Proton Magnetic Resonance Studies of Pentaerythritol

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 $E_R\text{\hspace{0.17em}=\hspace{0.17em}25.2\hspace{0.17em}±\hspace{0.17em}1.0}\mathrm{kcal}/\mathrm{mole}$ . 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 $E_D\text{\hspace{0.17em}=\hspace{0.17em}24.3\hspace{0.17em}±\hspace{0.17em}0.5}\mathrm{kcal}/\mathrm{mole}$ . 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 $E_R$ and $E_D$ 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.