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Crystal Field (crystal + field)
Selected AbstractsLow- and high-spin iron (II) complexes studied by effective crystal field method combined with molecular mechanicsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 14 2003M. B. Darkhovskii Abstract A computational method targeted to Werner-type complexes is developed on the basis of quantum mechanical effective Hamiltonian crystal field (EHCF) methodology (previously proposed for describing electronic structure of transition metal complexes) combined with the Gillespie,Kepert version of molecular mechanics (MM). It is a special version of the hybrid quantum/MM approach. The MM part is responsible for representing the whole molecule, including ligand atoms and metal ion coordination sphere, but leaving out the effects of the d -shell. The quantum mechanical EHCF part is limited to the metal ion d -shell. The method reproduces with reasonable accuracy geometry and spin states of the Fe(II) complexes with monodentate and polydentate aromatic ligands with nitrogen donor atoms. In this setting a single set of MM parameters set is shown to be sufficient for handling all spin states of the complexes under consideration. © 2003 Wiley Periodicals, Inc. J Comput Chem 14: 1703,1719, 2003 [source] Organic cyclic difluoramino-nitramines: infrared and Raman spectroscopy of 3,3,7,7-tetrakis(difluoramino)octahydro 1,5-dinitro-1,5-diazocine (HNFX)JOURNAL OF RAMAN SPECTROSCOPY, Issue 8 2009Philippe F. Weck Abstract We present the first vibrational structure investigation of 3,3,7,7-tetrakis(difluoramino)octahydro-1,5-dinitro- 1,5-diazocine (HNFX),and, more generally, of a member of the new class of gem -bis(difluoramino)-substituted heterocyclic nitramine energetic materials,using combined theoretical and experimental approaches. Optimized molecular structure and vibrational spectra of the Ci, symmetry conformer constituting the HNFX crystal were computed using density functional theory methods. Fourier transform infrared and Raman spectra of HNFX crystalline samples were also collected at ambient temperature and pressure. The average deviation of calculated structural parameters from X-ray diffraction data is ,1% at the B3LYP/6-311 + + G(d,p) level of theory, suggesting the absence of significant molecular distortion induced by the crystal field. Very good agreement was found between simulated and measured spectra, allowing reliable assignment of the fundamental normal modes of vibration of the HNFX crystal. Detailed analysis of the normal modes of the C,(NF2)2 and N,NO2 moieties was performed due to their critical importance in the initial steps of the molecular homolytic fragmentation process. Copyright © 2009 John Wiley & Sons, Ltd. [source] Electronic spectra of [(CH3)2NH2]5Cd2CuCl11 crystalsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2004V. Kapustianik Abstract The temperature evolution of Cu2+ ion environment in the solid solutions of ((CH3)2NH2)5Cd2CuCl11 is studied on the basis of absorption spectroscopy data. For the detailed analysis of experimental data the special program package Crys Tool 1.0 based on quantum-mechanical models, first of all on the model of normalized spherical harmonics (NSH), has been employed. It has been found that similarly to the crystal of ((CH3)2NH2)5Cd3Cl11 (DMACC) the investigated solid solution contains tetragonally distorted octahedral metal,halogen complexes of two types and the degree of their distortion is changed considerably at the temperatures of phase transitions (PTs). The parameters of crystal field, angular overlap model, as well as the copper,chlorine distances, show continuous changes at T1 = 176 K that should be related to the second-order transition, whereas the jump-like anomalies of the spectral parameters at T2 = 115 K (on cooling) are characteristic of the first-order PTs. Introduction of the copper ions into the structure of the host DMACC crystal induces the shifts of these PTs toward low temperatures by 3.5 and 5 K, respectively. The observed structural changes around T0 = 313 K are connected with a complex co-operative effect involving weakening of the hydrogen bonds and modification of the Jahn,Teller distortion with temperature. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] EPR study of Fe3+ centers in ternary layered TlInS2 single crystalPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2004F.A. Mikailov Abstract An iron doped TlInS2 single crystal has been studied at room temperature by Electron Paramagnetic Resonance (EPR) technique. The fine structure of EPR spectra of the paramagnetic Fe3+ ions was observed. The spectra were interpreted to correspond to the transitions among the spin multiplets (S=5/2, L=0) of the Fe3+ ion, which is split in the local ligand crystal field (CF) of orthorhombic symmetry. Four equivalent Fe3+ centers have been observed in the EPR spectra and the local symmetry of crystal field at the Fe3+ site and the CF parameters were determined. It was established that the symmetry axis of the axial component in the CF is making an angle of about 48 degree with the (ab) plane of TlInS2 crystal. Experimental results indicate that Fe ions substitute for In ions at the center of the InS4 tetrahedrons, and the rhombic distortion of the CF is caused by Tl ions located in the trigonal cavities between the tetrahedral complexes. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Experimental charge-density study of paracetamol , multipole refinement in the presence of a disordered methyl groupACTA CRYSTALLOGRAPHICA SECTION A, Issue 6 2009Joanna M. B A high-resolution single-crystal X-ray study of paracetamol has been performed at 85,K. Different approaches to modeling the experimental electron density (ED) were tested for the dynamically disordered portions of the molecule in order to check to what extent it is possible to obtain a proper ED distribution in the ordered part. Models were examined in which the methyl-group ED was built from pseudoatoms taken from the University at Buffalo Pseudoatom Databank or the Invariom database, with multipole parameters for the remaining atoms being obtained from free refinement. The ,, restricted multipolar model (KRMM) and free ,, refinements were compared; restriction of the ,, parameters was essential in order to obtain values of the