Fermi resonances adipic acids

Since it was admitted that Fermi resonances may play a role in crystalline adipic acid [102], they were incorporated in the model by using the Fourier transform of Eq. (304) in place of Eq. (279). Note that Yaremko et al. [105] also studied these spectra in a rather formal way with the aid of a general formalism [106-108], which is not without connection with that used by the Witkowski school. The left column of Fig. 24 reports the comparisons between the experimental line shapes measured by Auvert and Marechal [101] and the corresponding theoretical ones calculated by Eq. (279) in this way [98]. In this figure, the line shapes are reproduced up to down, for, respectively, 10, 100, 200, and 300 K. For each temperature, the right part is devoted to the O—H species and the left one to the O D isotopomer. The values of the parameters used for the calculations are reported in the caption of the figure. 

Figure 24. Effects of temperature and isotopic substitution on the spectral densities of crystalline adipic acid in the absence of Fermi resonance. Comparison between theoiy (Eq. (279)) (thick Line) and experiment [101] (grayed spectra). Left column calculations using the breaking of the IR selection rule (r)° = 0). Right column same calculations but without the breaking of the IR selection rule (r 0 = 0).

Figure 25. Second moments of the spectra of adipic acid in the absence of Fermi resonance corresponding to the left set of spectra of Fig. 24.

Figure 26 is devoted to the incorporation of Fermi resonances in the model of adipic acid. The values of the parameters involved in the calculations are reported in Table XIII. Comparison of Figs. 25 and 26 shows that the improvement in accuracy resulting from the incorporation of Fermi resonances is rather weak. Nevertheless, note that this incorporation allows us to stabilize the fitted parameters that ought to be the same, when passing from one temperature to another. From our viewpoint, this stabilization behavior is more revealing for the presence of Fermi resonances than the weak improvement of the fit. 

In conclusion, note that the quality of the theoretical SD with respect to the experimental line shape is not as good for crystalline adipic acid as for the gaseous and liquid carboxylic acids studied above. The reason is that if Fermi resonances seem to be unavoidable in order to reproduce all features of the experimental line shapes and to conserve a good stability of the basic physical parameters when changing the temperature, however, the way in which the Fermi resonances are taken into account is very sentitive to the used adiabatic and exchange approximations [82]. 

Figure 26. Adipic acid with four Fermi resonances and with r °. Comparison between theory (thick line) and experiment (grayed spectra). The theoretical parameters are given in Table XIII.

As it appears, and as seen in Section VII.B.l dealing with adipic acid, the introduction of Fermi resonances weakly improves the accuracy of the fitting and allows a stabilization of the physical parameters when passing from one temperature to another. 

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See also in sourсe #XX -- [ , , ]

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