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Proton and carbon‐13 NMR relaxation studies of 2,2‐dichloropropane and 2,2‐dibromopropane
Author(s) -
Aksnes D. W.,
Ramstad K.
Publication year - 1987
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.1260250616
Subject(s) - chemistry , arrhenius equation , activation energy , proton , relaxation (psychology) , spin–lattice relaxation , spin diffusion , proton nmr , atmospheric temperature range , proton spin crisis , crystallography , diffusion , thermodynamics , stereochemistry , psychology , social psychology , physics , quantum mechanics
Proton and 13 C line widths and spin‐lattice relaxation times, T 1 , have been measured for liquid and solid 2,2‐dichloropropane (1) and 2,2‐dibromopropane (2). Molecular self‐diffusion, with activation energies in the range 18‐24 kJ mol −1 , is responsible for the considerable line narrowing observed for the NMR spectra in solid I of these substances. The NMR data reveal a previously unreported phase transition point at 208 K for 2. An analysis of the proton and 13 C T 1 times in both the liquid and solid phases is reported. Effective correlation times, activation energies and pre‐exponential factors for the appropriate motions have been obtained from the Arrhenius equation. The proton and 13 C spin‐lattice relaxation mechanisms in the liquid and solid I phases are governed by overall molecular reorientations, with activation energies in the range 9–11 kJ mol −1 . The rotational correlation time increases slightly at the liquid → solid I boundary from 3.2 to 3.6 ps for 1 and from 5.1 to 5.4 ps for 2. The correlation time is observed to decrease by five orders of magnitude at the solid I → solid II transition point of 2. The dominant motion causing spin‐lattice relaxation in phase II is methyl group reorientation with an activation energy of 18 kJ mol −1 .