Naresh S. Dalal

Electronic Structure and Slow Magnetic Relaxation of Low-Coordinate Cyclic Alkyl(amino) Carbene Stabilized Iron(I) Complexes

Cyclic alkyl(amino) carbene stabilized two- and three-coordinate Fe(I) complexes, (cAAC)2FeCl (2) and [(cAAC)2Fe][B(C6F5)4] (3), respectively, were prepared and thoroughly studied by a bouquet of analytical techniques as well as theoretical calculations. Magnetic susceptibility and Mössbauer spectroscopy reveal the +1 oxidation state and S = 3/2 spin ground state of iron in both compounds. 2 and 3 show slow magnetic relaxation typical for single molecule magnets under an applied direct current magnetic field. The high-frequency EPR measurements confirm the S = 3/2 ground state with a large, positive zero-field splitting (∼20.4 cm(-1)) and reveal easy plane anisotropy for compound 2. CASSCF/CASPT2/RASSI-SO ab initio calculations using the MOLCAS program package support the experimental results.

Enhanced proton and electron reservoir abilities of polyoxometalate grafted on graphene for high-performance hydrogen evolution

A beneficial microenvironment effect on the efficiency of confined electrocatalysts is predicted by theory. However, examples of its experimental confirmation are scarce for catalysts based on polyoxometalates for the hydrogen evolution reaction (HER). For this purpose, the cyclic 48-tungsto-8-phosphate [H7P8W48O184](33) (P8W48) was fixed in a 3D configuration on reduced graphene oxide sheets (rGO) to boost its HER activity. The HRTEM imaging and the solid state P-31 NMR spectrum of P8W48/rGO reveal a strong interaction between individual P8W48 and transparent rGO sheets. The calculation of the interaction between P8W48 and graphene (G) sheets is difficult to perform within a reasonable period of time because of the large size and very high overall negative charge of P8W48. However, as P8W48 is symmetrical, a quarter of its structure [H2P2W12O48](12-) (P2W12) was extracted as a DFT calculation model. As P8W48 in P8W48/rGO is neutral, due to surrounding counter cations, the calculation model P2W12 is neutral with protons considering the affordable computational time. The adsorption energy for P2W12 on G (-1.55 eV) and the charge transfer between P2W12 and G (0.66 vertical bar e vertical bar) indicate that a strong interaction between P2W12 and G sheets exists. Kinetic studies show that the P8W48/rGO hybrids display excellent HER activity in acid, further confirmed by reproducible generation of hydrogen with quantitative faradaic yield and a high turnover frequency (11 s(-1) at 295 mV overpotential) for a noble metal-free electrocatalyst. Importantly, the overpotentials required for the HER compare well with those of the commercial Pt/C (20 wt% Pt), which indicates that P8W48/rGO is a promising cheap HER electrocatalyst. We demonstrate here the most convincing experimental evidence of the microenvironment effect on HER electrocatalysis by a polyoxometalate.

Molecular spin qubits based on lanthanide ions encapsulated in cubic polyoxopalladates: design criteria to enhance quantum coherence

The family of cubic polyoxopalladates encapsulating lanthanide ions [LnPd12(AsPh)8O32]5− where Ln = Tb, Dy, Ho, Er and Tm, is magnetically characterised and theoretically described by the Radial Effective Charge (REC) model and a phenomenological crystal-field approach using the full-hamiltonian, in the SIMPRE and CONDON packages respectively. The lack of anisotropy generates an extraordinarily rich energy level structure at low temperatures, which allows us to study how such a structure is affected by lifting the strict cubic symmetry and/or by applying an external magnetic field. In particular, we will explore the possibility of using these cubic Ln complexes as spin-qubits. We will focus on the Ho derivative. We find that it is possible to reach a regime where decoherence caused by the nuclear spin bath is quenched for moderate axial compression of the cube and small magnetic fields.

High field electron paramagnetic resonance characterization of electronic and structural environments for paramagnetic metal ions and organic free radicals in Deepwater Horizon oil spill tar balls.

In the first use of high-field electron paramagnetic resonance (EPR) spectroscopy to characterize paramagnetic metal-organic and free radical species from tar balls and weathered crude oil samples from the Gulf of Mexico (collected after the Deepwater Horizon oil spill) and an asphalt volcano sample collected off the coast of Santa Barbara, CA, we are able to identify for the first time the various paramagnetic species present in the native state of these samples and understand their molecular structures and bonding. The two tar ball and one asphalt volcano samples contain three distinct paramagnetic species: (i) an organic free radical, (ii) a [VO](2+) containing porphyrin, and (iii) a Mn(2+) containing complex. The organic free radical was found to have a disc-shaped or flat structure, based on its axially symmetric spectrum. The characteristic spectral features of the vanadyl species closely resemble those of pure vanadyl porphyrin; hence, its nuclear framework around the vanadyl ion must be similar to that of vanadyl octaethyl porphyrin (VOOEP). The Mn(2+) ion, essentially undetected by low-field EPR, yields a high-field EPR spectrum with well-resolved hyperfine features devoid of zero-field splitting, characteristic of tetrahedral or octahedral Mn-O bonding. Although the lower-field EPR signals from the organic free radicals in fossil fuel samples have been investigated over the last 5 decades, the observed signal was featureless. In contrast, high-field EPR (up to 240 GHz) reveals that the species is a disc-shaped hydrocarbon molecule in which the unpaired electron is extensively delocalized. We envisage that the measured g-value components will serve as a sensitive basis for electronic structure calculations. High-field electron nuclear double resonance experiments should provide an accurate picture of the spin density distribution for both the vanadyl-porphyrin and Mn(2+) complexes, as well as the organic free radical, and will be the focus of follow-up studies.

