Self-trapped N-H vibrational states in the polymorphs of glycine, L- and DL-alanine

The polarized Raman spectra of the oriented single crystals of L- and DL-alanine, alpha-, beta- and gamma-polymorphs of glycine have been studied at 3-300 K. Regularly spaced band packets have been observed in the spectral range of 2500-3000 cm(-1), with intensity decreasing noticeably on heating. These band packets were interpreted as the manifestations of the existence of N-H self-trapped states in these systems at low temperatures. The analysis of the polarized spectra has shown that the self-trapping is observed exclusively for the NH stretching vibration of the amino groups, which is related to the NH center dot center dot center dot O hydrogen bonds along the head-to-tail chains of zwitterions in the crystal structures. The wavenumber of this NH stretching vibration, however, was proposed to depend not solely on the length of this NH center dot center dot center dot O hydrogen bond, but also on the lengths of all the other NH center dot center dot center dot O hydrogen bonds formed by the NH3+ and the COO- groups in the structure linking the head-to-tail chains with each other. The arguments in favor of the hypothesis that the self-trapping in these systems can be mediated by zero-point quantum motions, and not by lattice phonons, are considered. The unusually low wavenumber (2500 cm(-1)) observed for the NH stretching vibration and indicating at the formation of a very strong NH center dot center dot center dot O bond is interpreted based on considering the effect of the crystalline environment on the formation and properties of the NH center dot center dot center dot O bonds in the head-to-tail chains of amino acid zwitterions. The results are interesting for understanding the factors determining the dynamics and structural instability of crystalline amino acids and also for biophysical chemistry, as the hydrogen bonded chains formed by amino acid zwitterions in the crystals can mimic the peptide chains. Copyright (C) 2009 John Wiley & Sons, Ltd.