Rotational isomeric state calculating chemical shift

The conformational characteristics of PVF are the subject of several studies (53,65). The rotational isomeric state (RIS) model has been used to calculate mean square end-to-end distance, dipole moments, and conformational entropies. C-nmr chemical shifts are in agreement with these predictions (66). The stiffness parameter (5) has been calculated (67) using the relationship between chain stiffness and cross-sectional area (68). In comparison to polyethylene, PVF has greater chain stiffness which decreases melting entropy, ie, (AS ) = 8.58 J/(molK) [2.05 cal/(molK)] versus... 

A common way for calculating the chemical shift of different stereosequences is based on the Rotational Isomeric State (RIS) approximation (8). In this case the effect of the conformation on the chemical shift 8 of the magnetic carbon C is ascribed to changes in populations of rotamers differing only for the C-C(a)-C(P)-C(y) dihedral angle. It can be translated into the following equation. 

To predict the chemical shifts observed for the methyl carbons in a-PP (see Fig. 20.10), which show sensitivity to heptad stereosequences, we simply have to calculate the trans and gauche probabdities for the backbone bonds in each of the 36 heptad stereosequences. When this is carried out with the Suter—Floiy rotational isomeric state (RIS) conformational model for PP [29] and the resultant probabilities of finding CH3 in a gauche arrangement with its y-substituents (CHs) are multiplied by ycHs—ch = 5.2ppm, we obtain the 5CH3S shown as the stick spec-... 

Having assigned all heptad stereosequence dependent C NMR resonances in a-PP [11,13], integration of the resonances provides us with a detailed accounting of how much of each stereosequence is present. Such information is needed to test various statistical models of PP polymerization [18]. Furthermore, the close agreement between observed and calculated chemical shifts provides strong confirmation of the Suter-Flory rotational isomeric state (RIS) conformational model for PP [29]. 

NMR experiments and ab initio theoretical chemical shift calculations, combined with rotational isomeric state (RIS) statistics of the P-N chain, gave the first assignment of the NMR spectra of P-N copolymers [95-97]. Cis-2,3-exo norbomene insertion is considered to occur into the metal-carbon bond as in... 

The configurational structure (stereoregularity) of 1-butene and of the higher polyolefins up to 1-nonene has been studied by NMR spectroscopy in solution [38, 39], interpreted with the aid of chemical shift calculations, consideration of the y effect and of the rotational isomeric state model of Flory. The evaluation of the results favors the bicatalytic sites model of polymerization [40] over simple Markovian statistics. In contrast to polypropylene, side-chain conformation also has to be considered. Comparison with alkane model compounds indicates that in meso-units of poly-1-butene, trans conformation of backbone is less favored than in isotactic polypropylene because of contiguous ethyl group interactions. Introduction of racemic units in both... 

The chemical shift calculation (y-effect method) based on the y-effect of the chemical shift and the rotational isomeric state model (RIS model) has been developed as a reliable method for predicting chemical shift differences among pentad, hexad, and heptad sequences in various polyolefins [47-49, 14, 50, 51]. chemical shift assignments of tactic pentad and heptad sequences in polypropylene have been provided by this method [47-49]. Hayashi and co-workers [45,46] confirmed that the chemical shift due to the y-effect is also sensitive to different comonomer sequences in ethylene-propylene copolymers. Asakura and co-workers [52] have demonstrated that... 

Aoki and co-workers [46] confirmed previous assignments of triad and tetrad sequences in this copolymer. Referring to the confirmed assignments, the chemical shift differences among comonomer sequences longer than pentad were predicted by the chemical shift calculation (the y-effect method) based on the y-effect of the chemical shift and Mark s rotational isomeric state model modified by considering the side-chain conformation in a 1-butene unit. Assignments provided in this study... 

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