Deciphering the abnormal IR spectral density of phthalic acid dimer crystals: Unveiling the role of the dynamical effects of the Davydov coupling and the mechanisms of relaxation.

To consistently determine the anomalous characteristics of phthalic acid crystal (PAC) derivatives, we performed quantum dynamics simulations of the infrared spectral density of the h-PAC and d-PAC isotopomers that show up in the H/D isotopic frequency domain at two different temperatures viz. 77 and 298 K. A theoretical framework explaining the dynamical cooperative interactions within the hydrogen bonds (HBs) in the PAC crystals across a simulation of IR spectral density of the stretching band was developed. The model presupposes HBs from the nearby (COOH) cycles to have exceptionally strong dynamical cooperative contacts. The approach precisely hinges upon on a model that consider the dimer as centrosymmetric in which the low-frequency intermonomer mode (that is, O⋯O) stretches anharmonically and committing the O-H vibrational mode, associated with the high-frequency mode, to stretch harmonically. The approach considers also the following effects: Davydov coupling arising from the interference between the two HBs in the dimer; strong anharmonic coupling between the two stretching modes within the same HB; direct relaxation mechanism of the high stretches in the HB; and indirect relaxation mechanism that is devoted to the high stretches via the slow stretches. Using this approach, the principal characteristics of the O-H(D) experimental bands of the isotopomers studied herein with their peculiarities are simulated by selecting physically sound parameters that uniquely identify the onset of the specific features of the IR spectral densities. The impacts of the important parameters, characterizing the different leading mechanisms within this approach on the spectra, are revealed. The contrast of the emerging simulated spectra with the experimental ones reveals decent agreement for the studied isotopomers. By congregating the different mechanisms employed herein within the pioneering context of the linear response theory of Kubo, the obtained results strongly demonstrate and identify the main effects responsible for the generation of the experimental O-H and O-D bands. Through an examination of the effect of each mechanism involved within the h -PAC and d-PAC isotopomers, it is possible to relate the emergence of IR spectral density to an interplay between the Davydov coupling and the relaxation dynamics.