Ions, active space

It is evident that the approach described so far to derive the electronic structure of lanthanide ions, based on perturbation theory, requires a large number of parameters to be determined. While state-of-the-art ab initio calculation procedures, based on complete active space self consistent field (CASSCF) approach, are reaching an extremely high degree of accuracy [34-37], the CF approach remains widely used, especially in spectroscopic studies. However, for low point symmetry, such as those commonly observed in molecular complexes, the number of CF... 

Table 6.1 Energies of the low-lying J-multiplets predicted within the CASSCF/RASSI approach for the free Ln3+ ions. ANO-RCC of single-zeta quality were employed. The active space of the CASSCF method included only n electrons spanning the 4f shell. 

A tandem-in-space mass spectrometer consists of an ion source, a precursor ion activation device, and at least two nontrapping mass analyzers. The first mass analyzer is used to select precursor ions within a narrow m/z range. Isolated precursor ions are allowed to enter the ion activation device, for example, a gas-filled collision cell, where they dissociate. Created fragments continue on to the second mass analyzer for analysis. The second mass analyzer can either acquire a full mass fragment spectrum or be set to monitor a selected, narrow, m/z range. In principle the second mass analyzer could be followed by more ion activation devices and mass analyzers for MSn experiments. However, due to rapidly decreasing transmission and increasing experimental... 

Ion traps are tandem-in-time instruments, i.e., they perform the steps of precursor ion selection, ion activation and acquisition of fragment ion spectra in the very same place. This advantageous property allows the multiple use of a single QTT to perform not only MS but also MS and higher order MS experiments - indeed a very economic concept. Depending on the abundance of the initial precursor ion, its fragmentation behavior - and of course, on the performance of the QIT - MS experiments are possible. [138] However, in contrast to tandem-in-space instruments, tandem-in-time instruments do not support constant neutral loss and precursor ion scans. 

As a consequence of the precipitation of calcium ions inside the vesicles, the product of the ion activities of calcium and the respective anions in the internal vesicular space is fixed by the solubility product of the precipitating salts (L). Since in the external solution changes of the ion activities of calcium (Ca0) and of the precipitating anions (A ) can be followed experimentally, the energy requirement for calcium uptake can be determined at every moment. The energy requirement is given by the expression ... 

TABLE 3 CASSCF and CASPT2 values of the magnetic coupling parameter J (in meV) for bulk TMO and fartheTMO (100) surface. The active space in the CASSCF is Formed by the open shell orbitals located on the I M ions and the unpaired electrons. CASPT2 correlates the TM-3s, 3p. 3d and die 0-2s, 2p electrons... 

Finally, the magnetic interactions in the fee TMO (TM=Cu, Ni, Co, Fe, Mn) have been analyzed. We find the magnetic interaction decreases with decreasing nuclear charge on the TM ion. A comparison of magnetic interactions between bulk atoms and between atoms located at the (100) surface shows that the interaction in all five compounds is lowered at the surface. The calculated value of the interaction in bulk NiO compares rather well with the measured one, especially when charge transfer effects are included in a more complete manner by extending the active space in CASSCF. 

The measurement of ion activities assumes chemical equilibrium between the PVC membrane and the electrolyte bearing solutions. The time domain chemical and dielectric space charge changes that occur are minimized by membrane composition and sensor design and are considered negligible during the measurement period. Hence, the potential dependence of the ion activity is characterized by the Nemst equation. The following thermodynamic expressions describe the potentials of the... 

Very few calculations have so far been performed for lanthanides and not much is known about the choice of the active space. However, most lanthanide complexes have the metal in oxidation state 3+. Furthermore, are the 4/ orbitals inert and do not interact strongly with the ligands. It is therefore likely that in such complexes only the 4/ orbitals have to be active unless the process studied includes charge transfer from the ligands to the metal. In systems with the metal in a lower oxidation state, the choice of the active space would show similar problems as in the actinides, in particular because the 5d orbitals may also take part in the bonding. As an example we might mention a recent study of the SmO molecule and positive ion where 13 active orbitals where shown to produce results of good accuracy [42],... 

The choice of the active orbital space for the CASSCF calculations is a crucial step, and has turned out to be especially difficult in these proteins and other systems containing a Cu-thiolate bond. From earlier studies it was known that in complexes with first-row transition metal ions with many 3d electrons, the active space should include one correlating orbital for each of the doubly occupied 3d orbitals [28]. Therefore the starting active space contains 10 orbitals 3d and 3d"). In addition, it is necessary to add the 3p orbitals on Scys to describe correctly the covalent character of the Cu-Scys bond and also 2p and 3p orbitals on nitrogen and sulphur to describe charge-transfer states. The final active space therefore contain 11 or 12 active orbitals (12 active orbitals are at present the upper limit for the CASPT2 method). 

The following ion-activation techniques have been used at one time or other to sequence peptides (1) fast atom bombardment (FAB) ionization, (2) CID—tandem MS (MS/MS), (3) ESI in-source CID, (4) MALDI ion-source decay, (5) MALDI postsource decay (PSD), (6) electron-capture dissociation (ECD) and electron-transfer dissociation, and (7) peptide ladder sequencing. Because of the lack of space, only (2) and (4) will be discussed further. 

Calcium ions are released into the cytosol from the endoplasmic reticulum and cytochrome c and other proteins from the mitochondrial intermembrane space. The calcium ions activate caspase-9, enhancing the activation of caspase-3, which is activated by caspase-8 in addition to BID. 

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