H/D Isotopic Recognition in Hydrogen-Bonded Systems: Strong Dynamical Coupling Effects in the Polarized IR Spectra of 3-Methylthioacetanilide and 4-Methylthioacetanilide Crystals

Abstract

This paper presents the investigation results of the polarized IR spectra of the hydrogen bond in crystals of 3- and 4-methylthioacetanilide. The spectra were measured at 293 and 77 K by a transmission method, with the use of polarized light. The main spectral properties of the crystals can be interpreted satisfactorily in terms of the "strong-coupling" theory, on the basis of the hydrogen bond centrosymmetric dimer model. The spectra revealed that the strongest vibrational exciton coupling involved the closely spaced hydrogen bonds, each belonging to a different chain of associated 3- and 4-methylthioacetanilide molecules. A weaker exciton coupling involved the adjacent hydrogen bonds in each individual chain. It was proven that a nonrandom distribution of the protons and deuterons took place in the lattices of isotopically diluted crystalline samples of 3- and 4-methylthioacetanilide. In each case, the H/D isotopic "self-organization" mechanism involved all four hydrogen bonds from each unit cell.

Towards a better comprehension of interactions in the crystalline N-acetylbenzylamine and its sulphur analogue N-benzyl-ethanethioamide. IR, Raman, DFT studies and Hirshfeld surfaces analysis

“…The single crystals of (1) and (2) have been obtained by crystallization from a melted substance (melting point of compound (1) and ( 2) is 333K [20] and 338-339K [21], respectively), which appears between two closely CaF 2 windows. The details of the experimental techniques of the preparing and the selection of single crystals from a mosaic are given in References [22][23][24][25][26]. Measurements of the spectra of monocrystalline fragments have been performed for two different orientations of the electric field vector "E" (polarization 0° and 90°).…”