Poly chemical shift calculation

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... 

Figure 30 Molecular structure and hydrogen bonding network of poly (vinylphosphonic acid) (PVPA) as used in the first-principles molecular dynamics simulations and chemical shift calculations. Intra- and inter-molecular hydrogen bonds are identified. Reproduced with permission from Lee, Y. J. Murakhtina, T. Sebastiani, D. Spiess, H. W. J. Am. Chem.

The ability of NMR to distinguish poly types will be greatly aided, as it was in the recent case discussed of SiC, by theoretical calculations of chemical shifts and CS A values (and for quadrupolar nuclei, NQCC values). Also, MAS-NMR may succeed in identifying low levels of polytypes or new ones that have not been seen by diffraction methods. 

Table 3. The comparison of calculated and observed chemical shifts for methylene and methine carbons in poly(ethylene-co-p-methylstyrene) and poly(ethylene-co-styrene).

The 13C chemical shift contour map for the Cp carbon of the L-alanine residue in peptides and polypeptides was made as a function of the dihedral angles(, W) by using the experimental data. Also, the corresponding calculated map was made by using the ab initio coupled Hartree-Fock method with the gauge included atomic orbitals(GIAO-CHF). From these results, it was found that the calculated map explains the chemical shift behavior of the a-helix and p-sheet forms in poly(L-alanine) and some proteins. This suggests that the calculated map is applicable to the structural analysis of proteins with complicated structure. 

From these results, it was found that the calculated map explains well the chemical shift behavior of the a-helix and p-sheet forms in poly(L-Ala) and some proteins. This suggests that the calculated map is applicable to the structural analysis of proteins with complicated structure. 

Recently, it was found that poly(phospholes) provide an even better perspective for such devices (06CRV4681). Indeed, the calculated aromaticity of the unsubstituted phosphole is quite low in comparison with that of pyrrole in terms of aromatic stabilization energy (ASE) and nucleus-independent chemical shift (NICS) pyrrole, ASE = 25.5 (kcal/mol), NICS =—15.1 (ppm) phosphole, ASE 7.0 (kcal/mol), NICS = -5.3 (ppm) (06CRV4681). 

Fig. 24. Experimental and simulated ( H)- C cross-polarization spectra of poly-5-caprolactone nanospheres in aqueous suspension after 12 h freezing. The calculated spectrum reproduces the carbon spectmm of four different methylene groups based on the chemical shift anisotropy listed in Table 1. It represents a superposition of two fractions of different isotropic mobility (80% T=0.1 ms 20% r=0.007 ms).

Figure 28 shows the two-dimensional H- V HETCOR NMR spectra of poly(L-prolines) (A) PPI and (B) PPII. The solid-state H- C HETCOR NMR measurements were performed on a Bruker DSX 300 spectrometer operating at 300 MHz, equipped with a 4 mm CP-MAS probe. The pulse sequence proposed by Burum et alJ was utilized for HETCOR, and the BLEW-24 pulse sequence was used for homonuclear decoupling of H. The 7r/2 pulse width was 2.8 ps for both C and H under CP conditions. The spinning speed was set to 5.0 kHz. The H chemical shift using BLEW-24 was calculated with a scaling factor 0.29 for all samples, which is a reasonable value. 

Fig. 19. (a) Observed 15N chemical shift diagram of poly(L-alanine) in the solid state, (b) Calculated 15N shielding diagram of A-acetyl-r.-alanine methylamide (taking hydrogen bonds with two formamide molecules), as a dipeptide model of poly(L-alanine), by means of the FPT-INDO method. 

NMR chemical shift prediction by quantum chemical calculation and the 7-gauche effect method has become a useful approach for stereochemical assignment of vinyl polymers. Recently C NMR assignments based on 7-effect and the rotational isomeric model were demonstrated successfully for a series of polyolefins, poly(l-pentene) to poly(l-octene).188... 

Fig. 17.4. (a) The calculated stick spectra of n-Si H2 + 2 (w = ca. 7-15) with various conformations by the FPT CNDO/2 method and (b) the observed stick spectra of poly(methylphenylsi-lane) in the solid state. The chemical shift is converted with respect to that of the TT conformation. The positive sign in the 5-scale means a high frequency shift. The stick height indicates the relative peak intensity. 

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