Category: Research Highlight

ESR analyses of picket fence MnII and 6th ligand coordinated FeIII porphyrins (S = 5/2) and a CoII(hfac) complex (S = 3/2) with sizable ZFS parameters revisited: a full spin Hamiltonian approach and quantum chemical calculations

Dalton Trans., 47, 16429-16444 (2018). (DOI:10.1039/C8DT02988A) The fictitious spin-1/2 (effective spin-1/2) Hamiltonian approach has been the putative method to analyze the conventional fine-structure/hyperfine ESR spectra of high spin metallocomplexes with sizable zero-field splitting (ZFS) tensors since the early 1950s, and the approach gives salient principal geff-values far from g = 2 without explicitly affording their ZFS values in most cases. The experimental geff-values thus determined, however, never agree with those (gtrue-values) of the true principal g-tensors, which are obtainable from reliable quantum chemical calculations. We have recently derived exact or extremely accurate analytical expressions for the geff/gtrue relationships for the spin quantum number S‘s up to S = 7/2 (T. […]

Reversible solution pi-dimerization and long multicenter bonding in a stable phenoxyl radical

Chemistry – A European Journal Chem. Eur. Journal, 24, pp.14906-14910 (2018). (DOI:10.1002/chem.201802204) Reversible solution p-dimerization is observed in the stable neutral phenoxyl radical 2,6-bis-(8-quinolylamino)-4-(tertbutyl) phenoxyl baqp and is spectroscopically characterized. This behavior, not previously observed for p-extended phenoxyl radicals, is relevant to the formation of long multicenter bonding in the p-dimer at low temperature akin to previously reported phenalenyl radicals. Our experimental data are supported in a quantitative manner by results from density functional theory (DFT) and ab initio molecular orbital theory calculations. Our theoretical results indicate that the solution dimer features strong bonding interactions between the two phenoxyl rings but that the stability of the dimer is also related […]

Fe-Transferrins or their homologues in ex-vivo mushrooms as identified by ESR spectroscopy and quantum chemical calculations: a full spin-Hamiltonian approach for the ferric sextet state with intermediate zero-field splitting parameters

Food Chemistry Volume 266, 15 November 2018, Pages 24-30 http://doi.org/10.1016/j.foodchem.2018.05.092 Fe-transferrins/their homologues in ex-vivo mushrooms were identified by ESR spectroscopy at liquid helium temperature, 4 K. The ESR fine-structure signals from Grifola frondosa were analyzed by spectral simulation with a full spin-Hamiltonian approach, determining the spin Hamiltonian parameters of the ferric iron species bound in the biological environment: S = 5/2, g = (2.045, 2.01, 2.235), |D| = 0.28 cm−1, |E/D| = 0.05. The zero-field splitting (ZFS) parameters, D– and E-values, are very close to the reported values, |D| = 0.25 cm−1 and |E/D| = 0.06, for an Fe-transferrin with oxalate anion, and to |D| = 0.25 cm−1 and |E/D| = 0.04 for one with malonate anion in human sera, suggesting that the Fe3+ species are from Fe-transferrins or their […]

Using optimal control methods with constraints to generate singlet states in NMR

J. Magn. Reson., 291, pp.14-22 (2018). DOI:10.1016/j.jmr.2018.03.005 A method is proposed for optimizing the performance of the APSOC (Adiabatic-Passage Spin Order Conversion) technique, which can be exploited in NMR experiments with singlet spin states. In this technique magnetization-to-singlet conversion (and singlet-to-magnetization conversion) is performed by using adiabatically ramped RF-fields. Optimization utilizes the GRAPE (Gradient Ascent Pulse Engineering) approach, in which for a fixed search area we assume monotonicity to the envelope of the RF-field. Such an approach allows one to achieve much better performance for APSOC; consequently, the efficiency of magnetization-to-singlet conversion is greatly improved as compared to simple model RF-ramps, e.g., linear ramps. We also demonstrate that the optimization […]

Behaviour of DFT-based approaches to the spin– orbit term of zero-field splitting tensors: a case study of metallocomplexes, MIII(acac)3 (M = V, Cr, Mn, Fe and Mo)

Phys. Chem. Chem. Phys., 19, pp.30128-30138 (2017). DOI: 10.1039/C7CP05533a Spin–orbit contributions to the zero-field splitting (ZFS) tensor (DSO tensor) of MIII(acac)3 complexes (M = V, Cr, Mn, Fe and Mo; acac = acetylacetonate anion) are evaluated by means of ab initio (a hybrid CASSCF/MRMP2) and DFT (Pederson–Khanna (PK) and natural orbital-based Pederson–Khanna (NOB-PK)) methods, focusing on the behaviour of DFT-based approaches to the DSO tensors against the valence d-electron configurations of the transition metal ions in octahedral coordination. Both the DFT-based approaches reproduce trends in the D tensors. Significantly, the differences between the theoretical and experimental D (D = DZZ – (DXX + DYY)/2) values are smaller in NOB-PK than in PK, emphasising the usefulness of […]

