H-BONDS OF ADIPIC ACID CRYSTALS AT LOW-TEMPERATURE .2. POLARIZATIONS OF THE VS AND 2GAMMAOH (2GAMMAOD) BANDS

We examine the theoretical implications of the results found in the preceding article, which are related to the variation of the polarizations of the νs bands of adipic acid crystals when passing from one side of the band to the other, and to the polarization of the 2γOH (2γOD) band. We discuss the possible origins of the variations of the polarizations of the νs bands and indicate that it is likely to be due to a coupling of the low frequency νσ mode of the H-bond with some low frequency bending mode of the same H-bond. We attribute the polarization of the 2γOHand 2γOD vibrations (parallel to that of νs) to a coupling of νs and γOH (γOD) of the same type as the coupling of νs with δOH which leads to anharmonic resonance interactions (Fermi resonances). We deduce the energy of interaction of such a coupling (≈70 cm-1) which we compare with that which we can predict by supposing a simple geometrical origin for this type of interaction, which simply reflects the change in the moment of inertia of the H-atom with respect to the O-atom when the OH length vibrates. From this comparison we suggest that Fermi resonances might well have their origins in this simple geometrical effect. © 1979.