Elucidating the theoretical evolution of the IR spectral density and the potential energy surfaces of hydrogen bonded complexes: A quantum Dynamical study

Concerted potentials are proposed herein for the description of vibrational modes in hydrogen bonded systems to consistently determine the evolution of the infrared absorption spectrum from weak to strong hydrogen bonds. A double well potential constructed using two crossed Morse curves is particularly employed to describe the high stretching mode whereas the H-bond Bridge is designated using harmonic potential. Within the circumstances of the strong anharmonic coupling theory, a strenuous coupling between the high frequency ( ω∘ ) and the H-bond Bridge ( ω∘∘ ) modes is introduced. We consider in this work theoretical IR spectral density of the high frequency stretching mode of hydrogen bonds of the type X-H…Y within the frame of the linear response theory. For this purpose, several experimental correlations are introduced, namely the two angular frequencies ω∘ (R) and ω∘∘ (R), the barrier height V∘ (R), the distance D(R) between the minima and the distance X-H, r e (R), which are correlated to the R = R X−Y distance between the two bulks X and Y. The relaxation of the fast mode is incorporated using the theory rationalized by Rosh and Ratner. Fourier transform of the autocorrelation function of the dipole moment operator of the fast mode is employed to determine the IR spectral density. The evolution of the infrared absorption spectrum as well as the evolution of the energy surface potential from weak to strong H-bonds are demonstrated. Realistic evolution of the spectral density is found when weak anharmonic coupling parameters between the fast and slow modes are used. Altogether, our findings highlight the importance of the usage of crossed Morse potentials sampling in the quantum dynamics simulations of the IR spectral density of hydrogen bonded systems with different strengths.