Skeleton vibrations

Symmetry mode Symmetry element E>egree of freedom Molecule Number of C—H M braiions Number of skeleton vibrations Activity of vibrations ... 

The skeleton vibrations. C3NSX, CjNSXj. C NSXY, or C NSXj (where X or Y is the monoatomic substituent or the atom of the substituent which is bonded to the ring for polyatomic substituents), have been classified into suites, numbered I to X. A suite is a set of absorption bands or diffusion lines assigned, to a first approximation, to a same mode of vibration for the different molecules. Suites I to VIII concern bands assigned to A symmetry vibrations, while suites IX and X describe bands assigned to A" symmetry vibrations. For each of these suites, the analysis of the various published works gives the limits of the observed frequencies (Table 1-29). 

Quantum-chemical calculations of PES for carbonic acid dimers [Meier et al. 1982] have shown that at fixed heavy-atom coordinates the barrier is higher than 30kcal/mol, and distance between O atoms is 2.61-2.71 A. Stretching skeleton vibrations reduce this distance in the transition state to 2.45-2.35 A, when the barrier height becomes less than 3 kcal/mol. Meier et al. [1982] have stressed that the transfer is possible only due to the skeleton deformation, which shortens the distances for the hydrogen atom tunneling from 0.6-0.7 A to 0.3 A. The effective tunneling mass exceeds 2mn-... 

In addition, the frequency cooo, as well as the tunneUng distance can also be extracted from the same empirical data. Thus all the information needed to construct a PES is available. Of course, this PES is a rather crude approximation, since all the skeleton vibrations are replaced by a single mode with effective frequency cooo and coupling parameter C. From the experimental data it is known that the strong hydrogen bond (roo < 2.6 A) is usually typical of intramolecular hydrogen transfer. 

Table 3 presents the values of the force constants corresponding to the C skeleton vibrations of s-trans-1,3-butadiene obtained at several levels of calculation. The computed values are very sensitive to the inclusion of electron correlation. Stretching C=C and C—C force constants decrease when electron correlation is taken into account. This effect is generally larger for basis sets without polarization functions than for those with polarization functions23. On the contrary, the values of the C=C/C—C and C=C/C=C coupling constants do not vary much upon increasing the level of calculation of electron correlation. 

The crystallinity of the so formed intermediate phases was checked by various physical methods. No XRD crystallinity was detected after the first 45 hours of heating, while other techniques such as Infrared (20,66,67), TG-DTA 01,32,67) or 13C NMR (32,33), which are sensitive to the presence of very small amounts of Pr N+ species occluded in ZSM-5 crystallites, confirm that very small size ZSM-5 particles are present in the early beginning of the synthesis process (Table III). Further IR studies of the ZSM-5 skeleton vibration bands (20,66) or catalytic tests (66) have confirmed their presence in B-type procedures. By contrast, and as expected, XRD and DTA techniques give identical crystallinity values in the case of synthesis A (Table III). 

IR spectral data for most known 1,2,4-triazines have been published. The absorption of the 1,2,4-triazines in the IR region are those expected for this system. The IR spectrum of the parent compound (1) shows three absorption bands for the C—H stretching vibrations at 3090, 3060 and 3030 cm-1, five bands for C=N and C=C stretching vibrations at 1560, 1529,1435,1380 and 1295 cm-1, three for the C—H in-plane deformations at 1163,1135 and 1113 cm-1, two for the characteristic ring skeleton vibrations at 1050 and 995 cm-1 and three bands for the C—H out-of-plane deformation vibrations at 851,768 and 713 cm-1 (68CB3952). These values are in good agreement with similar bands for pyridine, pyridazine, pyrimidine and pyrazine. Alkyl and aryl derivatives of 1,2,4-triazine show similar bands in their IR spectra, with additional bands from the substituents. 

Fig.3 presents IR absorption spectra of TCNQ ARS, the cations of which include longer alkyl radicals iso-C H - and iso-CsIIn. Here essential distinctions from the above-discussed spectra of conducting ARS (Fig. 2) are obvious. In particular, no EPI-caused peculiarities are observed since no lines are broadened in the range of pyrazinium skeleton vibrations and v(C=N) vibrations. Continuous absorption is also absent in all the studied range. This suggests the existence of a dielectric ground state in the [N-iso-... 

Figure 6.8-22 Calculated frequency dependence of some skeleton vibrations of the C(CH3)3 cation on the displacements of the methyl groups from the planar configuration.

Infrared bands at 1128 cm and 1097 cm in sulfonated polystyrene result from the in-plane skeleton vibrations of a disub-stltuted benzene ring and are strong] affected by the second substituent. An absorbance at 1128 cm Is observed only In hydrated samples. In the hydrated form, the proton of the sulfonlc add Is removed from the anion, yielding the sulfonate anion, SO, which... 

Fig. 14 RR intensities of nine skeleton vibrations and the metal-oxo stretch of Mo (tpc)(0), all overlaid on the UV-vis absorption spectrum. The Mo-O stretch is the most enhanced, and the blue shifting of the peak implies CT character in the high-energy portion of the Soret manifold. Reprinted with permission from [176]. Copyright 2009 American Chemical Society...

The parent 1,2.4-triazine has the following bands C —H stretching vibrations 3090, 3060, 3030cm-1 C = N and C = C stretching vibrations 1560, 1529. 1435, 1380, 1295 cm"1 C-H in-plane deformations 1163, 1135, 1113 cm"1 characteristic ring skeleton vibrations 1050, 995 cm"1 C-H out-of-plane deformation vibrations 851, 768, 713 cm"1.78 These values are in good agreement with similar bands for other azabenzenes, such as pyridine, pyridazine,... 

The anomalous decrease in the activation energies of n-alkanes cracking after Cie the C- sixteen effect[31]", could be explained in terms of the contribution from the internal vibrations of zeolites whose frequencies are in resonance with the skeleton vibrations of hydrocarbons at isocatal)4 ic temperature and has been described in detail elsewhere[32]. 

Figure 5.2 ATR-IR spectra of P4VP brushes in the as-grafted state fscdid and after con lexation of (dashed line/ A shoulder on the pyridine ring skeleton vibration at 1615 cnT appears after conydexation [3y.

Solid Ge(CH3)4 at — 190°C showed a very weak feature at 134 cm tentatively assigned to torsional modes [52]. Inelastic neutron scattering exhibited this mode at 142 cm" (the odd shape faintly indicating the 33—fi splitting) along with a skeleton vibration at 180 cm [80] (see Vs in Table 1) cf. barrier of rotation on p. 24. 

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