By Dr. R. Born, Prof. H. W. Spiess (auth.)
The elucidation of the constitution of amorphous platforms is without doubt one of the such a lot tough difficulties of condensed-matter learn. Our lack of awareness in regards to the certain constitution of disordered structures at the molecular point contrasts with the speedily becoming significance of such fabrics in technology and expertise. reliable country NMR is of key value during this sector, because it grants targeted information regarding internuclear distances and torsional angles in crystals and glasses alike. universal examples of amorphous platforms are glassy polymers. There, structural details is encoded within the chemical shifts of 13C-NMR spectra.
This quantity offers a robust new method of get to the bottom of this knowledge via combining polymer statistical types, quantum chemistry and strong nation NMR. It experiences the current cutting-edge, studies formerly unpublished effects, and shows methods for extra improvement within the future.
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Additional info for Ab Initio Calculations of Conformational Effects on 13C NMR Spectra of Amorphous Polymers
Local Origin/Local Gauge LORG has been developed by Hansen and Bouman , and like IGLO uses gauge dependent localized molecular orbitals. It differs from IGLO in a subtle way by violating a stationarity condition . Some new results are presented in . The three concepts (IGLO, GIAO, LORG) have been first implemented on the Hartree-Fock level; effects of electron correlation have been neglected. In the past few years, beyond-Hartree-Fock extensions have been developed: a Multi-Configuration Self-Consistent Field (MC-SCF) version ofIGLO [40,42], a perturbation treatment of correlation effects for GIAO [59, 60] and LORG .
Oc> 0 40 35 c5 [ppm] 30 Fig. 17. The methylene group in aPP displays a y-gauche effect as the chemical shift fJ is correlated with 1 8t 1 + 1 8-4 I. The seven horizontally aligned clusters stem from different combinations of the rotational isomeric states. 0 "U 0 240 180 ~ 120 + 60 0 0 0 0 0 000 0 «> 0 0 0000 (I) 0 0 00 38 36 34 15 [p pm] Fig. 18. For the cluster belonging to Uh = t, ih. = *, ~ = *,84 = t), the chemical shift of the methylene residue is correlated with I ih. I + I ~ I (a). , a, are relevant for the chemical shift.
This assertion is at variance with the observed experimental solid state spectra, that display broad resonances rather than sharp peaks (see Introduction). In Sects. 3 and 5 it is shown by quantum chemical calculations, that Eqs. 48) hold only approximately and have to be modified considerably for real systems. In fact, for polyCmethyl methacrylate) (PMMA) a complete breakdown of the y-gauche effect will be reported. In spite of the shortcomings of the phenomenological Eqs. 48), they can be used for a successful explanation of the configurational splitting in solution spectra [62,4,8].