By Dr. R. Born, Prof. H. W. Spiess (auth.)
The elucidation of the constitution of amorphous structures is likely one of the so much tough difficulties of condensed-matter learn. Our lack of know-how in regards to the distinct constitution of disordered structures at the molecular point contrasts with the speedily starting to be value of such fabrics in technological know-how and know-how. stable country NMR is of key significance during this zone, because it grants specific information regarding internuclear distances and torsional angles in crystals and glasses alike. universal examples of amorphous structures are glassy polymers. There, structural info is encoded within the chemical shifts of 13C-NMR spectra.
This quantity offers a robust new method of get to the bottom of this data through combining polymer statistical versions, quantum chemistry and stable country NMR. It experiences the current state-of-the-art, stories formerly unpublished effects, and shows methods for additional improvement within the future.
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Additional info for Ab Initio Calculations of Conformational Effects on 13C NMR Spectra of Amorphous Polymers
The y-gauche effect is only partly steric in origin (Fig. 20) since the combined distances of the y-neighbors do not fully explain the calculated chemical shift; the correlation of t5 with the y-neighbor distances is far from being perfect. One of the tacit assumptions of the phenomenological parametrization of the y-gauche effect is the strictly tetrahedral coordination of the carbon atoms involved. In reality, the bond angle 11' depends on the two neighboring dihedral angles and widens if these change from a trans to a gauche position (Fig.
21. The bonding situation in poly(propylene) strongly deviates from ideal tetrahedral coordination, as the bond angle tp deviates from the textbook value tp = 1090 and moreover depends on the dihedral angles 8) and 8']. in a systematic fashion (a). t) (Lg)(g .. g) (g .. t) 60 50 40 30 o [ppm] 20 10 Fig. 22. By correlating the theoretical chemical shifts with the underlying geometries, the subpeaks of the experimental spectrum can be assigned. The dots symbolize unspecified dihedral angles (t ..
E. the geometry dependence is essentially confined to these four molecular orbitals. Because of the correlation of the separate MO contributions with the total chemical shift, all of them display to a certain degree the same geometry dependence (Fig. c, for the y-gauche effect).