Infrared Spectra of H-Bonded Systems : Anharmonicity of the H-Bond Vibrations in Cyclic Dimers

Abstract

We present a theoretical simulation of the infrared spectra of the stretching vibrations of the H atom in hydrogen-bonded carboxylic acid dimers. We have refined a model previously proposed where the H-atom stretching vibration in an H-bond X–H···Y is coupled to the “hydrogen-bond vibration” X→–H→···Y← (we suppose that the H-atom vibration is harmonic with a frequency of oscillation depending on the X···Y distance), by the introduction of an anharmonic term (Morse potential) in the X→–H→···Y← motion. This model gives a value for the frequency of oscillation of the X→–H→···Y← motion in agreement with the observed value (in contrast to the previous model where the X→–H→···Y← motion was assumed to be harmonic), simulates in terms of delta functions the experimental spectrum rather well, and predicts the isotope effect completely. We conclude that the coupling of the H-atom stretching vibration with the hydrogen-bond vibration X→–H→···Y← together with the strong anharmonicity of the latter motion are characteristic features of stretching vibrations in hydrogen bonds, at least for weak and intermediate hydrogen bonds.

Polarized Infrared Spectra of the H(D) Bond in 2‐Thiophenic Acid Crystals: A Spectroscopic and Computational Study

“…The theoretical analysis of the ν XH IR band shape has been widely undertaken by assuming that the frequency of this stretching mode (fast mode) exhibits a strong linear dependence on the slow intermonomer motion (slow mode). From this frequency modulation follows an anharmonic coupling between the slow and fast modes, which is at the basis of the strong anharmonic coupling theory 3–6. Most of the features shown by the ν XH spectra may be explained by this assumption.…”

Anharmonic effects on theoretical IR line shapes of medium strong H(D) bonds

“…Several theoretical analysis of the υ S ( X − $\mathop H\limits^{\rightarrow}$ … Y ) infrared bandshape has been widely undertaken by assuming that the frequency of this stretching mode exhibits a strong linear dependence on the slow intermonomer motion X … Y (“slow mode”). From this frequency, modulation follows an anharmonic coupling between the slow and fast mode, which is at the basis of the strong anharmonic coupling theory 7–26 (see Table I). Most of the features shown by the υ S ( X − $\mathop H\limits^{\rightarrow}$ … Y ) spectra may be explained by this assumption: the band is shifted toward low frequencies (with respect to the same band without hydrogen bond), it spreads over a large range of frequencies, it is asymmetric and displays numerous subbands.…”

“…Most of the features shown by the υ S ( X − $\mathop H\limits^{\rightarrow}$ … Y ) spectra may be explained by this assumption: the band is shifted toward low frequencies (with respect to the same band without hydrogen bond), it spreads over a large range of frequencies, it is asymmetric and displays numerous subbands. However, the majority of works dealing with the strong anharmonic theory 7–26, with some exception 27, 28, used only the dependence up to order one of ω( Q ) and unaware the modulation of the equilibrium distance of the fast mode by the intermonomer coordinate Q . For weak H‐bond, the angular frequency of the fast mode is assumed to depend linearly on the position coordinate of the slow mode.…”

An equilibrium focused approach to calculating the Raman spectrum of the symmetric OH stretch in formic acid dimer

“…This is accompanied by a distinct change from a sharp line to a broad spectral feature which can extend over 1000 cm À1 . These effects are the subject of several works [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17].…”