Band assignments difficulty

In addition to being significantly less intense, NIR spectral features tend to be broad and overlapped. Historically, this made band assignments more difficult than in the MIR, and slowed the adoption of NIR relative to this technique. Despite these difficulties, however, as early as 1922-9, researchers at UCLA and Johns Hopkins... 

Since isomerically pure polymers were not available, three different kinds of BR, each relatively high in one of the three kinds of base units were used as standards [35]. The band near 1308 cm 1 was identified [38,39] with the cis isomer and used for analyses [43]. The 1308 cm 1 band is weak and relatively broad, with the appearance of an unresolved doublet (1306,1311 cm 1). The cis band at 730 cm 1 is more frequently used in spite of some difficulties. Relatively pure, crystalline stereoregular polymers have been prepared and structures were determined by X-ray diffraction for cis [44], trans [45] and syndiotactic vinyl [46] and isotactic vinyl [47]. Infrared spectra [48-50] have been published for the four stereoregular polybutadienes, with detailed analyses of the spectra and band assignments for cis [51], trans [51] and syndiotactic vinyl [51] polymers. For the spectrum of isotactic vinyl BR, bands at 1232, 1225, 1109, 943, 876, 807 and 695 cm"1... 

Infra-red dichroism Molecular bond directions can be specific to crystalline or noncrystalline regions, perhaps even fold bands co 5, cos 6cor(l> Usually assumes dipole moment change corresponds to bond direction—may not always be correct results only as good as band assignments Thin specimens required. Difficulties in overlapping bands base-line determination... 

The commonest modern method for determining the degree of hydration is to measure the intensity of the broad n- carbonyl absorption band at about 280 m/x, which disappears on hydration. Early measurements (Schou, 1926, 1929 Harold and Wolf, 1929, 1931) show considerable discrepancies, but the results of later workers are in reasonable agreement. The main uncertainty lies in the value to be assigned to the maximum extinction coefficient of the unhydrated carbonyl compound, which varies between 12 and 80 for different compounds. This is commonly taken as the value measured in a non-hydroxylic solvent such as hexane or cyclohexane, but this is not strictly valid, since the intensities of n-n- transitionsvary somewhat with the solvent (see e.g. Dertooz and Nasielki, 1961) moreover, since the shape of the band and the value of e are also solvent-dependent it may make some difference whether the extinction coefficients are compared at the same wavelength, at the respective maxima, or in terms of the band area. Special difficulties arise... 

It is sometimes found that the 0-0 band is not the most intense in the doublet absorption band. In Cr(NH3)83+ the 0-0 band is only the third strongest in the spectrum and in Cr(CN)e3 the 0-0 band is so weak it can be observed only with difficulty.15 Observation of a weak 0-0 band may be taken to indicate that at equilibrium the 2Eg state is significantly distorted relative to the iA2g state. The problem of using the observed structure of absorption and emission spectra to assign vibrational levels in complexes has been discussed by Porter and Schlafer.15... 

Chromium(II) chelates of 3-(N-2-furfuralideneimino)propionic acid (HFP), o-(JV-ar-furfurali-dene)benzoic acid (HFB), and o-(N-ar-furfuralideneimino)ethanesulfonic add (HFE)284-286 have been found to have magnetic moments of approximately 4.8 BM at room temperature. A band near 14500 cm-1 (CHC13 solution) has been assigned to the % 5T transition and formation constants have been measured. However, there is no indication of difficulties arising from the air sensitivity of chromium(II) solutions. 

It has been suggested [Tadokoro, Seki, and Nitta (218) Tadokoro (215b)] that this band arises from a symmetric C—C stretching mode, which would be consistent with its polarization parallel to the zig-zag chain, but this assignment presents difficulties too. In particular, it places such a skeletal mode at what seems to be much too high a frequency, and the skeletal vibration involved, viz., i>+(0), is already adequately assigned to the band at 915 cm-1. 

While most of the above assignments are satisfactory, it is necessary to point out that some difficulties do exist [Liang and Krimm (115)]. The assignment of the 1455 and 1473 cm-1 bands to 6 (CH2) modes is reasonable in view of the range observed in the polymers discussed so far, viz.,... 

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