Field correlation

In-place specific weight tests are used to correlate field compaction results with specified engineering requirements for specific weight. 

There is another set of values (called a values) that are also used to correlate field effects. 

The IR spectra of orthovanadates with trivalent cations are also well known as seen in Table 3 59, 60). In the IR spectra of the orthovanadates from Ce to Gd only one band less is observed as predicted and in no case is the vn band observed to split. On the contrary, for LaV04 more bands appear than are predicted from site symmetry rules due to strong correlation field effects. For the same reasons the Raman spectrum of this compound is difficult to interpret 60). 

Barium- and strontium permanganate show clearly the effects of the correlation field effect by well split vs bands. However, the vs band never splits in the predicted way for salts of hexaaquo cations. This again is most likely due to a decreased distortion of the Ta units because of screening by the [M(H20)6] + cations. 

There is another set of values (called [Pg.282]

The Correlation Field Approximation. In some cases it is not possible to explain experimental observations in terms of the site symmetry approximation, whereby the surroundings of a given molecule are treated as static. A clear example is provided by the crystalline form of the trans isomer of [(C5H5)Fe(CO)2]2, which has the centrosymmetric structure and the IR spectrum shown in Figure 10.13. The trans molecule (other isomers exist) has inherent Cy, symmetry when rotational orientation of the C5H5 rings about... 

This way of expressing the overall modes for the pair of molecular units is only approximate, and it assumes that intramolecular coupling exceeds in-termolecular coupling. The frequency difference between the two antisymmetric modes arising in the pair of molecules jointly will depend on both the intra- and intermolecular interaction force constants. Obviously the algebraic details are a bit complicated, but the idea of intermolecular coupling subject to the symmetry restrictions based on the symmetry of the entire unit cell is a simple and powerful one. It is this symmetry-restricted intermolecular correlation of the molecular vibrational modes which causes the correlation field splittings. 

The preceding discussion of correlation field effects is mathematically very superficial. The reader interested in further details should consult the paper by Vetter and Hornig cited in Appendix IX. 

The particular feature that defines the class, and contributes to useful characteristics and to limitations is the N - S bonds, a linkage which within the constraints of the generic formula is particularly susceptible to nucleophilic attack. This requires that useful members of the class possess low solubility, particularly aqueous, so that hydrolytic destruction may be maintained at a low practical limit. We have correlated field usefulness of members of this class to their chemical and physical properties as above discussed. 

In the gase phase, the infrared bands are broad (50 cm ), due to the rotational structure, overlapping vibrations, and hot transitions. In the solid state, the rotational motions are quenched, but due to intermolecular (hydrogen bond) and correlation field interactions, the band positions are shifted and the bands are even broader. The infrared absorptions of matrix-isolated molecules are close to the gas-phase frequencies and exhibit a sharp line-like character (half-widths 0.1 to 2 cm ). Hence the spectra of matrix-isolated molecules are less complicated, and, in comparison to gas phase or solid state spectra, the sensitivity and selectivity of detection increase by a factor of about 10 to 100. Closely spaced vibrations attributed to mixtures of similar molecules, such as conformers, rotamers, molecular complexes, or isotopic species, e.g., H C104 and H CI04, are easily distinguished. 

Concerning IR spectroscopy, it was demonstrated by high-pressure FTIR that the spectra of the non-interdigitated and those of the interdigitated phase exhibit marked differences in terms of splitting of the methylene scissoring band ()CH2 around 1468 cm and its correlation field component CH2. 

Intermolecular Coupling and Correlation Field Splitting of the Water Bands. ... 

The main features that must be considered are the magnitude and symmetry of the static potential in the H2O lattice site and intermolecular coupling (even between molecules of adjacent primitive unit cells) of the water bands. The former is shown by large frequency shifts of the water bands and decreased intramolecular coupling of the stretching vibrations (see Sect. 4.2.7) compared to those of free water molecules (see Table 3) and is discussed in Sects. 4.2-4.4 in more detail. The latter also produces frequency shifts and the so-called correlation field (Davydov) splitting of the water bands. 

Because of the relatively large halfwidths of water bands, correlation field splitting has not been observed very often in the spectra of solid hydrates (except for frequency... 

Table 3-6 distinguishes between correlational evidence from field observations and experimental evidence. Correlational evidence from field studies inherently has multiple sources of uncertainty, many of which are controlled in experimental studies. Observations that indicate a positive relation between environmental PCB exposure (sometimes represented by PCB concentration in tissue) and an adverse health effect in free-ranging wildlife are represented in Table 3-6 as correlational field observations. Effects that were observed in experimental studies under controlled or closely monitored exposure conditions were included in the table as experimental observations. However, no entry was made in Table 3-6 for responses that were reported in an experimental study to be equivocal, ambiguous, or not statistically significant. 

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