Vaida Arcisauskaite

Nuclear magnetic resonance shielding constants and chemical shifts in linear 199Hg compounds: A comparison of three relativistic computational methods

We investigate the importance of relativistic effects on NMR shielding constants and chemical shifts of linear HgL(2) (L = Cl, Br, I, CH(3)) compounds using three different relativistic methods: the fully relativistic four-component approach and the two-component approximations, linear response elimination of small component (LR-ESC) and zeroth-order regular approximation (ZORA). LR-ESC reproduces successfully the four-component results for the C shielding constant in Hg(CH(3))(2) within 6 ppm, but fails to reproduce the Hg shielding constants and chemical shifts. The latter is mainly due to an underestimation of the change in spin-orbit contribution. Even though ZORA underestimates the absolute Hg NMR shielding constants by ∼2100 ppm, the differences between Hg chemical shift values obtained using ZORA and the four-component approach without spin-density contribution to the exchange-correlation (XC) kernel are less than 60 ppm for all compounds using three different functionals, BP86, B3LYP, and PBE0. However, larger deviations (up to 366 ppm) occur for Hg chemical shifts in HgBr(2) and HgI(2) when ZORA results are compared with four-component calculations with non-collinear spin-density contribution to the XC kernel. For the ZORA calculations it is necessary to use large basis sets (QZ4P) and the TZ2P basis set may give errors of ∼500 ppm for the Hg chemical shifts, despite deceivingly good agreement with experimental data. A Gaussian nucleus model for the Coulomb potential reduces the Hg shielding constants by ∼100-500 ppm and the Hg chemical shifts by 1-143 ppm compared to the point nucleus model depending on the atomic number Z of the coordinating atom and the level of theory. The effect on the shielding constants of the lighter nuclei (C, Cl, Br, I) is, however, negligible.

Billion‐Fold Enhancement in Sensitivity of Nuclear Magnetic Resonance Spectroscopy for Magnesium Ions in Solution

β-nuclear magnetic resonance (NMR) spectroscopy is highly sensitive compared to conventional NMR spectroscopy, and may be applied for several elements across the periodic table. β-NMR has previously been successfully applied in the fields of nuclear and solid-state physics. In this work, β-NMR is applied, for the first time, to record an NMR spectrum for a species in solution. (31)Mg β-NMR spectra are measured for as few as 10(7) magnesium ions in ionic liquid (EMIM-Ac) within minutes, as a prototypical test case. Resonances are observed at 3882.9 and 3887.2 kHz in an external field of 0.3 T. The key achievement of the current work is to demonstrate that β-NMR is applicable for the analysis of species in solution, and thus represents a novel spectroscopic technique for use in general chemistry and potentially in biochemistry.

Electric field gradients in Hg compounds: molecular orbital (MO) analysis and comparison of 4-component and 2-component (ZORA) methods.

