The A—H Stretching Mode

Figure 2.2 Potential of the A-H stretching mode, Ulnstrating the enhanced anharmonicity cansed by formation of an A-H- -B bond.

IR and Raman studies of the A—H stretching mode (it is important to note that frequency, band width, and intensity are all useful criteria). 

Vibrational Spectral Behavior. Possibly the most unique manifestation of the H bond is the perturbation of the A—H stretching mode, Vi, Each of the three changes that occur—frequency shift, increase in band width, and enhancement of intensity—is unusual and characteristic. Since these effects are so unusual they must contain interesting information about the nature of the H bond. Despite a great deal of study, this information has not yet been extracted. 

Frequency Shift. The frequency shift of v, reveals the change brought about in the potential function for the A—H stretching mode when it participates in a H bond. Despite theoretical interpretation from several points of view, the frequency shifts have not provided definitive support for any particular model of the H bond. For electrostatic model estimates of Av, see Section 8.2.1. For quantum mechanical calculations of Av, see 407, 1915-1919, 465. 

Zengin et al. [41] characterized a polyaniline (PANI)/MWNT composite. The FTIR spectra of the composite film show benzoid and quinoid ring vibrations at 1500cm-1 and 1600 cm-1, respectively, which indicate the presence of emeraldine salt (ES) of polyaniline. A weak broad band near 3400 cm-1 is assigned to the N—H stretching mode. The strong band at 1150cm-1 is characteristic of PANI conductivity. The FTIR spectrum of PANI/MWNT composite in the ES form exhibits several clear differences from the spectrum of neat ES PANI (1) the composite spectrum shows an inverse... 

A number of other models were considered and tested (for example, direct B—H bonding). The most significant test was the IR vibrational spectrum, where a sharp absorption band at 1875 cm-1 was found, corresponding to the Si—H stretch mode softened by the proximity of the B-atom. Had the hydrogen been bonded to boron, a sharp absorption band at 2560 cm-1 would have been expected. Also, Johnson (1985) showed that deuteration produced the expected isotopic shift. The most definitive and elegant proof of the correctness of the Si-H-B bonding model was provided by Watkins and coworkers (1990), on the basis of a parametric vibrational interaction between the isotopes D and 10B. 

The BC configuration and the H motion of interest are shown in Fig. 23. The stress-induced-alignment technique described in Sec. II.3 was used by Stavola et al. (1988b) to measure the kinetics of such an H jump. The longitudinal H stretching mode that is used as a probe of the center s orientation lies at 1903 cm-1 at low temperature (Sec. III. 1). 

Intramolecular hydrogen bonding can be observed in dilute solutions of di and poly hydroxy compounds in CC14, where no intermolecular hydrogen bonds are formed. Under these conditions a number of cyclic and acyclic diols have two bonds and others have single band in the O-H stretching mode region. 

While the O—H stretching mode is a sensitive indicator of weak hydrogen bonds to fluorine and the hydrogen bond topology, there are other dynamical... 

Another example of the power of infrared spectroscopy in this context became evident in the investigations of the C—H stretch modes . Previously, it had always been assumed that methoxide, like free methanol, has a symmetric CH3 group. Around 3000cm the vibrational spectrum would... 

Figure 27. Evolution of the IR integrated absorption of the C—H stretching modes involving sp carbon atoms. The corresponding absorption band is indicated by the star in the IR spectra reported in the inset during a compression (full dots)-decompression (empty dots) cycle.

ETEROAROMATics FURAN AND THIOPHENE. The chemical transformation of thiophene at high pressure has not been studied in detail. However, an infrared [441,445] study has placed the onset of the reaction at 16 GPa when the sample becomes yellow-orange and the C—H stretching modes involving sp carbon atoms are observed. This reaction threshold is lower than in benzene, as expected for the lower stability of thiophene. The infrared spectrum of the recovered sample differs from that of polythiophene, and the spectral characteristics indicate that it is probably amorphous. Also, the thiophene reaction is extremely sensitive to photochemical effects as reported by Shimizu and Matsunami [446]. Thiophene was observed to transform into a dark red material above 8 GPa when irradiated with 50 mW of the 514.5-nm Ar+ laser line. The reaction was not observed without irradiation. This material was hypothesized to be polythiophene because the same coloration is reported for polymeric films prepared by electrochemical methods, but no further characterization was carried out. 

As at very low temperatures, for very high frequencies the thermal motion becomes temperature independent. For example, for the C—H stretching mode, with frequency in the 2700-3300 cm 1 range, the exponential in the denominator of Eq. (2.51) is very large for common temperatures and the second term in the square brackets is negligible. Using for m the reduced mass of the oscillator (0.9231 dalton for diatomic C—H) gives, with v = 3000 cm-1, a constant mean-square vibrational amplitude of 0.006 A2. 

Matrix studies of water suspended in solid nitrogen at 20°K reveal a similar pattern of frequency shifts of the 0—H stretching modes as a y-ion of multimer size. On analogous arguments to those used for -> K ianol it is concluded that water forms cyclic dimers also. 

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