Site splitting

The nitrogen complex had already been synthesized in a solid matrix, but its decomposition kinetics and its further photolysis could be studied only in solution. The liquid noble gas technique is superior to the solid matrix technique, especially for the synthesis of multiple substituted chromium carbonyl nitrogen complexes. Their IR spectra were extremely complex in matrices, due to "site splittings" which arise when different molecules are trapped in different matrix environments /18/. 

When pairs of lines of different intensity occur in a region where only a single one is expected on the basis of normal mode calculations (cf. below), these need not indicate a site splitting, but are often indicative of Fermi-resonances, that is, a (near) coincidence of an IR active fundamental with a combination band that borrows intensity from the former. Such phenomena can often only be detected by virtue of the high resolution that is available in matrix isolation IR spectroscopy. 

Fig. 5. Octahedral-site splitting of (a) the or-bitally fivefold-degenerate d1 manifold by a cubic crystalline field and (b) the high-spin Mn(III) configuration.

The 9o(i) sites of Fm3c split into two sets of 48(f), and the 192(i) sites split into four. The Na(l) 64(g) sites split into a set of special 16(c) sites and a general set, while the Na(3) 96(h) sites split into two sets of special 24(e) sites and one general set. The number of variable positional parameters thus increases from 15 to 51. This represents a formidable problem and therefore the structural analysis is reduced to refinement of the occupancies of the seven possible Na sites in Rp3c, while all the other positional parameters are constrained by the higher symmetry of Fm3c. 

The symmetry around the dimer is, however, usually low in crystals. The degeneracy of configurations (a) and (b) in Fig. 9, therefore, is removed by an environmental effect, which is sometimes called a site-splitting effect. The presence of two non-equivalent configurations in crystals has been confirmed by X-ray and neutron diffraction experiments. Although the... 

A difference Fourier map, calculated at this point, reveals an additional small electron density maximum in the tetrahedral cavity next to the partially occupied V2. Thus, it is reasonable to assume that the V2 site splits into two independent partially occupied positions with the coordinates, which distribute V atoms in a random fashion in two adjacent tetrahedral positions rather than being simply vanadium-deficient. We label these two sites as V2a (corresponding to the former V2) and V2b (corresponding to the Fourier peak). Refinement of this model slightly improves the fit. Subsequently, additional profile parameters (F, F , and sample displacement) were included in the refinement, followed by a typical procedure of refining the porosity in the Suortti approximation with fixed atomic coordinates and Ui o, and then fixing the porosity parameters for the remainder of the refinement. 

Hiemstra et al. [44] suggested a complicated electrostatic model, whieh was combined with the 1-pK concept with inert electrolyte binding (reactions (5.23), (5.44) and (5.45)). This model was used to explain the asymmetry in charging curves of aluminum (hydr)oxides. Different penetration of the surface by anions and cations of inert electrolyte was modeled by three different electrostatic planes. The counterions are adsorbed mostly in the 2 plane (which corresponds to the /9 plane), but cations of inert electrolyte are bound by one type of subsurface sites in the surface plane. In contrast, anions bound by these sites split their charge 50-50 between the surface plane and the 1 plane (placed between the surface and the 2 plane. Two capacitances (between the surface and the 1 plane on the one hand, and between the 1 and 2 planes on the other) were adjustable parameters of the electrostatic model. 

A common complication in ESR and ENDOR single crystal studies is the occurrence of different spectra due to identical paramagnetic species that are differently oriented due to crystal symmetry. The species are said to be located in different sites, and the overlap of spectra due to different orientations is referred to as site splitting. In the triclinic crystal system, e.g. for the malonic acid discussed in Chapter 2, there is only one molecule in the unit cell and no site-splitting occurs. Two symmetry-related sites with coordinates (x, y, z) and (-x, y, -z) give identical ESR spectra when the crystal is rotated about the y axis. The result is plausible since the magnetic field... 

Fig. 3.10 g-factor variation in a piane showing site-splitting due to a singie species trapped in two equivalent but differentiy oriented positions, sites in the crystai... 

Sni9.3Cu4 7P22I8 [24] a = 10.852 framework sites bulk material is not uniform containing domains with segregation of Te and Ge within the fourfold positions (S.G. P23) 24k site splits into 2... 

Sn24Pi9.6Br8 [31] a = 10.8142 24A site splits into 2, vacancy forms in 6c site... 

Assignment of the 2100 A transition has been difficult because the upper state is subject to fast nonradiative decay which broadens the vibrational structure to the point of diffuseness. This plus the normal hot band and sequence congestion characteristic of room temperature polyatomic spectra complicate the identification of the upper state ( Bi or by vibronic analysis. In turn, complexity due to site splittings has clouded the analysis of the spectrum obtained from benzene in low-temperature solid hosts where the removal of hot bands otherwise makes the spectrum more discrete. Recently, however, two studies have appeared, which together seem to point in favor of the assignment Bi -<- Ajg. 

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