We examine the performance of Density Functional Theory (DFT) approaches based on the Zeroth-Order Regular Approximation (ZORA) Hamiltonian (with and without inclusion of spin-orbit coupling) for predictions of electric field gradients (EFGs) at the heavy atom Hg nucleus. This is achieved by comparing with benchmark DFT and CCSD-T data (Arcisauskaite et al., Phys. Chem. Chem. Phys., 2012, 14, 2651-2657) obtained from 4-component Dirac-Coulomb Hamiltonian calculations. The investigated set of molecules comprises linear HgL(2) (L = Cl, Br, I, CH(3)) and bent HgCl(2) mercury compounds as well as the trigonal planar [HgCl(3)](-) system. In 4-component calculations we used the dyall.cv3z basis set for Hg, Br, I and the cc-pCVTZ basis set for H, C, Cl, whereas in ZORA calculations we used the QZ4P basis set for all the atoms. ZORA-4 reproduces the fully relativistic 4-component DFT reference values within 6% for all studied Hg compounds and employed functionals (BH&H, BP86, PBE0), whereas scalar relativistic (SR)-ZORA-4 results show deviations of up to 15%. Compared to our 4-component CCSD-T benchmark the BH&H functional performs best at both 4-component and ZORA levels. We furthermore observe that changes in the largest component of the diagonalised EFG tensor, V(zz), of linear HgCl(2) show a slightly stronger dependence than the r(-3) scaling upon bond length r(Hg-Cl) alterations. The 4-component/BH&H V(zz) value of -9.26 a.u. for a bent HgCl(2) (∠Cl-Hg-Cl = 120°) is close to -9.60 a.u. obtained for the linear HgCl(2) structure. Thus a point charge model for EFG calculations completely fails in this case. By means of a projection analysis of molecular orbital (MO) contributions to V(zz) in terms of the atomic constituents, we conclude that this is due to the increased importance of the Hg 5d orbitals upon bending HgCl(2) compared to the linear HgCl(2) structure. Changing ligand leads to only minor changes in V(zz) (from -9.60 a.u. (HgCl(2)) to -8.85 a.u. (HgI(2)) at the 4-component/BH&H level). This appears to be due to cancellation of contributions with opposite signs to V(zz) arising from: (i) increasing electron donation from occupied ligand orbitals to the formally empty Hg 6p orbitals and (ii) an increasing bond length and a decreasing negative charge on the ligand along the series.

Structural prediction, analysis and decomposition mechanism of solid M(NH2BH3)n (M = Mg, Ca and Al)

Multivalent Metal amidoboranes (MABs) are considered to be the important candidates for hydrogen storage materials. Whereas CaAB crystal structure has been determined experimentally, the crystal structures of MgAB and AlAB have not been obtained, thus hindering the further development of MABs. In this work, we determine the crystal and electronic structures of MABs (M = Mg, Ca, Al), and obtain their phononic density of states and thermodynamic properties. By means of M–N population analysis, we conclude that Al–N and Ca–N bonds have a covalent character whereas the Mg–N bond is ionic. We furthermore observe that HOMO–LUMO gaps and thus stability follow the trend MgAB ≈ AlAB > CaAB. Thermodynamic properties and their dependence on temperature seem to be very similar for AlAB and MgAB compounds compared to CaAB. CaAB has the lowest enthalpy and thus the lowest internal energy in the series of MABs (M = Mg, Ca, Al). We also propose the most probable dehydrogenation pathways during which 4n H2 molecules are released (in 4 steps) from MgAB and CaAB and 6n H2 (in 6 steps) from AlAB indicating that dehydrogenation from the same AB group twice in a row is less likely to occur. Furthermore, we reveal that the energy barriers for the 1st dehydrogenation step follow the trend CaAB > MgAB > AlAB. This is in agreement with the experiments showing that AlAB will start to dehydrogenate at the lowest temperature followed by MgAB. Finally, we propose that complete dehydrogenation rates at 400 °C follow the trend MgAB (1.73 × 10−14 min−1) ≈ CaAB (1.72 × 10−14 min−1) > AlAB (2.18 × 10−16 min−1).

Computational assignment of redox states to Coulomb blockade diamonds

With the advent of molecular transistors, electrochemistry can now be studied at the single-molecule level. Experimentally, the redox chemistry of the molecule manifests itself as features in the observed Coulomb blockade diamonds. We present a simple theoretical method for explicit construction of the Coulomb blockade diamonds of a molecule. A combined quantum mechanical/molecular mechanical method is invoked to calculate redox energies and polarizabilities of the molecules, including the screening effect of the metal leads. This direct approach circumvents the need for explicit modelling of the gate electrode. From the calculated parameters the Coulomb blockade diamonds are constructed using simple theory. We offer a theoretical tool for assignment of Coulomb blockade diamonds to specific redox states in particular, and a study of chemical details in the diamonds in general. With the ongoing experimental developments in molecular transistor experiments, our tool could find use in molecular electronics, electrochemistry, and electrocatalysis.

