A Raman spectroscopic temperature study of NH3+ torsional motion as related to hydrogen bonding in the L‐alanine crystal

Abstract

AbstractA Raman study of the NH3+ torsional mode in the amino acid L‐alanine has been performed. The temperature dependence of this mode has been followed in the temperature region 110–350 K. The activation energy for NH3+ reorientation obtained from a linewidth vs temperature investigation, Ea = 3100±400 cm−1, is in excellent agreement with corresponding NMR results, thus demonstrating that Raman spectroscopy can serve as a complementary method to NMR in obtaining torsional barrier heights. Based on an X‐ray investigation, crystal and hydrogen bond parameters have been calculated as a function of temperature. The increase of the NH3+ torsional barrier on going to lower temperatures is essentially attributed to the shortening of one specific hydrogen bond. The detection of the NH3+ torsional hot transition is important for the determination of the shape of the lower portion of the NH3+ torsional potential. The sixfold term V6 of the potential used, three‐plus sixfold, is thus very sensitive to the frequency difference between the NH3+ torsional fundamental and hot band. The optimum frequency fit gives, however, too large a value for the barrier height V3. The reason for this is interpreted as arising from the severe distortion of the hydrogen bonds at large torsional angles. The results presented here support recent findings that a revision of the dynamic picture of the hydrogen bonds in L‐alanine crystal, as reported in two earlier papers, is needed.