The following theories elaborated for the description of IR spectra of secondary amide crystals may be divided into two groups: 1. Theories of the first group tried to explain the mechanism of the generation of the CdO group stretching vibration bands in the IR spectra of peptides. The subsequent versions of Davydovs Solution theory belong to this group. In these models excitations were obtained as polaronic type solutions of a Hamiltonian describing the interaction of the amide I vibration quanta with low frequency lattice 40_44 2. Theories of the other group comprise models focused on the generation mechanisms of the fine structure pattern of the N_H proton stretching vibration bands in IR spectra of hydrogen bonded amide crystals.
A wide spectrum of theories was proposed from the models assuming Fermi resonance mechanism involving the proton stretching vibrations and some other vibrations of the hydrogen bonded molecule to theories assuming vibrational exciton N_H band fine structure pattern could not be explained in terms of the formalism of the Fermi resonance Apoptosis mechanism. On studying the temperature efects in polarized IR spectra of acetanilide and acetanilide 8d crystals and on the basis of the femtosecond infrared pump_probe experiments, they proposed the so called self trapping theory. In this model an exciton_ phonon coupling plays an essential role that leads to the vibra tional self trapping state. Within this theory, the lower frequency branch of the band is generated by the transition to a hypothetical metastable excited state of the proton stretching vibrations in the hydrogen bond lattice of the crystal, which anharmonically couple with the low frequency N 3 3 3 O hydrogen bridge stretching vibrations.
As the result of such a coupling, the absorption spectra in the band frequency range exhibit PF299804 shapes qualitatively resembling typical Franck_Condon type progressions, composed of one vibrational excitation quantum CdO modes. Edller and Hamm noticed that the generation of the several quanta of phonon excitation. This theory has been Figure 4. Impact of temperature on the polarized spectra of the most intense components of the PAM crystal: the ac plane case, the ab crystalline face case. proposed recently and is highly intuitive as well as being only of a qualitative character.
The model of the metastable state within the self trapping theory is totally abstracted from the state of art in the quantitative theories of the IR spectra of the hydrogen bond dimers and hydrogen bonded crystals. The authors of the self trapping theory have not considered the H/D isotopic CDK efects in the IR spectra of the hydrogen bond of amide crystals. This N_H band shapes characterizing crystals of diverse secondary amide systems. Moreover, to the authors knowledge no monograph dealing with the quantitative interpretation of IR spectra of PAM crystals has been published so far. 3. 2. Initial Studies of Vibrational Spectra of PAM Crystals. The N_H band in the IR spectra of PAM crystal consists of several intense, well resolved spectral lines. In Figure3 IR spectra of polycrystalline samples of the compound measured at 293K and 77 K are presented.
Also the Raman spectrum is shown to identify the lines attributed to the C_H bond stretching vibrations. The C_H bond stretching vibration lines CFTR facilitate identification of crystal faces developed during crystallization from melt. In the spectra of the PAM crystal the N_H band shift toward the lower frequencies, accompanying the formation of the hydrogen bond, is ca. equal to 250 cm. This fact indicates that hydrogen bonds are relatively strong. An identical conclusion can be drawn from the geometry of hydrogen bonds in the crystal approach also does not explain the diferences. 3. 1. Temperature Effects in IR Spectra of PAM. In Figure 4 the temperature effect in the spectra of the two forms of PAM crystals is shown. From these spectra it results that on a N_H band remains almost unchanged.
In these circumstances, the intensity of the band lower frequency branch increases. The temperature efects in the crystal spectra seem to be very complex. This efect probably is connected with the averaging of the bent structure of the hydrogen bridges toward the VEGF axial symmetry at growing temperatures. On the other hand, the equilibrium geometry of the hydrogen bonds is temperature dependent. This fact poses a problem for the theoretical models describing spectra of crystals with hydrogen bonds. 3. 3. Linear Dichroic Effects in the Spectra. Polarized IR spectra of the hydrogen bond in PAM crystals measured in the frequency range of the band for two individual crystal forms, each having developed another crystal plane, are shown in Figure 5. Spectra of the two different crystal forms differ from one another by the intensities of their lower frequency band branches and by relative intensities of the C_H bond stretching vibration lines. Although in the spectra of the two crystal forms some splitting efects accompanied by slight local linear dichroic efects can be temperature decrease the higher frequency branch of the N_H Figure 7.