electrostatic interaction energy consistent with the results of theoretical single-point periodic calculations. After simultaneous use of KRMM refinement and the databases to model the methyl group, the bond critical point properties and interaction electrostatic energy values were found to be closer to those obtained from theory. Additionally, some discrepancies in the ED distribution and dipole moment among transferred aspherical atom model refinements utilizing both theoretical databases and parameters from theoretical periodic calculations are shown. Including the influence of the crystal field in the periodic calculations increases the ED in the hydroxyl and amide groups, thus leading to higher values of the electrostatic interaction energy, changes in the electrostatic potential values mapped on the isodensity surface and changes in the shape of the anisotropic displacement parameters with respect to results found for both database models. [source] X-ray atomic orbital analysis.ACTA CRYSTALLOGRAPHICA SECTION A, Issue 4 2008The scattering unit of X-ray crystal structure analysis is changed from atoms to the subshell electrons by X-ray atomic orbital analysis (XAO). All the atoms in the unit cell are divided into groups of subshell electrons in the XAO analysis. Each subshell is treated as an independent pseudo-atom, which enables the atomic orbitals (AO's) and the electron population of each AO expressed as a linear combination of s/p/d/f orbitals in each subshell to be determined. When the environmental condition of the sample is varied, the electron transfer among the AO's in the crystal can be traced with XAO. It is applicable mainly to analyses of the electron-density distribution in ionic solids including those with a nonstoichiometric structure. The expansion coefficients of each AO are calculated with the perturbation theory putting a point charge on each atom in the unit cell. This automatically makes the perturbation potential have the point-group symmetry of the atom in the crystal field. Then the coefficients of each AO are refined to fit to the observed structure factors keeping the orthonormal relationships among the AO's. Complex basis functions with , or , spin as well as real ones are employed for heavy atoms and the relationships among the coefficients for the AO's of an electron in the crystal fields of the 32 point-group symmetries are derived for p, d and f orbitals. The AO's thus derived can be applicable to an anti-symmetrized multi-electron system, although X-ray diffraction cannot specify the atomic terms occupied when the crystal symmetry permits the atom to have many terms. [source] 5d and 4f electron configuration of CeB6 at 340 and 535,KACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2008Ryoko Makita X-ray atomic orbital (XAO) analysis revealed that at both temperatures the electrons are transferred from B 2px(= py) to Ce 5d and 4f orbitals. At 340,K 5d(j = 5/2),8 orbitals are occupied partially, but 4f(j = 5/2),8 orbitals are more populated than 4f(j = 5/2),7 orbitals, in contrast to our observation at 430,K [Makita et al. (2007). Acta Cryst. B63, 683,692]. At 535,K the XAO analysis revealed clearly that the order of the energy levels of 4f(j = 5/2),8 and ,7 states reversed again and is the same as that at room temperature. It also limited the possible 5d configurations to three models among the nine possible ones. However, the XAO analysis could not decide which of the three models was the best with the present accuracy of the measurement. Two of them have partially and fully occupied 5d(j = 5/2),7 orbitals and the remaining one has a fully occupied 5d(j = 3/2),8 orbital. Since the lobes of 5d(j = 3/2),8 or 5d(j = 5/2),7 orbitals do not overlap with the 4f(j = 5/2),8 orbitals as well as the 5d(j = 5/2),8 orbitals, the order of the energy levels of the 4f(j = 5/2) orbitals became the same as that at room temperature. These results indicate that the crystal field varies with temperature due to the electron transfer from B 2p to Ce 5d orbitals. The difference densities after the spherical-atom refinement at the three temperatures clearly revealed the different combinations of 4f and 5d orbitals which are occupied. In the present study positive peaks due to the 4f electrons appear near the Ce nucleus and those due to 5d orbitals are found in the area outside the 4f peaks. Between the two areas there is a negative area distributed spherically at 340,K. The negative area produced by the contraction of 4f(j = 5/2),8 orbitals seems to reduce the electron repulsion of the 5d(j = 5/2),8 orbitals and helps the 4f(j = 5/2),8 orbitals to remain as the ground state. [source] X-ray atomic orbital analysis.ACTA CRYSTALLOGRAPHICA SECTION A, Issue 4 2008The scattering unit of X-ray crystal structure analysis is changed from atoms to the subshell electrons by X-ray atomic orbital analysis (XAO). All the atoms in the unit cell are divided into groups of subshell electrons in the XAO analysis. Each subshell is treated as an independent pseudo-atom, which enables the atomic orbitals (AO's) and the electron population of each AO expressed as a linear combination of s/p/d/f orbitals in each subshell to be determined. When the environmental condition of the sample is varied, the electron transfer among the AO's in the crystal can be traced with XAO. It is applicable mainly to analyses of the electron-density distribution in ionic solids including those with a nonstoichiometric structure. The expansion coefficients of each AO are calculated with the perturbation theory putting a point charge on each atom in the unit cell. This automatically makes the perturbation potential have the point-group symmetry of the atom in the crystal field. Then the coefficients of each AO are refined to fit to the observed structure factors keeping the orthonormal relationships among the AO's. Complex basis functions with , or , spin as well as real ones are employed for heavy atoms and the relationships among the coefficients for the AO's of an electron in the crystal fields of the 32 point-group symmetries are derived for p, d and f orbitals. The AO's thus derived can be applicable to an anti-symmetrized multi-electron system, although X-ray diffraction cannot specify the atomic terms occupied when the crystal symmetry permits the atom to have many terms. [source] | |||||||||||||