Relativistic Quantum Chemical Methods Research Articles

New relativistic quantum chemical methods for understanding light-induced therapeutics.

The inorganic platinum complexes currently in clinical use for cancer treatment have severe side effects, and complexes with fewer side effects are required. One option is to use complexes that are inactive until they are light-activated. Theoretical chemistry can contribute to the design of these complexes, but most current theoretical methods lack explicit treatment of relativistic effects (since the target complexes often contain heavy elements). In particular, spin-orbit coupling is required for accurate predictions of the complexes' photo-physical properties. In this perspective, we summarize relativistic methods developed in recent years that can contribute to our understanding of light-induced reactivity and thereby help predict new, suitable complexes.

Ligands enhanced the Ac[triple bond, length as m-dash]Ac triple bond.

The multiple bonds between actinide atoms and their derivatives are computationally investigated extensively and compounds with an unsupported actinide-actinide bond, especially in low oxidation states, have attracted great attention. Herein, high level relativistic quantum chemical methods are used to probe the Ac-Ac bonding in compounds with a general formula LAcAcL (L = AsH3, PH3, NH3, H, CO, NO) at both scalar and spin-orbit coupling relativistic levels. H3AsAcAcAsH3, H3PAcAcPH3 and OCAcAcCO compounds show a type of zero valence Ac[triple bond, length as m-dash]Ac triple bond with a 1σ2g1π4u configuration, and H3AsAcAcAsH3 has been found to have the shortest Ac-Ac bond length of 3.012 Å reported so far. The Ac2 unit is very sensitive to the σ donor ligands and can form triple, double and even single bonds when suitable ligands are introduced, up to 3.652 Å with an Ac-Ac single bond in H3NAcAcNH3.

Variation of the Fine-Structure Constant in Model Systems for Singlet Fission.

Energetic conditions that are required for favorable singlet exciton fission, an intermolecular electron correlation effect, are studied with relativistic quantum chemical methods as to investigate the effect of a varying fine-structure constant. Ethylene and derivatives thereof serve as simple model systems, whereas pentacene and perfluoropentacene, which display singlet exciton fission experimentally, are used as specific examples for possible applications. Effects are estimated to be small in this class of compounds, but substitution with heavier halogens might lead to oppositely shifting energy levels and thereby improved sensitivity in narrow resonance situations.

BDF: A relativistic electronic structure program package.

The BDF (Beijing Density Functional) program package is in the first place a platform for theoretical and methodological developments, standing out particularly in relativistic quantum chemical methods for chemistry and physics of atoms, molecules, and periodic solids containing heavy elements. These include the whole spectrum of relativistic Hamiltonians and their combinations with density functional theory for the electronic structure of ground states as well as time-dependent and static density functional linear response theories for electronically excited states and electric/magnetic properties. However, not to be confused by its name, BDF nowadays comprises also of standard and novel wave function-based correlation methods for the ground and excited states of strongly correlated systems of electrons [e.g., multireference configuration interaction, static-dynamic-static configuration interaction, static-dynamic-static second-order perturbation theory, n-electron valence second-order perturbation theory, iterative configuration interaction (iCI), iCI with selection plus PT2, and equation-of-motion coupled-cluster]. Additional features of BDF include a maximum occupation method for finding excited states of Hartree-Fock/Kohn-Sham (HF/KS) equations, a very efficient localization of HF/KS and complete active space self-consistent field orbitals, and a unique solver for exterior and interior roots of large matrix eigenvalue problems.

Relativistic quantum chemical calculations show that the uranium molecule U2 has a quadruple bond

Understanding the bonding, reactivity and electronic structure of actinides is lagging behind that of the rest of the periodic table. This can be partly explained by the challenges that one faces in experimental studies of such radioactive compounds and also by the need to properly account for relativistic effects in theoretical studies. A further challenge is the very complicated electronic structures encountered in actinide chemistry, as vividly illustrated by the naked diuranium molecule U2. Here we report a computational study of this emblematic molecule using state-of-the-art relativistic quantum chemical methods. Notably, the variational inclusion of spin-orbit interactions leads not only to a different electronic ground state, but also to a lower bond multiplicity compared with those in previous studies.

