Profiles of hydrogen stretching ir bands of molecules with hydrogen bonds: A stochastic theory. I. Weak and medium strength hydrogen bonds

Abstract

A theory is proposed to describe the shape, in inert solutions, of hydrogen stretching ir bands of complexes with hydrogen bonds. The theory uses stochastic arguments and is related to general theories of ir band profiles of liquids. The cases of weak and medium strength hydrogen bond are treated separately. In the former case the band profile is, essentially, a broad asymmetrically distorted Gaussian produced by an anharmonic coupling between the high frequency AH stretching mode and the perturbed low frequency AH⋅⋅⋅B stretching mode. In the latter case the profile is, essentially, a very broad Gaussian intersected by a number of flat or deep, broad or narrow Evans-type transmission windows. Here, the band shaping mechanisms are the anharmonic coupling between the AH stretching and different low frequency external motions and the Fermi resonance between states involving the AH stretching and some other internal modes. Spectral manifestations of hydrogen bonded liquids represent the most extreme case of vibrational broadening and the presence of the Evans holes is its characteristic feature.

Inelastic neutron scattering, Raman, infrared and DFT theoretical studies on chloranilic acid

“…32,33 However, a lack of substructure could indicate that the low-frequency vibrations are strongly damped, which can justify the application of the stochastic model of H-bond IR spectra. 34,35 The application of such a model to mediumstrong OHÁ Á ÁO hydrogen-bonded systems leads to an asymmetric shape of the (OH) band exactly the same as in this case. 36 The maximum of the (OD) band is located at 2415 cm À1 , i.e.…”

Vibrational Dephasing Mechanisms in Hydrogen-Bonded Systems

“…Recent developments in timeresolved spectroscopy have allowed for, e.g., the measurement of homogeneous vibrational line shapes with IR transient hole-burning techniques [2,3] and the timeresolved observation of vibrational dephasing by means of a photon-echo experiment [4]. A great number of sophisticated models have been developed to describe vibrational line shapes [5][6][7][8][9][10], some specifically addressing the effect of hydrogen bonds [8][9][10]. The spectral bandwidth of a hydrogen-bonded complex is typically an order of magnitude larger than that of the unassociated molecule or oscillator, indicating that the vibrational dephasing rate is strongly influenced by hydrogen-bond formation.…”

“…The first model is based on those developed by Marechal and Witkowski [8] and by Bratos [9], which were of a static nature (slow modulation limit, t c !`). The key point of these theories was the recognition that dephasing of the n s (X-H) stretch vibration occurs through coupling to the hydrogen bridge stretching mode n s (XH· · · Y), directed along the same spatial coordinate as the n s (X-H) highfrequency mode itself; the coordinate of the n s ͑XH · · · Y) mode r s determines the frequency of the n s (X-H) mode.…”

Pressure-Induced Phase Transitions in l-Asparagine Crystal.

“…Modes with frequencies lower than 200 cm-1 are assigned as external modes related to hydrogen bonds V (A-H ---B) and structural deformations [8,9]. When hydrostatic pressure is applied over the material an accommodation of the skeletal structure of the molecule occurs as a consequence of the change in hydrogen bond length .…”