Enhancing the Magnetic Anisotropy of Linear Cr(II) Chain Compounds Using Heavy Metal Substitutions

Magnetic properties of the series of three linear, trimetallic chain compounds Cr2Cr(dpa)4Cl2, 1, Mo2Cr(dpa)4Cl2, 2, and W2Cr(dpa)4Cl2, 3 (dpa = 2,2-dipyridylamido), have been studied using variable-temperature dc and ac magnetometry and high-frequency EPR spectroscopy. All three compounds possess an S = 2 electronic ground state arising from the terminal Cr(2+) ion, which exhibits slow magnetic relaxation under an applied magnetic field, as evidenced by ac magnetic susceptibility and magnetization measurements. The slow relaxation stems from the existence of an easy-axis magnetic anisotropy, which is bolstered by the axial symmetry of the compounds and has been quantified through rigorous high-frequency EPR measurements. The magnitude of D in these compounds increases when heavier ions are substituted into the trimetallic chain; thus D = -1.640, -2.187, and -3.617 cm(-1) for Cr2Cr(dpa)4Cl2, Mo2Cr(dpa)4Cl2, and W2Cr(dpa)4Cl2, respectively. Additionally, the D value measured for W2Cr(dpa)4Cl2 is the largest yet reported for a high-spin Cr(2+) system. While earlier studies have demonstrated that ligands containing heavy atoms can enhance magnetic anisotropy, this is the first report of this phenomenon using heavy metal atoms as ligands.

Ion Environments in Mn2+-Doped Polyelectrolyte Complexes: Dilute Magnetic Saloplastics

Amorphous hydrated complexes of the polyelectrolytes poly(styrene sulfonate) (PSS) and poly(diallyldimethylammonium) were doped with the spin-5/2 ion Mn(2+). X-band electron paramagnetic resonance (EPR) measurements of the Mn(2+) spins within these stoichiometric polyelectrolyte complexes (PECs) revealed an octahedral coordination environment, similar to that observed in aqueous solutions of Mn(2+). This octahedral symmetry of the [Mn(H2O)6](2+) complexes, observed in fully hydrated PECs, is somewhat distorted because of the wide range of ion pairs possible with the sulfonate group on PSS. As the Mn(2+) concentration was increased, the linewidths broadened, indicating the dominance of dipolar broadening over exchange narrowing in determining the linewidths; that is, any exchange narrowing was masked by the large dipolar broadening. The calculated linewidths were used to estimate the strengths of the dipolar interactions, and hence the distances between the Mn(2+) spins, on the basis of a simple model of regularly spaced spins. The distances calculated by this method were roughly comparable to the geometric average distances calculated on the basis of the Mn(2+) concentrations and densities of the doped PEC samples. From a comparison of their EPR spectra, the ion environments in the doped, fully hydrated PECs were found to be similar to those in hydrated classical ion exchange resins. EPR spectra before and after drying of the PECs indicate the replacement of octahedrally coordinated water by oxide anions from the polyanion chain and the corresponding loss of the symmetric environment of Mn(2+) ions.

Characterization of PtIV-containing polyoxometalates by high-resolution solid-state 195Pt and 51V NMR spectroscopy

We report on the feasibility of applying high-resolution solid-state 195Pt MAS NMR spectroscopy for several PtIV-containing polyoxotungstates, and of 195Pt as well as 51V for a PtIV-containing polyoxovanadate. This method is particularly useful for polyanions which are unstable in solution and/or poorly soluble, as well as for systems exhibiting crystallographic disorder of Pt and W sites. We also report solution 195Pt and 183W NMR spectra of hexatungstoplatinate(IV) [H3PtW6O24]5− for the first time.

Magic angle spinning NMR study of the ferroelectric transition of KH2PO4: definitive evidence of a displacive component

Variable temperature magic angle spinning (MAS) NMR measurements are reported on 1H and 31P nuclei in KH2PO4 (KDP) in the vicinity of its paraelectric-ferroelectric phase transition temperature, T c, of 123u2009K, to examine the transition mechanism, in particular if this is a model order-disorder type or whether it also involves a displacive component. It has been well established that the temperature variation of the isotropic chemical shift, δ iso, in NMR measurements of the nuclei directly involved in the transition should remain constant or change smoothly through T c for an order-disorder type transition but it should show an anomalous change for a displacive one. Here we demonstrate that the δ iso for both 31P and 1H nuclei in KDP show clear anomalies as a function of temperature around KDPs T c, providing direct evidence of a displacive component for the phase transition of KDP in contrast to the generally accepted notion that it is a model order-disorder type.

Synthesis and Solid-State Structure of Cyclobutyltellurium(IV)-Containing Dimeric Tungstoarsenates(III)

The cage-like cyclobutyltellurium(IV)-containing tungstoarsenate(III) dimers [(C4H8Te-OH)2(C4H8Te)6{As2W17O61(H2O)}2]14− (1) and [{(C4H8Te)2W2O5(H2O)2As2W19O67(H2O)}2]16− (2) were synthesized in moderately acidic aqueous medium by reaction of C4H8TeI2 with the lacunary tungstoarsenates(III) [B-α-AsW9O33]9− and [As2W19O67(H2O)]14−, respectively. Polyanion 1 was isolated as a mixed cesium-guanidinium salt Cs8.5(C(NH2)3)5.5[(C4H8TeOH)2(C4H8Te)6{As2W17O61(H2O)}2]·60H2O (1a), whereas 2 crystallized as a mixed cesium-potassium salt Cs9K7[{(C4H8Te)2W2O5(H2O)2As2W19O67(H2O)}2]·90H2O (2a). Single crystal X-ray analysis demonstrated that 1a and 2a crystallized in the triclinic space group