Crystal field mode

The estimation of the crystallinity index (Cl) of bone is based on one of the four vibrational modes associated with the apatite phosphate group. In amorphous calcium phosphate, the absorption band at 550-600 cm-1 appears as a single broad peak, whilst in hydroxyapatite it is split into bands of unequal intensity by the apatite crystal field (Sillen and Parkington 1996). Based on the splitting factor introduced by Termine and Posner (1966), Weiner and Bar-Yosef (1990) proposed the use of a crystallinity index to measure the crystallinity of bone mineral. As illustrated in Fig. 4.7, the Cl is estimated by drawing a base line from 750 to 495 cm 1 and measuring the heights of the absorption peaks at 603 cm-1 (measurement a), 565 cm 1 (measurement b) and the distance from the base line to the lowest point between the two peaks (c). Cl is calculated from the formula ... 

An appreciation of the crystal field effect on the vibrations of the Bravais cell which is repeated to build the crystal is extremely important when interpreting the vibrational spectra of many substances, since in the presence of a crystal field influence the number of observed bands in the spectrum cannot be directly determined from the formula unit which goes to make up the unit cell. In other words, there is almost always a larger number of bands to account for when investigating solid state samples. The solid state effects often cause degenerate bands to split in the same degree as symmetric and antisymmetric stretching modes split. 

This expression is based on the approximation that the 15 normal modes of vibration of a FeN6 chromophore in each spin state may be reasonably well represented by one mean effective vibrational wave number VL and vH for the LS state and the HS state, respectively. The ej are the energy separations of the 15 electronic levels of the 5T2(Oh) term, split by crystal field distortion and eventually by spin-orbit coupling, from the Aj level. In a reasonable approximation the 15 Cj parameters may be reduced to only two, viz. the energy separation e between the baricenter of the ST2 term and the Ai level, and the trigonal distortion parameter St =... 

When a Jahn-Teller-distorted C q molecule is placed in a solid-state environment, strain or crystal-field perturbations may play a decisive role in the selection and/or enhancement of the Jahn-Teller-distorted configuration. In an attempt to investigate the Jahn-Teller-distorted C6 0 molecules in the solid state, C60-tetraphenylphosphonium iodide has been synthesized and studied [40]. The well-known Flu(l) and Flu(2) modes were found to split into doublets at room temperature indicating a D5d or D3d and not a D2u Jahn-Teller configuration. These results are consistent with a dynamic Jahn-Teller effect in the strong coupling limit or with a static distortion stabilized by the low-symmetry perturbations. 

The most Raman-active intramolecular modes are the two modes (ag) encountered in the absorption spectra of Fig. 2.8 397 and 1404 cm-With a resolution of 1 cm- it is difficult to observe any splitting in these modes in addition, their frequencies show very little sensitivity to temperature. All these observations are in good agreement with the fact that the intramolecular modes are little affected by the crystal field. In contrast, the weakest vibrations (torsions, flexions, and internal distortions, such as butterfly and twisting motions) contribute significantly to the determination of the intermolecular modes, securing the thermalization of all the molecular degrees of freedom 45... 

Detailed investigations have been made of the octahedral [UCle] ion. Its spectrum is largely vibronic in nature, with electronic transitions accompanied by vibrations of the complexion (odd-parity modes-the Tiu asymmetric stretch and and T a deformations). Here, as in other cases, overlap of bands from different states occurs because of the similarity in crystal-field and spin-orbit coupling effects. Its spectrum can be altered by destroying the centre of symmetry (e.g., by hydrogen bonding), which enables pure electronic transitions to be observed, and alters band patterns in multiplets. 

In the resonance Raman spectra of BEDT-TTF, an interesting line was observed around 100 cm which could be associated with the symmetric stretch mode of the I3 molecule shown in Fig. 4.8-15. At low temperature, the mode splits into three components, a behavior which has been attributed to crystal field interaction (Swietlik et al., 1987). 

The Ih symmetry of the 60-atoms molecule allows 2Ag and SH modes to be Raman active and 4T modes to be IR active. The four IR active modes are at 1430, 1185, 580 and 528 cm , respectively. The most important Raman modes are at 1469 (tangential bond alternation or pinch mode, Ag), 495 (radial breathing mode, T ) and 271 (squashing mode, Hg) cm , respectively. In the low temperatur phase degenerate modes split from a crystal field and Davidov interaction. Good reviews on the group theoretical analysis and on the line positions are given in (Dresselhaus et al., 1992 Matus and Kuzmany, 1993). 

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