Analyses of sizable ZFS and magnetic tensors of high spin metallocomplexes

Phys. Chem. Chem. Phys., 19, pp.24769-24791 (2017). DOI: 10.1039/C7CP03850J The fictitious spin-1/2 Hamiltonian approach is the putative method to analyze the fine-structure/hyperfine ESR spectra of high spin metallocomplexes having sizable zerofield splitting (ZFS), thus giving salient principal g-values far from around g = 2 without explicitly providing their ZFS parameters in most cases. Indeed, the significant departure of the g-values from g = 2 is indicative of the occurrence of their high spin states, but naturally they never agree with true g-values acquired by quantum chemical calculations such as sophisticated DFT or ab initio MO calculations. In this work, we propose facile approaches to determine the magnetic tensors of high spin metallocomplexes having sizable ZFS, instead of performing advanced high-field/high-frequency […]

Quantum Chemistry on Quantum Computers: A Polynomial-Time Quantum Algorithm for Constructing the Wave Functions of Open-Shell Molecules

J. Phys. Chem. A 120, pp.6459-6466 (2016). DOI: 10.1021/acs.jpca.6b04932 Quantum computers are capable to efficiently perform full configuration interaction (FCI) calculations of atoms and molecules by using the quantum phase estimation (QPE) algorithm. Because the success probability of the QPE depends on the overlap between approximate and exact wave functions, efficient methods to prepare accurate initial guess wave functions enough to have sufficiently large overlap with the exact ones are highly desired. Here, we propose a quantum algorithm to construct the wave function consisting of one configuration state function, which is suitable for the initial guess wave function in QPE-based FCI calculations of openshell molecules, based on the addition theorem of […]

Adiabatic Quantum Computing with Spin Qubits Hosted by Molecules

Phys. Chem. Chem. Phys., accepted. DOI:10.1039/C4CP04744C  A molecular spin quantum computer (MSQC) requires electron spin qubits which pulse-based electron spin/magnetic resonance (ESR/MR) techniques can afford to manipulate for implementing quantum gate operations in open shell molecular entities. Importantly, nuclear spins which are topologically connected, particularly in organic molecular spin systems are client qubits, while electron spins play a role of bus qubits. Here, we introduce the implementation for an adiabatic quantum algorithm, suggesting the possible utilization of molecular spins with optimized spin structures for MSQCs. We exemplify the utilization of an adiabatic factorization problem of 21, comparing with the corresponding nuclear magnetic resonance (NMR) case. Two molecular spins are selected: One […]

An ab initio MO study of heavy atom effects on the zero-field splitting tensors of high-spin nitrenes: how the spin–orbit contributions are affecte

Phys. Chem. Chem. Phys., 16, pp.9171-9181 (2014). DOI:10.1039/c4cp00822g    The CASSCF and the hybrid CASSCF–MRMP2 methods are applied to the calculations of spin–spin and spin–orbit contributions to the zero-field splitting tensors (D tensors) of the halogen-substituted spin- septet 2,4,6-trinitrenopyridines, focusing on the heavy atom effects on the spin–orbit term of the D ten- sors (DSO tensors). The calculations reproduced experimentally determined |D| values within an error of 15%. Halogen substitutions at the 3,5-positions are less influential in the spin–spin dipolar (DSS) term of 2,4,6-trinitrenopyridines, although the DSO terms are strongly affected by the introduction of heavier halogens. The absolute sign of the DSO value (D = DZZ  (DXX + DYY)/2) […]

Pulsed electron spin nutation spectroscopy for weakly exchange-coupled multi-spin molecular systems with nuclear hyperfine couplings: a general approach to bi- and triradicals and determination of their spin dipolar and exchange interactions

Molecular Physics, 111, published on the web on July 4, 2013 DOI:10.1080/00268976.2013.811304   Weakly exchange-coupled biradicals have attracted much attention in terms of their dynamic nuclear polarisation application in NMR spectroscopy for biological systems or the use of synthetic electron-spin qubits in quantum information processing/quantum-computing technology. Analogues multi-partite molecular systems are important in entering a new phase of the relevant fields. Many stable organic biradicals known so far have nitrogen nuclei at their electron spin sites, where singly occupied molecular orbitals are dominating and large hyperfine couplings occur. A salient feature of such weakly exchange-coupled molecular systems in terms of electronic spin structures is underlain by small zero-field splitting (ZFS) […]