Low-temperature Fourier transform infrared spectra and hydrogen bonding in polycrystalline l-alanine

Abstract

The 400 Á/4000 cm (1 FTIR spectra of pure NH and isotopically substituted (10 and 90% doped ND/NH) polycrystalline L-alanine were recorded in the temperature range 10 Á/300 K. The observed temperature dependence and isotopic shifts behavior enabled to identify, in the spectra of the doped crystals, three well-separated bands ascribable to either the NH or ND stretching vibrations associated with the three different types of hydrogen bonds existing in the crystal. The observed red shifts of these bands relative to the frequency of a reference ''free'' NH (or ND) stretching mode were found to correlate well with the H-bond distances found in the crystal and provide an indirect way of estimating the enthalpies associated with each type of H-bond found in the crystal. In the low-frequency deformation and torsional spectral region (below 2000 cm(1 ), several bands, which were found to be affected by isotopic substitution, were identified as belonging to the NH 3 ' group. Several bands show splitting at low temperatures, indicating the occurrence of a significant reorganization in the crystal structure, which with all probability results mainly from changes in the proton positions. Finally, the literature assignments of the IR spectra of both crystalline NH 3 ' and ND 3 ' L-alanine were revised taking into consideration their temperature dependence and behavior upon deuteration. #

Low-temperature infrared spectra and hydrogen bonding in polycrystalline dl-serine and deuterated derivatives

“…The red shift of n 1 and the blue shift of n 4 bands associated with a given H-bond proton correlate to each other as 10 À4 ½ðn 4 Þ 2 À ðn 0 4 Þ 2 and Dn 1 ¼ n 0 1 À n 1 is the red shift of the n 1 band (both n 1 and n 4 are expressed in cm À1 ) [36]. Taking the as n 0 1 and n 0 4 reference values respectively equal to 3450 and 1000 cm À1 , which correspond to rounded average values for free NH amine stretching and rocking vibrations in amines, amino acids and other compounds bearing amino or protonated amino groups [2,8,35,[37][38][39], the three out-of-plane uncoupled vibrations originated in individual NH protons are predicted to occur at 1131, 1144 and 1149 cm À1 . These frequencies can be associated with the three progressively stronger NHÁ Á ÁO bonds observed in the crystalline phase.…”

“…4C). However, the estimated frequency for the nOD vibration, that can be obtained from that predicted for nOH by using the isotopic shift relationship (nOH/nOD in the range 1.34-1.35 [2][3][4][5][6][7][8]), is ca. 2150 cm À1 , in excellent agreement with the average frequency of 2148 cm À1 , corresponding to the group of bands here assigned to the nOD uncoupled mode in the spectra of Ser90.…”

“…This expression was obtained taking into consideration data collected for a considerable number of H-bonded systems, including crystalline carbohydrates [12,13], nucleobases (cytosine [4], uracil and thymine [5]), nucleosides (cytidine [6], adenosine and uridine [7]) and several amino acids (glycine, alanine, glutamine, hystidine, tyrosine and threonine [2,8]). …”

“…In the last few years, we have been developing a novel experimental approach for systematic investigation of biologically relevant H-bonded systems in the solid state [1][2][3][4][5][6][7][8], which is based on the study of isotopically doped crystals at low temperature. In this technique, partially deuterated substituted molecules are diluted in crystalline samples of the compound at low 2-10% deuterium content (or unsubstituted molecules are embedded in crystals of the deuterated compound at the same doped concentration), and advantage is taken of working at a temperature of a few degrees Kelvin.…”

“…At low temperature (<30 K), the bandwidth associated with these decoupled vibrations changes typically from hundreds to tens of wavenumbers and, as a result, the spectral resolution of the IR spectra improves significantly, allowing the observation of proton-related bands ascribable to specific Hbonds in the crystal. This approach was previously applied successfully to study of the H-bonding structure in crystalline carbohydrates [12,13], nucleobases (cytosine [4], uracil and thymine [5]), nucleosides (cytidine [6], adenosine and uridine [7]) and several amino acids (glycine, alanine, glutamine, histidine, tyrosine and threonine [2,8]), and led to the establishment of very general correlations between the frequency shifts of the proton vibrations upon hydrogen-bond formation and the corresponding H-bond lengths and energies [1,3,8].…”