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14 N NMR spectroscopy of NH 4 + as a probe of ion‐exchange membranes
Author(s) -
Rankothge M.,
Hook J.,
van Gorkom L.,
Moran G.
Publication year - 1994
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1260320803
Subject(s) - chemistry , membrane , nafion , aqueous solution , ion , analytical chemistry (journal) , relaxation (psychology) , nuclear magnetic resonance spectroscopy , spectroscopy , spin–lattice relaxation , crystallography , ion exchange , nuclear magnetic resonance , stereochemistry , chromatography , organic chemistry , electrochemistry , psychology , social psychology , biochemistry , physics , electrode , quantum mechanics , nuclear quadrupole resonance
14 N NMR Spectroscopy of NH 4 + is a convenient and sensitive probe of structure and mobility in Nafion, Dow and Neosepta cation‐exchange membranes. NH 4 + ions bound to these commercially produced membranes give rise to small 14 N quadrupolar splittings ranging from 10 to 160 Hz. Moderate stretching of Nafion and Dow membranes induces much larger splittings of up to 2 kHz. The splitting has also been found to be dependent on the orientation of the membranes in the magnetic field B 0 . In Nafion it reaches a minimum at an angle of 45–55° between the direction of stretch and B 0 . The quadrupolar splittings also increase with increase in temperature. 14 N spin–lattice relaxation times in all three membranes at 300 K are about 20 times shorter than in aqueous solution, consistent with strongly bound NH 4 + ions. The apparent activation energies for 14 N relaxation, derived from the temperature dependence of the T 1 , data, show that reorientation of the bound NH 4 + ions requires ca. 20 kJ mol −1 in Nafion and Dow membranes and ca. 30 kJ mol −1 in Neosepta, compared with 6.5 kJ mol −1 for free NH 4 + in aqueous solution.