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

Abstract

ABSTRACT:We extend a quantum nonadiabatic treatment of damped H-bonds involving combined effects of anharmonicities of both the fast and the slow modes, Fermi resonances and relaxation [Rekik et al., J Mol Liq (in press)] in order to account for stronger H-bonds. For this purpose, we introduce, in the model, the quadratic modulation of both the angular frequency and the equilibrium position of the X − − → H . . . Y stretching mode on the intermonomer ← − X − H . . . − → Y motions to refine the structure of the spectrum, whereas we have considered in our previous work only the linear modulation of the angular frequency of the fast mode. In this approach, the strong anharmonic coupling theory is used through second-order expansion in the slow-mode coordinate Q of the angular frequency and the equilibrium position of the fast mode. The relaxations of the fast mode (direct damping) and of the H-bond bridge (indirect damping) are incorporated by aid of previous results [Rekik et al., J Mol Struct 2004, 687, 125]. The spectral density is obtained by Fourier transform of the autocorrelation function of the dipole moment of the fast stretching mode. The numerical calculation shows that the modulation of the angular frequency of the fast mode and its equilibrium position by the slow mode coordinate generate an improvement of the fine structure of the spectrum and also provide a direct evidence of the increase of the level density and the spectral broadening.

A density functional theory based estimation of the anharmonic contributions to the free energy of a polypeptide helix

“…38) that the integrand in Eq. (12) can be replaced by the canonical ensemble average of the potential energy U (λ):…”

“…10 Thermal vibrations along the weak/soft potential of the hydrogen bonds are expected to exhibit significant anharmonic contributions at elevated temperatures. [11][12][13][14][15][16][17][18] Using synchrotron radiation Xie et al have collected far-infra-red data on the low frequency spectrum of extended helical amino acid homopolymers. 15 They found a significant redshift in the low frequency spectrum collected at T = 395 K, with respect to the T = 100 K spectrum.…”

Theoretical Simulation of the Infrared Absorption Spectrum of the Strong Hydrogen and Deuterium Bond in 2-Pyridone Dimer

“…We have shown in recent [32] work that the fine structure of the IR υ X-H stretching band are connected with the new anharmonic coupling parameters (i.e. β, f˚ and g˚), whereas these parameters do not manifest markedly in effects of temperature and deuteration on the IR spectra.…”

“…To take them into account, one needs to know a great number of parameters. For this idea, we think that the small residual discrepancies between theory and experiment are related to the neglect of some of these parameters: 1) assumption of a linear dependence of the angular frequency of the fast mode on the coordinate of the H-bond bridge [32], 2) assumption of the independence of the equilibrium position of the fast mode on the coordinate of the H-bond bridge [32], 3) assumption of a constant term for the Davydov coupling parameter [52], 4) neglect of electrical anharmonicity, 5) neglect of eventual weak tunneling through the potential barrier separating the two H-bond bridge minima, and 6) neglect of eventual relaxation mechanisms of non-adiabatic nature.…”

Theoretical model of infrared spectra of hydrogen bonds in molecular crystals of 2-thiopheneacetic acid: Fermi resonance and Davdov coupling effects.

“…The properties of hydrogen bonds have been widely investigated theoretically and experimentally, and have been the subject of several monographs [6,7]. Many theoretical models have been proposed to describe the unusual features of hydrogen bond stretching bands in IR spectra [8][9][10][11][12][13][14].…”