, Cl, CN, NC, NCO, OCN, NCS and SCN). The bonding in UF 6 is analyzed and found to have some multiple-bond character, approaching at a theoretical limit a bond order of 1.5. In addition to these r and p orbital interactions, the electrostatic attraction is important. Evidence for p bonding in the other systems studied was also found. The triatomic pseudohalides as well as fluorine and chlorine are in this sense better ligands than cyanide. The -CN group is a r donor and p acceptor, as uranium itself, and hence is unfit to bond to U(VI). The r-bonded UH 6 is octahedral.

Chemical Communications, 2002

Theoretical Chemistry Accounts, 2003

Relativistic small-core pseudopotential B3LYP and CCSD(T) calculations and frozen-core PW91–PW91 ... more Relativistic small-core pseudopotential B3LYP and CCSD(T) calculations and frozen-core PW91–PW91 studies are reported for the series UF 4X 2 ( X=H, F, Cl, CN, NC, NCO, OCN, NCS and SCN). The bonding in UF 6 is analyzed and found to have some multiple-bond character, approaching at a theoretical limit a bond order of 1.5. In addition to these s and

The isotropic 129 Xe nuclear magnetic resonance (NMR) chemical shift (CS) in Xe@C 60 dissolved in... more The isotropic 129 Xe nuclear magnetic resonance (NMR) chemical shift (CS) in Xe@C 60 dissolved in liquid benzene was calculated by piecewise approximation to faithfully simulate the experimen-tal conditions and to evaluate the role of different physical fac-tors influencing the 129 Xe NMR CS. The 129 Xe shielding constant was obtained by averaging the 129 Xe nuclear magnetic shield-ings calculated for snapshots obtained from the molecular dy-namics trajectory of the Xe@C 60 system embedded in a periodic box of benzene molecules. Relativistic corrections were added at the Breit–Pauli perturbation theory (BPPT) level, included the sol-vent, and were dynamically averaged. It is demonstrated that the contribution of internal dynamics of the Xe@C 60 system represents about 8% of the total nonrelativistic NMR CS, whereas the effects of dynamical solvent add another 8%. The dynamically averaged relativistic effects contribute by 9% to the total calculated 129 Xe NMR CS. The final theoretic...

Physical Review E, 2007

We present constant-pressure Monte Carlo simulations of nuclear magnetic resonance (NMR) spectral... more We present constant-pressure Monte Carlo simulations of nuclear magnetic resonance (NMR) spectral parameters, nuclear magnetic shielding relative to the free atom as well as nuclear quadrupole coupling, for atomic xenon dissolved in a model thermotropic liquid crystal. The solvent is described by Gay-Berne (GB) molecules with parametrization kappa=4.4 , kappa'=20.0 , and mu=nu=1 . The reduced pressure of P*=2.0 is