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A Raman spectroscopic temperature study of NH 3 + torsional motion as related to hydrogen bonding in the L‐alanine crystal
Author(s) -
Forss S.
Publication year - 1982
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.1250120313
Subject(s) - raman spectroscopy , chemistry , hydrogen bond , crystal (programming language) , activation energy , hydrogen , deuterium , laser linewidth , crystallography , crystal structure , analytical chemistry (journal) , nuclear magnetic resonance , atomic physics , molecule , physics , optics , laser , organic chemistry , chromatography , computer science , programming language
A Raman study of the NH 3 + 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 NH 3 + reorientation obtained from a linewidth vs temperature investigation, E a = 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 NH 3 + torsional barrier on going to lower temperatures is essentially attributed to the shortening of one specific hydrogen bond. The detection of the NH 3 + torsional hot transition is important for the determination of the shape of the lower portion of the NH 3 + torsional potential. The sixfold term V 6 of the potential used, three‐plus sixfold, is thus very sensitive to the frequency difference between the NH 3 + torsional fundamental and hot band. The optimum frequency fit gives, however, too large a value for the barrier height V 3 . 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.