Modeling molecular crystals formed by spin-active metal complexes by atom-atom potentials

We apply the atom-atom potentials to molecular crystals of iron (II) complexes with bulky organic ligands. The crystals under study are formed by low-spin or high-spin molecules of Fe(phen)$_{2}$(NCS)$_{2}$ (phen = 1,10-phenanthroline), Fe(btz)$_{2}$(NCS)$_{2}$ (btz = 5,5$^{\prime }$,6,6$^{\prime}$-tetrahydro-4\textit{H},4$^{\prime}$\textit{H}-2,2$^{\prime }$-bi-1,3-thiazine), and Fe(bpz)$_{2}$(bipy) (bpz = dihydrobis(1-pyrazolil)borate, and bipy = 2,2$^{\prime}$-bipyridine). All molecular geometries are taken from the X-ray experimental data and assumed to be frozen. The unit cell dimensions and angles, positions of the centers of masses of molecules, and the orientations of molecules corresponding to the minimum energy at 1 atm and 1 GPa are calculated. The optimized crystal structures are in a good agreement with the experimental data. Sources of the residual discrepancies between the calculated and experimental structures are discussed. The intermolecular contributions to the enthalpy of the spin transitions are found to be comparable with its total experimental values. It demonstrates that the method of atom-atom potentials is very useful for modeling organometalic crystals undergoing the spin transitions

Classes of admissible exchange-correlation density functionals for pure spin and angular momentum states

We analyze the various approaches to construct exchange-correlation functionals which are able to describe states of definite spin multiplicity in the DFT realm and outline the characteristics of possible functionals consistent with the Kohn-Sham theory. To achieve this goal the unitary group technique is applied to label many-electron states of definite total spin and to calculate the corresponding analogs of the Roothaan coupling coefficients. The possibility of using range separated Coulomb potential of electron-electron interaction for constructing functionals discriminating multiplet states in the d-shells is explored and a tentative system of state-specific functionals, covering nontrivial correlations in d-shells of transition metal ions, is proposed for the Fe^{2+} ions.Comment: 7-th European Conference on Computational Chemistry, Venice, Italy, 12 - 15, Sept., 200

Influence of Bound Water on the Interaction of the Same-Nature and Different-Nature Chemical Groups of Drugs and Receptors

Role of water in thermodynamic of molecular recognition processes in the binding of a ligand to a receptor is extremely important. However, there is still a significant lack of specific knowledge to the extent of water influence on the process

Ligand field torque : a pi-type electronic driving force for determining ligand rotational preferences

Transition metal complexes with triply-degenerate T ground states are formally Jahn-Teller active but do not usually display the significant bond length distortions familiar from their E ground state counterparts like d(9) Cu(II). The electronic 'asymmetry' for T-state systems lies in the d(pi) orbitals, which interact with the ligands relatively weakly compared to the stronger sigma-type interactions for E-state systems. However, in combination with asymmetric M-L pi bonding, T-type systems have an additional mechanism for relieving the electronic strain. Density functional theory, ligand field theory and ligand field molecular mechanics calculations are used to show how rotations around the M-L bonds can affect their pi-pi (d(pi)-L-pi) interactions and lead to significant energy lowering. For example, d(6) [ Fe(OH2)(6)](2+), which has a T-5(g) state in cubic T-h symmetry, 'distorts' to an S-6 structure 4.4 kcal mol(-1) lower in energy (by DFT) but with six equal Fe-O distances via Fe-O rotations of similar to 20 degrees. and thus masquerades as an apparently regular geometry. Using model systems, we show that this effect is not restricted to formally Jahn-Teller active complexes. The combination of asymmetric p bonding and asymmetric d(pi) orbital occupations can generate an M-L 'torque' worth up to 6 kcal mol(-1) per bond which can 'lock' the ligand in a particular orientation relative to the partially-occupied d orbital(s). The effect is particularly marked for imidazole, the donor group of histidine, which, in a model low-spin d(5) Fe(III) system, shows almost no orientational preference in its neutral form but a very strong (similar to 6 kcal mol(-1)) orientational preference in its deprotonated form