ISSN:
1572-8943
Schlagwort(e):
chiral molecule
;
conformational disorder and motion
;
crystal
;
DSC
;
heat capacity
;
γ-gauche effect
;
glass
;
glass transition
;
melting transition
;
molecular mechanics computations
;
tetra[methyleneoxycarbonyl (2,4,4-trimethyl) pentyl] methane
;
solid state13C NMR
;
X-ray diffraction
Quelle:
Springer Online Journal Archives 1860-2000
Thema:
Chemie und Pharmazie
Notizen:
Abstract The symmetric neopolyol ester tetra[methyleneoxycarbonyl(2,4,4-trirnethyl)pentyl]methane (MOCPM) has been studied by variable-temperature solid-state13C NMR and X-ray powder diffraction and compared to molecular mechanics calculations of the molecular structure. Between melting and glass transition temperatures the material is semicrystalline, consisting of two conformationally and motionally distinguishable phases. The more mobile phase is liquid-like and is, thus attributed to an amorphous phase (≈16%). The branches of the molecules in the crystal exhibit two conformationally distinguishable behaviors. In one, the branches are well ordered (≈56%), in the other, the branches are conformationally disordered (≈28%). Different branches of the same molecule may show different conformational order. This unique character of the rigid phase is the reason for the deficit of the entropy of fusion observed earlier by DSC. In the melt, solid state NMR can identify two bonds that are rotationally immobile, even though the molecules as a whole have liquid-like mobility. This partial rigidity of the branches accounts quantitatively for the observed increase in heat capacity at the glass transition. The reason for this unique behavior of MOCPM, a small molecule, is the existence of one chiral centers in each of the four arms of the molecule. A statistical model assuming that at least two of the chiral centers must fit into the order of the crystal can explain the crystallization behavior and would require 12.5% amorphous phase, 28.1% conformational disorder, and 59.4% crystallinity, close to the observed maximum perfection.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1007/BF01983624
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