The role of π-blocking hydride ligands in a pressure-induced insulator-to-metal phase transition in SrVO 2 H

Transition-metal oxyhydrides are of considerable current interest due to the unique features of the hydride anion, most notably the absence of valence p orbitals. This feature distinguishes hydrides from all other anions, and gives rise to unprecedented properties in this new class of materials. Here we show via a high-pressure study of anion-ordered strontium vanadium oxyhydride SrVO2H that H− is extraordinarily compressible, and that pressure drives a transition from a Mott insulator to a metal at ~ 50 GPa. Density functional theory suggests that the band gap in the insulating state is reduced by pressure as a result of increased dispersion in the ab-plane due to enhanced Vdπ-Opπ-Vdπ overlap. Remarkably, dispersion along c is limited by the orthogonal Vdπ-H1s-Vdπ arrangement despite the greater c-axis compressibility, suggesting that the hydride anions act as π-blockers. The wider family of oxyhydrides may therefore give access to dimensionally reduced structures with novel electronic properties.Incorporating hydride anions into transition metal oxides can dramatically affect their structural and electronic properties. Here the authors reveal a pressure-induced insulator-to-metal transition in SrVO2H and show that the compressibility of hydride anions without π-symmetry valence orbitals causes them to act as π-blockers.

In search of structure–function relationships in transition-metal based rectifiers

Heterometallic chains have been proposed as potential current rectifiers in molecular electronics, their left–right asymmetry providing, at least in principle, a mechanism for differentiation of current flow in forward and reverse directions. Here we compare two known extended metal atom chains (EMACs), Ru2Ni(dpa)4(NCS)2 and Ru2Cu(dpa)4(NCS)2, both of which meet the first criterion for rectification in so much as they are physically asymmetric. In both cases the dominant transport channel is a doubly degenerate π* orbital localised, to a first approximation, on the Ru2 unit. As a result, current is limited by tunnelling across the Au–SCN–Ni/Cu junction. The paramagnetic Ni centre tunes the left–right delocalisation of the channel, making the minority-spin (β) channel more transparent than its spin-α counterpart and this difference provides the basis for asymmetry in the current under forward and reverse bias.

Electronic Properties of Endohedral Clusters of Group 14

The concept of stable “superatoms,” molecular species which mimic the shell closures emphasised by Lewis and Kossel, has become an important paradigm of stability in cluster chemistry. In this review we discuss recent work, both experimental and theoretical, on the family of endohedral clusters M@Ex, where M is a transition metal ion and E is a member of group 14 (Si, Ge, Sn, Pb). The structural chemistry within this family is very varied, ranging from deltahedral motifs for the heavier tetrels to open 3-connected structures such as the hexagonal prism in Cr@Si12. We explore the arguments that have been presented to rationalise these structural trends and their implications for chemical bonding.

Application of 204mPb perturbed angular correlation of γ-rays spectroscopy in coordination chemistry.

(204m)Pb perturbed angular correlation of γ-rays (PAC) spectroscopy has been applied successfully for the first time to detect the nuclear quadrupole interaction in a lead(II) coordination compound in a molecular crystal [tetraphenylarsonium lead(II) isomaleonitriledithiolate ([AsPh(4)](4)[Pb(2)(i-mnt)(4)])]. The recorded parameters from a powder crystalline sample are ν(Q) = 0.178(1) GHz and η = 0.970(7). The electric field gradient (EFG) was determined at the PW91/QZ4P level including relativistic effects using the two-component zeroth-order regular approximation method for both the [Pb(i-mnt)(2)](2-) monomer and the [Pb(2)(i-mnt)(4)](4-) dimer. Only the EFG for the latter compares favorably with the experimental data, indicating that the picture of this complex as a prototypical hemidirected coordination geometry with a stereochemically active lone pair on lead(II) is inadequate. Advantages and limitations of (204m)Pb PAC spectroscopy as a novel technique to elucidate the electronic and molecular structures of lead-containing complexes and biomolecules are presented.