H-Bonds of adipic acid crystals III : measurement of the various anharmonic couplings of the O-H (O-D) stretching bands

Open Access
Publisher Website
Google Scholar
Abstract
We show the evolutions with temperature of the first moments of the ν s bands (O-H... O or O-D... O) of adipic acid crystals and we establish theoretical relations giving the values of these first moments with the general assumption that the νs vibration of a single H-bond is strongly coupled to the νσ vibration (O-H ... O) of the same bond and is also coupled to binary combinations or overtones of other vibrations. Both these couplings are anharmonic. We also assume that the transition moment at the origin of the ν sband shows non negligible electrical anharmonicity coupling ν s with νσ and we give arguments for our neglecting, in a first approximation, harmonic resonance terms between two neighbouring ν s vibrations which do not give important temperature effects. From the comparison of experimental and theoretical values of these moments we determine the magnitudes of all these couplings. The νs-ν σ coupling is shown to decrease with temperature, which corresponds to an increase of the average O ... O distance of about 0.03 Å between 10 K and 300 K, which we attribute to a coupling of νσ with lower frequency vibrations of the H-bonds. The total energy of anharmonic resonance interactions (80 cm-1 for H-adipic acid and 50 cm -1 for D-adipic acid) is shown to be that which we can calculate if we suppose that these interactions originate from the variation of the moment of inertia of the H atom with respect to the O atom when the O-H length vibrates. This strongly suggests that this simple geometrical mechanism might be at the origin of Fermi resonances, thus defining a simple procedure for their calculation. Finally the magnitude of electrical anharmonicity is also measured and shown to be important