Polarized Infrared Spectra of the H(D) Bond in 2‐Thiophenic Acid Crystals: A Spectroscopic and Computational Study

Abstract

Polarized IR spectra of the hydrogen bond in 2-thiophenic acid crystals, isotopically neat and of mixed H/D isotopic content, are measured at 298 and 77 K in the "residual" nuO-H and nuO-D band frequency ranges. This crystalline system provides spectra in these band frequency ranges that differ considerably in intensity distribution from the spectra of other H-bonded centrosymmetric dimeric species. This change in the spectral properties of the crystals is probably due to the influence of the sulfur atoms from the thiophene aromatic rings, which are directly linked to the (COOH)2 or (COOD)2 cycles. The magnitude of this effect correlates with the net electronic charge distribution at the 2- and 3-positions of substituted thiophene rings, which in a different way influences the electron charge density in the hydrogen bonds of the two thiophenic acid isomers. The experimental results for spectral structures are compared to predictions obtained with theoretical calculations involving the combined effects of anharmonicities, Davydov coupling, Fermi resonances, and direct and indirect relaxations within the framework of the linear response theory. Numerical results show that mixing of all these effects allows satisfactory reproduction of the main features of the experimental IR line shapes of crystalline H- and D-bonded 2-thiophenic acid at room and liquid-nitrogen temperatures.

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.

“…We have selected for the dissociation energy of the H-bond bridge, the value D e ¼2417 cm À 1 (then β e ¼0.2175) for all dimers. This value is similar to those used previously in several computational studies of H-bonded dimers [15,17,64].…”

“…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.…”

“…It is surprising that spectra of cyclic hydrogen bond dimers in arylcarboxylic [15][16][17][18], arylacrylic acid [31,32] 3-hydroxy-4-methyl-2(3 H)-thiazolethione [38], 2-thiopyridone [39] and 2-pyridone [40] crystals fairly resemble by their intensity distribution patterns of the υ N-H bands the spectra of chain hydrogen bond systems in a particular group of molecular crystals, e.g., of pyrazole [41] and 4-thiopyridone [42].…”

“…b. In the case arylcarboxylic [15][16][17][18] and arylacrylic acids [31,32] dimers for which carboxyl groups are directly linked to π-electronic systems, the υ O-H band contours resemble a "mirror reflection" of the υ O-H band shapes of systems from the point "a". In this case the lower-frequency branch of the band is the most intense one.…”

“…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.…”