Theoretical elucidation of the IR spectra of 4-bromo-3, 5-dimethylpyrazole crystals and their deuterium-bonded analogues: Multi-objective analysis from a quantum modeling perspective.

In the field of medicine, compounds that contain pyrazole and their derivatives are recognized as some of the earliest anti-inflammatory and analgesic agents. Consequently, the development of novel pyrazole-containing synthons represents a promising avenue in the quest for new biologically active substances. The supramolecular aspect plays a crucial role in comprehending the chemistry of pyrazoles. The primary driving force behind the self-assembly of pyrazolyl molecules is attributed to hydrogen bonds. In this context, we propose a quantum theoretical approach to hydrogen bonding in order to elucidate the infrared (IR) spectra of 4-bromo-3,5-dimethylpyrazole (4-Br,3,5-DMPz) crystals. Our ultimate objective is to elucidate the hydrogen-bond dynamics and illustrate the main mechanisms that govern the generation of the IR band contours. The approach addresses the impact of the anharmonic vibrational coupling within the dimeric units, the Davydov coupling and the combined influences of damping mechanisms on theυSX-H IR band contours. The exchange interaction between the two hydrogen bonds of the dimeric units is analyzed through the non-adiabatic coupling framework. The direct damping mechanism is addressed within the theoretical framework established by Rösch and Ratner, while the indirect damping associated with the two hydrogen-bonded bridges is integrated using non-Hermitian Hamiltonians. The contours of the IR bands are obtained through linear response theory by applying a Fourier transform to the dipole moment operator autocorrelation function associated with the high-frequency mode. This theoretical framework reverts to the model proposed by Maréchal and Witkowski regarding Davydov coupling in damping lack, and aligns with the foundational quantum approach of indirect damping when Davydov coupling is not present. The methodology is applied to the study the IR spectra of crystals of 4-Br,3,5-DMPz and their deuterium-bonded analogues at 293 K and 77 K. Numerical simulations demonstrate that the congregated influence of Davydov coupling, the linear interaction within the dimeric units, and the various damping mechanisms can effectively account for the intricate characteristics of the experimental IR band contours. The notable distinctions identified in the fine structure patterns of theυSX-H(D) band contours, along with the temperature effects in these crystals, have been clarified and interpreted from a physical perspective.