Spin-orbit Effects on the Structure of Haloiodomethane Cations CH2XI+(X=F, Cl, Br, and I)

The importance of including spin-orbit interactions for the correct description of structures and vibrational frequencies of haloiodomethanes is demonstrated by density functional theory calculations with spin-orbit relativistic effective core potentials (SO-DFT). The vibrational frequencies and the molecular geometries obtained by SO-DFT calculations do not match with the experimental results as well as for other cations without significant relativistic effects. In this sense, the present data can be considered as a guideline in the development of the relativistic quantum chemical methods. The influence of spin-orbit effects on the bending frequency of the cation could well be recognized by comparing the experimental and calculated results for <TEX>$CH_2BrI$</TEX> and <TEX>$CH_2ClI$</TEX> cations. Spin-orbit effects on the geometries and vibrational frequencies of <TEX>$CH_2XI$</TEX> (X=F, Cl, Br, and I) neutral are negligible except that C-I bond lengths of haloiodomethane neutral is slightly increased by the inclusion of spin-orbit effects. The <TEX>$^2A^{\prime}$</TEX> and <TEX>$^2A^{{\prime}{\prime}}$</TEX> states were found in the cations of haloiodomethanes and mix due to the spin-orbit interactions and generate two <TEX>$^2E_{1/2}$</TEX> fine-structure states. The geometries of <TEX>$CH_2XI^+$</TEX> (X=F and Cl) from SO-DFT calculations are roughly in the middle of two cation geometries from DFT calculations since two cation states of <TEX>$CH_2XI$</TEX> (X=F and Cl) from DFT calculations are energetically close enough to mix two cation states. The geometries of <TEX>$CH_2XI^+$</TEX> (X=Br and I) from SO-DFT calculations are close to that of the most stable cation from DFT calculations since two cation states of <TEX>$CH_2XI$</TEX>(X=Br and I) from DFT calculations are energetically well separated near the fine-structure state minimum.

Analytic energy derivatives in relativistic quantum chemistry

In this review, we discuss the current status of analytic derivative theory in relativistic quantum chemistry. A brief overview of the basic theory for the available relativistic quantum chemical methods as well as the state‐of‐the‐art development of their analytic energy derivatives is given. Among the various relativistic quantum chemical methods, cost‐effective approaches based on spin separation and/or on the matrix representation of two‐component theory have been proven particularly promising for the accurate and efficient treatment of relativistic effects in electron correlation calculations. We highlight analytic derivative techniques for these cost‐effective relativistic quantum chemical approaches including direct perturbation theory, the spin‐free Dirac‐Coulomb approach, and the exact two‐component theory in its one‐electron variant. An outlook is given on future developments of analytic energy derivative techniques for relativistic quantum chemical methods, again with an emphasis on cost‐effective schemes. © 2014 Wiley Periodicals, Inc.

Closed-Cage Tungsten Oxide Clusters in the Gas Phase

During the course of a study on the clustering of W-Se and W-S mixtures in the gas phase using laser desorption ionization (LDI) mass spectrometry, we observed several anionic W-O clusters. Three distinct species, W(6)O(19)(-), W(13)O(29)(-), and W(14)O(32)(-), stand out as intense peaks in the regular mass spectral pattern of tungsten oxide clusters suggesting unusual stabilities for them. Moreover, these clusters do not fragment in the postsource decay analysis. While trying to understand the precursor material, which produced these clusters, we found the presence of nanoscale forms of tungsten oxide. The structure and thermodynamic parameters of tungsten clusters have been explored using relativistic quantum chemical methods. Our computed results of atomization energy are consistent with the observed LDI mass spectra. The computational results suggest that the clusters observed have closed-cage structure. These distinct W(13) and W(14) clusters were observed for the first time in the gas phase.

Relativistic Density Functional Study of the Dinuclear Uranyl Complex [(UO2)2(μ2‐OH)2Cl2(H2O)4] in Its Crystalline Environment

AbstractThe hexavalent dinuclear uranyl dichloride complex [(UO2)2(μ2‐OH)2Cl2(H2O)4] was studied computationally with an all‐electron scalar relativistic density functional method. This suggested hydrolysis product of uranyl in the presence of chlorine ions is one of the few polynuclear uranyl species for which a crystal structure is known. The calculated gas‐phase structure is similar to the experimental crystal geometry; any major deviations are due to hydrogen bonds in the crystal. If the eight strongest hydrogen bonds are included in a model of the complex’s crystalline environment, the calculated structure improves significantly. Based on this model, the hydrogen bond lengths and angles were determined, indicating that they are moderate and strong with an average binding energy of 39 kJ/mol. These computational results corroborate earlier suggestions based on experimental results concerning the location and strength of the hydrogen bonds. In addition, a valuable reference for relativistic quantum chemical methods is provided by the gas‐phase results. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2003)