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Molecular motion and transitions in solid tripalmitin measured by deuterium nuclear magnetic resonance
Author(s) -
Eads T. M.,
Blaurock A. E.,
Bryant R. G.,
Roy D. J.,
Croasmun W. R.
Publication year - 1992
Publication title -
journal of the american oil chemists' society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.512
H-Index - 117
eISSN - 1558-9331
pISSN - 0003-021X
DOI - 10.1007/bf02541038
Subject(s) - chemistry , orthorhombic crystal system , motional narrowing , triclinic crystal system , deuterium nmr , nuclear magnetic resonance , deuterium , methylene , relaxation (psychology) , crystallography , solid state nuclear magnetic resonance , nuclear magnetic resonance spectroscopy , nmr spectra database , quadrupole splitting , crystal (programming language) , spectral line , tripalmitin , crystal structure , atomic physics , stereochemistry , physics , organic chemistry , psychology , social psychology , mössbauer spectroscopy , astronomy , biochemistry , computer science , programming language
Deuterium nuclear magnetic resonance (NMR) quadrupole echo spectra of deuterated acyl chains and the glyceryl moiety of tripalmitin were found to depend on the crystal form. At 20°C, which is below melting points, line shapes indicate that molecular motions in the β form (triclinic subcell) are more restricted than in β′ (orthorhombic) or α (hexagonal). Motional rates in excess of about 20 kHz are responsible for the line shapes. Spin‐lattice relaxation, sensitive to motional frequencies in the tens of megahertz range, is much faster for methyl (CD 2 ) groups in α or β′ than in β , indicating that fast motions are also governed by crystal form. General theoretical considerations suggest that motion of methylene groups is dynamically heterogeneous and that motion of methyl (CD 3 ) groups may be averaged by motions other than rotation about the terminal C‐C bond. The isothermal solid‐state transition from α to β , induced by increasing the temperature to 35°C, was accompanied by NMR lineshape changes consistent with immobilization. The reversible transition of α to “sub‐ α ” upon cooling, accompanied by orthorhombic‐like Bragg spacings and other changes in the X‐ray pattern and by corresponding changes in the infrared spectrum, also produced a marked restriction in NMR‐detected mobility of the kind seen in β′ relative to α . The advantages of 2 H NMR for studies of motions and transitions in solid glycerides are discussed.