Proton Magnetic Resonance Studies of Pentaerythritol
- 15 April 1969
- journal article
- Published by AIP Publishing in The Journal of Chemical Physics
- Vol. 50 (8) , 3595-3605
- https://doi.org/10.1063/1.1671591
Abstract
Proton magnetic resonance (PMR) studies of pentaerythritol, C(CH2OH)4, have revealed two line-narrowing processes in the solid. One, at temperatures just below the transition to the “plastic” phase, is associated with a rather general reorientation of molecules about their centers of gravity, with activation energy ER = 25.2 ± 1.0 kcal/mole. In reorienting, the molecules must overcome the considerable energy of intermolecular hydrogen bonds. The second process corresponds to molecular self-diffusion in the plastic phase with activation energy ED = 24.3 ± 0.5 kcal/mole. Spin–lattice relaxation studies indicate that impurity diffusion may be occurring in the pentaerythritol lattice and that the low-temperature line narrowing may be the result of a succession of small amplitude, lower-energy processes which lead to and culminate in the 25-kcal/mole general molecular reorientation. The rigid-lattice second moment (22.9 G2) agrees within experimental error with the value calculated from Hvoslef's neutron-diffraction studies (after his proton coordinates have been corrected for a tabulation error). The PMR measurements of ER and ED for pentaerythritol and eight different molecular crystals are compared to thermal parameters and correlated with the Pople–Karasz theory of melting for molecular crystals. Of the nine substances, only two (phosphorous, P4, and pentaerythritol) fail to give qualitative agreement with the Pople–Karasz theory. The dissimilarity of P4 with the other molecules and the intermolecular hydrogen bonding of pentaerythritol are assumed to be the sources of disagreement for these two substances.Keywords
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