Current Inorganic Chemistry (v.3, #3)

Aspects of the theoretical and computational investigation of inorganic spectroscopy and photochemistry arepresented. We discuss the appearance of regions of strong non-adiabatic coupling such as Jahn-Teller couplings, pseudo-Jahn-Teller couplings, and conical intersections that are general features in the photodissociation, photostereochemistry,and photoisomerization reactions of coordination complexes. As well as reviewing important theoretical contributions tothe field we discuss four case studies that highlight aspects of the computational modeling of inorganic photochemical reactions.

The geometry of tetracyanoplatinate(II) (TCP) has been optimized with density functional theory (DFT)calculations in order to compare different computational strategies. Two approximate scalar relativistic methods, i.e. thescalar zeroth-order regular approximation (ZORA) and non-relativistic calculations with relativistic effective corepotentials (ECPs), were benchmarked against the four-component fully relativistic approach using the Dirac-CoulombHamiltonian and all-electron non-relativistic calculations. We find that the 5% contraction of the platinum-carbon bonddue to relativistic effects is almost quantitatively reproduced in the ZORA and ECP calculations. In addition, the effect ofthe exchange-correlation functional and one-electron basis set was studied by employing the two generalized gradientapproximation (GGA) functionals, BLYP and PBE, as well as their hybrid version B3LYP and PBE0 in combination withboth correlation consistent and Ahlrichs type basis sets. The platinum-carbon bond length (relativistic or non-relativistic)is approximately 1% shorter on using the PBE exchange-correlation functional compared to the BLYP functional butincluding exact exchange has no significant effect. For the C-N bond these trends are reversed and an order of magnitudesmaller. With respect to the basis set dependence we observed that a triple zeta basis set with polarization functions givesin general sufficiently converged results, but while for the Pt-C bond it is advantageous to include extra diffuse functions,this did not turn out to be important for the C-N bond.

Exploring Bonding in Heavy Atom Chemistry with Dirac-Exact Methods by Wenli Zou, Michael Filatov, Dieter Cremer (220-234).
The normalized elimination of the small component (NESC) method is a first principles 2-componentrelativistic approach that leads to the Dirac-exact description of one-electron systems. It is a powerful method to routinelyinvestigate chemical and physical properties of molecules containing relativistic atoms. The vibrational modes of mercuryhalides are investigated to derive via the corresponding local HgX (X = H, F, Cl, Br, I, At) stretching modes an appropriatemeasure for the HgX bond strength. It is shown that HgF bonding in HgF4 is stronger than that in HgF2, which is a resultof enhanced charge transfer from Hg to the four F atoms and the formation of electron deficient 2e-4c bonds with strongionic character. A generally applicable bonding model for HgX molecules is outlined.

Following a recently reported synthesis of linearly halide-bridged Cu(II) complexes [Inorg. Chem. 2012, 51,7966-7968] characterized by strong antiferromagnetic exchange couplings (J), we applied Difference DedicatedConfiguration Interaction (DDCI) and Broken-Symmetry Density Functional Theory (BS-DFT) approaches in order toanalyze theoretically the trend observed, in which the decreasing electronegativity of the central halide induces anexacerbed magnetic coupling. The importance of the magnetic orbitals in DDCI calculations is acknowledged. The use ofreduced molecular models is shown to lead to significant differences in J, in the case of chloride and bromide bridges.Finally, the BS-DFT decomposition of J in its main physical contributions confirms the importance of the kineticexchange mediated by the halide bridge, but also points towards an increasing core polarization contribution when goingfrom fluoride to bromide.

Using the study of the low-spin complex [Fe(bpy)3]2+ in the gas phase and in condensed phases as a guideline,we examine different aspects of the application of DFT to the study of transition metal complexes in the framework ofspin crossover or related phenomena.

Fundamental biochemical processes such as respiration and photosynthesis rely on electron transfer (ET) betweencofactors containing transition metal atoms. Donor-acceptor electronic coupling VDA is a key parameter that controlsthe ET rate. Accurate estimation of this parameter is challenging. In the paper, we consider different methods used tocalculate the coupling matrix element for ET between transition-metal centers in proteins and discuss their advantages andlimitations.

The usage of locally dense basis sets in the calculation of Electron Paramagnetic Resonance (EPR) hyperfinecoupling constants is investigated at the level of Density Functional Theory (DFT) for two model systems of biologicallyimportant transition metal complexes: One for the active site in the compound 0 intermediate of cytochrome P450cam,[Fe(OOH)(SH)(en)2]+, and one for the active site in coenzyme B12, [Co(NH3)(CN)(en)2]+. The Fermi contact, spin-dipolarand second order paramagnetic spin-orbit coupling contributions to the hyperfine coupling tensors of the metal and theligating ethylenediamine N atoms are calculated, and their dependence on the basis set for the remaining atoms are investigated.Core property basis sets are employed for the metals (aug-cc-pVTZ-Juc) and their equatorially coordinating Natoms (aug-cc-pVTZ-J or 6-31G-Juc analogues to the Pople style basis sets used for the remaining atoms), while smallercorrelation-consistent or Pople style basis sets are used for the remaining, so-called “non-coupled'', atoms. Most of the investigatedbasis set combinations are found to give results which differ by less than 1% from the results obtained withcore property basis sets on all atoms. We find thus for the cytochrome model system that using the small 6-31G(d) basisset on the non-coupled atoms together with core property basis sets on the Fe and N atoms gives essentially converged results.It is found to be mostly the second order paramagnetic spin-orbit interaction that demands the use of larger basis setson the non-coupled atoms. If, however, an error of less than 0.5MHz is sufficient any basis set can be used for the noncoupledatoms. For the cobalt containing model system the 6-31G(2d) basis set generally gives results within 1% of thereference value.

Relativistic Calculation of Hyperfine Parameters of Mercury Compounds by Michael Filatov, Wenli Zou, Dieter Cremer (284-290).
The normalized elimination of the small component (NESC) method is a Dirac-exact relativistic method thatleads to reliable first order response properties such as contact densities, Mössbauer shifts, electric field gradients,quadrupole coupling constants, or hyperfine structure constants for heavy atoms. In this review, the calculation of thesehyperfine parameters with a NESC analytical derivatives formalism is discussed and demonstrated for mercury containingmolecules. There is a distinct need for accurate calculated hyperfine parameters because the possibilities of experiment arelimited in a case such as mercury. This need can be fulfilled if, beside scalar relativistic effects, the influence of spin-orbitcoupling, electron correlation and the finite dimension of the nucleus are accounted for.