Effects of strong inter-hydrogen bond dynamical couplings in the polarized IR spectra of adipic acid crystals

Elucidating the Davydov-coupling mechanism in hydrogen bond dimers: Experimental and theoretical investigation of the polarized IR spectra of 3-thiopheneacetic and 3-thiopheneacrylic acid crystals.

“…In this case the exciton coupling energy is of the positive sign [29]. The sequence and the properties of the branches in the proton stretching vibration bands in the discussed case, is reverse to those observed in the IR spectra of the majority of hydrogen bond cyclic dimeric systems [19,20].…”

“…In the case of carboxylic acid molecules in which the aliphatic fragments are connected directly with carboxyl groups [19,20], the υ O-H and υ O-D bands differ by their intensity distribution patterns, when compared with the corresponding band properties in the IR spectra of arylcarboxylic acids [15][16][17][18]. In the first case the higher-frequency branch of the υ O-H band is more intense in relation to the intensity of the lower-frequency band branch.…”

“…Similar property characterizes spectra of other carboxylic acids with large π-electronic systems in their molecules, e.g., cinnamic acid [32], 2-naphthoic acid [16] and 1-naphthylacrylic acid [33]. c. For other carboxylic acid dimers, in which aromatic radicals are separated from carboxyl groups by fragments of aliphatic hydrocarbon chains (e.g., arylacetic acids [33,34] and styrylacetic acid [35], the υ O-H band contour shapes fairly resemble the corresponding band contours from the point "a", i.e., the corresponding spectra of aliphatic carboxylic acids [19,20],…”

“…For instance, π-electronic systems of aromatic rings or other larger conjugated π-electronic systems, linked directly to carboxyl groups, may strongly change the spectral properties of carboxylic acid dimers in crystals [15][16][17][18] in comparison with the analogous properties of aliphatic carboxylic acid dimers formed in the solid-state [19,20]. It seems that the generation mechanism of these effects still remains obscure.…”

Long-distance inter-hydrogen bond coupling effects in the polarized IR spectra of succinic acid crystals

“…According to the familiar fact that the inner structures of carboxylic acid crystals most frequently contain cyclic hydrogen-bonded molecular dimers, formed by associated carboxyl groups, those molecular crystals comprise to be the optimum group of chemical compounds for the systematic study of interactions between hydrogen bonds involved in the dimeric structures. According to the tendency of aliphatic dicarboxylic acid molecules to form crystals characterized by well-defined physical properties they are considered to be interesting subjects for the spectral studies in IR of hydrogenbonded cyclic dimeric systems [6][7][8][9][10][11]. In contrast, for monocarboxylic acids, the low-molecular weight homologous compounds are liquids, while those including long-chain alkyl radicals are of a wax-like consistence.…”